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 //===- Twine.h - Fast Temporary String Concatenation ------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_ADT_TWINE_H
 #define LLVM_ADT_TWINE_H
 
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/ErrorHandling.h"
 #include <cassert>
 #include <cstdint>
 #include <string>
 #include <string_view>
 
 namespace llvm {
 
   class formatv_object_base;
   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 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
   ///
   /// 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
   ///
   /// 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 : unsigned char {
       /// An empty string; the result of concatenating anything with it is also
       /// empty.
       NullKind,
 
       /// The empty string.
       EmptyKind,
 
       /// A pointer to a Twine instance.
       TwineKind,
 
       /// A pointer to a C string instance.
       CStringKind,
 
       /// A pointer to an std::string instance.
       StdStringKind,
 
       /// A Pointer and Length representation. Used for std::string_view,
       /// StringRef, and SmallString.  Can't use a StringRef here
       /// because they are not trivally constructible.
       PtrAndLengthKind,
 
       /// A pointer and length representation that's also null-terminated.
       /// Guaranteed to be constructed from a compile-time string literal.
       StringLiteralKind,
 
       /// A pointer to a formatv_object_base instance.
       FormatvObjectKind,
 
       /// A char value, to render as a character.
       CharKind,
 
       /// An unsigned int value, to render as an unsigned decimal integer.
       DecUIKind,
 
       /// An int value, to render as a signed decimal integer.
       DecIKind,
 
       /// A pointer to an unsigned long value, to render as an unsigned decimal
       /// integer.
       DecULKind,
 
       /// A pointer to a long value, to render as a signed decimal integer.
       DecLKind,
 
       /// A pointer to an unsigned long long value, to render as an unsigned
       /// decimal integer.
       DecULLKind,
 
       /// A pointer to a long long value, to render as a signed decimal integer.
       DecLLKind,
 
       /// A pointer to a uint64_t value, to render as an unsigned hexadecimal
       /// integer.
       UHexKind
     };
 
     union Child
     {
       const Twine *twine;
       const char *cString;
       const std::string *stdString;
       struct {
         const char *ptr;
         size_t length;
       } ptrAndLength;
       const formatv_object_base *formatvObject;
       char character;
       unsigned int decUI;
       int decI;
       const unsigned long *decUL;
       const long *decL;
       const unsigned long long *decULL;
       const long long *decLL;
       const uint64_t *uHex;
     };
 
     /// LHS - The prefix in the concatenation, which may be uninitialized for
     /// Null or Empty kinds.
     Child LHS;
 
     /// RHS - The suffix in the concatenation, which may be uninitialized for
     /// Null or Empty kinds.
     Child RHS;
 
     /// LHSKind - The NodeKind of the left hand side, \see getLHSKind().
     NodeKind LHSKind = EmptyKind;
 
     /// RHSKind - The NodeKind of the right hand side, \see getRHSKind().
     NodeKind RHSKind = EmptyKind;
 
     /// Construct a nullary twine; the kind must be NullKind or EmptyKind.
     explicit Twine(NodeKind Kind) : LHSKind(Kind) {
       assert(isNullary() && "Invalid kind!");
     }
 
     /// Construct a binary twine.
     explicit Twine(const Twine &LHS, const Twine &RHS)
         : LHSKind(TwineKind), RHSKind(TwineKind) {
       this->LHS.twine = &LHS;
       this->RHS.twine = &RHS;
       assert(isValid() && "Invalid twine!");
     }
 
     /// Construct a twine from explicit values.
     explicit Twine(Child LHS, NodeKind LHSKind, Child RHS, NodeKind RHSKind)
         : LHS(LHS), RHS(RHS), LHSKind(LHSKind), RHSKind(RHSKind) {
       assert(isValid() && "Invalid twine!");
     }
 
     /// Check for the null twine.
     bool isNull() const {
       return getLHSKind() == NullKind;
     }
 
     /// Check for the empty twine.
     bool isEmpty() const {
       return getLHSKind() == EmptyKind;
     }
 
     /// Check if this is a nullary twine (null or empty).
     bool isNullary() const {
       return isNull() || isEmpty();
     }
 
     /// Check if this is a unary twine.
     bool isUnary() const {
       return getRHSKind() == EmptyKind && !isNullary();
     }
 
     /// Check if this is a binary twine.
     bool isBinary() const {
       return getLHSKind() != NullKind && getRHSKind() != EmptyKind;
     }
 
     /// 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 &&
           !LHS.twine->isBinary())
         return false;
       if (getRHSKind() == TwineKind &&
           !RHS.twine->isBinary())
         return false;
 
       return true;
     }
 
     /// Get the NodeKind of the left-hand side.
     NodeKind getLHSKind() const { return LHSKind; }
 
     /// Get the NodeKind of the right-hand side.
     NodeKind getRHSKind() const { return RHSKind; }
 
     /// Print one child from a twine.
     void printOneChild(raw_ostream &OS, Child Ptr, NodeKind Kind) const;
 
     /// Print the representation of one child from a twine.
     void printOneChildRepr(raw_ostream &OS, Child Ptr,
                            NodeKind Kind) const;
 
   public:
     /// @name Constructors
     /// @{
 
     /// Construct from an empty string.
     /*implicit*/ Twine() {
       assert(isValid() && "Invalid twine!");
     }
 
     Twine(const Twine &) = default;
 
     /// 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) {
       if (Str[0] != '\0') {
         LHS.cString = Str;
         LHSKind = CStringKind;
       } else
         LHSKind = EmptyKind;
 
       assert(isValid() && "Invalid twine!");
     }
     /// Delete the implicit conversion from nullptr as Twine(const char *)
     /// cannot take nullptr.
     /*implicit*/ Twine(std::nullptr_t) = delete;
 
     /// Construct from an std::string.
     /*implicit*/ Twine(const std::string &Str) : LHSKind(StdStringKind) {
       LHS.stdString = &Str;
       assert(isValid() && "Invalid twine!");
     }
 
     /// Construct from an std::string_view by converting it to a pointer and
     /// length.  This handles string_views on a pure API basis, and avoids
     /// storing one (or a pointer to one) inside a Twine, which avoids problems
     /// when mixing code compiled under various C++ standards.
     /*implicit*/ Twine(const std::string_view &Str)
         : LHSKind(PtrAndLengthKind) {
       LHS.ptrAndLength.ptr = Str.data();
       LHS.ptrAndLength.length = Str.length();
       assert(isValid() && "Invalid twine!");
     }
 
     /// Construct from a StringRef.
     /*implicit*/ Twine(const StringRef &Str) : LHSKind(PtrAndLengthKind) {
       LHS.ptrAndLength.ptr = Str.data();
       LHS.ptrAndLength.length = Str.size();
       assert(isValid() && "Invalid twine!");
     }
 
     /// Construct from a StringLiteral.
     /*implicit*/ Twine(const StringLiteral &Str)
         : LHSKind(StringLiteralKind) {
       LHS.ptrAndLength.ptr = Str.data();
       LHS.ptrAndLength.length = Str.size();
       assert(isValid() && "Invalid twine!");
     }
 
     /// Construct from a SmallString.
     /*implicit*/ Twine(const SmallVectorImpl<char> &Str)
         : LHSKind(PtrAndLengthKind) {
       LHS.ptrAndLength.ptr = Str.data();
       LHS.ptrAndLength.length = Str.size();
       assert(isValid() && "Invalid twine!");
     }
 
     /// Construct from a formatv_object_base.
     /*implicit*/ Twine(const formatv_object_base &Fmt)
         : LHSKind(FormatvObjectKind) {
       LHS.formatvObject = &Fmt;
       assert(isValid() && "Invalid twine!");
     }
 
     /// Construct from a char.
     explicit Twine(char Val) : LHSKind(CharKind) {
       LHS.character = Val;
     }
 
     /// Construct from a signed char.
     explicit Twine(signed char Val) : LHSKind(CharKind) {
       LHS.character = static_cast<char>(Val);
     }
 
     /// Construct from an unsigned char.
     explicit Twine(unsigned char Val) : LHSKind(CharKind) {
       LHS.character = static_cast<char>(Val);
     }
 
     /// Construct a twine to print \p Val as an unsigned decimal integer.
     explicit Twine(unsigned Val) : LHSKind(DecUIKind) {
       LHS.decUI = Val;
     }
 
     /// Construct a twine to print \p Val as a signed decimal integer.
     explicit Twine(int Val) : LHSKind(DecIKind) {
       LHS.decI = Val;
     }
 
     /// Construct a twine to print \p Val as an unsigned decimal integer.
     explicit Twine(const unsigned long &Val) : LHSKind(DecULKind) {
       LHS.decUL = &Val;
     }
 
     /// Construct a twine to print \p Val as a signed decimal integer.
     explicit Twine(const long &Val) : LHSKind(DecLKind) {
       LHS.decL = &Val;
     }
 
     /// Construct a twine to print \p Val as an unsigned decimal integer.
     explicit Twine(const unsigned long long &Val) : LHSKind(DecULLKind) {
       LHS.decULL = &Val;
     }
 
     /// Construct a twine to print \p Val as a signed decimal integer.
     explicit Twine(const long long &Val) : LHSKind(DecLLKind) {
       LHS.decLL = &Val;
     }
 
     // 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)
         : LHSKind(CStringKind), RHSKind(PtrAndLengthKind) {
       this->LHS.cString = LHS;
       this->RHS.ptrAndLength.ptr = RHS.data();
       this->RHS.ptrAndLength.length = RHS.size();
       assert(isValid() && "Invalid twine!");
     }
 
     /// Construct as the concatenation of a StringRef and a C string.
     /*implicit*/ Twine(const StringRef &LHS, const char *RHS)
         : LHSKind(PtrAndLengthKind), RHSKind(CStringKind) {
       this->LHS.ptrAndLength.ptr = LHS.data();
       this->LHS.ptrAndLength.length = LHS.size();
       this->RHS.cString = RHS;
       assert(isValid() && "Invalid twine!");
     }
 
     /// Since the intended use of twines is as temporary objects, assignments
     /// when concatenating might cause undefined behavior or stack corruptions
     Twine &operator=(const Twine &) = delete;
 
     /// Create a 'null' string, which is an empty string that always
     /// concatenates to form another empty string.
     static Twine createNull() {
       return Twine(NullKind);
     }
 
     /// @}
     /// @name Numeric Conversions
     /// @{
 
     // Construct a twine to print \p Val as an unsigned hexadecimal integer.
     static Twine utohexstr(const uint64_t &Val) {
       Child LHS, RHS;
       LHS.uHex = &Val;
       RHS.twine = nullptr;
       return Twine(LHS, UHexKind, RHS, EmptyKind);
     }
 
     /// @}
     /// @name Predicate Operations
     /// @{
 
     /// Check if this twine is trivially empty; a false return value does not
     /// necessarily mean the twine is empty.
     bool isTriviallyEmpty() const {
       return isNullary();
     }
 
     /// Check if this twine is guaranteed to refer to single string literal.
     bool isSingleStringLiteral() const {
       return isUnary() && getLHSKind() == StringLiteralKind;
     }
 
     /// Return true if this twine can be dynamically accessed as a single
     /// StringRef value with getSingleStringRef().
     bool isSingleStringRef() const {
       if (getRHSKind() != EmptyKind) return false;
 
       switch (getLHSKind()) {
       case EmptyKind:
       case CStringKind:
       case StdStringKind:
       case PtrAndLengthKind:
       case StringLiteralKind:
         return true;
       default:
         return false;
       }
     }
 
     /// @}
     /// @name String Operations
     /// @{
 
     Twine concat(const Twine &Suffix) const;
 
     /// @}
     /// @name Output & Conversion.
     /// @{
 
     /// Return the twine contents as a std::string.
     std::string str() const;
 
     /// Append the concatenated string into the given SmallString or SmallVector.
     void toVector(SmallVectorImpl<char> &Out) const;
 
     /// This returns the twine as a single StringRef.  This method is only valid
     /// if isSingleStringRef() is true.
     StringRef getSingleStringRef() const {
       assert(isSingleStringRef() &&"This cannot be had as a single stringref!");
       switch (getLHSKind()) {
       default: llvm_unreachable("Out of sync with isSingleStringRef");
       case EmptyKind:
         return StringRef();
       case CStringKind:
         return StringRef(LHS.cString);
       case StdStringKind:
         return StringRef(*LHS.stdString);
       case PtrAndLengthKind:
       case StringLiteralKind:
         return StringRef(LHS.ptrAndLength.ptr, LHS.ptrAndLength.length);
       }
     }
 
     /// This returns the twine as a single StringRef if it can be
     /// represented as such. Otherwise the twine is written into the given
     /// SmallVector and a StringRef to the SmallVector's data is returned.
     StringRef toStringRef(SmallVectorImpl<char> &Out) const {
       if (isSingleStringRef())
         return getSingleStringRef();
       toVector(Out);
       return StringRef(Out.data(), Out.size());
     }
 
     /// This returns the twine as a single null terminated StringRef if it
     /// can be represented as such. Otherwise the twine is written into the
     /// given SmallVector and a StringRef to the SmallVector's data is returned.
     ///
     /// The returned StringRef's size does not include the null terminator.
     StringRef toNullTerminatedStringRef(SmallVectorImpl<char> &Out) const;
 
     /// Write the concatenated string represented by this twine to the
     /// stream \p OS.
     void print(raw_ostream &OS) const;
 
     /// Dump the concatenated string represented by this twine to stderr.
     void dump() const;
 
     /// Write the representation of this twine to the stream \p OS.
     void printRepr(raw_ostream &OS) const;
 
     /// 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.
     Child NewLHS, NewRHS;
     NewLHS.twine = this;
     NewRHS.twine = &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;
   }
 
   /// @}
 
 } // end namespace llvm
 
 #endif // LLVM_ADT_TWINE_H

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