rippled/include/xrpl/basics/StructuredLogging.h

#pragma once

#include <fmt/format.h>

#include <concepts>
#include <iterator>
#include <string>
#include <string_view>
#include <type_traits>

namespace xrpl {

namespace detail {

/**
 * @brief Concept that detects whether @c to_string(t) is a valid expression
 * returning something convertible to @c std::string.
 *
 * Because this concept lives inside @c namespace @c xrpl, unqualified lookup
 * will find every @c xrpl::to_string overload (e.g. for @c Number,
 * @c Currency, @c AccountID, @c XRPAmount, @c IOUAmount, @c Asset, etc.).
 * ADL further extends the search to any namespace associated with @p T.
 */
template <typename T>
concept HasToString = requires(std::remove_cvref_t<T> const& t) {
    { to_string(t) } -> std::convertible_to<std::string>;
};

/**
 * @brief Append a value formatted as a JSON fragment to @p dest.
 *
 * - @c bool \u2192 unquoted @c true / @c false
 * - integral (non-bool) \u2192 unquoted number
 * - floating-point \u2192 unquoted number
 * - types with @c to_string \u2192 quoted string via @c to_string
 * - everything else \u2192 quoted string obtained via @c fmt::format
 *
 * @tparam T The value type.
 * @param dest The string to append the JSON fragment to.
 * @param value The value to format.
 */
template <typename T>
void
appendJsonValue(std::string& dest, T const& value)
{
    if constexpr (std::is_same_v<T, bool>)
    {
        dest += value ? "true" : "false";
    }
    else if constexpr (std::is_integral_v<T>)
    {
        dest += std::to_string(value);
    }
    else if constexpr (std::is_floating_point_v<T>)
    {
        dest += std::to_string(value);
    }
    else if constexpr (HasToString<T>)
    {
        fmt::format_to(std::back_inserter(dest), "\"{}\"", to_string(value));
    }
    else
    {
        fmt::format_to(std::back_inserter(dest), "\"{}\"", value);
    }
}

/**
 * @brief Append a @c "key":value JSON field to @p dest.
 *
 * Inserts a comma separator when @p dest already contains fields.
 * The value is formatted by @ref appendJsonValue.
 */
template <typename T>
void
appendJsonField(std::string& dest, std::string_view key, T const& value)
{
    if (!dest.empty())
        dest += ',';
    fmt::format_to(std::back_inserter(dest), "\"{}\":", key);
    appendJsonValue(dest, value);
}

/**
 * @brief Append a @c key=value plain-text field to @p dest.
 *
 * Inserts a space separator when @p dest already contains fields.
 */
template <typename T>
void
appendTextField(std::string& dest, std::string_view key, T const& value)
{
    if (!dest.empty())
        dest += ' ';
    if constexpr (HasToString<T>)
    {
        fmt::format_to(std::back_inserter(dest), "{}={}", key, to_string(value));
    }
    else
    {
        fmt::format_to(std::back_inserter(dest), "{}={}", key, value);
    }
}

/** @brief Internal builder used by @ref buildJsonPattern.
 *
 *  Not part of the public API — use @ref buildJsonPattern instead.
 */
class JsonLoggingPatternBuilder
{
    std::string fields_;

    static constexpr std::string_view kSUFFIX = ", \"message\": %v }";

    static std::string
    extractFields(std::string_view pattern)
    {
        if (!pattern.empty() && pattern.front() == '{')
            pattern.remove_prefix(1);
        if (auto pos = pattern.rfind(kSUFFIX); pos != std::string_view::npos)
            pattern = pattern.substr(0, pos);
        return std::string(pattern);
    }

public:
    JsonLoggingPatternBuilder() = default;

    explicit JsonLoggingPatternBuilder(std::string_view existingPattern)
        : fields_(extractFields(existingPattern))
    {
    }

    /** @brief Add a field. Delegates to @ref appendJsonField. */
    template <typename T>
    JsonLoggingPatternBuilder&
    add(std::string_view key, T const& value)
    {
        appendJsonField(fields_, key, value);
        return *this;
    }

    [[nodiscard]] std::string
    build() const
    {
        return "{" + fields_ + std::string(kSUFFIX);
    }
};

}  // namespace detail

namespace log {

/**
 * @brief Holds a named log parameter with its value.
 *
 * Constructed via the @ref param helper function. Designed to be passed
 * into Logger::Pump (the integration with Pump will be added later).
 *
 * @tparam T The value type.
 */
template <typename T>
class Parameter
{
    std::string name_;
    T value_;

public:
    Parameter(std::string_view name, T const& value) : name_(name), value_(value)
    {
    }

    /** @brief Get the parameter name. */
    [[nodiscard]] std::string_view
    name() const
    {
        return name_;
    }

    /** @brief Get the parameter value. */
    [[nodiscard]] T const&
    value() const
    {
        return value_;
    }
};

/**
 * @brief Create a named log parameter.
 *
 * @tparam T The value type (deduced).
 * @param name The parameter name (JSON key).
 * @param value The parameter value.
 * @return A Parameter holding the name and value.
 */
template <typename T>
[[nodiscard]] Parameter<std::decay_t<T>>
param(std::string_view name, T&& value)
{
    return Parameter<std::decay_t<T>>{name, value};
}

}  // namespace log

/**
 * @brief Build or extend a JSON log pattern from @ref log::Parameter objects.
 *
 * Each parameter's name becomes a JSON key and its value becomes the
 * corresponding JSON value in the spdlog pattern string.  String-like
 * values are treated as spdlog pattern flags (e.g. @c "%%n", @c "%%l");
 * numeric and boolean values are embedded as literals.
 *
 * @code
 *     // Build from scratch (empty existing pattern)
 *     auto pattern = buildJsonPattern("",
 *         log::param("channel", std::string_view("%n")),
 *         log::param("level",   std::string_view("%l")));
 *
 *     // Extend an existing pattern with extra fields
 *     auto extended = buildJsonPattern(pattern,
 *         log::param("trace_id", traceId));
 * @endcode
 *
 * @tparam Ts  Value types of the parameters.
 * @param existingPattern  A pattern previously produced by build() or an
 *                         empty string to start fresh.
 * @param params  The parameters to add as JSON fields.
 * @return The spdlog-compatible JSON pattern string.
 */
template <typename... Ts>
[[nodiscard]] std::string
buildJsonPattern(std::string_view existingPattern, log::Parameter<Ts> const&... params)
{
    detail::JsonLoggingPatternBuilder builder(existingPattern);
    (builder.add(params.name(), params.value()), ...);
    return builder.build();
}

}  // namespace xrpl