/*
 * (C) Copyright 2025- ECMWF and individual contributors.
 *
 * This software is licensed under the terms of the Apache Licence Version 2.0
 * which can be obtained at http://www.apache.org/licenses/LICENSE-2.0.
 * In applying this licence, ECMWF does not waive the privileges and immunities
 * granted to it by virtue of its status as an intergovernmental organisation nor
 * does it submit to any jurisdiction.
 */

///
/// @file derivedEncoding.h
/// @brief Implementation of the GRIB `derived` concept operation.
///
/// This header defines the applicability rules and execution logic for the
/// **derived concept** within the mars2grib backend.
///
/// The concept is responsible for populating GRIB keys related to
/// *derived ensemble products*, including:
///
/// - `derivedForecast`
/// - `numberOfForecastsInEnsemble`
///
/// These keys are encoded in the Product Definition Section and are used
/// to describe ensemble-derived statistical products (e.g. means, spreads,
/// probabilities).
///
/// The concept implementation follows the standard mars2grib concept model:
/// - Compile-time applicability via `derivedApplicable`
/// - Runtime structural validation of the Product Definition Section
/// - Delegation of semantic deduction to dedicated deduction functions
/// - Strict error handling with contextual concept exceptions
///
/// @note
/// The namespace name `concepts_` is intentionally used instead of `concepts`
/// to avoid ambiguity and potential conflicts with the C++20 `concept` language
/// feature and related standard headers.
///
/// This is a deliberate design choice and must not be changed.
///
/// @ingroup mars2grib_backend_concepts
///
#pragma once

// System includes
#include <string>

// Core concept includes
#include "metkit/mars2grib/backend/compile-time-registry-engine/common.h"
#include "metkit/mars2grib/backend/concepts/derived/derivedEnum.h"
#include "metkit/mars2grib/utils/generalUtils.h"

// Deductions
#include "metkit/mars2grib/backend/deductions/channel.h"
#include "metkit/mars2grib/backend/deductions/derivedForecast.h"
#include "metkit/mars2grib/backend/deductions/numberOfForecastsInEnsemble.h"
#include "metkit/mars2grib/backend/deductions/numberOfFrequencies.h"

// Tables
#include "metkit/mars2grib/backend/tables/derivedForecast.h"

// Checks
#include "metkit/mars2grib/backend/checks/checkDerivedProductDefinitionSection.h"

// Utils
#include "metkit/config/LibMetkit.h"
#include "metkit/mars2grib/utils/logUtils.h"
#include "metkit/mars2grib/utils/mars2gribExceptions.h"

namespace metkit::mars2grib::backend::concepts_ {

///
/// @brief Compile-time applicability predicate for the `derived` concept.
///
/// This predicate determines whether the `derived` concept is applicable
/// for a given combination of:
/// - encoding stage
/// - GRIB section
/// - concept variant
///
/// Applicability is evaluated entirely at compile time and is used by the
/// concept dispatcher to ensure that only valid concept instantiations occur.
///
/// @tparam Stage   Encoding stage (compile-time constant)
/// @tparam Section GRIB section index (compile-time constant)
/// @tparam Variant Derived concept variant
///
/// @return `true` if the concept is applicable for the given parameters,
/// `false` otherwise.
///
/// @note The default applicability rule enables the concept only when:
/// - `Stage == StagePreset`
/// - `Section == SecProductDefinitionSection`
///
/// @todo [owner: mds][scope: concept][reason: correctness][prio: medium]
/// - Refine applicability rules once derived product usage is fully constrained
/// by stage, section, or variant.
///
template <std::size_t Stage, std::size_t Section, DerivedType Variant>
constexpr bool derivedApplicable() {
    return (Stage == StagePreset) && ((Section == SecProductDefinitionSection) || (Section == SecLocalUseSection));
}

///
/// @brief Execute the `derived` concept operation.
///
/// This function implements the runtime logic of the GRIB `derived` concept.
/// When applicable, it:
///
/// 1. Validates that the Product Definition Section is compatible with
/// derived ensemble products.
/// 2. Deduces the type of derived forecast from MARS and parameter dictionaries.
/// 3. Deduces the number of ensemble members involved.
/// 4. Encodes the corresponding GRIB keys in the output dictionary.
///
/// The concept acts as a **coordination layer**:
/// - Structural validation is performed explicitly.
/// - Semantic deduction is delegated to backend deductions.
/// - Value correctness is guaranteed by GRIB table-backed enumerations.
///
/// If the concept is invoked when not applicable, a
/// `Mars2GribConceptException` is thrown.
///
/// @tparam Stage      Encoding stage (compile-time constant)
/// @tparam Section    GRIB section index (compile-time constant)
/// @tparam Variant    Derived concept variant
/// @tparam MarsDict_t Type of the MARS input dictionary
/// @tparam ParDict_t  Type of the parameter dictionary
/// @tparam OptDict_t  Type of the options dictionary
/// @tparam OutDict_t  Type of the GRIB output dictionary
///
/// @param[in]  mars MARS input dictionary
/// @param[in]  par  Parameter dictionary
/// @param[in]  opt  Options dictionary
/// @param[out] out  Output GRIB dictionary to be populated
///
/// @throws metkit::mars2grib::utils::exceptions::Mars2GribConceptException
/// If:
/// - structural validation of the Product Definition Section fails
/// - a deduction fails
/// - a GRIB key cannot be set
///
/// @note
/// - All runtime errors are wrapped with full concept context
/// (concept name, variant, stage, section).
/// - This concept does not rely on any pre-existing GRIB header state.
///
/// @see derivedApplicable
///
template <std::size_t Stage, std::size_t Section, DerivedType Variant, class MarsDict_t, class ParDict_t,
          class OptDict_t, class OutDict_t>
void DerivedOp(const MarsDict_t& mars, const ParDict_t& par, const OptDict_t& opt, OutDict_t& out) {

    using metkit::mars2grib::utils::dict_traits::set_or_throw;
    using metkit::mars2grib::utils::exceptions::Mars2GribConceptException;

    if constexpr (derivedApplicable<Stage, Section, Variant>()) {

        try {

            MARS2GRIB_LOG_CONCEPT(derived);

            if constexpr (Variant == DerivedType::Default) {
                if constexpr (Section == SecProductDefinitionSection && Stage == StagePreset) {
                    // Structural validation
                    validation::check_DerivedProductDefinitionSection_or_throw(opt, out);

                    // Deductions
                    tables::DerivedForecast derivedForecast =
                        deductions::resolve_DerivedForecast_or_throw(mars, par, opt);
                    long numberOfForecastsInEnsemble =
                        deductions::resolve_NumberOfForecastsInEnsemble_or_throw(mars, par, opt);

                    // Encoding
                    set_or_throw<long>(out, "derivedForecast", static_cast<long>(derivedForecast));
                    set_or_throw<long>(out, "numberOfForecastsInEnsemble", numberOfForecastsInEnsemble);
                }
            }

            if constexpr (Variant == DerivedType::ShiftOfTails) {
                if constexpr (Section == SecProductDefinitionSection && Stage == StagePreset) {
                    // Structural validation
                    // validation::check_DerivedProductDefinitionSection_or_throw(opt, out);

                    // Deductions
                    tables::DerivedForecast derivedForecast =
                        deductions::resolve_DerivedForecast_or_throw(mars, par, opt);
                    // long numberOfForecastsInEnsemble =
                    //    deductions::resolve_NumberOfForecastsInEnsemble_or_throw(mars, par, opt);

                    // Encoding
                    set_or_throw<long>(out, "derivedForecast", static_cast<long>(derivedForecast));
                    // set_or_throw<long>(out, "numberOfForecastsInEnsemble", numberOfForecastsInEnsemble);
                }
            }
        }
        catch (...) {

            MARS2GRIB_CONCEPT_RETHROW(derived, "Unable to set `derived` concept...");
        }

        return;
    }

    // Concept invoked outside its applicability domain
    MARS2GRIB_CONCEPT_THROW(derived, "Concept called when not applicable...");

    // Remove compiler warning
    mars2gribUnreachable();
}

}  // namespace metkit::mars2grib::backend::concepts_
