#include <expected>
#include <memory>
#include <stdexcept>
#include <string>
#include <variant>

#include <catch2/catch_test_macros.hpp>

#include "serif/utils/dependencies/computation_cache.hpp"

namespace {
    namespace dependencies = serif::utils::dependencies;

    struct Source {};
    struct CheckedInput {};
    struct PhysicsResult {};
    struct Output {};

    /** @brief Preserve the original invalid argument without converting all failures into text. */
    struct InputFailure {
        int value;
        std::string message;
    };

    /** @brief Preserve downstream numerical diagnostics independently of input validation. */
    struct PhysicsFailure {
        double value;
        int sample;
        std::string message;
    };

    /** @brief A cache error may own diagnostic data that cannot be copied. */
    struct OwnedFailure {
        std::unique_ptr<int> value;
    };
}

TEST_CASE("Explicit error unions preserve each computation's structured failure and transactional recovery", "[unit][utils][dependencies]") {
    using Failure = std::variant<InputFailure, PhysicsFailure>;

    int input = 2;
    int input_calls = 0;
    int physics_calls = 0;
    int output_calls = 0;
    const auto context = dependencies::make_dependency_context(dependencies::borrow<Source>(input));
    const auto checked = dependencies::make_computation<CheckedInput, dependencies::InputList<Source>>(
        [&](int value) -> std::expected<double, InputFailure> {
            ++input_calls;
            if (value < 0) {
                return std::unexpected(InputFailure{.value = value, .message = "The input must be nonnegative."});
            }
            return 2.0 * value;
        }
    );
    const auto physics = dependencies::make_computation<PhysicsResult, dependencies::InputList<CheckedInput>>(
        [&](double value) -> std::expected<double, PhysicsFailure> {
            ++physics_calls;
            if (value > 10.0) {
                return std::unexpected(PhysicsFailure{.value = value, .sample = 3, .message = "The physical state exceeds the admissible range."});
            }
            return value + 0.5;
        }
    );
    const auto output = dependencies::make_computation<Output, dependencies::InputList<PhysicsResult>>(
        [&](double value) {
            ++output_calls;
            return 3.0 * value;
        }
    );
    auto cache = dependencies::make_computation_cache<Failure>(context, checked, physics, output);
    REQUIRE(cache.try_prepare(context));
    CHECK(cache.get<Output>() == 13.5);
    CHECK(cache.preparation_count() == 1);

    input = -1;
    const auto rejected_input = cache.try_prepare(context);
    REQUIRE_FALSE(rejected_input);
    CHECK(rejected_input.error().computation_index == 0);
    const InputFailure* input_failure = std::get_if<InputFailure>(&rejected_input.error().error);
    REQUIRE(input_failure != nullptr);
    CHECK(input_failure->value == -1);
    CHECK(input_failure->message == "The input must be nonnegative.");
    CHECK_FALSE(cache.ready());
    CHECK(cache.preparation_count() == 1);
    CHECK(input_calls == 2);
    CHECK(physics_calls == 1);
    CHECK(output_calls == 1);
    CHECK_THROWS_AS(cache.get<CheckedInput>(), std::logic_error);
    CHECK_THROWS_AS(cache.get<PhysicsResult>(), std::logic_error);
    CHECK_THROWS_AS(cache.get<Output>(), std::logic_error);

    input = 3;
    const auto recovered_input = cache.try_prepare(context);
    REQUIRE(recovered_input);
    CHECK(recovered_input->computed_count() == 3);
    CHECK(cache.get<Output>() == 19.5);
    CHECK(cache.preparation_count() == 2);

    input = 6;
    const auto rejected_physics = cache.try_prepare(context);
    REQUIRE_FALSE(rejected_physics);
    CHECK(rejected_physics.error().computation_index == 1);
    const PhysicsFailure* physics_failure = std::get_if<PhysicsFailure>(&rejected_physics.error().error);
    REQUIRE(physics_failure != nullptr);
    CHECK(physics_failure->value == 12.0);
    CHECK(physics_failure->sample == 3);
    CHECK(physics_failure->message == "The physical state exceeds the admissible range.");
    CHECK_FALSE(cache.ready());
    CHECK(cache.preparation_count() == 2);
    CHECK(input_calls == 4);
    CHECK(physics_calls == 3);
    CHECK(output_calls == 2);
    CHECK_THROWS_AS(cache.get<CheckedInput>(), std::logic_error);
    CHECK_THROWS_AS(cache.get<Output>(), std::logic_error);

    // The rejected candidate must not replace the last accepted source or intermediate result.
    input = 3;
    const auto recovered_physics = cache.try_prepare(context);
    REQUIRE(recovered_physics);
    CHECK(recovered_physics->computed_count() == 0);
    CHECK(cache.get<CheckedInput>() == 6.0);
    CHECK(cache.get<PhysicsResult>() == 6.5);
    CHECK(cache.get<Output>() == 19.5);
    CHECK(input_calls == 4);
    CHECK(physics_calls == 3);
    CHECK(output_calls == 2);
}

TEST_CASE("Requested results avoid unrelated errors in a graph with an explicit failure union", "[unit][utils][dependencies]") {
    using Failure = std::variant<InputFailure, PhysicsFailure>;

    int input = 7;
    int physics_calls = 0;
    const auto context = dependencies::make_dependency_context(dependencies::borrow<Source>(input));
    const auto checked = dependencies::make_computation<CheckedInput, dependencies::InputList<Source>>(
        [](int value) -> std::expected<int, InputFailure> {
            return value;
        }
    );
    const auto physics = dependencies::make_computation<PhysicsResult, dependencies::InputList<CheckedInput>>(
        [&](int value) -> std::expected<int, PhysicsFailure> {
            ++physics_calls;
            return std::unexpected(PhysicsFailure{.value = static_cast<double>(value), .sample = 2, .message = "This downstream result is deliberately unavailable."});
        }
    );
    auto cache = dependencies::make_computation_cache<Failure>(context, checked, physics);
    const auto requested = cache.try_prepare(context, dependencies::RequestedResults<CheckedInput>{});
    REQUIRE(requested);
    CHECK(requested->computed_count() == 1);
    CHECK(requested->status<PhysicsResult>() == dependencies::PreparationStatus::not_required);
    CHECK(physics_calls == 0);
    CHECK(cache.get<CheckedInput>() == 7);
    CHECK_FALSE(cache.contains<PhysicsResult>());

    const auto rejected = cache.try_prepare(context);
    REQUIRE_FALSE(rejected);
    CHECK(std::holds_alternative<PhysicsFailure>(rejected.error().error));
    CHECK_FALSE(cache.ready());
    REQUIRE(cache.try_prepare(context, dependencies::RequestedResults<CheckedInput>{}));
    CHECK(cache.get<CheckedInput>() == 7);
    CHECK(physics_calls == 1);
}

TEST_CASE("Explicit failure unions transfer move-only diagnostic payloads without copying", "[unit][utils][dependencies]") {
    using Failure = std::variant<InputFailure, OwnedFailure>;

    int input = -4;
    const auto context = dependencies::make_dependency_context(dependencies::borrow<Source>(input));
    const auto computation = dependencies::make_computation<Output, dependencies::InputList<Source>>(
        [](int value) -> std::expected<int, OwnedFailure> {
            if (value < 0) {
                return std::unexpected(OwnedFailure{.value = std::make_unique<int>(value)});
            }
            return value;
        }
    );
    auto cache = dependencies::make_computation_cache<Failure>(context, computation);
    const auto rejected = cache.try_prepare(context);
    REQUIRE_FALSE(rejected);
    const OwnedFailure* failure = std::get_if<OwnedFailure>(&rejected.error().error);
    REQUIRE(failure != nullptr);
    REQUIRE(failure->value != nullptr);
    CHECK(*failure->value == -4);
    CHECK_FALSE(cache.ready());
    input = 4;
    REQUIRE(cache.try_prepare(context));
    CHECK(cache.get<Output>() == 4);
    CHECK(*failure->value == -4);
}