json_escape.h

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#pragma once
#include <cstddef>
#include <cstdint>
#include <span>
 
#include "esphome/core/helpers.h"
#include "esphome/core/string_ref.h"
 
namespace esphome::captive_portal {
 
static constexpr size_t JSON_ESCAPE_MAX_EXPANSION = 6;
 
inline const char *json_escape_into_buffer(std::span<char> buf, StringRef value) {
  if (buf.empty())
    return "";
  // Reserve one byte for the null terminator.
  const size_t limit = buf.size() - 1;
  size_t pos = 0;
  for (char ch : value) {
    auto c = static_cast<unsigned char>(ch);
    // Every short form is a backslash followed by a single character, so only that character is needed here. Keeping
    // it a char rather than a string avoids putting the sequences in read only data, which is RAM on the ESP8266.
    char escape = '\0';
    switch (c) {
      case '"':
        escape = '"';
        break;
      case '\\':
        escape = '\\';
        break;
      case '\n':
        escape = 'n';
        break;
      case '\r':
        escape = 'r';
        break;
      case '\t':
        escape = 't';
        break;
      case '\b':
        escape = 'b';
        break;
      case '\f':
        escape = 'f';
        break;
      default:
        break;
    }
    if (escape != '\0') {
      if (pos + 2 > limit)
        break;
      buf[pos++] = '\\';
      buf[pos++] = escape;
    } else if (c < 0x20) {
      // Remaining control characters have no short form and must be written as \u00XX. The value is below 0x20, so
      // the two high hex digits are always zero.
      if (pos + JSON_ESCAPE_MAX_EXPANSION > limit)
        break;
      buf[pos++] = '\\';
      buf[pos++] = 'u';
      buf[pos++] = '0';
      buf[pos++] = '0';
      buf[pos++] = format_hex_char(static_cast<uint8_t>(c >> 4));
      buf[pos++] = format_hex_char(static_cast<uint8_t>(c & 0x0F));
    } else {
      if (pos + 1 > limit)
        break;
      buf[pos++] = static_cast<char>(c);
    }
  }
  buf[pos] = '\0';
  return buf.data();
}
 
}  // namespace esphome::captive_portal