path: root/src/json.cpp

#include "cepis/json.hpp"

#include <cmath>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <string>

namespace cepis {
namespace {

const std::string& empty_string() {
  static const std::string value;
  return value;
}

const Json::Array& empty_array() {
  static const Json::Array value;
  return value;
}

const Json::Object& empty_object() {
  static const Json::Object value;
  return value;
}

void append_utf8(std::string& out, std::uint32_t code_point) {
  if (code_point <= 0x7F) {
    out.push_back(static_cast<char>(code_point));
  } else if (code_point <= 0x7FF) {
    out.push_back(static_cast<char>(0xC0 | (code_point >> 6)));
    out.push_back(static_cast<char>(0x80 | (code_point & 0x3F)));
  } else if (code_point <= 0xFFFF) {
    out.push_back(static_cast<char>(0xE0 | (code_point >> 12)));
    out.push_back(static_cast<char>(0x80 | ((code_point >> 6) & 0x3F)));
    out.push_back(static_cast<char>(0x80 | (code_point & 0x3F)));
  } else {
    out.push_back(static_cast<char>(0xF0 | (code_point >> 18)));
    out.push_back(static_cast<char>(0x80 | ((code_point >> 12) & 0x3F)));
    out.push_back(static_cast<char>(0x80 | ((code_point >> 6) & 0x3F)));
    out.push_back(static_cast<char>(0x80 | (code_point & 0x3F)));
  }
}

class Parser {
 public:
  explicit Parser(std::string_view text) : text_(text) {}

  std::optional<Json> run() {
    skip_whitespace();
    std::optional<Json> value = parse_value(0);
    if (!value) {
      return std::nullopt;
    }
    skip_whitespace();
    if (pos_ != text_.size()) {
      return std::nullopt;
    }
    return value;
  }

 private:
  static constexpr int kMaxDepth = 64;

  [[nodiscard]] bool at_end() const { return pos_ >= text_.size(); }
  [[nodiscard]] char peek() const { return text_[pos_]; }

  void skip_whitespace() {
    while (!at_end()) {
      const char c = peek();
      if (c == ' ' || c == '\t' || c == '\n' || c == '\r') {
        ++pos_;
      } else {
        break;
      }
    }
  }

  bool consume(char expected) {
    if (at_end() || peek() != expected) {
      return false;
    }
    ++pos_;
    return true;
  }

  bool consume(std::string_view literal) {
    if (text_.compare(pos_, literal.size(), literal) != 0) {
      return false;
    }
    pos_ += literal.size();
    return true;
  }

  std::optional<Json> parse_value(int depth) {
    if (depth > kMaxDepth || at_end()) {
      return std::nullopt;
    }
    switch (peek()) {
      case '{':
        return parse_object(depth);
      case '[':
        return parse_array(depth);
      case '"': {
        std::optional<std::string> text = parse_string();
        if (!text) {
          return std::nullopt;
        }
        return Json::string(std::move(*text));
      }
      case 't':
        return consume("true") ? std::optional<Json>{Json::boolean(true)} : std::nullopt;
      case 'f':
        return consume("false") ? std::optional<Json>{Json::boolean(false)} : std::nullopt;
      case 'n':
        return consume("null") ? std::optional<Json>{Json{}} : std::nullopt;
      default:
        return parse_number();
    }
  }

  std::optional<Json> parse_object(int depth) {
    ++pos_;
    Json result = Json::object();
    skip_whitespace();
    if (consume('}')) {
      return result;
    }
    while (true) {
      skip_whitespace();
      if (at_end() || peek() != '"') {
        return std::nullopt;
      }
      std::optional<std::string> key = parse_string();
      if (!key) {
        return std::nullopt;
      }
      skip_whitespace();
      if (!consume(':')) {
        return std::nullopt;
      }
      skip_whitespace();
      std::optional<Json> value = parse_value(depth + 1);
      if (!value) {
        return std::nullopt;
      }
      result.set(std::move(*key), std::move(*value));
      skip_whitespace();
      if (consume(',')) {
        continue;
      }
      if (consume('}')) {
        return result;
      }
      return std::nullopt;
    }
  }

  std::optional<Json> parse_array(int depth) {
    ++pos_;
    Json result = Json::array();
    skip_whitespace();
    if (consume(']')) {
      return result;
    }
    while (true) {
      skip_whitespace();
      std::optional<Json> value = parse_value(depth + 1);
      if (!value) {
        return std::nullopt;
      }
      result.push_back(std::move(*value));
      skip_whitespace();
      if (consume(',')) {
        continue;
      }
      if (consume(']')) {
        return result;
      }
      return std::nullopt;
    }
  }

  std::optional<std::string> parse_string() {
    ++pos_;
    std::string out;
    while (true) {
      if (at_end()) {
        return std::nullopt;
      }
      const char c = text_[pos_++];
      if (c == '"') {
        return out;
      }
      if (static_cast<unsigned char>(c) < 0x20) {
        return std::nullopt;
      }
      if (c != '\\') {
        out.push_back(c);
        continue;
      }
      if (at_end()) {
        return std::nullopt;
      }
      const char escape = text_[pos_++];
      switch (escape) {
        case '"':
          out.push_back('"');
          break;
        case '\\':
          out.push_back('\\');
          break;
        case '/':
          out.push_back('/');
          break;
        case 'b':
          out.push_back('\b');
          break;
        case 'f':
          out.push_back('\f');
          break;
        case 'n':
          out.push_back('\n');
          break;
        case 'r':
          out.push_back('\r');
          break;
        case 't':
          out.push_back('\t');
          break;
        case 'u': {
          std::optional<std::uint32_t> unit = parse_hex4();
          if (!unit) {
            return std::nullopt;
          }
          std::uint32_t code_point = *unit;
          if (code_point >= 0xD800 && code_point <= 0xDBFF) {
            if (text_.compare(pos_, 2, "\\u") == 0) {
              const std::size_t saved = pos_;
              pos_ += 2;
              std::optional<std::uint32_t> low = parse_hex4();
              if (low && *low >= 0xDC00 && *low <= 0xDFFF) {
                code_point = 0x10000 + ((code_point - 0xD800) << 10) + (*low - 0xDC00);
              } else {
                pos_ = saved;
                code_point = 0xFFFD;
              }
            } else {
              code_point = 0xFFFD;
            }
          } else if (code_point >= 0xDC00 && code_point <= 0xDFFF) {
            code_point = 0xFFFD;
          }
          append_utf8(out, code_point);
          break;
        }
        default:
          return std::nullopt;
      }
    }
  }

  std::optional<std::uint32_t> parse_hex4() {
    if (pos_ + 4 > text_.size()) {
      return std::nullopt;
    }
    std::uint32_t value = 0;
    for (int i = 0; i < 4; ++i) {
      const char c = text_[pos_++];
      value <<= 4;
      if (c >= '0' && c <= '9') {
        value |= static_cast<std::uint32_t>(c - '0');
      } else if (c >= 'a' && c <= 'f') {
        value |= static_cast<std::uint32_t>(c - 'a' + 10);
      } else if (c >= 'A' && c <= 'F') {
        value |= static_cast<std::uint32_t>(c - 'A' + 10);
      } else {
        return std::nullopt;
      }
    }
    return value;
  }

  std::optional<Json> parse_number() {
    const std::size_t start = pos_;
    if (!at_end() && peek() == '-') {
      ++pos_;
    }
    if (at_end() || !is_digit(peek())) {
      return std::nullopt;
    }
    if (peek() == '0') {
      ++pos_;
    } else {
      while (!at_end() && is_digit(peek())) {
        ++pos_;
      }
    }
    if (!at_end() && peek() == '.') {
      ++pos_;
      if (at_end() || !is_digit(peek())) {
        return std::nullopt;
      }
      while (!at_end() && is_digit(peek())) {
        ++pos_;
      }
    }
    if (!at_end() && (peek() == 'e' || peek() == 'E')) {
      ++pos_;
      if (!at_end() && (peek() == '+' || peek() == '-')) {
        ++pos_;
      }
      if (at_end() || !is_digit(peek())) {
        return std::nullopt;
      }
      while (!at_end() && is_digit(peek())) {
        ++pos_;
      }
    }
    const std::string literal(text_.substr(start, pos_ - start));
    return Json::number(std::strtod(literal.c_str(), nullptr));
  }

  static bool is_digit(char c) { return c >= '0' && c <= '9'; }

  std::string_view text_;
  std::size_t pos_ = 0;
};

void dump_string(const std::string& value, std::string& out) {
  out.push_back('"');
  for (const char c : value) {
    switch (c) {
      case '"':
        out += "\\\"";
        break;
      case '\\':
        out += "\\\\";
        break;
      case '\b':
        out += "\\b";
        break;
      case '\f':
        out += "\\f";
        break;
      case '\n':
        out += "\\n";
        break;
      case '\r':
        out += "\\r";
        break;
      case '\t':
        out += "\\t";
        break;
      default:
        if (static_cast<unsigned char>(c) < 0x20) {
          char buffer[7];
          std::snprintf(buffer, sizeof(buffer), "\\u%04x", static_cast<unsigned char>(c));
          out += buffer;
        } else {
          out.push_back(c);
        }
    }
  }
  out.push_back('"');
}

void dump_number(double value, std::string& out) {
  if (!std::isfinite(value)) {
    out += "null";
    return;
  }
  if (value == static_cast<double>(static_cast<std::int64_t>(value)) && std::fabs(value) < 9.0e15) {
    out += std::to_string(static_cast<std::int64_t>(value));
    return;
  }
  char buffer[40];
  for (const int digits : {15, 16, 17}) {
    std::snprintf(buffer, sizeof(buffer), "%.*g", digits, value);
    if (std::strtod(buffer, nullptr) == value) {
      break;
    }
  }
  out += buffer;
}

void dump_value(const Json& value, std::string& out) {
  switch (value.kind()) {
    case Json::Kind::Null:
      out += "null";
      break;
    case Json::Kind::Bool:
      out += value.as_bool() ? "true" : "false";
      break;
    case Json::Kind::Number:
      dump_number(value.as_number(), out);
      break;
    case Json::Kind::String:
      dump_string(value.as_string(), out);
      break;
    case Json::Kind::Array: {
      out.push_back('[');
      bool first = true;
      for (const Json& element : value.as_array()) {
        if (!first) {
          out.push_back(',');
        }
        first = false;
        dump_value(element, out);
      }
      out.push_back(']');
      break;
    }
    case Json::Kind::Object: {
      out.push_back('{');
      bool first = true;
      for (const auto& [key, member] : value.as_object()) {
        if (!first) {
          out.push_back(',');
        }
        first = false;
        dump_string(key, out);
        out.push_back(':');
        dump_value(member, out);
      }
      out.push_back('}');
      break;
    }
  }
}

}  // namespace

Json Json::boolean(bool value) {
  Json result;
  result.kind_ = Kind::Bool;
  result.bool_ = value;
  return result;
}

Json Json::number(double value) {
  Json result;
  result.kind_ = Kind::Number;
  result.number_ = value;
  return result;
}

Json Json::string(std::string value) {
  Json result;
  result.kind_ = Kind::String;
  result.string_ = std::move(value);
  return result;
}

Json Json::array(Array value) {
  Json result;
  result.kind_ = Kind::Array;
  result.array_ = std::move(value);
  return result;
}

Json Json::object(Object value) {
  Json result;
  result.kind_ = Kind::Object;
  result.object_ = std::move(value);
  return result;
}

std::optional<Json> Json::parse(std::string_view text) { return Parser(text).run(); }

std::string Json::dump() const {
  std::string out;
  dump_value(*this, out);
  return out;
}

bool Json::as_bool(bool fallback) const noexcept { return kind_ == Kind::Bool ? bool_ : fallback; }

double Json::as_number(double fallback) const noexcept {
  return kind_ == Kind::Number ? number_ : fallback;
}

const std::string& Json::as_string() const noexcept {
  return kind_ == Kind::String ? string_ : empty_string();
}

const Json::Array& Json::as_array() const noexcept {
  return kind_ == Kind::Array ? array_ : empty_array();
}

const Json::Object& Json::as_object() const noexcept {
  return kind_ == Kind::Object ? object_ : empty_object();
}

const Json* Json::find(std::string_view key) const noexcept {
  if (kind_ != Kind::Object) {
    return nullptr;
  }
  for (const auto& member : object_) {
    if (member.first == key) {
      return &member.second;
    }
  }
  return nullptr;
}

void Json::set(std::string key, Json value) {
  if (kind_ != Kind::Object) {
    kind_ = Kind::Object;
    object_.clear();
  }
  for (auto& member : object_) {
    if (member.first == key) {
      member.second = std::move(value);
      return;
    }
  }
  object_.emplace_back(std::move(key), std::move(value));
}

void Json::push_back(Json value) {
  if (kind_ != Kind::Array) {
    kind_ = Kind::Array;
    array_.clear();
  }
  array_.push_back(std::move(value));
}

}  // namespace cepis