html_escape.h

#ifndef SOURCEMETA_CORE_HTML_ESCAPE_H_
#define SOURCEMETA_CORE_HTML_ESCAPE_H_
 
#ifndef SOURCEMETA_CORE_HTML_EXPORT
#include <sourcemeta/core/html_export.h>
#endif
 
#include <sourcemeta/core/html_buffer.h>
#include <sourcemeta/core/preprocessor.h>
 
#include <array>       // std::array
#include <concepts>    // std::same_as
#include <cstddef>     // std::size_t
#include <cstdint>     // std::uint8_t, std::uint64_t
#include <cstring>     // std::memcpy
#include <string>      // std::string
#include <string_view> // std::string_view
 
namespace sourcemeta::core {

SOURCEMETA_CORE_HTML_EXPORT
auto html_escape(std::string &text) -> void;

template <typename Output>
  requires std::same_as<Output, std::string> || std::same_as<Output, HTMLBuffer>
inline auto html_escape_append(Output &output, const std::string_view input)
    -> void {
  // The bytes that escaping may replace, which are the quotation mark, the
  // ampersand, the apostrophe, the angle brackets, and the first byte of the
  // UTF-8 encoding of the no-break space
  static constexpr std::array<std::uint8_t, 256> SPECIAL_BYTES{{
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0x00
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0x10
      0, 0, 1, 0, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, // 0x20
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 1, 0, // 0x30
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0x40
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0x50
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0x60
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0x70
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0x80
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0x90
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0xA0
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0xB0
      0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0xC0
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0xD0
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0xE0
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0  // 0xF0
  }};
  constexpr std::uint64_t LOW_BITS{0x0101010101010101ULL};
  constexpr std::uint64_t HIGH_BITS{0x8080808080808080ULL};
 
  const auto size{input.size()};
  // Whatever stays as it is goes out in runs, so that text with nothing to
  // escape takes a single append
  std::size_t run_start{0};
  std::size_t position{0};
  while (position < size) {
    auto end{size};
    if (size - position >= 8) {
      std::uint64_t word{0};
      std::memcpy(&word, input.data() + position, 8);
      // A byte of a word equals another when subtracting from their difference
      // borrows, which also flags the byte right after a genuine match, and
      // that is harmless when only asking whether the word has a match at all.
      // Setting the lowest bit of every byte makes the ampersand match the
      // apostrophe, and setting the second lowest bit makes the less-than sign
      // match the greater-than sign, so four comparisons cover the six bytes
      const auto quotations{word ^ (LOW_BITS * '"')};
      const auto apostrophes{(word | LOW_BITS) ^ (LOW_BITS * '\'')};
      const auto angles{(word | (LOW_BITS * 2U)) ^ (LOW_BITS * '>')};
      const auto leads{word ^ (LOW_BITS * 0xC2)};
      const auto matches{((quotations - LOW_BITS) & ~quotations) |
                         ((apostrophes - LOW_BITS) & ~apostrophes) |
                         ((angles - LOW_BITS) & ~angles) |
                         ((leads - LOW_BITS) & ~leads)};
      if ((matches & HIGH_BITS) == 0) {
        position += 8;
        continue;
      }
 
      end = position + 8;
    }
 
    while (position < end) {
      if (SPECIAL_BYTES[static_cast<unsigned char>(input[position])] == 0) {
        position += 1;
        continue;
      }
 
      std::string_view replacement;
      std::size_t consumed{1};
      switch (input[position]) {
        case '&':
          replacement = "&amp;";
          break;
        case '<':
          replacement = "&lt;";
          break;
        case '>':
          replacement = "&gt;";
          break;
        case '"':
          replacement = "&quot;";
          break;
        case '\'':
          replacement = "&#39;";
          break;
        default:
          // The no-break space is replaced by its named entity (HTML Living
          // Standard "escaping a string" step 2)
          if (static_cast<unsigned char>(input[position]) == 0xC2 &&
              position + 1 < size &&
              static_cast<unsigned char>(input[position + 1]) == 0xA0) {
            replacement = "&nbsp;";
            consumed = 2;
          }
 
          break;
      }
 
      if (replacement.empty()) {
        position += 1;
        continue;
      }
 
      output.append(input.substr(run_start, position - run_start));
      output.append(replacement);
      position += consumed;
      run_start = position;
    }
  }
 
  output.append(input.substr(run_start));
}
 
} // namespace sourcemeta::core
 
#endif