package bnp

import (
	"archive/tar"
	"fmt"
	"os"
	"path/filepath"
	"sort"
	"strings"

	"github.com/klauspost/compress/zstd"
)

// packEntry is one file destined for the artifact: its path inside the .bnp
// (always forward-slash relative) and either literal Data or a Source file to
// stream from.
type packEntry struct {
	ArtifactPath string
	Data         []byte
	Source       string
}

// packArtifact writes entries as a zstd-compressed tar to outPath. Entries are
// sorted by artifact path for a deterministic archive. Directory entries are
// synthesized as needed so the reader's dirExists checks fire for
// migrations/, schemas/, assets/, web/.
func packArtifact(outPath string, entries []packEntry) (int64, error) {
	sort.Slice(entries, func(i, j int) bool { return entries[i].ArtifactPath < entries[j].ArtifactPath })

	out, err := os.Create(outPath)
	if err != nil {
		return 0, fmt.Errorf("create %s: %w", outPath, err)
	}
	defer func() { _ = out.Close() }()

	enc, err := zstd.NewWriter(out, zstd.WithEncoderLevel(zstd.SpeedDefault))
	if err != nil {
		return 0, err
	}
	tw := tar.NewWriter(enc)

	seenDir := map[string]bool{}
	writeDir := func(dir string) error {
		if dir == "" || dir == "." || seenDir[dir] {
			return nil
		}
		seenDir[dir] = true
		return tw.WriteHeader(&tar.Header{
			Name:     dir + "/",
			Typeflag: tar.TypeDir,
			Mode:     0o755,
		})
	}

	var total int64
	for _, e := range entries {
		name := filepath.ToSlash(e.ArtifactPath)
		// Ensure parent directories exist as explicit entries.
		for _, d := range parentDirs(name) {
			if err := writeDir(d); err != nil {
				return 0, fmt.Errorf("write dir header %q: %w", d, err)
			}
		}

		var data []byte
		if e.Source != "" {
			data, err = os.ReadFile(e.Source)
			if err != nil {
				return 0, fmt.Errorf("read %s: %w", e.Source, err)
			}
		} else {
			data = e.Data
		}
		if err := tw.WriteHeader(&tar.Header{
			Name:     name,
			Typeflag: tar.TypeReg,
			Mode:     0o644,
			Size:     int64(len(data)),
		}); err != nil {
			return 0, fmt.Errorf("write header %q: %w", name, err)
		}
		if _, err := tw.Write(data); err != nil {
			return 0, fmt.Errorf("write %q: %w", name, err)
		}
		total += int64(len(data))
	}

	if err := tw.Close(); err != nil {
		return 0, fmt.Errorf("close tar: %w", err)
	}
	if err := enc.Close(); err != nil {
		return 0, fmt.Errorf("close zstd: %w", err)
	}
	return total, nil
}

// parentDirs returns the ancestor directories of a forward-slash path, from
// shallowest to deepest ("a/b/c.txt" → ["a", "a/b"]).
func parentDirs(name string) []string {
	var dirs []string
	parts := strings.Split(name, "/")
	for i := 1; i < len(parts); i++ {
		dirs = append(dirs, strings.Join(parts[:i], "/"))
	}
	return dirs
}

// collectDir returns pack entries for every regular file under root, mapped
// under artifactPrefix. Symlinks and irregular files are skipped. Returns
// (entries, hadFiles).
func collectDir(root, artifactPrefix string) ([]packEntry, bool, error) {
	info, err := os.Stat(root)
	if err != nil || !info.IsDir() {
		return nil, false, nil //nolint:nilerr // absent optional dir is not an error
	}
	var entries []packEntry
	walkErr := filepath.WalkDir(root, func(path string, d os.DirEntry, err error) error {
		if err != nil {
			return err
		}
		if d.IsDir() || !d.Type().IsRegular() {
			return nil
		}
		rel, err := filepath.Rel(root, path)
		if err != nil {
			return err
		}
		entries = append(entries, packEntry{
			ArtifactPath: artifactPrefix + "/" + filepath.ToSlash(rel),
			Source:       path,
		})
		return nil
	})
	if walkErr != nil {
		return nil, false, fmt.Errorf("walk %s: %w", root, walkErr)
	}
	return entries, len(entries) > 0, nil
}