src/__support/CPP/string_view.h

//===-- Standalone implementation std::string_view --------------*- 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_LIBC_SRC_SUPPORT_CPP_STRINGVIEW_H
#define LLVM_LIBC_SRC_SUPPORT_CPP_STRINGVIEW_H

#include "src/__support/common.h"

#include <stddef.h>

namespace __llvm_libc {
namespace cpp {

// This is very simple alternate of the std::string_view class. There is no
// bounds check performed in any of the methods. The callers are expected to
// do the checks before invoking the methods.
//
// This class will be extended as needed in future.
class string_view {
private:
  const char *Data;
  size_t Len;

  static size_t min(size_t A, size_t B) { return A <= B ? A : B; }

  LIBC_INLINE static int compareMemory(const char *Lhs, const char *Rhs,
                                       size_t Length) {
    for (size_t I = 0; I < Length; ++I)
      if (int Diff = (int)Lhs[I] - (int)Rhs[I])
        return Diff;
    return 0;
  }

  LIBC_INLINE static constexpr size_t length(const char *Str) {
    for (const char *End = Str;; ++End)
      if (*End == '\0')
        return End - Str;
  }

  LIBC_INLINE bool equals(string_view Other) const {
    return (Len == Other.Len &&
            compareMemory(Data, Other.Data, Other.Len) == 0);
  }

public:
  using value_type = char;
  using size_type = size_t;
  using difference_type = ptrdiff_t;
  using pointer = char *;
  using const_pointer = const char *;
  using reference = char &;
  using const_reference = const char &;
  using const_iterator = char *;
  using iterator = const_iterator;

  // special value equal to the maximum value representable by the type
  // size_type.
  inline static constexpr size_t npos = -1;

  LIBC_INLINE constexpr string_view() : Data(nullptr), Len(0) {}

  // Assumes Str is a null-terminated string. The length of the string does
  // not include the terminating null character.
  // Preconditions: [Str, Str + ​length(Str)) is a valid range.
  LIBC_INLINE constexpr string_view(const char *Str)
      : Data(Str), Len(length(Str)) {}

  // Preconditions: [Str, Str + N) is a valid range.
  LIBC_INLINE constexpr string_view(const char *Str, size_t N)
      : Data(Str), Len(N) {}

  LIBC_INLINE constexpr const char *data() const { return Data; }

  // Returns the size of the string_view.
  LIBC_INLINE constexpr size_t size() const { return Len; }

  // Returns whether the string_view is empty.
  LIBC_INLINE constexpr bool empty() const { return Len == 0; }

  // Returns an iterator to the first character of the view.
  LIBC_INLINE const char *begin() const { return Data; }

  // Returns an iterator to the character following the last character of the
  // view.
  LIBC_INLINE const char *end() const { return Data + Len; }

  // Returns a const reference to the character at specified location pos.
  // No bounds checking is performed: the behavior is undefined if pos >=
  // size().
  LIBC_INLINE constexpr const char &operator[](size_t Index) const {
    return Data[Index];
  }

  /// compare - Compare two strings; the result is -1, 0, or 1 if this string
  /// is lexicographically less than, equal to, or greater than the \p Other.
  LIBC_INLINE int compare(string_view Other) const {
    // Check the prefix for a mismatch.
    if (int Res = compareMemory(Data, Other.Data, min(Len, Other.Len)))
      return Res < 0 ? -1 : 1;
    // Otherwise the prefixes match, so we only need to check the lengths.
    if (Len == Other.Len)
      return 0;
    return Len < Other.Len ? -1 : 1;
  }

  LIBC_INLINE bool operator==(string_view Other) const { return equals(Other); }
  LIBC_INLINE bool operator!=(string_view Other) const {
    return !(*this == Other);
  }
  LIBC_INLINE bool operator<(string_view Other) const {
    return compare(Other) == -1;
  }
  LIBC_INLINE bool operator<=(string_view Other) const {
    return compare(Other) != 1;
  }
  LIBC_INLINE bool operator>(string_view Other) const {
    return compare(Other) == 1;
  }
  LIBC_INLINE bool operator>=(string_view Other) const {
    return compare(Other) != -1;
  }

  // Moves the start of the view forward by n characters.
  // The behavior is undefined if n > size().
  LIBC_INLINE void remove_prefix(size_t N) {
    Len -= N;
    Data += N;
  }

  // Moves the end of the view back by n characters.
  // The behavior is undefined if n > size().
  LIBC_INLINE void remove_suffix(size_t N) { Len -= N; }

  // Check if this string starts with the given Prefix.
  LIBC_INLINE bool starts_with(string_view Prefix) const {
    return Len >= Prefix.Len &&
           compareMemory(Data, Prefix.Data, Prefix.Len) == 0;
  }

  // Check if this string starts with the given Prefix.
  LIBC_INLINE bool starts_with(const char Prefix) const {
    return !empty() && front() == Prefix;
  }

  // Check if this string ends with the given Prefix.
  LIBC_INLINE bool ends_with(const char Suffix) const {
    return !empty() && back() == Suffix;
  }

  // Check if this string ends with the given Suffix.
  LIBC_INLINE bool ends_with(string_view Suffix) const {
    return Len >= Suffix.Len &&
           compareMemory(end() - Suffix.Len, Suffix.Data, Suffix.Len) == 0;
  }

  // Return a reference to the substring from [Start, Start + N).
  //
  // Start The index of the starting character in the substring; if the index is
  // npos or greater than the length of the string then the empty substring will
  // be returned.
  //
  // N The number of characters to included in the substring. If N exceeds the
  // number of characters remaining in the string, the string suffix (starting
  // with Start) will be returned.
  LIBC_INLINE string_view substr(size_t Start, size_t N = npos) const {
    Start = min(Start, Len);
    return string_view(Data + Start, min(N, Len - Start));
  }

  // front - Get the first character in the string.
  LIBC_INLINE char front() const { return Data[0]; }

  // back - Get the last character in the string.
  LIBC_INLINE char back() const { return Data[Len - 1]; }

  // Finds the first occurence of c in this view, starting at position From.
  LIBC_INLINE size_t find_first_of(const char c, size_t From = 0) const {
    for (size_t Pos = From; Pos < size(); ++Pos)
      if ((*this)[Pos] == c)
        return Pos;
    return npos;
  }

  // Finds the last occurence of c in this view, ending at position End.
  LIBC_INLINE size_t find_last_of(const char c, size_t End = npos) const {
    End = End >= size() ? size() : End + 1;
    for (; End > 0; --End)
      if ((*this)[End - 1] == c)
        return End - 1;
    return npos;
  }
};

} // namespace cpp
} // namespace __llvm_libc

#endif // LLVM_LIBC_SRC_SUPPORT_CPP_STRINGVIEW_H