/*
 * Copyright (C) 2015 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.android.car.view;

import android.content.Context;
import android.graphics.PointF;
import android.support.annotation.IntDef;
import android.support.annotation.NonNull;
import android.support.v7.widget.LinearSmoothScroller;
import android.support.v7.widget.RecyclerView;
import android.util.DisplayMetrics;
import android.util.Log;
import android.view.View;
import android.view.ViewGroup;
import android.view.animation.AccelerateInterpolator;
import android.view.animation.DecelerateInterpolator;
import android.view.animation.Interpolator;

import com.android.car.stream.ui.R;

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.util.ArrayList;

/**
 * Custom {@link RecyclerView.LayoutManager} that behaves similar to LinearLayoutManager except that
 * it has a few tricks up its sleeve.
 * <ol>
 *    <li>In a normal ListView, when views reach the top of the list, they are clipped. In
 *        CarLayoutManager, views have the option of flying off of the top of the screen as the
 *        next row settles in to place. This functionality can be enabled or disabled with
 *        {@link #setOffsetRows(boolean)}.
 *    <li>Standard list physics is disabled. Instead, when the user scrolls, it will settle
 *        on the next page. {@link #FLING_THRESHOLD_TO_PAGINATE} and
 *        {@link #DRAG_DISTANCE_TO_PAGINATE} can be set to have the list settle on the next item
 *        instead of the next page for small gestures.
 *    <li>Items can scroll past the bottom edge of the screen. This helps with pagination so that
 *        the last page can be properly aligned.
 * </ol>
 *
 * This LayoutManger should be used with {@link CarRecyclerView}.
 */
public class CarLayoutManager extends RecyclerView.LayoutManager {
    private static final String TAG = "CarLayoutManager";
    private static final boolean DEBUG = false;

    /**
     * Any fling below the threshold will just scroll to the top fully visible row. The units is
     * whatever {@link android.widget.Scroller} would return.
     *
     * A reasonable value is ~200
     *
     * This can be disabled by setting the threshold to -1.
     */
    private static final int FLING_THRESHOLD_TO_PAGINATE = -1;

    /**
     * Any fling shorter than this threshold (in px) will just scroll to the top fully visible row.
     *
     * A reasonable value is 15.
     *
     * This can be disabled by setting the distance to -1.
     */
    private static final int DRAG_DISTANCE_TO_PAGINATE = -1;

    /**
     * If you scroll really quickly, you can hit the end of the laid out rows before Android has a
     * chance to layout more. To help counter this, we can layout a number of extra rows past
     * wherever the focus is if necessary.
     */
    private static final int NUM_EXTRA_ROWS_TO_LAYOUT_PAST_FOCUS = 2;

    /**
     * Scroll bar calculation is a bit complicated. This basically defines the granularity we want
     * our scroll bar to move. Set this to 1 means our scrollbar will have really jerky movement.
     * Setting it too big will risk an overflow (although there is no performance impact). Ideally
     * we want to set this higher than the height of our list view. We can't use our list view
     * height directly though because we might run into situations where getHeight() returns 0, for
     * example, when the view is not yet measured.
     */
    private static final int SCROLL_RANGE = 1000;

    @ScrollStyle private final int SCROLL_TYPE = MARIO;

    @Retention(RetentionPolicy.SOURCE)
    @IntDef({MARIO, SUPER_MARIO})
    private @interface ScrollStyle {}
    private static final int MARIO = 0;
    private static final int SUPER_MARIO = 1;

    @Retention(RetentionPolicy.SOURCE)
    @IntDef({BEFORE, AFTER})
    private @interface LayoutDirection {}
    private static final int BEFORE = 0;
    private static final int AFTER = 1;

    @Retention(RetentionPolicy.SOURCE)
    @IntDef({ROW_OFFSET_MODE_INDIVIDUAL, ROW_OFFSET_MODE_PAGE})
    public @interface RowOffsetMode {}
    public static final int ROW_OFFSET_MODE_INDIVIDUAL = 0;
    public static final int ROW_OFFSET_MODE_PAGE = 1;

    public interface OnItemsChangedListener {
        void onItemsChanged();
    }

    private final AccelerateInterpolator mDanglingRowInterpolator = new AccelerateInterpolator(2);
    private final Context mContext;

    /** Determines whether or not rows will be offset as they slide off screen **/
    private boolean mOffsetRows = false;
    /** Determines whether rows will be offset individually or a page at a time **/
    @RowOffsetMode private int mRowOffsetMode = ROW_OFFSET_MODE_PAGE;

    /**
     * The LayoutManager only gets {@link #onScrollStateChanged(int)} updates. This enables the
     * scroll state to be used anywhere.
     */
    private int mScrollState = RecyclerView.SCROLL_STATE_IDLE;
    /**
     * Used to inspect the current scroll state to help with the various calculations.
     **/
    private CarSmoothScroller mSmoothScroller;
    private OnItemsChangedListener mItemsChangedListener;

    /** The distance that the list has actually scrolled in the most recent drag gesture **/
    private int mLastDragDistance = 0;
    /** True if the current drag was limited/capped because it was at some boundary **/
    private boolean mReachedLimitOfDrag;
    /**
     * The values are continuously updated to keep track of where the current page boundaries are
     * on screen. The anchor page break is the page break that is currently within or at the
     * top of the viewport. The Upper page break is the page break before it and the lower page
     * break is the page break after it.
     *
     * A page break will be set to -1 if it is unknown or n/a.
     * @see #updatePageBreakPositions()
     */
    private int mItemCountDuringLastPageBreakUpdate;
    // The index of the first item on the current page
    private int mAnchorPageBreakPosition = 0;
    // The index of the first item on the previous page
    private int mUpperPageBreakPosition = -1;
    // The index of the first item on the next page
    private int mLowerPageBreakPosition = -1;
    /** Used in the bookkeeping of mario style scrolling to prevent extra calculations. **/
    private int mLastChildPositionToRequestFocus = -1;
    private int mSampleViewHeight = -1;

    /**
     * Set the anchor to the following position on the next layout pass.
     */
    private int mPendingScrollPosition = -1;

    public CarLayoutManager(Context context) {
        mContext = context;
    }

    @Override
    public RecyclerView.LayoutParams generateDefaultLayoutParams() {
        return new RecyclerView.LayoutParams(
                ViewGroup.LayoutParams.MATCH_PARENT,
                ViewGroup.LayoutParams.WRAP_CONTENT);
    }

    @Override
    public boolean canScrollVertically() {
        return true;
    }

    /**
     * onLayoutChildren is sort of like a "reset" for the layout state. At a high level, it should:
     * <ol>
     *    <li>Check the current views to get the current state of affairs
     *    <li>Detach all views from the window (a lightweight operation) so that rows
     *        not re-added will be removed after onLayoutChildren.
     *    <li>Re-add rows as necessary.
     * </ol>
     *
     * @see super#onLayoutChildren(RecyclerView.Recycler, RecyclerView.State)
     */
    @Override
    public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
        /**
         * The anchor view is the first fully visible view on screen at the beginning
         * of onLayoutChildren (or 0 if there is none). This row will be laid out first. After that,
         * layoutNextRow will layout rows above and below it until the boundaries of what should
         * be laid out have been reached. See {@link #shouldLayoutNextRow(View, int)} for
         * more information.
         */
        int anchorPosition = 0;
        int anchorTop = -1;
        if (mPendingScrollPosition == -1) {
            View anchor = getFirstFullyVisibleChild();
            if (anchor != null) {
                anchorPosition = getPosition(anchor);
                anchorTop = getDecoratedTop(anchor);
            }
        } else {
            anchorPosition = mPendingScrollPosition;
            mPendingScrollPosition = -1;
            mAnchorPageBreakPosition = anchorPosition;
            mUpperPageBreakPosition = -1;
            mLowerPageBreakPosition = -1;
        }

        if (DEBUG) {
            Log.v(TAG, String.format(
                    ":: onLayoutChildren anchorPosition:%s, anchorTop:%s,"
                            + " mPendingScrollPosition: %s, mAnchorPageBreakPosition:%s,"
                            + " mUpperPageBreakPosition:%s, mLowerPageBreakPosition:%s",
                    anchorPosition, anchorTop, mPendingScrollPosition, mAnchorPageBreakPosition,
                    mUpperPageBreakPosition, mLowerPageBreakPosition));
        }

        /**
         * Detach all attached view for 2 reasons:
         * <ol>
         *     <li> So that views are put in the scrap heap. This enables us to call
         *          {@link RecyclerView.Recycler#getViewForPosition(int)} which will either return
         *          one of these detached views if it is in the scrap heap, one from the
         *          recycled pool (will only call onBind in the adapter), or create an entirely new
         *          row if needed (will call onCreate and onBind in the adapter).
         *     <li> So that views are automatically removed if they are not manually re-added.
         * </ol>
         */
        detachAndScrapAttachedViews(recycler);

        // Layout new rows.
        View anchor = layoutAnchor(recycler, anchorPosition, anchorTop);
        if (anchor != null) {
            View adjacentRow = anchor;
            while (shouldLayoutNextRow(state, adjacentRow, BEFORE)) {
                adjacentRow = layoutNextRow(recycler, adjacentRow, BEFORE);
            }
            adjacentRow = anchor;
            while (shouldLayoutNextRow(state, adjacentRow, AFTER)) {
                adjacentRow = layoutNextRow(recycler, adjacentRow, AFTER);
            }
        }

        updatePageBreakPositions();
        offsetRows();

        if (DEBUG&& getChildCount() > 1) {
            Log.v(TAG, "Currently showing " + getChildCount() + " views " +
                    getPosition(getChildAt(0)) + " to " +
                    getPosition(getChildAt(getChildCount() - 1)) + " anchor " + anchorPosition);
        }
    }

    /**
     * scrollVerticallyBy does the work of what should happen when the list scrolls in addition
     * to handling cases where the list hits the end. It should be lighter weight than
     * onLayoutChildren. It doesn't have to detach all views. It only looks at the end of the list
     * and removes views that have gone out of bounds and lays out new ones that scroll in.
     *
     * @param dy The amount that the list is supposed to scroll.
     *               > 0 means the list is scrolling down.
     *               < 0 means the list is scrolling up.
     * @param recycler The recycler that enables views to be reused or created as they scroll in.
     * @param state Various information about the current state of affairs.
     * @return The amount the list actually scrolled.
     *
     * @see super#scrollVerticallyBy(int, RecyclerView.Recycler, RecyclerView.State)
     */
    @Override
    public int scrollVerticallyBy(
            int dy, @NonNull RecyclerView.Recycler recycler, @NonNull RecyclerView.State state) {
        // If the list is empty, we can prevent the overscroll glow from showing by just
        // telling RecycerView that we scrolled.
        if (getItemCount() == 0) {
            return dy;
        }

        // Prevent redundant computations if there is definitely nowhere to scroll to.
        if (getChildCount() <= 1 || dy == 0) {
            return 0;
        }

        View firstChild = getChildAt(0);
        if (firstChild == null) {
            return 0;
        }
        int firstChildPosition = getPosition(firstChild);
        RecyclerView.LayoutParams firstChildParams = getParams(firstChild);
        int firstChildTopWithMargin = getDecoratedTop(firstChild) - firstChildParams.topMargin;

        View lastFullyVisibleView = getChildAt(getLastFullyVisibleChildIndex());
        if (lastFullyVisibleView == null) {
            return 0;
        }
        boolean isLastViewVisible = getPosition(lastFullyVisibleView) == getItemCount() - 1;

        View firstFullyVisibleChild = getFirstFullyVisibleChild();
        if (firstFullyVisibleChild == null) {
            return 0;
        }
        int firstFullyVisiblePosition = getPosition(firstFullyVisibleChild);
        RecyclerView.LayoutParams firstFullyVisibleChildParams = getParams(firstFullyVisibleChild);
        int topRemainingSpace = getDecoratedTop(firstFullyVisibleChild)
                - firstFullyVisibleChildParams.topMargin - getPaddingTop();

        if (isLastViewVisible && firstFullyVisiblePosition == mAnchorPageBreakPosition
                && dy > topRemainingSpace && dy > 0) {
            // Prevent dragging down more than 1 page. As a side effect, this also prevents you
            // from dragging past the bottom because if you are on the second to last page, it
            // prevents you from dragging past the last page.
            dy = topRemainingSpace;
            mReachedLimitOfDrag = true;
        } else if (dy < 0 && firstChildPosition == 0
                && firstChildTopWithMargin + Math.abs(dy) > getPaddingTop()) {
            // Prevent scrolling past the beginning
            dy = firstChildTopWithMargin - getPaddingTop();
            mReachedLimitOfDrag = true;
        } else {
            mReachedLimitOfDrag = false;
        }

        boolean isDragging = mScrollState == RecyclerView.SCROLL_STATE_DRAGGING;
        if (isDragging) {
            mLastDragDistance += dy;
        }
        // We offset by -dy because the views translate in the opposite direction that the
        // list scrolls (think about it.)
        offsetChildrenVertical(-dy);

        // The last item in the layout should never scroll above the viewport
        View view = getChildAt(getChildCount() - 1);
        if (view.getTop() < 0) {
            view.setTop(0);
        }

        // This is the meat of this function. We remove views on the trailing edge of the scroll
        // and add views at the leading edge as necessary.
        View adjacentRow;
        if (dy > 0) {
            recycleChildrenFromStart(recycler);
            adjacentRow = getChildAt(getChildCount() - 1);
            while (shouldLayoutNextRow(state, adjacentRow, AFTER)) {
                adjacentRow = layoutNextRow(recycler, adjacentRow, AFTER);
            }
        } else {
            recycleChildrenFromEnd(recycler);
            adjacentRow = getChildAt(0);
            while (shouldLayoutNextRow(state, adjacentRow, BEFORE)) {
                adjacentRow = layoutNextRow(recycler, adjacentRow, BEFORE);
            }
        }
        // Now that the correct views are laid out, offset rows as necessary so we can do whatever
        // fancy animation we want such as having the top view fly off the screen as the next one
        // settles in to place.
        updatePageBreakPositions();
        offsetRows();

        if (getChildCount() >  1) {
            if (DEBUG) {
                Log.v(TAG, String.format("Currently showing  %d views (%d to %d)",
                        getChildCount(), getPosition(getChildAt(0)),
                        getPosition(getChildAt(getChildCount() - 1))));
            }
        }

        return dy;
    }

    @Override
    public void scrollToPosition(int position) {
        mPendingScrollPosition = position;
        requestLayout();
    }

    @Override
    public void smoothScrollToPosition(
            RecyclerView recyclerView, RecyclerView.State state, int position) {
        /**
         * startSmoothScroll will handle stopping the old one if there is one.
         * We only keep a copy of it to handle the translation of rows as they slide off the screen
         * in {@link #offsetRowsWithPageBreak()}
         */
        mSmoothScroller = new CarSmoothScroller(mContext, position);
        mSmoothScroller.setTargetPosition(position);
        startSmoothScroll(mSmoothScroller);
    }

    /**
     * Miscellaneous bookkeeping.
     */
    @Override
    public void onScrollStateChanged(int state) {
        if (DEBUG) {
            Log.v(TAG, ":: onScrollStateChanged " + state);
        }
        if (state == RecyclerView.SCROLL_STATE_IDLE) {
            // If the focused view is off screen, give focus to one that is.
            // If the first fully visible view is first in the list, focus the first item.
            // Otherwise, focus the second so that you have the first item as scrolling context.
            View focusedChild = getFocusedChild();
            if (focusedChild != null
                    && (getDecoratedTop(focusedChild) >= getHeight() - getPaddingBottom()
                    || getDecoratedBottom(focusedChild) <= getPaddingTop())) {
                focusedChild.clearFocus();
                requestLayout();
            }

        } else if (state == RecyclerView.SCROLL_STATE_DRAGGING) {
            mLastDragDistance = 0;
        }

        if (state != RecyclerView.SCROLL_STATE_SETTLING) {
            mSmoothScroller = null;
        }

        mScrollState = state;
        updatePageBreakPositions();
    }

    @Override
    public void onItemsChanged(RecyclerView recyclerView) {
        super.onItemsChanged(recyclerView);
        if (mItemsChangedListener != null) {
            mItemsChangedListener.onItemsChanged();
        }
        // When item changed, our sample view height is no longer accurate, and need to be
        // recomputed.
        mSampleViewHeight = -1;
    }

    /**
     * Gives us the opportunity to override the order of the focused views.
     * By default, it will just go from top to bottom. However, if there is no focused views, we
     * take over the logic and start the focused views from the middle of what is visible and move
     * from there until the end of the laid out views in the specified direction.
     */
    @Override
    public boolean onAddFocusables(
            RecyclerView recyclerView, ArrayList<View> views, int direction, int focusableMode) {
        View focusedChild = getFocusedChild();
        if (focusedChild != null) {
            // If there is a view that already has focus, we can just return false and the normal
            // Android addFocusables will work fine.
            return false;
        }

        // Now we know that there isn't a focused view. We need to set up focusables such that
        // instead of just focusing the first item that has been laid out, it focuses starting
        // from a visible item.

        int firstFullyVisibleChildIndex = getFirstFullyVisibleChildIndex();
        if (firstFullyVisibleChildIndex == -1) {
            // Somehow there is a focused view but there is no fully visible view. There shouldn't
            // be a way for this to happen but we'd better stop here and return instead of
            // continuing on with -1.
            Log.w(TAG, "There is a focused child but no first fully visible child.");
            return false;
        }
        View firstFullyVisibleChild = getChildAt(firstFullyVisibleChildIndex);
        int firstFullyVisibleChildPosition = getPosition(firstFullyVisibleChild);

        int firstFocusableChildIndex = firstFullyVisibleChildIndex;
        if (firstFullyVisibleChildPosition > 0 && firstFocusableChildIndex + 1 < getItemCount()) {
            // We are somewhere in the middle of the list. Instead of starting focus on the first
            // item, start focus on the second item to give some context that we aren't at
            // the beginning.
            firstFocusableChildIndex++;
        }

        if (direction == View.FOCUS_FORWARD) {
            // Iterate from the first focusable view to the end.
            for (int i = firstFocusableChildIndex; i < getChildCount(); i++) {
                views.add(getChildAt(i));
            }
            return true;
        } else if (direction == View.FOCUS_BACKWARD) {
            // Iterate from the first focusable view to the beginning.
            for (int i = firstFocusableChildIndex; i >= 0; i--) {
                views.add(getChildAt(i));
            }
            return true;
        }
        return false;
    }

    @Override
    public View onFocusSearchFailed(View focused, int direction, RecyclerView.Recycler recycler,
                                    RecyclerView.State state) {
        return null;
    }

    /**
     * This is the function that decides where to scroll to when a new view is focused.
     * You can get the position of the currently focused child through the child parameter.
     * Once you have that, determine where to smooth scroll to and scroll there.
     *
     * @param parent The RecyclerView hosting this LayoutManager
     * @param state Current state of RecyclerView
     * @param child Direct child of the RecyclerView containing the newly focused view
     * @param focused The newly focused view. This may be the same view as child or it may be null
     * @return true if the default scroll behavior should be suppressed
     */
    @Override
    public boolean onRequestChildFocus(RecyclerView parent, RecyclerView.State state,
                                       View child, View focused) {
        if (child == null) {
            Log.w(TAG, "onRequestChildFocus with a null child!");
            return true;
        }

        if (DEBUG) {
            Log.v(TAG, String.format(":: onRequestChildFocus child: %s, focused: %s", child,
                    focused));
        }

        // We have several distinct scrolling methods. Each implementation has been delegated
        // to its own method.
        if (SCROLL_TYPE == MARIO) {
            return onRequestChildFocusMarioStyle(parent, child);
        } else if (SCROLL_TYPE == SUPER_MARIO) {
            return onRequestChildFocusSuperMarioStyle(parent, state, child);
        } else {
            throw new IllegalStateException("Unknown scroll type (" + SCROLL_TYPE + ")");
        }
    }

    /**
     * Goal: the scrollbar maintains the same size throughout scrolling and that the scrollbar
     * reaches the bottom of the screen when the last item is fully visible. This is because
     * there are multiple points that could be considered the bottom since the last item can scroll
     * past the bottom edge of the screen.
     *
     * To find the extent, we divide the number of items that can fit on screen by the number of
     * items in total.
     */
    @Override
    public int computeVerticalScrollExtent(RecyclerView.State state) {
        if (getChildCount() <= 1) {
            return 0;
        }

        int sampleViewHeight = getSampleViewHeight();
        int availableHeight = getAvailableHeight();
        int sampleViewsThatCanFitOnScreen = availableHeight / sampleViewHeight;

        if (state.getItemCount() <= sampleViewsThatCanFitOnScreen) {
            return SCROLL_RANGE;
        } else {
            return SCROLL_RANGE * sampleViewsThatCanFitOnScreen / state.getItemCount();
        }
    }

    /**
     * The scrolling offset is calculated by determining what position is at the top of the list.
     * However, instead of using fixed integer positions for each row, the scroll position is
     * factored in and the position is recalculated as a float that takes in to account the
     * current scroll state. This results in a smooth animation for the scrollbar when the user
     * scrolls the list.
     */
    @Override
    public int computeVerticalScrollOffset(RecyclerView.State state) {
        View firstChild = getFirstFullyVisibleChild();
        if (firstChild == null) {
            return 0;
        }

        RecyclerView.LayoutParams params = getParams(firstChild);
        int firstChildPosition = getPosition(firstChild);

        // Assume the previous view is the same height as the current one.
        float percentOfPreviousViewShowing = (getDecoratedTop(firstChild) - params.topMargin)
                / (float) (getDecoratedMeasuredHeight(firstChild)
                + params.topMargin + params.bottomMargin);
        // If the previous view is actually larger than the current one then this the percent
        // can be greater than 1.
        percentOfPreviousViewShowing = Math.min(percentOfPreviousViewShowing, 1);

        float currentPosition = (float) firstChildPosition - percentOfPreviousViewShowing;

        int sampleViewHeight = getSampleViewHeight();
        int availableHeight = getAvailableHeight();
        int numberOfSampleViewsThatCanFitOnScreen = availableHeight / sampleViewHeight;
        int positionWhenLastItemIsVisible =
                state.getItemCount() - numberOfSampleViewsThatCanFitOnScreen;

        if (positionWhenLastItemIsVisible <= 0) {
            return 0;
        }

        if (currentPosition >= positionWhenLastItemIsVisible) {
            return SCROLL_RANGE;
        }

        return (int) (SCROLL_RANGE * currentPosition / positionWhenLastItemIsVisible);
    }

    /**
     * The range of the scrollbar can be understood as the granularity of how we want the
     * scrollbar to scroll.
     */
    @Override
    public int computeVerticalScrollRange(RecyclerView.State state) {
        return SCROLL_RANGE;
    }

    /**
     * @return The first view that starts on screen. It assumes that it fully fits on the screen
     *         though. If the first fully visible child is also taller than the screen then it will
     *         still be returned. However, since the LayoutManager snaps to view starts, having
     *         a row that tall would lead to a broken experience anyways.
     */
    public int getFirstFullyVisibleChildIndex() {
        for (int i = 0; i < getChildCount(); i++) {
            View child = getChildAt(i);
            RecyclerView.LayoutParams params = getParams(child);
            if (getDecoratedTop(child) - params.topMargin >= getPaddingTop()) {
                return i;
            }
        }
        return -1;
    }

    public View getFirstFullyVisibleChild() {
        int firstFullyVisibleChildIndex = getFirstFullyVisibleChildIndex();
        View firstChild = null;
        if (firstFullyVisibleChildIndex != -1) {
            firstChild = getChildAt(firstFullyVisibleChildIndex);
        }
        return firstChild;
    }

    /**
     * @return The last view that ends on screen. It assumes that the start is also on screen
     *         though. If the last fully visible child is also taller than the screen then it will
     *         still be returned. However, since the LayoutManager snaps to view starts, having
     *         a row that tall would lead to a broken experience anyways.
     */
    public int getLastFullyVisibleChildIndex() {
        for (int i = getChildCount() - 1; i >= 0; i--) {
            View child = getChildAt(i);
            RecyclerView.LayoutParams params = getParams(child);
            int childBottom = getDecoratedBottom(child) + params.bottomMargin;
            int listBottom = getHeight() - getPaddingBottom();
            if (childBottom <= listBottom) {
                return i;
            }
        }
        return -1;
    }

    /**
     * @return Whether or not the first view is fully visible.
     */
    public boolean isAtTop() {
        // getFirstFullyVisibleChildIndex() can return -1 which indicates that there are no views
        // and also means that the list is at the top.
        return getFirstFullyVisibleChildIndex() <= 0;
    }

    /**
     * @return Whether or not the last view is fully visible.
     */
    public boolean isAtBottom() {
        int lastFullyVisibleChildIndex = getLastFullyVisibleChildIndex();
        if (lastFullyVisibleChildIndex == -1) {
            return true;
        }
        View lastFullyVisibleChild = getChildAt(lastFullyVisibleChildIndex);
        return getPosition(lastFullyVisibleChild) == getItemCount() - 1;
    }

    public void setOffsetRows(boolean offsetRows) {
        mOffsetRows = offsetRows;
        if (offsetRows) {
            offsetRows();
        } else {
            int childCount = getChildCount();
            for (int i = 0; i < childCount; i++) {
                getChildAt(i).setTranslationY(0);
            }
        }
    }

    public void setRowOffsetMode(@RowOffsetMode int mode) {
        if (mode == mRowOffsetMode) {
            return;
        }
        mRowOffsetMode = mode;
        offsetRows();
    }

    public void setItemsChangedListener(OnItemsChangedListener listener) {
        mItemsChangedListener = listener;
    }

    /**
     * Finish the pagination taking into account where the gesture started (not where we are now).
     *
     * @return Whether the list was scrolled as a result of the fling.
     */
    public boolean settleScrollForFling(RecyclerView parent, int flingVelocity) {
        if (getChildCount() == 0) {
            return false;
        }

        if (mReachedLimitOfDrag) {
            return false;
        }

        // If the fling was too slow or too short, settle on the first fully visible row instead.
        if (Math.abs(flingVelocity) <= FLING_THRESHOLD_TO_PAGINATE
                || Math.abs(mLastDragDistance) <= DRAG_DISTANCE_TO_PAGINATE) {
            int firstFullyVisibleChildIndex = getFirstFullyVisibleChildIndex();
            if (firstFullyVisibleChildIndex != -1) {
                int scrollPosition = getPosition(getChildAt(firstFullyVisibleChildIndex));
                parent.smoothScrollToPosition(scrollPosition);
                return true;
            }
            return false;
        }

        // Finish the pagination taking into account where the gesture
        // started (not where we are now).
        boolean isDownGesture = flingVelocity > 0
                || (flingVelocity == 0 && mLastDragDistance >= 0);
        boolean isUpGesture = flingVelocity < 0
                || (flingVelocity == 0 && mLastDragDistance < 0);
        if (isDownGesture && mLowerPageBreakPosition != -1) {
            // If the last view is fully visible then only settle on the first fully visible view
            // instead of the original page down position. However, don't page down if the last
            // item has come fully into view.
            parent.smoothScrollToPosition(mAnchorPageBreakPosition);
            return true;
        } else if (isUpGesture && mUpperPageBreakPosition != -1) {
            parent.smoothScrollToPosition(mUpperPageBreakPosition);
            return true;
        } else {
            Log.e(TAG, "Error setting scroll for fling! flingVelocity: \t" + flingVelocity +
                    "\tlastDragDistance: " + mLastDragDistance + "\tpageUpAtStartOfDrag: " +
                    mUpperPageBreakPosition + "\tpageDownAtStartOfDrag: " +
                    mLowerPageBreakPosition);
            // As a last resort, at the last smooth scroller target position if there is one.
            if (mSmoothScroller != null) {
                parent.smoothScrollToPosition(mSmoothScroller.getTargetPosition());
                return true;
            }
        }
        return false;
    }

    /**
     * @return The position that paging up from the current position would settle at.
     */
    public int getPageUpPosition() {
        return mUpperPageBreakPosition;
    }

    /**
     * @return The position that paging down from the current position would settle at.
     */
    public int getPageDownPosition() {
        return mLowerPageBreakPosition;
    }

    /**
     * Layout the anchor row. The anchor row is the first fully visible row.
     *
     * @param anchorTop The decorated top of the anchor. If it is not known or should be reset
     *                  to the top, pass -1.
     */
    private View layoutAnchor(RecyclerView.Recycler recycler, int anchorPosition, int anchorTop) {
        if (anchorPosition > getItemCount() - 1) {
            return null;
        }
        View anchor = recycler.getViewForPosition(anchorPosition);
        RecyclerView.LayoutParams params = getParams(anchor);
        measureChildWithMargins(anchor, 0, 0);
        int left = getPaddingLeft() + params.leftMargin;
        int top = (anchorTop == -1) ? params.topMargin : anchorTop;
        int right = left + getDecoratedMeasuredWidth(anchor);
        int bottom = top + getDecoratedMeasuredHeight(anchor);
        layoutDecorated(anchor, left, top, right, bottom);
        addView(anchor);
        return anchor;
    }

    /**
     * Lays out the next row in the specified direction next to the specified adjacent row.
     *
     * @param recycler The recycler from which a new view can be created.
     * @param adjacentRow The View of the adjacent row which will be used to position the new one.
     * @param layoutDirection The side of the adjacent row that the new row will be laid out on.
     *
     * @return The new row that was laid out.
     */
    private View layoutNextRow(RecyclerView.Recycler recycler, View adjacentRow,
                               @LayoutDirection int layoutDirection) {

        int adjacentRowPosition = getPosition(adjacentRow);
        int newRowPosition = adjacentRowPosition;
        if (layoutDirection == BEFORE) {
            newRowPosition = adjacentRowPosition - 1;
        } else if (layoutDirection == AFTER) {
            newRowPosition = adjacentRowPosition + 1;
        }

        // Because we detach all rows in onLayoutChildren, this will often just return a view from
        // the scrap heap.
        View newRow = recycler.getViewForPosition(newRowPosition);

        measureChildWithMargins(newRow, 0, 0);
        RecyclerView.LayoutParams newRowParams =
                (RecyclerView.LayoutParams) newRow.getLayoutParams();
        RecyclerView.LayoutParams adjacentRowParams =
                (RecyclerView.LayoutParams) adjacentRow.getLayoutParams();
        int left = getPaddingLeft() + newRowParams.leftMargin;
        int right = left + getDecoratedMeasuredWidth(newRow);
        int top, bottom;
        if (layoutDirection == BEFORE) {
            bottom = adjacentRow.getTop() - adjacentRowParams.topMargin - newRowParams.bottomMargin;
            top = bottom - getDecoratedMeasuredHeight(newRow);
        } else {
            top = getDecoratedBottom(adjacentRow) +
                    adjacentRowParams.bottomMargin + newRowParams.topMargin;
            bottom = top + getDecoratedMeasuredHeight(newRow);
        }
        layoutDecorated(newRow, left, top, right, bottom);

        if (layoutDirection == BEFORE) {
            addView(newRow, 0);
        } else {
            addView(newRow);
        }

        return newRow;
    }

    /**
     * @return Whether another row should be laid out in the specified direction.
     */
    private boolean shouldLayoutNextRow(RecyclerView.State state, View adjacentRow,
                                        @LayoutDirection int layoutDirection) {
        int adjacentRowPosition = getPosition(adjacentRow);

        if (layoutDirection == BEFORE) {
            if (adjacentRowPosition == 0) {
                // We already laid out the first row.
                return false;
            }
        } else if (layoutDirection == AFTER) {
            if (adjacentRowPosition >= state.getItemCount() - 1) {
                // We already laid out the last row.
                return false;
            }
        }

        // If we are scrolling layout views until the target position.
        if (mSmoothScroller != null) {
            if (layoutDirection == BEFORE
                    && adjacentRowPosition >= mSmoothScroller.getTargetPosition()) {
                return true;
            } else if (layoutDirection == AFTER
                    && adjacentRowPosition <= mSmoothScroller.getTargetPosition()) {
                return true;
            }
        }

        View focusedRow = getFocusedChild();
        if (focusedRow != null) {
            int focusedRowPosition = getPosition(focusedRow);
            if (layoutDirection == BEFORE && adjacentRowPosition
                    >= focusedRowPosition - NUM_EXTRA_ROWS_TO_LAYOUT_PAST_FOCUS) {
                return true;
            } else if (layoutDirection == AFTER && adjacentRowPosition
                    <= focusedRowPosition + NUM_EXTRA_ROWS_TO_LAYOUT_PAST_FOCUS) {
                return true;
            }
        }

        RecyclerView.LayoutParams params = getParams(adjacentRow);
        int adjacentRowTop = getDecoratedTop(adjacentRow) - params.topMargin;
        int adjacentRowBottom = getDecoratedBottom(adjacentRow) - params.bottomMargin;
        if (layoutDirection == BEFORE
                && adjacentRowTop < getPaddingTop() - getHeight()) {
            // View is more than 1 page past the top of the screen and also past where the user has
            // scrolled to. We want to keep one page past the top to make the scroll up calculation
            // easier and scrolling smoother.
            return false;
        } else if (layoutDirection == AFTER
                && adjacentRowBottom > getHeight() - getPaddingBottom()) {
            // View is off of the bottom and also past where the user has scrolled to.
            return false;
        }

        return true;
    }

    /**
     * Remove and recycle views that are no longer needed.
     */
    private void recycleChildrenFromStart(RecyclerView.Recycler recycler) {
        // Start laying out children one page before the top of the viewport.
        int childrenStart = getPaddingTop() - getHeight();

        int focusedChildPosition = Integer.MAX_VALUE;
        View focusedChild = getFocusedChild();
        if (focusedChild != null) {
            focusedChildPosition = getPosition(focusedChild);
        }

        // Count the number of views that should be removed.
        int detachedCount = 0;
        int childCount = getChildCount();
        for (int i = 0; i < childCount; i++) {
            final View child = getChildAt(i);
            int childEnd = getDecoratedBottom(child);
            int childPosition = getPosition(child);

            if (childEnd >= childrenStart || childPosition >= focusedChildPosition - 1) {
                break;
            }

            detachedCount++;
        }

        // Remove the number of views counted above. Done by removing the first child n times.
        while (--detachedCount >= 0) {
            final View child = getChildAt(0);
            removeAndRecycleView(child, recycler);
        }
    }

    /**
     * Remove and recycle views that are no longer needed.
     */
    private void recycleChildrenFromEnd(RecyclerView.Recycler recycler) {
        // Layout views until the end of the viewport.
        int childrenEnd = getHeight();

        int focusedChildPosition = Integer.MIN_VALUE + 1;
        View focusedChild = getFocusedChild();
        if (focusedChild != null) {
            focusedChildPosition = getPosition(focusedChild);
        }

        // Count the number of views that should be removed.
        int firstDetachedPos = 0;
        int detachedCount = 0;
        int childCount = getChildCount();
        for (int i = childCount - 1; i >= 0; i--) {
            final View child = getChildAt(i);
            int childStart = getDecoratedTop(child);
            int childPosition = getPosition(child);

            if (childStart <= childrenEnd || childPosition <= focusedChildPosition - 1) {
                break;
            }

            firstDetachedPos = i;
            detachedCount++;
        }

        while (--detachedCount >= 0) {
            final View child = getChildAt(firstDetachedPos);
            removeAndRecycleView(child, recycler);
        }
    }

    /**
     * Offset rows to do fancy animations. If {@link #mOffsetRows} is false, this will do nothing.
     *
     * @see #offsetRowsIndividually()
     * @see #offsetRowsByPage()
     */
    public void offsetRows() {
        if (!mOffsetRows) {
            return;
        }

        if (mRowOffsetMode == ROW_OFFSET_MODE_PAGE) {
            offsetRowsByPage();
        } else if (mRowOffsetMode == ROW_OFFSET_MODE_INDIVIDUAL) {
            offsetRowsIndividually();
        }
    }

    /**
     * Offset the single row that is scrolling off the screen such that by the time the next row
     * reaches the top, it will have accelerated completely off of the screen.
     */
    private void offsetRowsIndividually() {
        if (getChildCount() == 0) {
            if (DEBUG) {
                Log.d(TAG, ":: offsetRowsIndividually getChildCount=0");
            }
            return;
        }

        // Identify the dangling row. It will be the first row that is at the top of the
        // list or above.
        int danglingChildIndex = -1;
        for (int i = getChildCount() - 1; i >= 0; i--) {
            View child = getChildAt(i);
            if (getDecoratedTop(child) - getParams(child).topMargin <= getPaddingTop()) {
                danglingChildIndex = i;
                break;
            }
        }

        mAnchorPageBreakPosition = danglingChildIndex;

        if (DEBUG) {
            Log.v(TAG, ":: offsetRowsIndividually danglingChildIndex: " + danglingChildIndex);
        }

        // Calculate the total amount that the view will need to scroll in order to go completely
        // off screen.
        RecyclerView rv = (RecyclerView) getChildAt(0).getParent();
        int[] locs = new int[2];
        rv.getLocationInWindow(locs);
        int listTopInWindow = locs[1] + rv.getPaddingTop();
        int maxDanglingViewTranslation;

        int childCount = getChildCount();
        for (int i = 0; i < childCount; i++) {
            View child = getChildAt(i);
            RecyclerView.LayoutParams params = getParams(child);

            maxDanglingViewTranslation = listTopInWindow;
            // If the child has a negative margin, we'll actually need to translate the view a
            // little but further to get it completely off screen.
            if (params.topMargin < 0) {
                maxDanglingViewTranslation -= params.topMargin;
            }
            if (params.bottomMargin < 0) {
                maxDanglingViewTranslation -= params.bottomMargin;
            }

            if (i < danglingChildIndex) {
                child.setAlpha(0f);
            } else if (i > danglingChildIndex) {
                child.setAlpha(1f);
                child.setTranslationY(0);
            } else {
                int totalScrollDistance = getDecoratedMeasuredHeight(child) +
                        params.topMargin + params.bottomMargin;

                int distanceLeftInScroll = getDecoratedBottom(child) +
                        params.bottomMargin - getPaddingTop();
                float percentageIntoScroll = 1 - distanceLeftInScroll / (float) totalScrollDistance;
                float interpolatedPercentage =
                        mDanglingRowInterpolator.getInterpolation(percentageIntoScroll);

                child.setAlpha(1f);
                child.setTranslationY(-(maxDanglingViewTranslation * interpolatedPercentage));
            }
        }
    }

    /**
     * When the list scrolls, the entire page of rows will offset in one contiguous block. This
     * significantly reduces the amount of extra motion at the top of the screen.
     */
    private void offsetRowsByPage() {
        View anchorView = findViewByPosition(mAnchorPageBreakPosition);
        if (anchorView == null) {
            if (DEBUG) {
                Log.d(TAG, ":: offsetRowsByPage anchorView null");
            }
            return;
        }
        int anchorViewTop = getDecoratedTop(anchorView) - getParams(anchorView).topMargin;

        View upperPageBreakView = findViewByPosition(mUpperPageBreakPosition);
        int upperViewTop = getDecoratedTop(upperPageBreakView)
                - getParams(upperPageBreakView).topMargin;

        int scrollDistance = upperViewTop - anchorViewTop;

        int distanceLeft = anchorViewTop - getPaddingTop();
        float scrollPercentage = (Math.abs(scrollDistance) - distanceLeft)
                / (float) Math.abs(scrollDistance);

        if (DEBUG) {
            Log.d(TAG, String.format(
                    ":: offsetRowsByPage scrollDistance:%s, distanceLeft:%s, scrollPercentage:%s",
                    scrollDistance, distanceLeft, scrollPercentage));
        }

        // Calculate the total amount that the view will need to scroll in order to go completely
        // off screen.
        RecyclerView rv = (RecyclerView) getChildAt(0).getParent();
        int[] locs = new int[2];
        rv.getLocationInWindow(locs);
        int listTopInWindow = locs[1] + rv.getPaddingTop();

        int childCount = getChildCount();
        for (int i = 0; i < childCount; i++) {
            View child = getChildAt(i);
            int position = getPosition(child);
            if (position < mUpperPageBreakPosition) {
                child.setAlpha(0f);
                child.setTranslationY(-listTopInWindow);
            } else if (position < mAnchorPageBreakPosition) {
                // If the child has a negative margin, we need to offset the row by a little bit
                // extra so that it moves completely off screen.
                RecyclerView.LayoutParams params = getParams(child);
                int extraTranslation = 0;
                if (params.topMargin < 0) {
                    extraTranslation -= params.topMargin;
                }
                if (params.bottomMargin < 0) {
                    extraTranslation -= params.bottomMargin;
                }
                int translation = (int) ((listTopInWindow + extraTranslation)
                        * mDanglingRowInterpolator.getInterpolation(scrollPercentage));
                child.setAlpha(1f);
                child.setTranslationY(-translation);
            } else {
                child.setAlpha(1f);
                child.setTranslationY(0);
            }
        }
    }

    /**
     * Update the page break positions based on the position of the views on screen. This should
     * be called whenever view move or change such as during a scroll or layout.
     */
    private void updatePageBreakPositions() {
        if (getChildCount() == 0) {
            if (DEBUG) {
                Log.d(TAG, ":: updatePageBreakPosition getChildCount: 0");
            }
            return;
        }

        if (DEBUG) {
            Log.v(TAG, String.format(":: #BEFORE updatePageBreakPositions " +
                            "mAnchorPageBreakPosition:%s, mUpperPageBreakPosition:%s, "
                            + "mLowerPageBreakPosition:%s",
                    mAnchorPageBreakPosition, mUpperPageBreakPosition, mLowerPageBreakPosition));
        }

        // If the item count has changed, our page boundaries may no longer be accurate. This will
        // force the page boundaries to reset around the current view that is closest to the top.
        if (getItemCount() != mItemCountDuringLastPageBreakUpdate) {
            if (DEBUG) {
                Log.d(TAG, "Item count changed. Resetting page break positions.");
            }
            mAnchorPageBreakPosition = getPosition(getFirstFullyVisibleChild());
        }
        mItemCountDuringLastPageBreakUpdate = getItemCount();

        if (mAnchorPageBreakPosition == -1) {
            Log.w(TAG, "Unable to update anchor positions. There is no anchor position.");
            return;
        }

        View anchorPageBreakView = findViewByPosition(mAnchorPageBreakPosition);
        if (anchorPageBreakView == null) {
            return;
        }
        int topMargin = getParams(anchorPageBreakView).topMargin;
        int anchorTop = getDecoratedTop(anchorPageBreakView) - topMargin;
        View upperPageBreakView = findViewByPosition(mUpperPageBreakPosition);
        int upperPageBreakTop = upperPageBreakView == null ? Integer.MIN_VALUE :
                getDecoratedTop(upperPageBreakView) - getParams(upperPageBreakView).topMargin;

        if (DEBUG) {
            Log.v(TAG, String.format(":: #MID updatePageBreakPositions topMargin:%s, anchorTop:%s"
                            + " mAnchorPageBreakPosition:%s, mUpperPageBreakPosition:%s, "
                            + "mLowerPageBreakPosition:%s", topMargin, anchorTop,
                    mAnchorPageBreakPosition, mUpperPageBreakPosition, mLowerPageBreakPosition));
        }

        if (anchorTop < getPaddingTop()) {
            // The anchor has moved above the viewport. We are now on the next page. Shift the page
            // break positions and calculate a new lower one.
            mUpperPageBreakPosition = mAnchorPageBreakPosition;
            mAnchorPageBreakPosition = mLowerPageBreakPosition;
            mLowerPageBreakPosition = calculateNextPageBreakPosition(mAnchorPageBreakPosition);
        } else if (mAnchorPageBreakPosition > 0 && upperPageBreakTop >= getPaddingTop()) {
            // The anchor has moved below the viewport. We are now on the previous page. Shift
            // the page break positions and calculate a new upper one.
            mLowerPageBreakPosition = mAnchorPageBreakPosition;
            mAnchorPageBreakPosition = mUpperPageBreakPosition;
            mUpperPageBreakPosition = calculatePreviousPageBreakPosition(mAnchorPageBreakPosition);
        } else {
            mUpperPageBreakPosition = calculatePreviousPageBreakPosition(mAnchorPageBreakPosition);
            mLowerPageBreakPosition = calculateNextPageBreakPosition(mAnchorPageBreakPosition);
        }

        if (DEBUG) {
            Log.v(TAG, String.format(":: #AFTER updatePageBreakPositions " +
                            "mAnchorPageBreakPosition:%s, mUpperPageBreakPosition:%s, "
                            + "mLowerPageBreakPosition:%s",
                    mAnchorPageBreakPosition, mUpperPageBreakPosition, mLowerPageBreakPosition));
        }
    }

    /**
     * @return The page break position of the page before the anchor page break position. However,
     *         if it reaches the end of the laid out children or position 0, it will just return
     *         that.
     */
    private int calculatePreviousPageBreakPosition(int position) {
        if (position == -1) {
            return -1;
        }
        View referenceView = findViewByPosition(position);
        int referenceViewTop = getDecoratedTop(referenceView) - getParams(referenceView).topMargin;

        int previousPagePosition = position;
        while (previousPagePosition > 0) {
            previousPagePosition--;
            View child = findViewByPosition(previousPagePosition);
            if (child == null) {
                // View has not been laid out yet.
                return previousPagePosition + 1;
            }

            int childTop = getDecoratedTop(child) - getParams(child).topMargin;

            if (childTop < referenceViewTop - getHeight()) {
                return previousPagePosition + 1;
            }
        }
        // Beginning of the list.
        return 0;
    }

    /**
     * @return The page break position of the next page after the anchor page break position.
     *         However, if it reaches the end of the laid out children or end of the list, it will
     *         just return that.
     */
    private int calculateNextPageBreakPosition(int position) {
        if (position == -1) {
            return -1;
        }

        View referenceView = findViewByPosition(position);
        if (referenceView == null) {
            return position;
        }
        int referenceViewTop = getDecoratedTop(referenceView) - getParams(referenceView).topMargin;

        int nextPagePosition = position;

        // Search for the first child item after the referenceView that didn't fully fit on to the
        // screen. The next page should start from the item before this child, so that users have
        // a visual anchoring point of the page change.
        while (position < getItemCount() - 1) {
            nextPagePosition++;
            View child = findViewByPosition(nextPagePosition);
            if (child == null) {
                // The next view has not been laid out yet.
                return nextPagePosition - 1;
            }

            int childBottom = getDecoratedBottom(child) + getParams(child).bottomMargin;
            if (childBottom - referenceViewTop > getHeight() - getPaddingTop()) {
                // If choosing the previous child causes the view to snap back to the referenceView
                // position, then skip that and go directly to the child. This avoids the case
                // where a tall card in the layout causes the view to constantly snap back to
                // the top when scrolled.
                return nextPagePosition - 1 == position ? nextPagePosition : nextPagePosition - 1;
            }
        }
        // End of the list.
        return nextPagePosition;
    }

    /**
     * In this style, the focus will scroll down to the middle of the screen and lock there
     * so that moving in either direction will move the entire list by 1.
     */
    private boolean onRequestChildFocusMarioStyle(RecyclerView parent, View child) {
        int focusedPosition = getPosition(child);
        if (focusedPosition == mLastChildPositionToRequestFocus) {
            return true;
        }
        mLastChildPositionToRequestFocus = focusedPosition;

        int availableHeight = getAvailableHeight();
        int focusedChildTop = getDecoratedTop(child);
        int focusedChildBottom = getDecoratedBottom(child);

        int childIndex = parent.indexOfChild(child);
        // Iterate through children starting at the focused child to find the child above it to
        // smooth scroll to such that the focused child will be as close to the middle of the screen
        // as possible.
        for (int i = childIndex; i >= 0; i--) {
            View childAtI = getChildAt(i);
            if (childAtI == null) {
                Log.e(TAG, "Child is null at index " + i);
                continue;
            }
            // We haven't found a view that is more than half of the recycler view height above it
            // but we've reached the top so we can't go any further.
            if (i == 0) {
                parent.smoothScrollToPosition(getPosition(childAtI));
                break;
            }

            // Because we want to scroll to the first view that is less than half of the screen
            // away from the focused view, we "look ahead" one view. When the look ahead view
            // is more than availableHeight / 2 away, the current child at i is the one we want to
            // scroll to. However, sometimes, that view can be null (ie, if the view is in
            // transition). In that case, just skip that view.

            View childBefore = getChildAt(i - 1);
            if (childBefore == null) {
                continue;
            }
            int distanceToChildBeforeFromTop = focusedChildTop - getDecoratedTop(childBefore);
            int distanceToChildBeforeFromBottom = focusedChildBottom - getDecoratedTop(childBefore);

            if (distanceToChildBeforeFromTop > availableHeight / 2
                    || distanceToChildBeforeFromBottom > availableHeight) {
                parent.smoothScrollToPosition(getPosition(childAtI));
                break;
            }
        }
        return true;
    }

    /**
     * In this style, you can free scroll in the middle of the list but if you get to the edge,
     * the list will advance to ensure that there is context ahead of the focused item.
     */
    private boolean onRequestChildFocusSuperMarioStyle(RecyclerView parent,
                                                       RecyclerView.State state, View child) {
        int focusedPosition = getPosition(child);
        if (focusedPosition == mLastChildPositionToRequestFocus) {
            return true;
        }
        mLastChildPositionToRequestFocus = focusedPosition;

        int bottomEdgeThatMustBeOnScreen;
        int focusedIndex = parent.indexOfChild(child);
        // The amount of the last card at the end that must be showing to count as visible.
        int peekAmount = mContext.getResources()
                .getDimensionPixelSize(R.dimen.car_last_card_peek_amount);
        if (focusedPosition == state.getItemCount() - 1) {
            // The last item is focused.
            bottomEdgeThatMustBeOnScreen = getDecoratedBottom(child);
        } else if (focusedIndex == getChildCount() - 1) {
            // The last laid out item is focused. Scroll enough so that the next card has at least
            // the peek size visible
            ViewGroup.MarginLayoutParams params =
                    (ViewGroup.MarginLayoutParams) child.getLayoutParams();
            // We add params.topMargin as an estimate because we don't actually know the top margin
            // of the next row.
            bottomEdgeThatMustBeOnScreen = getDecoratedBottom(child) +
                    params.bottomMargin + params.topMargin + peekAmount;
        } else {
            View nextChild = getChildAt(focusedIndex + 1);
            bottomEdgeThatMustBeOnScreen = getDecoratedTop(nextChild) + peekAmount;
        }

        if (bottomEdgeThatMustBeOnScreen > getHeight()) {
            // We're going to have to scroll because the bottom edge that must be on screen is past
            // the bottom.
            int topEdgeToFindViewUnder = getPaddingTop() +
                    bottomEdgeThatMustBeOnScreen - getHeight();

            View nextChild = null;
            for (int i = 0; i < getChildCount(); i++) {
                View potentialNextChild = getChildAt(i);
                RecyclerView.LayoutParams params = getParams(potentialNextChild);
                float top = getDecoratedTop(potentialNextChild) - params.topMargin;
                if (top >= topEdgeToFindViewUnder) {
                    nextChild = potentialNextChild;
                    break;
                }
            }

            if (nextChild == null) {
                Log.e(TAG, "There is no view under " + topEdgeToFindViewUnder);
                return true;
            }
            int nextChildPosition = getPosition(nextChild);
            parent.smoothScrollToPosition(nextChildPosition);
        } else {
            int firstFullyVisibleIndex = getFirstFullyVisibleChildIndex();
            if (focusedIndex <= firstFullyVisibleIndex) {
                parent.smoothScrollToPosition(Math.max(focusedPosition - 1, 0));
            }
        }
        return true;
    }

    /**
     * We don't actually know the size of every single view, only what is currently laid out.
     * This makes it difficult to do accurate scrollbar calculations. However, lists in the car
     * often consist of views with identical heights. Because of that, we can use
     * a single sample view to do our calculations for. The main exceptions are in the first items
     * of a list (hero card, last call card, etc) so if the first view is at position 0, we pick
     * the next one.
     *
     * @return The decorated measured height of the sample view plus its margins.
     */
    private int getSampleViewHeight() {
        if (mSampleViewHeight != -1) {
            return mSampleViewHeight;
        }
        int sampleViewIndex = getFirstFullyVisibleChildIndex();
        View sampleView = getChildAt(sampleViewIndex);
        if (getPosition(sampleView) == 0 && sampleViewIndex < getChildCount() - 1) {
            sampleView = getChildAt(++sampleViewIndex);
        }
        RecyclerView.LayoutParams params = getParams(sampleView);
        int height =
                getDecoratedMeasuredHeight(sampleView) + params.topMargin + params.bottomMargin;
        if (height == 0) {
            // This can happen if the view isn't measured yet.
            Log.w(TAG, "The sample view has a height of 0. Returning a dummy value for now " +
                    "that won't be cached.");
            height = mContext.getResources().getDimensionPixelSize(R.dimen.car_sample_row_height);
        } else {
            mSampleViewHeight = height;
        }
        return height;
    }

    /**
     * @return The height of the RecyclerView excluding padding.
     */
    private int getAvailableHeight() {
        return getHeight() - getPaddingTop() - getPaddingBottom();
    }

    /**
     * @return {@link RecyclerView.LayoutParams} for the given view or null if it isn't a child
     *         of {@link RecyclerView}.
     */
    private static RecyclerView.LayoutParams getParams(View view) {
        return (RecyclerView.LayoutParams) view.getLayoutParams();
    }

    /**
     * Custom {@link LinearSmoothScroller} that has:
     *     a) Custom control over the speed of scrolls.
     *     b) Scrolling snaps to start. All of our scrolling logic depends on that.
     *     c) Keeps track of some state of the current scroll so that can aid in things like
     *        the scrollbar calculations.
     */
    private final class CarSmoothScroller extends LinearSmoothScroller {
        /** This value (150) was hand tuned by UX for what felt right. **/
        private static final float MILLISECONDS_PER_INCH = 150f;
        /** This value (0.45) was hand tuned by UX for what felt right. **/
        private static final float DECELERATION_TIME_DIVISOR = 0.45f;
        private static final int NON_TOUCH_MAX_DECELERATION_MS = 1000;

        /** This value (1.8) was hand tuned by UX for what felt right. **/
        private final Interpolator mInterpolator = new DecelerateInterpolator(1.8f);

        private final boolean mHasTouch;
        private final int mTargetPosition;


        public CarSmoothScroller(Context context, int targetPosition) {
            super(context);
            mTargetPosition = targetPosition;
            mHasTouch = mContext.getResources().getBoolean(R.bool.car_true_for_touch);
        }

        @Override
        public PointF computeScrollVectorForPosition(int i) {
            if (getChildCount() == 0) {
                return null;
            }
            final int firstChildPos = getPosition(getChildAt(getFirstFullyVisibleChildIndex()));
            final int direction = (mTargetPosition < firstChildPos) ? -1 : 1;
            return new PointF(0, direction);
        }

        @Override
        protected int getVerticalSnapPreference() {
            // This is key for most of the scrolling logic that guarantees that scrolling
            // will settle with a view aligned to the top.
            return LinearSmoothScroller.SNAP_TO_START;
        }

        @Override
        protected void onTargetFound(View targetView, RecyclerView.State state, Action action) {
            int dy = calculateDyToMakeVisible(targetView, SNAP_TO_START);
            if (dy == 0) {
                if (DEBUG) {
                    Log.d(TAG, "Scroll distance is 0");
                }
                return;
            }

            final int time = calculateTimeForDeceleration(dy);
            if (time > 0) {
                action.update(0, -dy, time, mInterpolator);
            }
        }

        @Override
        protected float calculateSpeedPerPixel(DisplayMetrics displayMetrics) {
            return MILLISECONDS_PER_INCH / displayMetrics.densityDpi;
        }

        @Override
        protected int calculateTimeForDeceleration(int dx) {
            int time = (int) Math.ceil(calculateTimeForScrolling(dx) / DECELERATION_TIME_DIVISOR);
            return mHasTouch ? time : Math.min(time, NON_TOUCH_MAX_DECELERATION_MS);
        }

        public int getTargetPosition() {
            return mTargetPosition;
        }
    }
}