SkRTree.h revision 6c778164a743f8760dca251524d51848548b436f
1 2/* 3 * Copyright 2012 Google Inc. 4 * 5 * Use of this source code is governed by a BSD-style license that can be 6 * found in the LICENSE file. 7 */ 8 9#ifndef SkRTree_DEFINED 10#define SkRTree_DEFINED 11 12#include "SkRect.h" 13#include "SkTDArray.h" 14#include "SkChunkAlloc.h" 15#include "SkBBoxHierarchy.h" 16 17/** 18 * An R-Tree implementation. In short, it is a balanced n-ary tree containing a hierarchy of 19 * bounding rectangles. 20 * 21 * Much like a B-Tree it maintains balance by enforcing minimum and maximum child counts, and 22 * splitting nodes when they become overfull. Unlike B-trees, however, we're using spatial data; so 23 * there isn't a canonical ordering to use when choosing insertion locations and splitting 24 * distributions. A variety of heuristics have been proposed for these problems; here, we're using 25 * something resembling an R*-tree, which attempts to minimize area and overlap during insertion, 26 * and aims to minimize a combination of margin, overlap, and area when splitting. 27 * 28 * One detail that is thus far unimplemented that may improve tree quality is attempting to remove 29 * and reinsert nodes when they become full, instead of immediately splitting (nodes that may have 30 * been placed well early on may hurt the tree later when more nodes have been added; removing 31 * and reinserting nodes generally helps reduce overlap and make a better tree). Deletion of nodes 32 * is also unimplemented. 33 * 34 * For more details see: 35 * 36 * Beckmann, N.; Kriegel, H. P.; Schneider, R.; Seeger, B. (1990). "The R*-tree: 37 * an efficient and robust access method for points and rectangles" 38 * 39 * It also supports bulk-loading from a batch of bounds and values; if you don't require the tree 40 * to be usable in its intermediate states while it is being constructed, this is significantly 41 * quicker than individual insertions and produces more consistent trees. 42 */ 43class SkRTree : public SkBBoxHierarchy { 44public: 45 46 /** 47 * Create a new R-Tree with specified min/max child counts. 48 * The child counts are valid iff: 49 * - (max + 1) / 2 >= min (splitting an overfull node must be enough to populate 2 nodes) 50 * - min < max 51 * - min > 0 52 * - max < SK_MaxU16 53 */ 54 static SkRTree* Create(int minChildren, int maxChildren); 55 virtual ~SkRTree(); 56 57 /** 58 * Insert a node, consisting of bounds and a data value into the tree, if we don't immediately 59 * need to use the tree; we may allow the insert to be deferred (this can allow us to bulk-load 60 * a large batch of nodes at once, which tends to be faster and produce a better tree). 61 * @param data The data value 62 * @param bounds The corresponding bounding box 63 * @param defer Can this insert be deferred? (this may be ignored) 64 */ 65 virtual void insert(void* data, const SkIRect& bounds, bool defer = false); 66 67 /** 68 * If any inserts have been deferred, this will add them into the tree 69 */ 70 virtual void flushDeferredInserts(); 71 72 /** 73 * Given a query rectangle, populates the passed-in array with the elements it intersects 74 */ 75 virtual void search(const SkIRect& query, SkTDArray<void*>* results); 76 77 virtual void clear(); 78 bool isEmpty() const { return 0 == fCount; } 79 int getDepth() const { return this->isEmpty() ? 0 : fRoot.fChild.subtree->fLevel + 1; } 80 81 /** 82 * This gets the insertion count (rather than the node count) 83 */ 84 virtual int getCount() const { return fCount; } 85 86private: 87 88 struct Node; 89 90 /** 91 * A branch of the tree, this may contain a pointer to another interior node, or a data value 92 */ 93 struct Branch { 94 union { 95 Node* subtree; 96 void* data; 97 } fChild; 98 SkIRect fBounds; 99 }; 100 101 /** 102 * A node in the tree, has between fMinChildren and fMaxChildren (the root is a special case) 103 */ 104 struct Node { 105 uint16_t fNumChildren; 106 uint16_t fLevel; 107 bool isLeaf() { return 0 == fLevel; } 108 // Since we want to be able to pick min/max child counts at runtime, we assume the creator 109 // has allocated sufficient space directly after us in memory, and index into that space 110 Branch* child(size_t index) { 111 return reinterpret_cast<Branch*>(this + 1) + index; 112 } 113 }; 114 115 typedef int32_t SkIRect::*SortSide; 116 117 // Helper for sorting our children arrays by sides of their rects 118 static bool RectLessThan(SortSide const& side, const Branch lhs, const Branch rhs) { 119 return lhs.fBounds.*side < rhs.fBounds.*side; 120 } 121 122 static bool RectLessX(int&, const Branch lhs, const Branch rhs) { 123 return ((lhs.fBounds.fRight - lhs.fBounds.fLeft) >> 1) < 124 ((rhs.fBounds.fRight - lhs.fBounds.fLeft) >> 1); 125 } 126 127 static bool RectLessY(int&, const Branch lhs, const Branch rhs) { 128 return ((lhs.fBounds.fBottom - lhs.fBounds.fTop) >> 1) < 129 ((rhs.fBounds.fBottom - lhs.fBounds.fTop) >> 1); 130 } 131 132 SkRTree(int minChildren, int maxChildren); 133 134 /** 135 * Recursively descend the tree to find an insertion position for 'branch', updates 136 * bounding boxes on the way up. 137 */ 138 Branch* insert(Node* root, Branch* branch, uint16_t level = 0); 139 140 int chooseSubtree(Node* root, Branch* branch); 141 SkIRect computeBounds(Node* n); 142 int distributeChildren(Branch* children); 143 void search(Node* root, const SkIRect query, SkTDArray<void*>* results) const; 144 145 /** 146 * This performs a bottom-up bulk load using the STR (sort-tile-recursive) algorithm, this 147 * seems to generally produce better, more consistent trees at significantly lower cost than 148 * repeated insertions. 149 * 150 * This consumes the input array. 151 * 152 * TODO: Experiment with other bulk-load algorithms (in particular the Hilbert pack variant, 153 * which groups rects by position on the Hilbert curve, is probably worth a look). There also 154 * exist top-down bulk load variants (VAMSplit, TopDownGreedy, etc). 155 */ 156 Branch bulkLoad(SkTDArray<Branch>* branches, int level = 0); 157 158 void validate(); 159 int validateSubtree(Node* root, SkIRect bounds, bool isRoot = false); 160 161 const int fMinChildren; 162 const int fMaxChildren; 163 const size_t fNodeSize; 164 165 // This is the count of data elements (rather than total nodes in the tree) 166 size_t fCount; 167 168 Branch fRoot; 169 SkChunkAlloc fNodes; 170 SkTDArray<Branch> fDeferredInserts; 171 172 Node* allocateNode(uint16_t level); 173 174}; 175 176#endif 177 178