llvm/ADT/Twine.h

//===-- Twine.h - Fast Temporary String Concatenation -----------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_ADT_TWINE_H
#define LLVM_ADT_TWINE_H

#include "llvm/ADT/StringRef.h"
#include <cassert>
#include <string>

namespace llvm {
  template <typename T>
  class SmallVectorImpl;
  class StringRef;
  class raw_ostream;

  /// Twine - A lightweight data structure for efficiently representing the
  /// concatenation of temporary values as strings.
  ///
  /// A Twine is a kind of rope, it represents a concatenated string using a
  /// binary-tree, where the string is the preorder of the nodes. Since the
  /// Twine can be efficiently rendered into a buffer when its result is used,
  /// it avoids the cost of generating temporary values for intermediate string
  /// results -- particularly in cases when the Twine result is never
  /// required. By explicitly tracking the type of leaf nodes, we can also avoid
  /// the creation of temporary strings for conversions operations (such as
  /// appending an integer to a string).
  ///
  /// A Twine is not intended for use directly and should not be stored, its
  /// implementation relies on the ability to store pointers to temporary stack
  /// objects which may be deallocated at the end of a statement. Twines should
  /// only be used accepted as const references in arguments, when an API wishes
  /// to accept possibly-concatenated strings.
  ///
  /// Twines support a special 'null' value, which always concatenates to form
  /// itself, and renders as an empty string. This can be returned from APIs to
  /// effectively nullify any concatenations performed on the result.
  ///
  /// \b Implementation \n
  ///
  /// Given the nature of a Twine, it is not possible for the Twine's
  /// concatenation method to construct interior nodes; the result must be
  /// represented inside the returned value. For this reason a Twine object
  /// actually holds two values, the left- and right-hand sides of a
  /// concatenation. We also have nullary Twine objects, which are effectively
  /// sentinel values that represent empty strings.
  ///
  /// Thus, a Twine can effectively have zero, one, or two children. The \see
  /// isNullary(), \see isUnary(), and \see isBinary() predicates exist for
  /// testing the number of children.
  ///
  /// We maintain a number of invariants on Twine objects (FIXME: Why):
  ///  - Nullary twines are always represented with their Kind on the left-hand
  ///    side, and the Empty kind on the right-hand side.
  ///  - Unary twines are always represented with the value on the left-hand
  ///    side, and the Empty kind on the right-hand side.
  ///  - If a Twine has another Twine as a child, that child should always be
  ///    binary (otherwise it could have been folded into the parent).
  ///
  /// These invariants are check by \see isValid().
  ///
  /// \b Efficiency Considerations \n
  ///
  /// The Twine is designed to yield efficient and small code for common
  /// situations. For this reason, the concat() method is inlined so that
  /// concatenations of leaf nodes can be optimized into stores directly into a
  /// single stack allocated object.
  ///
  /// In practice, not all compilers can be trusted to optimize concat() fully,
  /// so we provide two additional methods (and accompanying operator+
  /// overloads) to guarantee that particularly important cases (cstring plus
  /// StringRef) codegen as desired.
  class Twine {
    /// NodeKind - Represent the type of an argument.
    enum NodeKind {
      /// An empty string; the result of concatenating anything with it is also
      /// empty.
      NullKind,

      /// The empty string.
      EmptyKind,

      /// A pointer to a C string instance.
      CStringKind,

      /// A pointer to an std::string instance.
      StdStringKind,

      /// A pointer to a StringRef instance.
      StringRefKind,

      /// A pointer to a Twine instance.
      TwineKind
    };

  private:
    /// LHS - The prefix in the concatenation, which may be uninitialized for
    /// Null or Empty kinds.
    const void *LHS;
    /// RHS - The suffix in the concatenation, which may be uninitialized for
    /// Null or Empty kinds.
    const void *RHS;
    /// LHSKind - The NodeKind of the left hand side, \see getLHSKind().
    NodeKind LHSKind : 8;
    /// RHSKind - The NodeKind of the left hand side, \see getLHSKind().
    NodeKind RHSKind : 8;

  private:
    /// Construct a nullary twine; the kind must be NullKind or EmptyKind.
    explicit Twine(NodeKind Kind)
      : LHSKind(Kind), RHSKind(EmptyKind) {
      assert(isNullary() && "Invalid kind!");
    }

    /// Construct a binary twine.
    explicit Twine(const Twine &_LHS, const Twine &_RHS)
      : LHS(&_LHS), RHS(&_RHS), LHSKind(TwineKind), RHSKind(TwineKind) {
      assert(isValid() && "Invalid twine!");
    }

    /// Construct a twine from explicit values.
    explicit Twine(const void *_LHS, NodeKind _LHSKind,
                   const void *_RHS, NodeKind _RHSKind)
      : LHS(_LHS), RHS(_RHS), LHSKind(_LHSKind), RHSKind(_RHSKind) {
      assert(isValid() && "Invalid twine!");
    }

    /// isNull - Check for the null twine.
    bool isNull() const {
      return getLHSKind() == NullKind;
    }

    /// isEmpty - Check for the empty twine.
    bool isEmpty() const {
      return getLHSKind() == EmptyKind;
    }

    /// isNullary - Check if this is a nullary twine (null or empty).
    bool isNullary() const {
      return isNull() || isEmpty();
    }

    /// isUnary - Check if this is a unary twine.
    bool isUnary() const {
      return getRHSKind() == EmptyKind && !isNullary();
    }

    /// isBinary - Check if this is a binary twine.
    bool isBinary() const {
      return getLHSKind() != NullKind && getRHSKind() != EmptyKind;
    }

    /// isValid - Check if this is a valid twine (satisfying the invariants on
    /// order and number of arguments).
    bool isValid() const {
      // Nullary twines always have Empty on the RHS.
      if (isNullary() && getRHSKind() != EmptyKind)
        return false;

      // Null should never appear on the RHS.
      if (getRHSKind() == NullKind)
        return false;

      // The RHS cannot be non-empty if the LHS is empty.
      if (getRHSKind() != EmptyKind && getLHSKind() == EmptyKind)
        return false;

      // A twine child should always be binary.
      if (getLHSKind() == TwineKind &&
          !static_cast<const Twine*>(LHS)->isBinary())
        return false;
      if (getRHSKind() == TwineKind &&
          !static_cast<const Twine*>(RHS)->isBinary())
        return false;

      return true;
    }

    /// getLHSKind - Get the NodeKind of the left-hand side.
    NodeKind getLHSKind() const { return LHSKind; }

    /// getRHSKind - Get the NodeKind of the left-hand side.
    NodeKind getRHSKind() const { return RHSKind; }

    /// printOneChild - Print one child from a twine.
    void printOneChild(raw_ostream &OS, const void *Ptr, NodeKind Kind) const;

    /// printOneChildRepr - Print the representation of one child from a twine.
    void printOneChildRepr(raw_ostream &OS, const void *Ptr,
                           NodeKind Kind) const;

  public:
    /// @name Constructors
    /// @{

    /// Construct from an empty string.
    /*implicit*/ Twine() : LHSKind(EmptyKind), RHSKind(EmptyKind) {
      assert(isValid() && "Invalid twine!");
    }

    /// Construct from a C string.
    ///
    /// We take care here to optimize "" into the empty twine -- this will be
    /// optimized out for string constants. This allows Twine arguments have
    /// default "" values, without introducing unnecessary string constants.
    /*implicit*/ Twine(const char *Str)
      : RHSKind(EmptyKind) {
      if (Str[0] != '\0') {
        LHS = Str;
        LHSKind = CStringKind;
      } else
        LHSKind = EmptyKind;

      assert(isValid() && "Invalid twine!");
    }

    /// Construct from an std::string.
    /*implicit*/ Twine(const std::string &Str)
      : LHS(&Str), LHSKind(StdStringKind), RHSKind(EmptyKind) {
      assert(isValid() && "Invalid twine!");
    }

    /// Construct from a StringRef.
    /*implicit*/ Twine(const StringRef &Str)
      : LHS(&Str), LHSKind(StringRefKind), RHSKind(EmptyKind) {
      assert(isValid() && "Invalid twine!");
    }

    /// Create a 'null' string, which is an empty string that always
    /// concatenates to form another empty string.
    static Twine createNull() {
      return Twine(NullKind);
    }

    // FIXME: Unfortunately, to make sure this is as efficient as possible we
    // need extra binary constructors from particular types. We can't rely on
    // the compiler to be smart enough to fold operator+()/concat() down to the
    // right thing. Yet.

    /// Construct as the concatenation of a C string and a StringRef.
    /*implicit*/ Twine(const char *_LHS, const StringRef &_RHS)
      : LHS(_LHS), RHS(&_RHS), LHSKind(CStringKind), RHSKind(StringRefKind) {
      assert(isValid() && "Invalid twine!");
    }

    /// Construct as the concatenation of a StringRef and a C string.
    /*implicit*/ Twine(const StringRef &_LHS, const char *_RHS)
      : LHS(&_LHS), RHS(_RHS), LHSKind(StringRefKind), RHSKind(CStringKind) {
      assert(isValid() && "Invalid twine!");
    }

    /// @}
    /// @name String Operations
    /// @{

    Twine concat(const Twine &Suffix) const;

    /// @}
    /// @name Output & Conversion.
    /// @{

    /// str - Return the twine contents as a std::string.
    std::string str() const;

    /// toVector - Write the concatenated string into the given SmallString or
    /// SmallVector.
    void toVector(SmallVectorImpl<char> &Out) const;

    /// print - Write the concatenated string represented by this twine to the
    /// stream \arg OS.
    void print(raw_ostream &OS) const;

    /// dump - Dump the concatenated string represented by this twine to stderr.
    void dump() const;

    /// print - Write the representation of this twine to the stream \arg OS.
    void printRepr(raw_ostream &OS) const;

    /// dumpRepr - Dump the representation of this twine to stderr.
    void dumpRepr() const;

    /// @}
  };

  /// @name Twine Inline Implementations
  /// @{

  inline Twine Twine::concat(const Twine &Suffix) const {
    // Concatenation with null is null.
    if (isNull() || Suffix.isNull())
      return Twine(NullKind);

    // Concatenation with empty yields the other side.
    if (isEmpty())
      return Suffix;
    if (Suffix.isEmpty())
      return *this;

    // Otherwise we need to create a new node, taking care to fold in unary
    // twines.
    const void *NewLHS = this, *NewRHS = &Suffix;
    NodeKind NewLHSKind = TwineKind, NewRHSKind = TwineKind;
    if (isUnary()) {
      NewLHS = LHS;
      NewLHSKind = getLHSKind();
    }
    if (Suffix.isUnary()) {
      NewRHS = Suffix.LHS;
      NewRHSKind = Suffix.getLHSKind();
    }

    return Twine(NewLHS, NewLHSKind, NewRHS, NewRHSKind);
  }

  inline Twine operator+(const Twine &LHS, const Twine &RHS) {
    return LHS.concat(RHS);
  }

  /// Additional overload to guarantee simplified codegen; this is equivalent to
  /// concat().

  inline Twine operator+(const char *LHS, const StringRef &RHS) {
    return Twine(LHS, RHS);
  }

  /// Additional overload to guarantee simplified codegen; this is equivalent to
  /// concat().

  inline Twine operator+(const StringRef &LHS, const char *RHS) {
    return Twine(LHS, RHS);
  }

  inline raw_ostream &operator<<(raw_ostream &OS, const Twine &RHS) {
    RHS.print(OS);
    return OS;
  }

  /// @}
}

#endif