/*
 * (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 levelEncoding.h
/// @brief Implementation of the GRIB `level` concept operation.
///
/// This header defines the applicability rules and execution logic for the
/// **level concept** within the mars2grib backend.
///
/// The level concept is responsible for encoding GRIB keys related to the
/// *vertical coordinate system* of the data, including:
///
/// - `typeOfLevel`
/// - `level`
/// - model-level coordinates and PV array (multi-level model layouts only)
/// - layered levels expressed via `topLevel` / `bottomLevel`
///
/// The encoder behaviour is governed by three orthogonal compile-time
/// predicates over the variant:
///
/// - `needPv<Variant>()`        : the variant requires a PV array
///                                (currently only `ModelMultipleLevel`).
/// - `needLevel<Variant>()`     : the variant carries a numeric `level`
///                                value to be written.
/// - `needTopBottomLevel<Variant>()` : the variant is expressed as a
///                                layer with explicit `topLevel` and
///                                `bottomLevel` GRIB keys.
///
/// These three axes are independent: any subset may apply to a given
/// variant. A small number of variants additionally have hard-coded
/// special cases inside `LevelOp` (for example the `*At2M` / `*At10M`
/// shortcuts and the `IsobaricInHpa` Pa->hPa conversion).
///
/// Depending on the selected level variant, the concept may:
/// - set only the level type,
/// - set both level type and numeric level,
/// - allocate and populate the PV array (multi-level model layouts).
///
/// The implementation follows the standard mars2grib concept model:
/// - Compile-time applicability via `levelApplicable`
/// - Stage-aware behavior (allocation vs preset/runtime)
/// - Explicit handling of hybrid vertical coordinates
/// - 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>
#include <vector>

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

// Deductions
#include "metkit/mars2grib/backend/deductions/level.h"
#include "metkit/mars2grib/backend/deductions/pvArray.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 predicate indicating whether a PV array is required.
///
/// Only multi-level model-level layouts require a PV array describing
/// the vertical hybrid coordinate transformation. Single-level model
/// fields share the GRIB `typeOfLevel="hybrid"` string but do not carry
/// a vertical column and therefore do not need a PV array.
///
/// @tparam Variant Level concept variant
///
/// @return `true` if a PV array is required,
/// `false` otherwise.
///
template <LevelType Variant>
constexpr bool needPv() {
    if constexpr (Variant == LevelType::ModelMultipleLevel) {
        return true;
    }
    else {
        return false;
    }

    // Remove compiler warning
    mars2gribUnreachable();
}

///
/// @brief Compile-time predicate indicating whether a numeric `level` value is required.
///
/// Some level types require an associated numeric level (e.g. pressure,
/// height, model-level number). Both model-level variants
/// (`ModelSingleLevel` and `ModelMultipleLevel`) require it; they differ
/// only in whether a PV array is also needed.
///
/// `AbstractLevel` carries an opaque numeric `level` value (in contrast
/// to `AbstractSingleLevel` and `AbstractMultipleLevel`, which do not).
///
/// @tparam Variant Level concept variant
///
/// @return `true` if a numeric `level` value must be set,
/// `false` otherwise.
///
template <LevelType Variant>
constexpr bool needLevel() {
    if constexpr (Variant == LevelType::HeightAboveGroundAt10M || Variant == LevelType::HeightAboveGroundAt2M ||
                  Variant == LevelType::HeightAboveGround || Variant == LevelType::HeightAboveSeaAt10M ||
                  Variant == LevelType::HeightAboveSeaAt2M || Variant == LevelType::HeightAboveSea ||
                  Variant == LevelType::ModelSingleLevel || Variant == LevelType::ModelMultipleLevel ||
                  Variant == LevelType::IsobaricInHpa || Variant == LevelType::IsobaricInPa ||
                  Variant == LevelType::Isothermal || Variant == LevelType::PotentialVorticity ||
                  Variant == LevelType::Theta || Variant == LevelType::OceanModel ||
                  Variant == LevelType::AbstractLevel || Variant == LevelType::FlightLevel) {

        return true;
    }
    else {
        return false;
    }

    // Remove compiler warning
    mars2gribUnreachable();
}

///
/// @brief Compile-time predicate indicating whether a numeric `topLevel` and `bottomLevel` value is required.
///
/// Some level types require two associated numeric levels (e.g. soilLayer),
/// while others encode only the level type and/or level.
///
/// @tparam Variant Level concept variant
///
/// @return `true` if a numeric `topLevel` and `bottomLevel` value must be set,
/// `false` otherwise.
///
template <LevelType Variant>
constexpr bool needTopBottomLevel() {
    if constexpr (Variant == LevelType::SoilLayer || Variant == LevelType::SeaIceLayer ||
                  Variant == LevelType::SnowLayer || Variant == LevelType::OceanModelLayer) {
        return true;
    }
    else {
        return false;
    }

    // Remove compiler warning
    mars2gribUnreachable();
}


///
/// @brief Compile-time applicability predicate for the `level` concept.
///
/// This predicate determines whether the level concept is applicable for a given
/// combination of:
/// - encoding stage
/// - GRIB section
/// - level variant
///
/// Applicability is evaluated entirely at compile time and is used by the
/// concept dispatcher to control instantiation and execution.
///
/// Multi-level model layouts require special handling:
/// - during allocation stage to reserve space for the PV array,
/// - during preset/runtime stages to set the level type and parameters.
///
/// @tparam Stage   Encoding stage (compile-time constant)
/// @tparam Section GRIB section index (compile-time constant)
/// @tparam Variant Level concept variant
///
/// @return `true` if the concept is applicable for the given parameters,
/// `false` otherwise.
///
template <std::size_t Stage, std::size_t Section, LevelType Variant>
constexpr bool levelApplicable() {

    if constexpr (Section == SecProductDefinitionSection && needPv<Variant>()) {
        // pvArray needs to be allocated at allocation stage
        return true;
    }

    if constexpr (Section == SecProductDefinitionSection && !needPv<Variant>()) {
        return (Stage != StageAllocate);
    }

    return false;
}


///
/// @brief Execute the `level` concept operation.
///
/// This function implements the runtime logic of the GRIB `level` concept.
/// When applicable, it:
///
/// - allocates and sets the PV array for hybrid levels during allocation stage,
/// - sets the GRIB `typeOfLevel` key,
/// - sets the numeric `level` key when required.
///
/// The behavior is explicitly stage-dependent:
/// - `StageAllocate` is used for memory allocation (PV array),
/// - `StagePreset` and `StageRuntime` are used for semantic encoding.
///
/// 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    Level 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:
/// - required deductions fail,
/// - invalid stage/variant combinations are invoked,
/// - any GRIB key cannot be set.
///
/// @note
/// - All runtime errors are wrapped with full concept context
/// (concept name, variant, stage, section).
/// - The concept does not rely on pre-existing GRIB header state.
/// - Setting of typeOfLevel is happening at both preset and runtime stages because
/// sometimes due to sideeffects in eccodes the typeOfLevel set at preset stage
/// can be overwritten before runtime stage.
///
///
/// @see levelApplicable
/// @see needLevel
/// @see needPv
///
template <std::size_t Stage, std::size_t Section, LevelType Variant, class MarsDict_t, class ParDict_t, class OptDict_t,
          class OutDict_t>
void LevelOp(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 (levelApplicable<Stage, Section, Variant>()) {

        try {

            MARS2GRIB_LOG_CONCEPT(level);

            // =============================================================
            // Allocation stage (PV array)
            // =============================================================
            if constexpr (Stage == StageAllocate && needPv<Variant>()) {

                // Allocate space for pv array
                std::vector<double> pv_array = deductions::resolve_PvArray_or_throw(mars, par, opt);

                // Set the PV array
                set_or_throw<long>(out, "PVPresent", 1L);
                set_or_throw<std::vector<double>>(out, "pv", pv_array);
            }


            // =============================================================
            // Preset / runtime stage
            // =============================================================
            if constexpr (Stage == StagePreset || Stage == StageRuntime) {

                // Set level type (and level)
                if constexpr (Variant == LevelType::HeightAboveGroundAt2M) {
                    set_or_throw<std::string>(out, "typeOfLevel", "heightAboveGround");
                    set_or_throw<long>(out, "level", 2L);
                }
                else if constexpr (Variant == LevelType::HeightAboveGroundAt10M) {
                    set_or_throw<std::string>(out, "typeOfLevel", "heightAboveGround");
                    set_or_throw<long>(out, "level", 10L);
                }
                else if constexpr (Variant == LevelType::HeightAboveSeaAt2M) {
                    set_or_throw<std::string>(out, "typeOfLevel", "heightAboveSea");
                    set_or_throw<long>(out, "level", 2L);
                }
                else if constexpr (Variant == LevelType::HeightAboveSeaAt10M) {
                    set_or_throw<std::string>(out, "typeOfLevel", "heightAboveSea");
                    set_or_throw<long>(out, "level", 10L);
                }
                else if constexpr (Variant == LevelType::IsobaricInHpa) {
                    long levelVal = deductions::resolve_Level_or_throw(mars, par, opt);
                    set_or_throw<std::string>(out, "typeOfLevel", "isobaricInhPa");
                    set_or_throw<long>(out, "level", levelVal / 100);
                }
                else {
                    set_or_throw<std::string>(out, "typeOfLevel", std::string(levelTypeName<Variant>()));
                    if constexpr (needLevel<Variant>()) {
                        long levelVal = deductions::resolve_Level_or_throw(mars, par, opt);
                        set_or_throw<long>(out, "level", levelVal);
                    }
                    if constexpr (needTopBottomLevel<Variant>()) {
                        long levelVal    = deductions::resolve_Level_or_throw(mars, par, opt);
                        long topLevel    = levelVal - 1;
                        long bottomLevel = levelVal;
                        set_or_throw<long>(out, "topLevel", topLevel);
                        set_or_throw<long>(out, "bottomLevel", bottomLevel);
                    }
                }
            }
        }
        catch (...) {

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

        // Successful operation
        return;
    }

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

    // Remove compiler warning
    mars2gribUnreachable();
}

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