package apt

import (
	"bytes"
	"compress/gzip"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"io"
	"os"
	"path/filepath"
)

// InitEmptyRepo creates the directory structure for an APT mirror and writes an
// empty Release file so APT clients don't get a 404 before the first clone.
func InitEmptyRepo(localDir string, cfg Config) error {
	if err := os.MkdirAll(filepath.Join(localDir, "pool"), 0o755); err != nil {
		return err
	}
	for _, component := range cfg.Components {
		for _, arch := range cfg.Architectures {
			dir := filepath.Join(localDir, "dists", cfg.Suite, component, "binary-"+arch)
			if err := os.MkdirAll(dir, 0o755); err != nil {
				return err
			}
		}
	}
	releasePath := filepath.Join(localDir, "dists", cfg.Suite, "Release")
	if _, err := os.Stat(releasePath); os.IsNotExist(err) {
		releaseData := GenerateRelease(cfg.Suite, cfg.Suite, cfg.Components, cfg.Architectures, nil)
		if err := atomicWrite(releasePath, releaseData); err != nil {
			return fmt.Errorf("write empty Release: %w", err)
		}
	}
	return nil
}

// RegenerateMetadata rebuilds local Packages, Packages.gz, and Release files
// for all configured component/arch pairs, keeping only packages present on disk.
func RegenerateMetadata(localDir string, cfg Config) error {
	suite := cfg.Suite
	var releaseEntries []ReleaseEntry

	for _, component := range cfg.Components {
		for _, arch := range cfg.Architectures {
			entries, err := regenerateComponentArch(localDir, suite, component, arch)
			if err != nil {
				return fmt.Errorf("regenerate %s/%s: %w", component, arch, err)
			}
			releaseEntries = append(releaseEntries, entries...)
		}
	}

	// Write dists/{suite}/Release.
	releaseData := GenerateRelease(suite, suite, cfg.Components, cfg.Architectures, releaseEntries)
	releasePath := filepath.Join(localDir, "dists", suite, "Release")
	if err := os.MkdirAll(filepath.Dir(releasePath), 0o755); err != nil {
		return fmt.Errorf("create dists dir: %w", err)
	}
	if err := atomicWrite(releasePath, releaseData); err != nil {
		return fmt.Errorf("write Release: %w", err)
	}

	return nil
}

// regenerateComponentArch regenerates Packages and Packages.gz for one component/arch
// and returns the two ReleaseEntry values (plain then gz) for the Release file.
func regenerateComponentArch(localDir, suite, component, arch string) ([]ReleaseEntry, error) {
	binDir := filepath.Join(localDir, "dists", suite, component, "binary-"+arch)
	upstreamGzPath := filepath.Join(binDir, upstreamPackagesGz)

	// 1. Read upstream Packages.gz.
	gzData, err := os.ReadFile(upstreamGzPath)
	if err != nil {
		// Not cloned yet — skip silently.
		return nil, nil
	}

	// 2. Decompress.
	gz, err := gzip.NewReader(bytes.NewReader(gzData))
	if err != nil {
		return nil, fmt.Errorf("open gzip %s: %w", upstreamGzPath, err)
	}
	rawPackages, err := io.ReadAll(gz)
	gz.Close()
	if err != nil {
		return nil, fmt.Errorf("decompress %s: %w", upstreamGzPath, err)
	}

	// 3. Parse all stanzas.
	allPkgs, err := ParsePackages(rawPackages)
	if err != nil {
		return nil, fmt.Errorf("parse upstream Packages: %w", err)
	}

	// 4. Filter: keep only packages present on disk.
	checker := diskPresenceChecker(localDir)
	localPkgs := FilterPackages(allPkgs, checker)

	// 5. Re-emit filtered stanzas.
	plainBytes := emitPackages(localPkgs)

	// 6. Gzip the filtered Packages.
	var gzBuf bytes.Buffer
	gzw := gzip.NewWriter(&gzBuf)
	if _, err := gzw.Write(plainBytes); err != nil {
		return nil, fmt.Errorf("gzip write: %w", err)
	}
	if err := gzw.Close(); err != nil {
		return nil, fmt.Errorf("gzip close: %w", err)
	}
	gzBytes := gzBuf.Bytes()

	// 7. Write Packages and Packages.gz.
	if err := os.MkdirAll(binDir, 0o755); err != nil {
		return nil, err
	}
	plainPath := filepath.Join(binDir, "Packages")
	if err := atomicWrite(plainPath, plainBytes); err != nil {
		return nil, fmt.Errorf("write Packages: %w", err)
	}
	gzPath := filepath.Join(binDir, "Packages.gz")
	if err := atomicWrite(gzPath, gzBytes); err != nil {
		return nil, fmt.Errorf("write Packages.gz: %w", err)
	}

	// 8. Compute SHA256 hashes.
	plainSum := sha256.Sum256(plainBytes)
	gzSum := sha256.Sum256(gzBytes)

	// Release paths are relative to dists/{suite}/.
	relBase := component + "/binary-" + arch + "/"

	return []ReleaseEntry{
		{
			Hash: hex.EncodeToString(plainSum[:]),
			Size: int64(len(plainBytes)),
			Path: relBase + "Packages",
		},
		{
			Hash: hex.EncodeToString(gzSum[:]),
			Size: int64(len(gzBytes)),
			Path: relBase + "Packages.gz",
		},
	}, nil
}