/*
 * (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 marsEncoding.h
/// @brief Implementation of the GRIB `mars` concept operation.
///
/// This header defines the applicability rules and execution logic for the
/// **mars concept** within the mars2grib backend.
///
/// The mars concept is responsible for encoding core MARS identity metadata
/// into the GRIB *Local Use Section*, specifically:
///
/// - `class`
/// - `type`
/// - `stream`
/// - `expver`
///
/// These fields collectively define the provenance and classification of the
/// encoded product and are required by downstream systems and workflows.
///
/// The implementation follows the standard mars2grib concept model:
/// - Compile-time applicability via `marsApplicable`
/// - Runtime structural validation of the Local Use Section
/// - Explicit deduction of all required MARS identity fields
/// - 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/mars/marsEnum.h"
#include "metkit/mars2grib/utils/generalUtils.h"

// Deductions
#include "metkit/mars2grib/backend/deductions/class.h"
#include "metkit/mars2grib/backend/deductions/expver.h"
#include "metkit/mars2grib/backend/deductions/stream.h"
#include "metkit/mars2grib/backend/deductions/type.h"

// checks
#include "metkit/mars2grib/backend/checks/checkLocalUseSection.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 `mars` concept.
///
/// This predicate determines whether the mars 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 invocations
/// are instantiated.
///
/// @tparam Stage   Encoding stage (compile-time constant)
/// @tparam Section GRIB section index (compile-time constant)
/// @tparam Variant Mars 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:
/// - `Variant == MarsType::Default`
/// - `Stage == StagePreset`
/// - `Section == SecLocalUseSection`
///
template <std::size_t Stage, std::size_t Section, MarsType Variant>
constexpr bool marsApplicable() {

    // Confitions to apply concept
    return ((Variant == MarsType::Default) && (Stage == StagePreset) && (Section == SecLocalUseSection));
}


///
/// @brief Execute the `mars` concept operation.
///
/// This function implements the runtime logic of the GRIB `mars` concept.
/// When applicable, it:
///
/// 1. Validates the structural integrity of the GRIB Local Use Section.
/// 2. Deduces core MARS identity fields from the input dictionaries.
/// 3. Encodes the corresponding GRIB keys in the output dictionary.
///
/// The concept establishes the fundamental identity of the GRIB message
/// and is typically a prerequisite for other Local Use Section concepts.
///
/// 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    Mars 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:
/// - the Local Use Section is structurally invalid,
/// - any required MARS field cannot be deduced,
/// - any GRIB key cannot be set,
/// - the concept is invoked when not applicable.
///
/// @note
/// - All runtime errors are wrapped with full concept context
/// (concept name, variant, stage, section).
/// - This concept does not rely on pre-existing GRIB header state.
///
/// @note
/// The keywords [marsClass, marsType, marsStream] correspond to *raw GRIB keys*
/// and are written directly without triggering additional logic.
///
/// In contrast, the high-level keywords [class, type, stream] are **ecCodes
/// concepts**. Setting them may implicitly modify multiple underlying GRIB
/// keys in order to maintain internal consistency.
///
/// As a consequence, assigning high-level keywords can have side effects.
/// Examples (non-exhaustive) include:
/// - setting "type" may implicitly update "typeOfProcessedData"
/// - setting "stream" may implicitly change the product definition template number
///
/// @see marsApplicable
///
template <std::size_t Stage, std::size_t Section, MarsType Variant, class MarsDict_t, class ParDict_t, class OptDict_t,
          class OutDict_t>
void MarsOp(const MarsDict_t& mars, const ParDict_t& par, const OptDict_t& opt, OutDict_t& out) {

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

    // eccodes/definitions/grib2/local.98.36.def
    if constexpr (marsApplicable<Stage, Section, Variant>()) {

        try {

            MARS2GRIB_LOG_CONCEPT(mars);

            // Preconditions/contracts
            validation::check_LocalUseSection_or_throw(opt, out);


            // Deductions
            std::string marsClassVal  = deductions::resolve_Class_or_throw(mars, par, opt);
            std::string marsTypeVal   = deductions::resolve_Type_or_throw(mars, par, opt);
            std::string marsStreamVal = deductions::resolve_Stream_or_throw(mars, par, opt);
            std::string marsExpverVal = deductions::resolve_Expver_or_throw(mars, par, opt);

            // Encoding
            set_or_throw<std::string>(out, "marsClass", marsClassVal);
            set_or_throw<std::string>(out, "marsType", marsTypeVal);
            set_or_throw<std::string>(out, "marsStream", marsStreamVal);
            set_or_throw<std::string>(out, "expver", marsExpverVal);
        }
        catch (...) {
            MARS2GRIB_CONCEPT_RETHROW(mars, "Unable to set `mars` concept...");
        }

        // Successful operation
        return;
    }

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

    // Remove compiler warning
    mars2gribUnreachable();
}

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