json11.h

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/* json11
 *
 * json11 is a tiny JSON library for C++11, providing JSON parsing and serialization.
 *
 * The core object provided by the library is json11::Json. A Json object represents any JSON
 * value: null, bool, number (int or double), string (std::string), array (std::vector), or
 * object (std::map).
 *
 * Json objects act like values: they can be assigned, copied, moved, compared for equality or
 * order, etc. There are also helper methods Json::dump, to serialize a Json to a string, and
 * Json::parse (static) to parse a std::string as a Json object.
 *
 * Internally, the various types of Json object are represented by the JsonValue class
 * hierarchy.
 *
 * A note on numbers - JSON specifies the syntax of number formatting but not its semantics,
 * so some JSON implementations distinguish between integers and floating-point numbers, while
 * some don't. In json11, we choose the latter. Because some JSON implementations (namely
 * Javascript itself) treat all numbers as the same type, distinguishing the two leads
 * to JSON that will be *silently* changed by a round-trip through those implementations.
 * Dangerous! To avoid that risk, json11 stores all numbers as double internally, but also
 * provides integer helpers.
 *
 * Fortunately, double-precision IEEE754 ('double') can precisely store any integer in the
 * range +/-2^53, which includes every 'int' on most systems. (Timestamps often use int64
 * or long long to avoid the Y2038K problem; a double storing microseconds since some epoch
 * will be exact for +/- 275 years.)
 */
 
/* Copyright (c) 2013 Dropbox, Inc.
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */
 
#pragma once
 
#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <initializer_list>
#include <iosfwd>
#include <limits>
#include <map>
#include <memory>
#include <string>
#include <type_traits>
#include <utility>
#include <vector>
 
#ifdef _MSC_VER
#pragma warning(disable : 4244)
#if _MSC_VER <= 1800  // VS 2013
#ifndef noexcept
#define noexcept throw()
#endif
 
#ifndef snprintf
#define snprintf _snprintf_s
#endif
#endif
#endif
 
namespace json11 {
 
enum JsonParse { STANDARD, COMMENTS };
 
class JsonValue;
 
class Json final {
public:
    // Types
    enum Type { NUL, NUMBER, BOOL, STRING, ARRAY, OBJECT };
 
    // Array and object typedefs
    using array = std::vector<Json>;
    using object = std::map<std::string, Json>;
 
    // Constructors for the various types of JSON value.
    Json() noexcept;                 // NUL
    Json(std::nullptr_t) noexcept;   // NUL
    Json(double value);              // NUMBER
    Json(long long value);           // NUMBER
    Json(bool value);                // BOOL
    Json(const std::string& value);  // STRING
    Json(std::string&& value);       // STRING
    Json(const char* value);         // STRING
    Json(const array& values);       // ARRAY
    Json(array&& values);            // ARRAY
    Json(const object& values);      // OBJECT
    Json(object&& values);           // OBJECT
 
    // Implicit constructor: anything with a to_json() function.
    template <class T, class = decltype(&T::to_json)>
    Json(const T& t) : Json(t.to_json()) {}
 
    // Implicit constructor: map-like objects (std::map, std::unordered_map, etc)
    template <class M,
            typename std::enable_if<
                    std::is_constructible<std::string, decltype(std::declval<M>().begin()->first)>::value &&
                            std::is_constructible<Json, decltype(std::declval<M>().begin()->second)>::value,
                    int>::type = 0>
    Json(const M& m) : Json(object(m.begin(), m.end())) {}
 
    // Implicit constructor: vector-like objects (std::list, std::vector, std::set, etc)
    template <class V,
            typename std::enable_if<std::is_constructible<Json, decltype(*std::declval<V>().begin())>::value,
                    int>::type = 0>
    Json(const V& v) : Json(array(v.begin(), v.end())) {}
 
    // This prevents Json(some_pointer) from accidentally producing a bool. Use
    // Json(bool(some_pointer)) if that behavior is desired.
    Json(void*) = delete;
 
    // Accessors
    Type type() const;
 
    bool is_null() const {
        return type() == NUL;
    }
    bool is_number() const {
        return type() == NUMBER;
    }
    bool is_bool() const {
        return type() == BOOL;
    }
    bool is_string() const {
        return type() == STRING;
    }
    bool is_array() const {
        return type() == ARRAY;
    }
    bool is_object() const {
        return type() == OBJECT;
    }
 
    // Return the enclosed value if this is a number, 0 otherwise. Note that json11 does not
    // distinguish between integer and non-integer numbers - number_value() and int_value()
    // can both be applied to a NUMBER-typed object.
    double number_value() const;
    int int_value() const;
    long long long_value() const;
 
    // Return the enclosed value if this is a boolean, false otherwise.
    bool bool_value() const;
    // Return the enclosed string if this is a string, "" otherwise.
    const std::string& string_value() const;
    // Return the enclosed std::vector if this is an array, or an empty vector otherwise.
    const array& array_items() const;
    // Return the enclosed std::map if this is an object, or an empty map otherwise.
    const object& object_items() const;
 
    // Return a reference to arr[i] if this is an array, Json() otherwise.
    const Json& operator[](size_t i) const;
    // Return a reference to obj[key] if this is an object, Json() otherwise.
    const Json& operator[](const std::string& key) const;
 
    // Serialize.
    void dump(std::string& out) const;
    std::string dump() const {
        std::string out;
        dump(out);
        return out;
    }
 
    // Parse. If parse fails, return Json() and assign an error message to err.
    static Json parse(const std::string& in, std::string& err, JsonParse strategy = JsonParse::STANDARD);
 
    static Json parse(const char* in, std::string& err, JsonParse strategy = JsonParse::STANDARD) {
        if (in == nullptr) {
            err = "null input";
            return nullptr;
        }
        return parse(std::string(in), err, strategy);
    }
    // Parse multiple objects, concatenated or separated by whitespace
    static std::vector<Json> parse_multi(const std::string& in, std::string::size_type& parser_stop_pos,
            std::string& err, JsonParse strategy = JsonParse::STANDARD);
 
    static inline std::vector<Json> parse_multi(
            const std::string& in, std::string& err, JsonParse strategy = JsonParse::STANDARD) {
        std::string::size_type parser_stop_pos;
        return parse_multi(in, parser_stop_pos, err, strategy);
    }
 
    bool operator==(const Json& rhs) const;
    bool operator<(const Json& rhs) const;
    bool operator!=(const Json& rhs) const {
        return !(*this == rhs);
    }
    bool operator<=(const Json& rhs) const {
        return !(rhs < *this);
    }
    bool operator>(const Json& rhs) const {
        return (rhs < *this);
    }
    bool operator>=(const Json& rhs) const {
        return !(*this < rhs);
    }
 
    /* has_shape(types, err)
     *
     * Return true if this is a JSON object and, for each item in types, has a field of
     * the given type. If not, return false and set err to a descriptive message.
     */
    using shape = std::initializer_list<std::pair<std::string, Type>>;
    bool has_shape(const shape& types, std::string& err) const {
        if (!is_object()) {
            err = "expected JSON object, got " + dump();
            return false;
        }
 
        for (auto& item : types) {
            if ((*this)[item.first].type() != item.second) {
                err = "bad type for " + item.first + " in " + dump();
                return false;
            }
        }
 
        return true;
    }
 
private:
    std::shared_ptr<JsonValue> m_ptr;
};
 
// Internal class hierarchy - JsonValue objects are not exposed to users of this API.
class JsonValue {
protected:
    friend class Json;
    friend class JsonInt;
    friend class JsonDouble;
    virtual Json::Type type() const = 0;
    virtual bool equals(const JsonValue* other) const = 0;
    virtual bool less(const JsonValue* other) const = 0;
    virtual void dump(std::string& out) const = 0;
    virtual double number_value() const;
    virtual int int_value() const;
    virtual long long long_value() const;
    virtual bool bool_value() const;
    virtual const std::string& string_value() const;
    virtual const Json::array& array_items() const;
    virtual const Json& operator[](size_t i) const;
    virtual const Json::object& object_items() const;
    virtual const Json& operator[](const std::string& key) const;
    virtual ~JsonValue() = default;
};
 
static const int max_depth = 200;
 
/* Helper for representing null - just a do-nothing struct, plus comparison
 * operators so the helpers in JsonValue work. We can't use nullptr_t because
 * it may not be orderable.
 */
struct NullStruct {
    bool operator==(NullStruct) const {
        return true;
    }
    bool operator<(NullStruct) const {
        return false;
    }
};
 
/* * * * * * * * * * * * * * * * * * * *
 * Serialization
 */
 
static void dump(NullStruct, std::string& out) {
    out += "null";
}
 
static void dump(double value, std::string& out) {
    if (std::isfinite(value)) {
        char buf[32];
        snprintf(buf, sizeof buf, "%.17g", value);
        out += buf;
    } else {
        out += "null";
    }
}
 
static void dump(long long value, std::string& out) {
    char buf[32];
    snprintf(buf, sizeof buf, "%lld", value);
    out += buf;
}
 
static void dump(bool value, std::string& out) {
    out += value ? "true" : "false";
}
 
static void dump(const std::string& value, std::string& out) {
    out += '"';
    for (size_t i = 0; i < value.length(); i++) {
        const char ch = value[i];
        if (ch == '\\') {
            out += "\\\\";
        } else if (ch == '"') {
            out += "\\\"";
        } else if (ch == '\b') {
            out += "\\b";
        } else if (ch == '\f') {
            out += "\\f";
        } else if (ch == '\n') {
            out += "\\n";
        } else if (ch == '\r') {
            out += "\\r";
        } else if (ch == '\t') {
            out += "\\t";
        } else if (static_cast<uint8_t>(ch) <= 0x1f) {
            char buf[8];
            snprintf(buf, sizeof buf, "\\u%04x", ch);
            out += buf;
        } else if (static_cast<uint8_t>(ch) == 0xe2 && static_cast<uint8_t>(value[i + 1]) == 0x80 &&
                   static_cast<uint8_t>(value[i + 2]) == 0xa8) {
            out += "\\u2028";
            i += 2;
        } else if (static_cast<uint8_t>(ch) == 0xe2 && static_cast<uint8_t>(value[i + 1]) == 0x80 &&
                   static_cast<uint8_t>(value[i + 2]) == 0xa9) {
            out += "\\u2029";
            i += 2;
        } else {
            out += ch;
        }
    }
    out += '"';
}
 
static void dump(const Json::array& values, std::string& out) {
    bool first = true;
    out += "[";
    for (const auto& value : values) {
        if (!first) out += ", ";
        value.dump(out);
        first = false;
    }
    out += "]";
}
 
static void dump(const Json::object& values, std::string& out) {
    bool first = true;
    out += "{";
    for (const auto& kv : values) {
        if (!first) out += ", ";
        dump(kv.first, out);
        out += ": ";
        kv.second.dump(out);
        first = false;
    }
    out += "}";
}
 
inline void Json::dump(std::string& out) const {
    m_ptr->dump(out);
}
 
/* * * * * * * * * * * * * * * * * * * *
 * Value wrappers
 */
 
template <Json::Type tag, typename T>
class Value : public JsonValue {
protected:
    // Constructors
    explicit Value(T value) : m_value(std::move(value)) {}
 
    // Get type tag
    Json::Type type() const override {
        return tag;
    }
 
    // Comparisons
    bool equals(const JsonValue* other) const override {
        return m_value == static_cast<const Value<tag, T>*>(other)->m_value;
    }
    bool less(const JsonValue* other) const override {
        return m_value < static_cast<const Value<tag, T>*>(other)->m_value;
    }
 
    const T m_value;
    void dump(std::string& out) const override {
        json11::dump(m_value, out);
    }
};
 
class JsonDouble final : public Value<Json::NUMBER, double> {
    double number_value() const override {
        return m_value;
    }
    int int_value() const override {
        return static_cast<int>(m_value);
    }
    long long long_value() const override {
        return static_cast<long long>(m_value);
    }
    bool equals(const JsonValue* other) const override {
        return m_value == other->number_value();
    }
    bool less(const JsonValue* other) const override {
        return m_value < other->number_value();
    }
 
public:
    explicit JsonDouble(double value) : Value(value) {}
};
 
class JsonInt final : public Value<Json::NUMBER, long long> {
    double number_value() const override {
        return m_value;
    }
    int int_value() const override {
        return m_value;
    }
    long long long_value() const override {
        return static_cast<long long>(m_value);
    }
    bool equals(const JsonValue* other) const override {
        return m_value == other->number_value();
    }
    bool less(const JsonValue* other) const override {
        return m_value < other->number_value();
    }
 
public:
    explicit JsonInt(int value) : Value(value) {}
};
 
class JsonBoolean final : public Value<Json::BOOL, bool> {
    bool bool_value() const override {
        return m_value;
    }
 
public:
    explicit JsonBoolean(bool value) : Value(value) {}
};
 
class JsonString final : public Value<Json::STRING, std::string> {
    const std::string& string_value() const override {
        return m_value;
    }
 
public:
    explicit JsonString(const std::string& value) : Value(value) {}
    explicit JsonString(std::string&& value) : Value(std::move(value)) {}
};
 
class JsonArray final : public Value<Json::ARRAY, Json::array> {
    const Json::array& array_items() const override {
        return m_value;
    }
    const Json& operator[](size_t i) const override;
 
public:
    explicit JsonArray(const Json::array& value) : Value(value) {}
    explicit JsonArray(Json::array&& value) : Value(std::move(value)) {}
};
 
class JsonObject final : public Value<Json::OBJECT, Json::object> {
    const Json::object& object_items() const override {
        return m_value;
    }
    const Json& operator[](const std::string& key) const override;
 
public:
    explicit JsonObject(const Json::object& value) : Value(value) {}
    explicit JsonObject(Json::object&& value) : Value(std::move(value)) {}
};
 
class JsonNull final : public Value<Json::NUL, NullStruct> {
public:
    JsonNull() : Value({}) {}
};
 
/* * * * * * * * * * * * * * * * * * * *
 * Static globals - static-init-safe
 */
struct Statics {
    const std::shared_ptr<JsonValue> null = std::make_shared<JsonNull>();
    const std::shared_ptr<JsonValue> t = std::make_shared<JsonBoolean>(true);
    const std::shared_ptr<JsonValue> f = std::make_shared<JsonBoolean>(false);
    const std::string empty_string{};
    const std::vector<Json> empty_vector{};
    const std::map<std::string, Json> empty_map{};
    Statics() = default;
};
 
static const Statics& statics() {
    static const Statics s{};
    return s;
}
 
static const Json& static_null() {
    // This has to be separate, not in Statics, because Json() accesses statics().null.
    static const Json json_null;
    return json_null;
}
 
/* * * * * * * * * * * * * * * * * * * *
 * Constructors
 */
 
inline Json::Json() noexcept : m_ptr(statics().null) {}
inline Json::Json(std::nullptr_t) noexcept : m_ptr(statics().null) {}
inline Json::Json(double value) : m_ptr(std::make_shared<JsonDouble>(value)) {}
inline Json::Json(long long value) : m_ptr(std::make_shared<JsonInt>(value)) {}
inline Json::Json(bool value) : m_ptr(value ? statics().t : statics().f) {}
inline Json::Json(const std::string& value) : m_ptr(std::make_shared<JsonString>(value)) {}
inline Json::Json(std::string&& value) : m_ptr(std::make_shared<JsonString>(std::move(value))) {}
inline Json::Json(const char* value) : m_ptr(std::make_shared<JsonString>(value)) {}
inline Json::Json(const Json::array& values) : m_ptr(std::make_shared<JsonArray>(values)) {}
inline Json::Json(Json::array&& values) : m_ptr(std::make_shared<JsonArray>(std::move(values))) {}
inline Json::Json(const Json::object& values) : m_ptr(std::make_shared<JsonObject>(values)) {}
inline Json::Json(Json::object&& values) : m_ptr(std::make_shared<JsonObject>(std::move(values))) {}
 
/* * * * * * * * * * * * * * * * * * * *
 * Accessors
 */
 
inline Json::Type Json::type() const {
    return m_ptr->type();
}
inline double Json::number_value() const {
    return m_ptr->number_value();
}
inline int Json::int_value() const {
    return m_ptr->int_value();
}
inline long long Json::long_value() const {
    return m_ptr->long_value();
}
inline bool Json::bool_value() const {
    return m_ptr->bool_value();
}
inline const std::string& Json::string_value() const {
    return m_ptr->string_value();
}
inline const std::vector<Json>& Json::array_items() const {
    return m_ptr->array_items();
}
inline const std::map<std::string, Json>& Json::object_items() const {
    return m_ptr->object_items();
}
inline const Json& Json::operator[](size_t i) const {
    return (*m_ptr)[i];
}
inline const Json& Json::operator[](const std::string& key) const {
    return (*m_ptr)[key];
}
 
inline double JsonValue::number_value() const {
    return 0;
}
inline int JsonValue::int_value() const {
    return 0;
}
inline long long JsonValue::long_value() const {
    return 0;
}
inline bool JsonValue::bool_value() const {
    return false;
}
inline const std::string& JsonValue::string_value() const {
    return statics().empty_string;
}
inline const std::vector<Json>& JsonValue::array_items() const {
    return statics().empty_vector;
}
inline const std::map<std::string, Json>& JsonValue::object_items() const {
    return statics().empty_map;
}
inline const Json& JsonValue::operator[](size_t) const {
    return static_null();
}
inline const Json& JsonValue::operator[](const std::string&) const {
    return static_null();
}
 
inline const Json& JsonObject::operator[](const std::string& key) const {
    auto iter = m_value.find(key);
    return (iter == m_value.end()) ? static_null() : iter->second;
}
inline const Json& JsonArray::operator[](size_t i) const {
    if (i >= m_value.size()) {
        return static_null();
    }
    return m_value[i];
}
 
/* * * * * * * * * * * * * * * * * * * *
 * Comparison
 */
 
inline bool Json::operator==(const Json& other) const {
    if (m_ptr == other.m_ptr) {
        return true;
    }
    if (m_ptr->type() != other.m_ptr->type()) {
        return false;
    }
 
    return m_ptr->equals(other.m_ptr.get());
}
 
inline bool Json::operator<(const Json& other) const {
    if (m_ptr == other.m_ptr) {
        return false;
    }
    if (m_ptr->type() != other.m_ptr->type()) {
        return m_ptr->type() < other.m_ptr->type();
    }
 
    return m_ptr->less(other.m_ptr.get());
}
 
/* * * * * * * * * * * * * * * * * * * *
 * Parsing
 */
 
/* esc(c)
 *
 * Format char c suitable for printing in an error message.
 */
static inline std::string esc(char c) {
    char buf[12];
    if (static_cast<uint8_t>(c) >= 0x20 && static_cast<uint8_t>(c) <= 0x7f) {
        snprintf(buf, sizeof buf, "'%c' (%d)", c, c);
    } else {
        snprintf(buf, sizeof buf, "(%d)", c);
    }
    return std::string(buf);
}
 
static inline bool in_range(long x, long lower, long upper) {
    return (x >= lower && x <= upper);
}
 
namespace {
/* JsonParser
 *
 * Object that tracks all state of an in-progress parse.
 */
struct JsonParser final {
    /* State
     */
    const std::string& str;
    size_t i;
    std::string& err;
    bool failed;
    const JsonParse strategy;
 
    /* fail(msg, err_ret = Json())
     *
     * Mark this parse as failed.
     */
    Json fail(std::string&& msg) {
        return fail(std::move(msg), Json());
    }
 
    template <typename T>
    T fail(std::string&& msg, T err_ret) {
        if (!failed) {
            err = std::move(msg);
        }
        failed = true;
        return err_ret;
    }
 
    /* consume_whitespace()
     *
     * Advance until the current character is non-whitespace.
     */
    void consume_whitespace() {
        while (str[i] == ' ' || str[i] == '\r' || str[i] == '\n' || str[i] == '\t') {
            i++;
        }
    }
 
    /* consume_comment()
     *
     * Advance comments (c-style inline and multiline).
     */
    bool consume_comment() {
        bool comment_found = false;
        if (str[i] == '/') {
            i++;
            if (i == str.size()) {
                return fail("unexpected end of input after start of comment", false);
            }
            if (str[i] == '/') {  // inline comment
                i++;
                // advance until next line, or end of input
                while (i < str.size() && str[i] != '\n') {
                    i++;
                }
                comment_found = true;
            } else if (str[i] == '*') {  // multiline comment
                i++;
                if (i > str.size() - 2)
                    return fail("unexpected end of input inside multi-line comment", false);
                // advance until closing tokens
                while (!(str[i] == '*' && str[i + 1] == '/')) {
                    i++;
                    if (i > str.size() - 2)
                        return fail("unexpected end of input inside multi-line comment", false);
                }
                i += 2;
                comment_found = true;
            } else {
                return fail("malformed comment", false);
            }
        }
        return comment_found;
    }
 
    /* consume_garbage()
     *
     * Advance until the current character is non-whitespace and non-comment.
     */
    void consume_garbage() {
        consume_whitespace();
        if (strategy == JsonParse::COMMENTS) {
            bool comment_found = false;
            do {
                comment_found = consume_comment();
                if (failed) {
                    return;
                }
                consume_whitespace();
            } while (comment_found);
        }
    }
 
    /* get_next_token()
     *
     * Return the next non-whitespace character. If the end of the input is reached,
     * flag an error and return 0.
     */
    char get_next_token() {
        consume_garbage();
        if (failed) {
            return static_cast<char>(0);
        }
        if (i == str.size()) {
            return fail("unexpected end of input", static_cast<char>(0));
        }
 
        return str[i++];
    }
 
    /* encode_utf8(pt, out)
     *
     * Encode pt as UTF-8 and add it to out.
     */
    void encode_utf8(long pt, std::string& out) {
        if (pt < 0) {
            return;
        }
 
        if (pt < 0x80) {
            out += static_cast<char>(pt);
        } else if (pt < 0x800) {
            out += static_cast<char>((pt >> 6) | 0xC0);
            out += static_cast<char>((pt & 0x3F) | 0x80);
        } else if (pt < 0x10000) {
            out += static_cast<char>((pt >> 12) | 0xE0);
            out += static_cast<char>(((pt >> 6) & 0x3F) | 0x80);
            out += static_cast<char>((pt & 0x3F) | 0x80);
        } else {
            out += static_cast<char>((pt >> 18) | 0xF0);
            out += static_cast<char>(((pt >> 12) & 0x3F) | 0x80);
            out += static_cast<char>(((pt >> 6) & 0x3F) | 0x80);
            out += static_cast<char>((pt & 0x3F) | 0x80);
        }
    }
 
    /* parse_string()
     *
     * Parse a std::string, starting at the current position.
     */
    std::string parse_string() {
        std::string out;
        long last_escaped_codepoint = -1;
        while (true) {
            if (i == str.size()) return fail("unexpected end of input in std::string", "");
 
            char ch = str[i++];
 
            if (ch == '"') {
                encode_utf8(last_escaped_codepoint, out);
                return out;
            }
 
            if (in_range(ch, 0, 0x1f)) {
                return fail("unescaped " + esc(ch) + " in std::string", "");
            }
 
            // The usual case: non-escaped characters
            if (ch != '\\') {
                encode_utf8(last_escaped_codepoint, out);
                last_escaped_codepoint = -1;
                out += ch;
                continue;
            }
 
            // Handle escapes
            if (i == str.size()) {
                return fail("unexpected end of input in std::string", "");
            }
 
            ch = str[i++];
 
            if (ch == 'u') {
                // Extract 4-byte escape sequence
                std::string esc = str.substr(i, 4);
                // Explicitly check length of the substring. The following loop
                // relies on std::string returning the terminating NUL when
                // accessing str[length]. Checking here reduces brittleness.
                if (esc.length() < 4) {
                    return fail("bad \\u escape: " + esc, "");
                }
                for (size_t j = 0; j < 4; j++) {
                    if (!in_range(esc[j], 'a', 'f') && !in_range(esc[j], 'A', 'F') &&
                            !in_range(esc[j], '0', '9'))
                        return fail("bad \\u escape: " + esc, "");
                }
 
                long codepoint = strtol(esc.data(), nullptr, 16);
 
                // JSON specifies that characters outside the BMP shall be encoded as a pair
                // of 4-hex-digit \u escapes encoding their surrogate pair components. Check
                // whether we're in the middle of such a beast: the previous codepoint was an
                // escaped lead (high) surrogate, and this is a trail (low) surrogate.
                if (in_range(last_escaped_codepoint, 0xD800, 0xDBFF) && in_range(codepoint, 0xDC00, 0xDFFF)) {
                    // Reassemble the two surrogate pairs into one astral-plane character, per
                    // the UTF-16 algorithm.
                    encode_utf8((((last_escaped_codepoint - 0xD800) << 10) | (codepoint - 0xDC00)) + 0x10000,
                            out);
                    last_escaped_codepoint = -1;
                } else {
                    encode_utf8(last_escaped_codepoint, out);
                    last_escaped_codepoint = codepoint;
                }
 
                i += 4;
                continue;
            }
 
            encode_utf8(last_escaped_codepoint, out);
            last_escaped_codepoint = -1;
 
            if (ch == 'b') {
                out += '\b';
            } else if (ch == 'f') {
                out += '\f';
            } else if (ch == 'n') {
                out += '\n';
            } else if (ch == 'r') {
                out += '\r';
            } else if (ch == 't') {
                out += '\t';
            } else if (ch == '"' || ch == '\\' || ch == '/') {
                out += ch;
            } else {
                return fail("invalid escape character " + esc(ch), "");
            }
        }
    }
 
    /* parse_number()
     *
     * Parse a double.
     */
    Json parse_number() {
        size_t start_pos = i;
 
        if (str[i] == '-') {
            i++;
        }
 
        // Integer part
        if (str[i] == '0') {
            i++;
            if (in_range(str[i], '0', '9')) {
                return fail("leading 0s not permitted in numbers");
            }
        } else if (in_range(str[i], '1', '9')) {
            i++;
            while (in_range(str[i], '0', '9')) {
                i++;
            }
        } else {
            return fail("invalid " + esc(str[i]) + " in number");
        }
 
        if (str[i] != '.' && str[i] != 'e' && str[i] != 'E' &&
                (i - start_pos) <= static_cast<size_t>(std::numeric_limits<int>::digits10)) {
            return std::atoll(str.c_str() + start_pos);
        }
 
        // Decimal part
        if (str[i] == '.') {
            i++;
            if (!in_range(str[i], '0', '9')) {
                return fail("at least one digit required in fractional part");
            }
 
            while (in_range(str[i], '0', '9')) {
                i++;
            }
        }
 
        // Exponent part
        if (str[i] == 'e' || str[i] == 'E') {
            i++;
 
            if (str[i] == '+' || str[i] == '-') {
                i++;
            }
 
            if (!in_range(str[i], '0', '9')) {
                return fail("at least one digit required in exponent");
            }
 
            while (in_range(str[i], '0', '9')) {
                i++;
            }
        }
 
        return std::strtod(str.c_str() + start_pos, nullptr);
    }
 
    /* expect(str, res)
     *
     * Expect that 'str' starts at the character that was just read. If it does, advance
     * the input and return res. If not, flag an error.
     */
    Json expect(const std::string& expected, Json res) {
        assert(i != 0);
        i--;
        if (str.compare(i, expected.length(), expected) == 0) {
            i += expected.length();
            return res;
        } else {
            return fail("parse error: expected " + expected + ", got " + str.substr(i, expected.length()));
        }
    }
 
    /* parse_json()
     *
     * Parse a JSON object.
     */
    Json parse_json(int depth) {
        if (depth > max_depth) {
            return fail("exceeded maximum nesting depth");
        }
 
        char ch = get_next_token();
        if (failed) {
            return Json();
        }
 
        if (ch == '-' || (ch >= '0' && ch <= '9')) {
            i--;
            return parse_number();
        }
 
        if (ch == 't') {
            return expect("true", true);
        }
 
        if (ch == 'f') {
            return expect("false", false);
        }
 
        if (ch == 'n') {
            return expect("null", Json());
        }
 
        if (ch == '"') {
            return parse_string();
        }
 
        if (ch == '{') {
            std::map<std::string, Json> data;
            ch = get_next_token();
            if (ch == '}') {
                return data;
            }
 
            while (true) {
                if (ch != '"') return fail("expected '\"' in object, got " + esc(ch));
 
                std::string key = parse_string();
                if (failed) {
                    return Json();
                }
 
                ch = get_next_token();
                if (ch != ':') {
                    return fail("expected ':' in object, got " + esc(ch));
                }
 
                data[std::move(key)] = parse_json(depth + 1);
                if (failed) {
                    return Json();
                }
 
                ch = get_next_token();
                if (ch == '}') {
                    break;
                }
                if (ch != ',') {
                    return fail("expected ',' in object, got " + esc(ch));
                }
 
                ch = get_next_token();
            }
            return data;
        }
 
        if (ch == '[') {
            std::vector<Json> data;
            ch = get_next_token();
            if (ch == ']') {
                return data;
            }
 
            while (true) {
                i--;
                data.push_back(parse_json(depth + 1));
                if (failed) {
                    return Json();
                }
 
                ch = get_next_token();
                if (ch == ']') {
                    break;
                }
                if (ch != ',') {
                    return fail("expected ',' in list, got " + esc(ch));
                }
 
                ch = get_next_token();
                (void)ch;
            }
            return data;
        }
 
        return fail("expected value, got " + esc(ch));
    }
};
}  // namespace
 
inline Json Json::parse(const std::string& in, std::string& err, JsonParse strategy) {
    JsonParser parser{in, 0, err, false, strategy};
    Json result = parser.parse_json(0);
 
    // Check for any trailing garbage
    parser.consume_garbage();
    if (parser.failed) {
        return Json();
    }
    if (parser.i != in.size()) {
        return parser.fail("unexpected trailing " + esc(in[parser.i]));
    }
 
    return result;
}
 
inline std::vector<Json> parse_multi(const std::string& in, std::string::size_type& parser_stop_pos,
        std::string& err, JsonParse strategy) {
    JsonParser parser{in, 0, err, false, strategy};
    parser_stop_pos = 0;
    std::vector<Json> json_vec;
    while (parser.i != in.size() && !parser.failed) {
        json_vec.push_back(parser.parse_json(0));
        if (parser.failed) {
            break;
        }
 
        // Check for another object
        parser.consume_garbage();
        if (parser.failed) {
            break;
        }
        parser_stop_pos = parser.i;
    }
    return json_vec;
}
 
#ifdef _MSC_VER
#pragma warning(default : 4244)
#endif  // _MSC_VER
 
}  // namespace json11