abseil-cpp/absl/strings/str_cat.h

//
// Copyright 2017 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// -----------------------------------------------------------------------------
// File: str_cat.h
// -----------------------------------------------------------------------------
//
// This package contains functions for efficiently concatenating and appending
// strings: `StrCat()` and `StrAppend()`. Most of the work within these routines
// is actually handled through use of a special AlphaNum type, which was
// designed to be used as a parameter type that efficiently manages conversion
// to strings and avoids copies in the above operations.
//
// Any routine accepting either a std::string or a number may accept `AlphaNum`.
// The basic idea is that by accepting a `const AlphaNum &` as an argument
// to your function, your callers will automagically convert bools, integers,
// and floating point values to strings for you.
//
// NOTE: Use of `AlphaNum` outside of the //absl/strings package is unsupported
// except for the specific case of function parameters of type `AlphaNum` or
// `const AlphaNum &`. In particular, instantiating `AlphaNum` directly as a
// stack variable is not supported.
//
// Conversion from 8-bit values is not accepted because, if it were, then an
// attempt to pass ':' instead of ":" might result in a 58 ending up in your
// result.
//
// Bools convert to "0" or "1".
//
// Floating point numbers are formatted with six-digit precision, which is
// the default for "std::cout <<" or printf "%g" (the same as "%.6g").
//
//
// You can convert to hexadecimal output rather than decimal output using the
// `Hex` type contained here. To do so, pass `Hex(my_int)` as a parameter to
// `StrCat()` or `StrAppend()`. You may specify a minimum hex field width using
// a `PadSpec` enum, so the equivalent of `StringPrintf("%04x", my_int)` is
// `absl::StrCat(absl::Hex(my_int, absl::kZeroPad4))`.
//
// -----------------------------------------------------------------------------

#ifndef ABSL_STRINGS_STR_CAT_H_
#define ABSL_STRINGS_STR_CAT_H_

#include <array>
#include <cstdint>
#include <string>
#include <type_traits>

#include "absl/base/port.h"
#include "absl/strings/numbers.h"
#include "absl/strings/string_view.h"

namespace absl {

namespace strings_internal {
// AlphaNumBuffer allows a way to pass a std::string to StrCat without having to do
// memory allocation.  It is simply a pair of a fixed-size character array, and
// a size.  Please don't use outside of absl, yet.
template <size_t max_size>
struct AlphaNumBuffer {
  std::array<char, max_size> data;
  size_t size;
};

}  // namespace strings_internal

// Enum that specifies the number of significant digits to return in a `Hex`
// conversion and fill character to use. A `kZeroPad2` value, for example, would
// produce hexadecimal strings such as "0A","0F" and 'kSpacePad5' value would
// produce hexadecimal strings such as "    A","    F".
enum PadSpec {
  kNoPad = 1,
  kZeroPad2,
  kZeroPad3,
  kZeroPad4,
  kZeroPad5,
  kZeroPad6,
  kZeroPad7,
  kZeroPad8,
  kZeroPad9,
  kZeroPad10,
  kZeroPad11,
  kZeroPad12,
  kZeroPad13,
  kZeroPad14,
  kZeroPad15,
  kZeroPad16,

  kSpacePad2 = kZeroPad2 + 64,
  kSpacePad3,
  kSpacePad4,
  kSpacePad5,
  kSpacePad6,
  kSpacePad7,
  kSpacePad8,
  kSpacePad9,
  kSpacePad10,
  kSpacePad11,
  kSpacePad12,
  kSpacePad13,
  kSpacePad14,
  kSpacePad15,
  kSpacePad16,
};

// -----------------------------------------------------------------------------
// Hex
// -----------------------------------------------------------------------------
//
// `Hex` stores a set of hexadecimal std::string conversion parameters for use
// within `AlphaNum` std::string conversions.
struct Hex {
  uint64_t value;
  uint8_t width;
  char fill;

  template <typename Int>
  explicit Hex(Int v, PadSpec spec = absl::kNoPad,
               typename std::enable_if<sizeof(Int) == 1>::type* = nullptr)
      : Hex(spec, static_cast<uint8_t>(v)) {}
  template <typename Int>
  explicit Hex(Int v, PadSpec spec = absl::kNoPad,
               typename std::enable_if<sizeof(Int) == 2>::type* = nullptr)
      : Hex(spec, static_cast<uint16_t>(v)) {}
  template <typename Int>
  explicit Hex(Int v, PadSpec spec = absl::kNoPad,
               typename std::enable_if<sizeof(Int) == 4>::type* = nullptr)
      : Hex(spec, static_cast<uint32_t>(v)) {}
  template <typename Int>
  explicit Hex(Int v, PadSpec spec = absl::kNoPad,
               typename std::enable_if<sizeof(Int) == 8>::type* = nullptr)
      : Hex(spec, static_cast<uint64_t>(v)) {}

 private:
  Hex(PadSpec spec, uint64_t v)
      : value(v),
        width(spec == absl::kNoPad
                  ? 1
                  : spec >= absl::kSpacePad2 ? spec - absl::kSpacePad2 + 2
                                             : spec - absl::kZeroPad2 + 2),
        fill(spec >= absl::kSpacePad2 ? ' ' : '0') {}
};

// -----------------------------------------------------------------------------
// AlphaNum
// -----------------------------------------------------------------------------
//
// The `AlphaNum` class acts as the main parameter type for `StrCat()` and
// `StrAppend()`, providing efficient conversion of numeric, boolean, and
// hexadecimal values (through the `Hex` type) into strings.

class AlphaNum {
 public:
  // No bool ctor -- bools convert to an integral type.
  // A bool ctor would also convert incoming pointers (bletch).

  AlphaNum(int x)  // NOLINT(runtime/explicit)
      : piece_(digits_,
               numbers_internal::FastIntToBuffer(x, digits_) - &digits_[0]) {}
  AlphaNum(unsigned int x)  // NOLINT(runtime/explicit)
      : piece_(digits_,
               numbers_internal::FastIntToBuffer(x, digits_) - &digits_[0]) {}
  AlphaNum(long x)  // NOLINT(*)
      : piece_(digits_,
               numbers_internal::FastIntToBuffer(x, digits_) - &digits_[0]) {}
  AlphaNum(unsigned long x)  // NOLINT(*)
      : piece_(digits_,
               numbers_internal::FastIntToBuffer(x, digits_) - &digits_[0]) {}
  AlphaNum(long long x)  // NOLINT(*)
      : piece_(digits_,
               numbers_internal::FastIntToBuffer(x, digits_) - &digits_[0]) {}
  AlphaNum(unsigned long long x)  // NOLINT(*)
      : piece_(digits_,
               numbers_internal::FastIntToBuffer(x, digits_) - &digits_[0]) {}

  AlphaNum(float f)  // NOLINT(runtime/explicit)
      : piece_(digits_, numbers_internal::SixDigitsToBuffer(f, digits_)) {}
  AlphaNum(double f)  // NOLINT(runtime/explicit)
      : piece_(digits_, numbers_internal::SixDigitsToBuffer(f, digits_)) {}

  AlphaNum(Hex hex);  // NOLINT(runtime/explicit)

  template <size_t size>
  AlphaNum(  // NOLINT(runtime/explicit)
      const strings_internal::AlphaNumBuffer<size>& buf)
      : piece_(&buf.data[0], buf.size) {}

  AlphaNum(const char* c_str) : piece_(c_str) {}  // NOLINT(runtime/explicit)
  AlphaNum(absl::string_view pc) : piece_(pc) {}  // NOLINT(runtime/explicit)
  template <typename Allocator>
  AlphaNum(  // NOLINT(runtime/explicit)
      const std::basic_string<char, std::char_traits<char>, Allocator>& str)
      : piece_(str) {}

  // Use std::string literals ":" instead of character literals ':'.
  AlphaNum(char c) = delete;  // NOLINT(runtime/explicit)

  AlphaNum(const AlphaNum&) = delete;
  AlphaNum& operator=(const AlphaNum&) = delete;

  absl::string_view::size_type size() const { return piece_.size(); }
  const char* data() const { return piece_.data(); }
  absl::string_view Piece() const { return piece_; }

  // Normal enums are already handled by the integer formatters.
  // This overload matches only scoped enums.
  template <typename T,
            typename = typename std::enable_if<
                std::is_enum<T>{} && !std::is_convertible<T, int>{}>::type>
  AlphaNum(T e)  // NOLINT(runtime/explicit)
      : AlphaNum(static_cast<typename std::underlying_type<T>::type>(e)) {}

 private:
  absl::string_view piece_;
  char digits_[numbers_internal::kFastToBufferSize];
};

// -----------------------------------------------------------------------------
// StrCat()
// -----------------------------------------------------------------------------
//
// Merges given strings or numbers, using no delimiter(s).
//
// `StrCat()` is designed to be the fastest possible way to construct a std::string
// out of a mix of raw C strings, string_views, strings, bool values,
// and numeric values.
//
// Don't use `StrCat()` for user-visible strings. The localization process
// works poorly on strings built up out of fragments.
//
// For clarity and performance, don't use `StrCat()` when appending to a
// std::string. Use `StrAppend()` instead. In particular, avoid using any of these
// (anti-)patterns:
//
//   str.append(StrCat(...))
//   str += StrCat(...)
//   str = StrCat(str, ...)
//
// The last case is the worst, with a potential to change a loop
// from a linear time operation with O(1) dynamic allocations into a
// quadratic time operation with O(n) dynamic allocations.
//
// See `StrAppend()` below for more information.

namespace strings_internal {

// Do not call directly - this is not part of the public API.
std::string CatPieces(std::initializer_list<absl::string_view> pieces);
void AppendPieces(std::string* dest,
                  std::initializer_list<absl::string_view> pieces);

}  // namespace strings_internal

ABSL_MUST_USE_RESULT inline std::string StrCat() { return std::string(); }

ABSL_MUST_USE_RESULT inline std::string StrCat(const AlphaNum& a) {
  return std::string(a.data(), a.size());
}

ABSL_MUST_USE_RESULT std::string StrCat(const AlphaNum& a, const AlphaNum& b);
ABSL_MUST_USE_RESULT std::string StrCat(const AlphaNum& a, const AlphaNum& b,
                                   const AlphaNum& c);
ABSL_MUST_USE_RESULT std::string StrCat(const AlphaNum& a, const AlphaNum& b,
                                   const AlphaNum& c, const AlphaNum& d);

// Support 5 or more arguments
template <typename... AV>
ABSL_MUST_USE_RESULT inline std::string StrCat(const AlphaNum& a, const AlphaNum& b,
                                          const AlphaNum& c, const AlphaNum& d,
                                          const AlphaNum& e,
                                          const AV&... args) {
  return strings_internal::CatPieces(
      {a.Piece(), b.Piece(), c.Piece(), d.Piece(), e.Piece(),
       static_cast<const AlphaNum&>(args).Piece()...});
}

// -----------------------------------------------------------------------------
// StrAppend()
// -----------------------------------------------------------------------------
//
// Appends a std::string or set of strings to an existing std::string, in a similar
// fashion to `StrCat()`.
//
// WARNING: `StrAppend(&str, a, b, c, ...)` requires that none of the
// a, b, c, parameters be a reference into str. For speed, `StrAppend()` does
// not try to check each of its input arguments to be sure that they are not
// a subset of the std::string being appended to. That is, while this will work:
//
//   std::string s = "foo";
//   s += s;
//
// This output is undefined:
//
//   std::string s = "foo";
//   StrAppend(&s, s);
//
// This output is undefined as well, since `absl::string_view` does not own its
// data:
//
//   std::string s = "foobar";
//   absl::string_view p = s;
//   StrAppend(&s, p);

inline void StrAppend(std::string*) {}
void StrAppend(std::string* dest, const AlphaNum& a);
void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b);
void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b,
               const AlphaNum& c);
void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b,
               const AlphaNum& c, const AlphaNum& d);

// Support 5 or more arguments
template <typename... AV>
inline void StrAppend(std::string* dest, const AlphaNum& a, const AlphaNum& b,
                      const AlphaNum& c, const AlphaNum& d, const AlphaNum& e,
                      const AV&... args) {
  strings_internal::AppendPieces(
      dest, {a.Piece(), b.Piece(), c.Piece(), d.Piece(), e.Piece(),
             static_cast<const AlphaNum&>(args).Piece()...});
}

// Helper function for the future StrCat default floating-point format, %.6g
// This is fast.
inline strings_internal::AlphaNumBuffer<
    numbers_internal::kSixDigitsToBufferSize>
SixDigits(double d) {
  strings_internal::AlphaNumBuffer<numbers_internal::kSixDigitsToBufferSize>
      result;
  result.size = numbers_internal::SixDigitsToBuffer(d, &result.data[0]);
  return result;
}

}  // namespace absl

#endif  // ABSL_STRINGS_STR_CAT_H_