/*
 RCC-Project

 Copyright (C) 2026 Richard Qin

 This file is part of RCC-Project.

 RCC-Project is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free
 Software Foundation; either version 3 of the License, or (at your option) any
 later version.

 RCC-Project is distributed in the hope that it will be useful, but WITHOUT ANY
 WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
 PARTICULAR PURPOSE. See the GNU General Public License for more details.

 You should have received a copy of the GNU General Public License along with
 RCC-Project; see the file COPYING. If not see
 <http://www.gnu.org/licenses/>.
 */

//===- core/adt/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 LIBBASE_ADT_TWINE_H
#define LIBBASE_ADT_TWINE_H

#include "core/adt/SmallVector.h"
#include "core/compiler/Compiler.h"
#include "core/error/ErrorHandling.h"
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <string>
#include <string_view>

namespace libbase
{

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
/// string view) 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 and
    /// SmallVector. It stores the pointer and length directly.
    PtrAndLengthKind,

    /// 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,

    /// An unsigned long value, to render as an unsigned decimal integer.
    DecULKind,

    /// A long value, to render as a signed decimal integer.
    DecLKind,

    /// An unsigned long long value, to render as an unsigned decimal integer.
    DecULLKind,

    /// A long long value, to render as a signed decimal integer.
    DecLLKind,

    /// 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;
    unsigned long decUL;
    long decL;
    unsigned long long decULL;
    long long decLL;
    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 (std::string_view Str) : LHSKind (PtrAndLengthKind)
  {
    LHS.ptrAndLength.ptr = Str.data ();
    LHS.ptrAndLength.length = Str.length ();
    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 (unsigned long Val) : LHSKind (DecULKind)
  {
    LHS.decUL = Val;
  }

  /// Construct a twine to print \p Val as a signed decimal integer.
  explicit Twine (long Val) : LHSKind (DecLKind)
  {
    LHS.decL = Val;
  }

  /// Construct a twine to print \p Val as an unsigned decimal integer.
  explicit Twine (unsigned long long Val) : LHSKind (DecULLKind)
  {
    LHS.decULL = Val;
  }

  /// Construct a twine to print \p Val as a signed decimal integer.
  explicit Twine (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 string view.
  /*implicit*/ Twine (const char *LHS, std::string_view 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 string view and a C string.
  /*implicit*/ Twine (std::string_view 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 (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 ();
  }

  /// Return true if this twine can be dynamically accessed as a single
  /// string view with getSingleStringView().
  bool
  isSingleStringView () const
  {
    if (getRHSKind () != EmptyKind)
    {
      return false;
    }

    switch (getLHSKind ())
    {
    case EmptyKind:
    case CStringKind:
    case StdStringKind:
    case PtrAndLengthKind:
      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.
  LIBBASE_ABI std::string
  str () const;

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

  /// This returns the twine as a single string view. This method is only valid
  /// if isSingleStringView() is true.
  std::string_view
  getSingleStringView () const
  {
    assert (isSingleStringView () &&
            "This cannot be had as a single string view!");
    switch (getLHSKind ())
    {
    default:
      libbase_unreachable ("Out of sync with isSingleStringView");
    case EmptyKind:
      return std::string_view ();
    case CStringKind:
      return std::string_view (LHS.cString);
    case StdStringKind:
      return std::string_view (*LHS.stdString);
    case PtrAndLengthKind:
      return std::string_view (LHS.ptrAndLength.ptr, LHS.ptrAndLength.length);
    }
  }

  /// This returns the twine as a single string view if it can be
  /// represented as such. Otherwise the twine is written into the given
  /// SmallVector and a string view to the SmallVector's data is returned.
  std::string_view
  toStringView (SmallVectorImpl<char> &Out) const
  {
    if (isSingleStringView ())
    {
      return getSingleStringView ();
    }
    toVector (Out);
    return std::string_view (Out.data (), Out.size ());
  }

  /// This returns the twine as a single null terminated string view if it
  /// can be represented as such. Otherwise the twine is written into the
  /// given SmallVector and a string view to the SmallVector's data is returned.
  ///
  /// The returned string view's size does not include the null terminator.
  LIBBASE_ABI std::string_view
  toNullTerminatedStringView (SmallVectorImpl<char> &Out) const;

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

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

#if !defined(NDEBUG) || defined(LIBBASE_ENABLE_DUMP)
  /// Dump the concatenated string represented by this twine to stderr.
  LIBBASE_DUMP_METHOD void
  dump () const;

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

  /// @}
};

/// @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, std::string_view RHS)
{
  return Twine (LHS, RHS);
}

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

inline Twine
operator+ (std::string_view 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 libbase

#endif // LIBBASE_ADT_TWINE_H