// mcpp.bmi_cache — cross-project persistent BMI cache (M3.2).
//
// Layout (per docs/26-bmi-cache.md):
//   $MCPP_HOME/bmi/<fingerprint>/deps/<indexName>/<pkgName>@<version>/
//     {gcm,pcm}.cache/<module>.{gcm,pcm}
//     obj/<file>.m.o + <file>.o
//     manifest.txt    (sentinel + file list)
//
// fingerprint already covers compiler / flags / stdlib / mcpp version /
// dep-lock hash etc. (docs/06), so different fingerprints get independent
// cache directories — no risk of stale-BMI cross-contamination.
//
// M4 #9: populate is wrapped in an advisory exclusive flock(2) on
// <cacheDir>/.lock so two concurrent mcpp builds can race to the same
// dep cache without trashing manifest.txt (docs/26 §5.4 V2).

module;

export module mcpp.bmi_cache;

import std;
import mcpp.platform;

export namespace mcpp::bmi_cache {

struct CacheKey {
    std::filesystem::path mcppHome;
    std::string fingerprint;
    std::string indexName;       // "mcpplibs" / "xim" / ...
    std::string packageName;     // "mcpplibs.cmdline"
    std::string version;         // "0.0.1"
    std::string bmiDirName   = "gcm.cache"; // "gcm.cache" | "pcm.cache"
    std::string manifestTag  = "gcm";       // "gcm" | "pcm"

    std::filesystem::path dir() const {
        return mcppHome / "bmi" / fingerprint / "deps"
             / indexName / std::format("{}@{}", packageName, version);
    }

    std::filesystem::path manifestFile() const { return dir() / "manifest.txt"; }
    std::filesystem::path bmiDir()      const { return dir() / bmiDirName; }
    std::filesystem::path objDir()      const { return dir() / "obj"; }
};

// File names (basename only) that belong to one dep package's cache entry.
struct DepArtifacts {
    std::vector<std::string> bmiFiles;   // basenames in bmiDir/
    std::vector<std::string> objFiles;   // basenames in obj/
};

// Cache hit if manifest.txt exists AND every listed file is present.
bool is_cached(const CacheKey& key);

// Read manifest.txt → DepArtifacts.
std::expected<DepArtifacts, std::string>
read_manifest(const CacheKey& key);

// Copy missing cached files into projectTarget/{bmiDirName,obj}. Existing
// project outputs are left untouched: BMIs may differ byte-for-byte between
// equivalent builds, and overwriting them would dirty downstream modules.
std::expected<DepArtifacts, std::string>
stage_into(const CacheKey& key,
           const std::filesystem::path& projectTargetDir);

// Copy fresh build outputs from projectTarget/{bmiDirName,obj} → cache dir
// and write manifest.txt last (atomic-ish sentinel).
std::expected<void, std::string>
populate_from(const CacheKey& key,
              const std::filesystem::path& projectTargetDir,
              const DepArtifacts& artifacts);

} // namespace mcpp::bmi_cache

namespace mcpp::bmi_cache {

namespace {

void touch_now(const std::filesystem::path& p) {
    std::error_code ec;
    auto t = std::chrono::file_clock::now() + std::chrono::seconds(1);
    std::filesystem::last_write_time(p, t, ec);
}

bool copy_one(const std::filesystem::path& from,
              const std::filesystem::path& to,
              std::error_code& ec)
{
    std::filesystem::create_directories(to.parent_path(), ec);
    std::filesystem::copy_file(from, to,
        std::filesystem::copy_options::overwrite_existing, ec);
    return !ec;
}

std::string serialize_manifest(std::string_view tag, const DepArtifacts& a) {
    std::string out = "# Auto-generated by mcpp bmi_cache. Do not edit.\n";
    for (auto& g : a.bmiFiles) out += std::format("{}: {}\n", tag, g);
    for (auto& o : a.objFiles) out += std::format("obj: {}\n", o);
    return out;
}

std::expected<DepArtifacts, std::string>
parse_manifest(const std::filesystem::path& p) {
    std::ifstream is(p);
    if (!is) return std::unexpected(std::format("cannot open '{}'", p.string()));
    DepArtifacts a;
    std::string line;
    while (std::getline(is, line)) {
        while (!line.empty() && (line.back() == '\r' || line.back() == ' ')) line.pop_back();
        if (line.empty() || line[0] == '#') continue;
        if (line.starts_with("gcm: "))      a.bmiFiles.push_back(line.substr(5));
        else if (line.starts_with("pcm: ")) a.bmiFiles.push_back(line.substr(5));
        else if (line.starts_with("obj: ")) a.objFiles.push_back(line.substr(5));
    }
    return a;
}

} // namespace

bool is_cached(const CacheKey& key) {
    auto mf = key.manifestFile();
    if (!std::filesystem::exists(mf)) return false;
    auto arts = parse_manifest(mf);
    if (!arts) return false;
    // Verify every listed file actually exists on disk.
    for (auto& g : arts->bmiFiles) {
        if (!std::filesystem::exists(key.bmiDir() / g)) return false;
    }
    for (auto& o : arts->objFiles) {
        if (!std::filesystem::exists(key.objDir() / o)) return false;
    }
    return true;
}

std::expected<DepArtifacts, std::string>
read_manifest(const CacheKey& key) {
    return parse_manifest(key.manifestFile());
}

std::expected<DepArtifacts, std::string>
stage_into(const CacheKey& key,
           const std::filesystem::path& projectTargetDir)
{
    auto arts = parse_manifest(key.manifestFile());
    if (!arts) return std::unexpected(arts.error());

    auto projectBmi = projectTargetDir / key.bmiDirName;
    auto projectObj = projectTargetDir / "obj";
    std::error_code ec;
    std::filesystem::create_directories(projectBmi, ec);
    std::filesystem::create_directories(projectObj, ec);

    for (auto& g : arts->bmiFiles) {
        auto from = key.bmiDir() / g;
        auto to   = projectBmi   / g;
        if (std::filesystem::exists(to, ec)) {
            ec.clear();
            continue;
        }
        ec.clear();
        if (!copy_one(from, to, ec)) {
            return std::unexpected(std::format(
                "stage bmi '{}': {}", g, ec.message()));
        }
        touch_now(to);
    }
    for (auto& o : arts->objFiles) {
        auto from = key.objDir() / o;
        auto to   = projectObj   / o;
        if (std::filesystem::exists(to, ec)) {
            ec.clear();
            continue;
        }
        ec.clear();
        if (!copy_one(from, to, ec)) {
            return std::unexpected(std::format(
                "stage obj '{}': {}", o, ec.message()));
        }
        touch_now(to);
    }
    return arts;
}

namespace {
} // namespace

std::expected<void, std::string>
populate_from(const CacheKey& key,
              const std::filesystem::path& projectTargetDir,
              const DepArtifacts& arts)
{
    auto cacheDir = key.dir();
    auto lock = mcpp::platform::fs::FileLock::try_acquire(cacheDir);
    if (!lock) {
        // Another writer holds the lock; treat as success (they'll do it).
        return {};
    }

    auto cacheBmi = key.bmiDir();
    auto cacheObj = key.objDir();
    std::error_code ec;
    std::filesystem::create_directories(cacheBmi, ec);
    std::filesystem::create_directories(cacheObj, ec);

    auto projectBmi = projectTargetDir / key.bmiDirName;
    auto projectObj = projectTargetDir / "obj";

    for (auto& g : arts.bmiFiles) {
        auto from = projectBmi / g;
        auto to   = cacheBmi   / g;
        if (!std::filesystem::exists(from)) {
            return std::unexpected(std::format(
                "expected build output missing: {}", from.string()));
        }
        if (!copy_one(from, to, ec)) {
            return std::unexpected(std::format(
                "populate bmi '{}': {}", g, ec.message()));
        }
    }
    for (auto& o : arts.objFiles) {
        auto from = projectObj / o;
        auto to   = cacheObj   / o;
        if (!std::filesystem::exists(from)) {
            return std::unexpected(std::format(
                "expected build output missing: {}", from.string()));
        }
        if (!copy_one(from, to, ec)) {
            return std::unexpected(std::format(
                "populate obj '{}': {}", o, ec.message()));
        }
    }
    // Write manifest.txt LAST as the sentinel — atomic via temp + rename.
    {
        auto tmp = key.manifestFile();
        tmp += ".tmp";
        {
            std::ofstream os(tmp);
            os << serialize_manifest(key.manifestTag, arts);
        }
        std::filesystem::rename(tmp, key.manifestFile(), ec);
        if (ec) {
            return std::unexpected(std::format(
                "populate manifest rename: {}", ec.message()));
        }
    }
    return {};
}

} // namespace mcpp::bmi_cache