package pipeline

import (
	"crypto/sha1"
	"encoding/json"
	"fmt"
	"os"
	"os/exec"
	"path/filepath"
	"regexp"
	"sort"
	"strings"

	"gitea.westgate.pw/ShadowsOverWestgate/sow-tools/internal/erf"
	"gitea.westgate.pw/ShadowsOverWestgate/sow-tools/internal/project"
)

const (
	maxMusicStemLen    = 16
	musicCreditsHeader = "# NWN Music Pack Credits\n\nOriginal rights remain with the respective composers and licensors.\n\n## Processed Files\n\n| Artist | Title | Output File | Original File | Album | Date | License / Copyright | Notes |\n|---|---|---|---|---|---|---|---|\n"
)

var (
	musicConvertExtensions = map[string]struct{}{
		".mp3": {},
		".ogg": {},
	}
	musicPassthroughExtensions = map[string]struct{}{
		".bmu": {},
		".wav": {},
	}
	musicDropWords = map[string]struct{}{
		"mp3": {}, "wav": {}, "flac": {}, "ogg": {}, "m4a": {},
		"music": {}, "track": {}, "audio": {}, "song": {}, "soundtrack": {},
		"final": {}, "version": {}, "ver": {}, "v": {}, "mix": {}, "master": {}, "remaster": {},
		"free": {}, "royalty": {}, "copyright": {}, "background": {}, "bgm": {},
		"loop": {}, "loops": {}, "loopable": {}, "seamless": {},
		"official": {}, "download": {}, "preview": {},
		"the": {}, "a": {}, "an": {}, "and": {}, "of": {}, "to": {}, "in": {}, "for": {}, "with": {}, "by": {},
	}
	musicBracketRE = regexp.MustCompile(`\[[^\]]*\]|\([^)]*\)|\{[^}]*\}`)
	musicSplitRE   = regexp.MustCompile(`[^A-Za-z0-9]+`)
	musicYearRE    = regexp.MustCompile(`^\d{4,}$`)
	musicVowelRE   = regexp.MustCompile(`[aeiou]`)
)

type preparedMusicAssets struct {
	Generated     []assetResource
	SkipSourceRel map[string]struct{}
	Artifacts     []string
	TempRoots     []string
}

type musicMetadata struct {
	Title     string
	Artist    string
	Album     string
	Date      string
	Rights    string
	Notes     string
	Comment   string
	Copyright string
	License   string
}

type creditsEntry struct {
	Artist       string `json:"artist"`
	Title        string `json:"title"`
	OutputFile   string `json:"output_file"`
	OriginalFile string `json:"original_file"`
	Album        string `json:"album"`
	Date         string `json:"date"`
	Rights       string `json:"rights"`
	Notes        string `json:"notes"`
}

type creditsSource struct {
	Path    string         `json:"path"`
	Kind    string         `json:"kind"`
	Entries []creditsEntry `json:"entries"`
}

type creditsInventory struct {
	Sources []creditsSource `json:"sources"`
}

type creditsOverlay struct {
	ByOriginal map[string]creditsEntry
	ByOutput   map[string]creditsEntry
	Entries    []creditsEntry
}

func prepareMusicAssets(p *project.Project) (*preparedMusicAssets, error) {
	usedNames := make(map[string]struct{})
	dirSources := make(map[string][]string)
	for _, rel := range p.Inventory.AssetFiles {
		rel = filepath.ToSlash(rel)
		ext := strings.ToLower(filepath.Ext(rel))
		name := strings.ToLower(strings.TrimSuffix(filepath.Base(rel), filepath.Ext(rel)))
		if _, convertible := musicConvertExtensions[ext]; convertible && isMusicAssetPath(rel) {
			dir := filepath.ToSlash(filepath.Dir(rel))
			dirSources[dir] = append(dirSources[dir], rel)
			continue
		}
		if name != "" {
			usedNames[name] = struct{}{}
		}
	}

	result := &preparedMusicAssets{
		SkipSourceRel: make(map[string]struct{}),
	}
	if len(dirSources) == 0 {
		artifactPaths, err := writeCreditsArtifacts(p, nil)
		if err != nil {
			return nil, err
		}
		result.Artifacts = artifactPaths
		return result, nil
	}

	dirs := make([]string, 0, len(dirSources))
	for dir := range dirSources {
		dirs = append(dirs, dir)
	}
	sort.Strings(dirs)

	generatedByDir := make(map[string][]creditsEntry)
	stageRoot := filepath.Join(p.Root, ".cache", "music")
	result.TempRoots = append(result.TempRoots, stageRoot)

	ffmpegPath, err := resolveFFmpegBinary()
	if err != nil {
		return nil, err
	}
	ffprobePath, err := resolveFFprobeBinary()
	if err != nil {
		return nil, err
	}

	for _, dir := range dirs {
		sources := append([]string(nil), dirSources[dir]...)
		sort.Strings(sources)
		prefix := musicPrefixForDir(p, dir)
		overlayPath := filepath.Join(p.AssetsDir(), filepath.FromSlash(dir), "CREDITS.md")
		overlay, err := parseCreditsOverlay(overlayPath)
		if err != nil {
			return nil, fmt.Errorf("parse credits overlay %s: %w", overlayPath, err)
		}

		entries := make([]creditsEntry, 0, len(sources))
		for _, rel := range sources {
			base := filepath.Base(rel)
			manual := overlay.match(base, "")
			outputFile := ""
			switch {
			case manual != nil && strings.TrimSpace(manual.OutputFile) != "":
				outputFile = strings.ToLower(strings.TrimSpace(manual.OutputFile))
				if err := validateManualOutputFile(outputFile); err != nil {
					return nil, fmt.Errorf("manual credits output for %s: %w", rel, err)
				}
				if err := reserveMusicName(strings.TrimSuffix(outputFile, filepath.Ext(outputFile)), usedNames); err != nil {
					return nil, fmt.Errorf("manual credits output for %s: %w", rel, err)
				}
			default:
				stem, err := generateMusicStem(base, prefix, usedNames)
				if err != nil {
					return nil, fmt.Errorf("generate music name for %s: %w", rel, err)
				}
				outputFile = stem + ".bmu"
			}
			outputRel := filepath.ToSlash(filepath.Join(dir, outputFile))
			metadata, err := probeMusicMetadata(ffprobePath, filepath.Join(p.AssetsDir(), filepath.FromSlash(rel)))
			if err != nil {
				return nil, fmt.Errorf("probe music metadata for %s: %w", rel, err)
			}
			entry := creditsEntry{
				Artist:       metadata.Artist,
				Title:        firstNonEmpty(metadata.Title, strings.TrimSuffix(base, filepath.Ext(base))),
				OutputFile:   outputFile,
				OriginalFile: base,
				Album:        metadata.Album,
				Date:         metadata.Date,
				Rights:       firstNonEmpty(metadata.Rights, joinNonEmpty("; ", metadata.License, metadata.Copyright)),
				Notes:        firstNonEmpty(metadata.Notes, metadata.Comment),
			}
			applyCreditsOverlay(&entry, manual)

			stagePath := filepath.Join(stageRoot, filepath.FromSlash(outputRel))
			if err := os.MkdirAll(filepath.Dir(stagePath), 0o755); err != nil {
				return nil, fmt.Errorf("create music stage dir: %w", err)
			}
			if err := convertMusicSource(ffmpegPath, filepath.Join(p.AssetsDir(), filepath.FromSlash(rel)), stagePath); err != nil {
				return nil, fmt.Errorf("convert music %s: %w", rel, err)
			}
			resourceInfo, err := assetResourceFromPath(stagePath, outputRel, lfsAssetInfo{}, false)
			if err != nil {
				return nil, fmt.Errorf("load converted music %s: %w", outputRel, err)
			}
			info, err := os.Stat(stagePath)
			if err != nil {
				return nil, fmt.Errorf("stat converted music %s: %w", stagePath, err)
			}
			result.Generated = append(result.Generated, assetResource{
				Rel:       outputRel,
				Resource:  resourceInfo.Resource,
				Size:      erf.ArchiveSize([]erf.Resource{resourceInfo.Resource}),
				CreatedAt: info.ModTime(),
				ContentID: resourceInfo.ContentID,
			})
			result.SkipSourceRel[rel] = struct{}{}
			entries = append(entries, entry)
		}
		if err := validateOverlayEntries(dir, overlay, entries); err != nil {
			return nil, err
		}
		generatedByDir[dir] = entries
	}

	artifactPaths, err := writeCreditsArtifacts(p, generatedByDir)
	if err != nil {
		return nil, err
	}
	result.Artifacts = artifactPaths
	sort.Slice(result.Generated, func(i, j int) bool {
		return compareResourceKeys(result.Generated[i].Resource, result.Generated[j].Resource) < 0
	})
	return result, nil
}

func writeCreditsArtifacts(p *project.Project, generatedByDir map[string][]creditsEntry) ([]string, error) {
	creditsRoot := filepath.Join(p.Root, ".cache", "credits")
	if err := os.RemoveAll(creditsRoot); err != nil {
		return nil, fmt.Errorf("reset credits dir: %w", err)
	}
	if err := os.MkdirAll(creditsRoot, 0o755); err != nil {
		return nil, fmt.Errorf("create credits dir: %w", err)
	}

	sources := make([]creditsSource, 0)
	artifacts := make([]string, 0)
	if generatedByDir != nil {
		dirs := make([]string, 0, len(generatedByDir))
		for dir := range generatedByDir {
			dirs = append(dirs, dir)
		}
		sort.Strings(dirs)
		for _, dir := range dirs {
			entries := append([]creditsEntry(nil), generatedByDir[dir]...)
			sort.Slice(entries, func(i, j int) bool {
				if strings.ToLower(entries[i].Artist) != strings.ToLower(entries[j].Artist) {
					return strings.ToLower(entries[i].Artist) < strings.ToLower(entries[j].Artist)
				}
				if strings.ToLower(entries[i].Title) != strings.ToLower(entries[j].Title) {
					return strings.ToLower(entries[i].Title) < strings.ToLower(entries[j].Title)
				}
				return strings.ToLower(entries[i].OutputFile) < strings.ToLower(entries[j].OutputFile)
			})
			outputPath := filepath.Join(creditsRoot, filepath.FromSlash(dir), "CREDITS.md")
			if err := os.MkdirAll(filepath.Dir(outputPath), 0o755); err != nil {
				return nil, fmt.Errorf("create generated credits dir: %w", err)
			}
			if err := os.WriteFile(outputPath, []byte(renderCreditsMarkdown(entries)), 0o644); err != nil {
				return nil, fmt.Errorf("write generated credits %s: %w", outputPath, err)
			}
			artifacts = append(artifacts, outputPath)
			sources = append(sources, creditsSource{
				Path:    filepath.ToSlash(outputPath),
				Kind:    "generated_music",
				Entries: entries,
			})
		}
	}

	err := filepath.WalkDir(p.AssetsDir(), func(path string, d os.DirEntry, walkErr error) error {
		if walkErr != nil {
			return walkErr
		}
		if d.IsDir() {
			return nil
		}
		if strings.ToLower(d.Name()) != "credits.md" {
			return nil
		}
		entries, err := parseCreditsMarkdown(path)
		if err != nil {
			return fmt.Errorf("parse credits %s: %w", path, err)
		}
		rel, err := filepath.Rel(p.Root, path)
		if err != nil {
			return err
		}
		sources = append(sources, creditsSource{
			Path:    filepath.ToSlash(rel),
			Kind:    "authored",
			Entries: entries,
		})
		return nil
	})
	if err != nil {
		return nil, err
	}

	sort.Slice(sources, func(i, j int) bool { return sources[i].Path < sources[j].Path })
	inventoryPath := filepath.Join(creditsRoot, "credits.json")
	payload, err := json.MarshalIndent(creditsInventory{Sources: sources}, "", "  ")
	if err != nil {
		return nil, fmt.Errorf("marshal credits inventory: %w", err)
	}
	payload = append(payload, '\n')
	if err := os.WriteFile(inventoryPath, payload, 0o644); err != nil {
		return nil, fmt.Errorf("write credits inventory: %w", err)
	}
	artifacts = append(artifacts, inventoryPath)
	return artifacts, nil
}

func cleanupPreparedMusicAssets(prepared *preparedMusicAssets) error {
	if prepared == nil {
		return nil
	}
	for _, root := range prepared.TempRoots {
		if strings.TrimSpace(root) == "" {
			continue
		}
		if err := os.RemoveAll(root); err != nil {
			return fmt.Errorf("remove temporary music artifact root %s: %w", root, err)
		}
	}
	return nil
}

func parseCreditsOverlay(path string) (creditsOverlay, error) {
	entries, err := parseCreditsMarkdown(path)
	if err != nil {
		return creditsOverlay{}, err
	}
	overlay := creditsOverlay{
		ByOriginal: make(map[string]creditsEntry),
		ByOutput:   make(map[string]creditsEntry),
		Entries:    entries,
	}
	for _, entry := range entries {
		if entry.OriginalFile != "" {
			key := strings.ToLower(strings.TrimSpace(entry.OriginalFile))
			if _, exists := overlay.ByOriginal[key]; exists {
				return creditsOverlay{}, fmt.Errorf("duplicate manual credits row for original file %s", entry.OriginalFile)
			}
			overlay.ByOriginal[key] = entry
		}
		if entry.OutputFile != "" {
			key := strings.ToLower(strings.TrimSpace(entry.OutputFile))
			if _, exists := overlay.ByOutput[key]; exists {
				return creditsOverlay{}, fmt.Errorf("duplicate manual credits row for output file %s", entry.OutputFile)
			}
			overlay.ByOutput[key] = entry
		}
	}
	return overlay, nil
}

func (o creditsOverlay) match(originalFile, outputFile string) *creditsEntry {
	if entry, ok := o.ByOutput[strings.ToLower(strings.TrimSpace(outputFile))]; ok {
		entryCopy := entry
		return &entryCopy
	}
	if entry, ok := o.ByOriginal[strings.ToLower(strings.TrimSpace(originalFile))]; ok {
		entryCopy := entry
		return &entryCopy
	}
	return nil
}

func validateOverlayEntries(dir string, overlay creditsOverlay, generated []creditsEntry) error {
	if len(overlay.Entries) == 0 {
		return nil
	}
	validOriginal := make(map[string]struct{}, len(generated))
	validOutput := make(map[string]struct{}, len(generated))
	for _, entry := range generated {
		validOriginal[strings.ToLower(entry.OriginalFile)] = struct{}{}
		validOutput[strings.ToLower(entry.OutputFile)] = struct{}{}
	}
	for _, entry := range overlay.Entries {
		if entry.OriginalFile != "" {
			if _, ok := validOriginal[strings.ToLower(entry.OriginalFile)]; !ok {
				return fmt.Errorf("manual credits entry in %s references unknown original file %s", dir, entry.OriginalFile)
			}
		}
		if entry.OutputFile != "" {
			if _, ok := validOutput[strings.ToLower(entry.OutputFile)]; !ok {
				return fmt.Errorf("manual credits entry in %s references unknown output file %s", dir, entry.OutputFile)
			}
		}
	}
	return nil
}

func parseCreditsMarkdown(path string) ([]creditsEntry, error) {
	raw, err := os.ReadFile(path)
	if err != nil {
		if os.IsNotExist(err) {
			return nil, nil
		}
		return nil, err
	}
	lines := strings.Split(strings.ReplaceAll(string(raw), "\r\n", "\n"), "\n")
	header := []string(nil)
	entries := make([]creditsEntry, 0)
	for _, line := range lines {
		line = strings.TrimSpace(line)
		if !strings.HasPrefix(line, "|") {
			continue
		}
		cells := parseMarkdownTableRow(line)
		if len(cells) == 0 {
			continue
		}
		if header == nil {
			header = make([]string, len(cells))
			for i, cell := range cells {
				header[i] = normalizeCreditsHeader(cell)
			}
			continue
		}
		if isMarkdownSeparatorRow(cells) {
			continue
		}
		entry := creditsEntry{}
		for i, key := range header {
			if i >= len(cells) {
				continue
			}
			value := cleanCreditsCell(cells[i])
			switch key {
			case "artist":
				entry.Artist = value
			case "title":
				entry.Title = value
			case "output_file":
				entry.OutputFile = value
			case "original_file":
				entry.OriginalFile = value
			case "album":
				entry.Album = value
			case "date":
				entry.Date = value
			case "rights":
				entry.Rights = value
			case "notes":
				entry.Notes = value
			}
		}
		if entry.Artist == "" && entry.Title == "" && entry.OutputFile == "" && entry.OriginalFile == "" {
			continue
		}
		entries = append(entries, entry)
	}
	return entries, nil
}

func parseMarkdownTableRow(line string) []string {
	trimmed := strings.TrimSpace(line)
	trimmed = strings.TrimPrefix(trimmed, "|")
	trimmed = strings.TrimSuffix(trimmed, "|")
	parts := strings.Split(trimmed, "|")
	cells := make([]string, 0, len(parts))
	for _, part := range parts {
		cells = append(cells, strings.TrimSpace(strings.ReplaceAll(part, `\|`, "|")))
	}
	return cells
}

func isMarkdownSeparatorRow(cells []string) bool {
	if len(cells) == 0 {
		return false
	}
	for _, cell := range cells {
		cell = strings.TrimSpace(cell)
		cell = strings.ReplaceAll(cell, "-", "")
		cell = strings.ReplaceAll(cell, ":", "")
		if cell != "" {
			return false
		}
	}
	return true
}

func normalizeCreditsHeader(value string) string {
	switch strings.ToLower(cleanCreditsCell(value)) {
	case "artist":
		return "artist"
	case "title":
		return "title"
	case "output file":
		return "output_file"
	case "original file":
		return "original_file"
	case "album":
		return "album"
	case "date":
		return "date"
	case "license / copyright":
		return "rights"
	case "notes":
		return "notes"
	default:
		return ""
	}
}

func cleanCreditsCell(value string) string {
	value = strings.TrimSpace(value)
	value = strings.Trim(value, "`")
	value = strings.ReplaceAll(value, "<br>", "\n")
	return strings.TrimSpace(value)
}

func renderCreditsMarkdown(entries []creditsEntry) string {
	var builder strings.Builder
	builder.WriteString(musicCreditsHeader)
	for _, entry := range entries {
		builder.WriteString("| ")
		builder.WriteString(escapeMarkdownCell(entry.Artist))
		builder.WriteString(" | ")
		builder.WriteString(escapeMarkdownCell(entry.Title))
		builder.WriteString(" | ")
		builder.WriteString(escapeMarkdownCell(wrapCodeCell(entry.OutputFile)))
		builder.WriteString(" | ")
		builder.WriteString(escapeMarkdownCell(wrapCodeCell(entry.OriginalFile)))
		builder.WriteString(" | ")
		builder.WriteString(escapeMarkdownCell(entry.Album))
		builder.WriteString(" | ")
		builder.WriteString(escapeMarkdownCell(entry.Date))
		builder.WriteString(" | ")
		builder.WriteString(escapeMarkdownCell(entry.Rights))
		builder.WriteString(" | ")
		builder.WriteString(escapeMarkdownCell(entry.Notes))
		builder.WriteString(" |\n")
	}
	return builder.String()
}

func wrapCodeCell(value string) string {
	if strings.TrimSpace(value) == "" {
		return ""
	}
	return "`" + value + "`"
}

func escapeMarkdownCell(value string) string {
	value = strings.ReplaceAll(value, `\`, `\\`)
	value = strings.ReplaceAll(value, "|", `\|`)
	value = strings.ReplaceAll(value, "\n", "<br>")
	return value
}

func applyCreditsOverlay(entry *creditsEntry, overlay *creditsEntry) {
	if overlay == nil {
		return
	}
	entry.Artist = firstNonEmpty(overlay.Artist, entry.Artist)
	entry.Title = firstNonEmpty(overlay.Title, entry.Title)
	entry.Album = firstNonEmpty(overlay.Album, entry.Album)
	entry.Date = firstNonEmpty(overlay.Date, entry.Date)
	entry.Rights = firstNonEmpty(overlay.Rights, entry.Rights)
	entry.Notes = firstNonEmpty(overlay.Notes, entry.Notes)
}

func firstNonEmpty(values ...string) string {
	for _, value := range values {
		if strings.TrimSpace(value) != "" {
			return strings.TrimSpace(value)
		}
	}
	return ""
}

func joinNonEmpty(sep string, values ...string) string {
	parts := make([]string, 0, len(values))
	for _, value := range values {
		value = strings.TrimSpace(value)
		if value != "" {
			parts = append(parts, value)
		}
	}
	return strings.Join(parts, sep)
}

func resolveFFmpegBinary() (string, error) {
	if configured := strings.TrimSpace(os.Getenv("SOW_FFMPEG")); configured != "" {
		return configured, nil
	}
	path, err := exec.LookPath("ffmpeg")
	if err != nil {
		return "", fmt.Errorf("ffmpeg not found; install ffmpeg or set SOW_FFMPEG")
	}
	return path, nil
}

func resolveFFprobeBinary() (string, error) {
	if configured := strings.TrimSpace(os.Getenv("SOW_FFPROBE")); configured != "" {
		return configured, nil
	}
	path, err := exec.LookPath("ffprobe")
	if err != nil {
		return "", fmt.Errorf("ffprobe not found; install ffprobe or set SOW_FFPROBE")
	}
	return path, nil
}

func probeMusicMetadata(ffprobePath, sourcePath string) (musicMetadata, error) {
	cmd := exec.Command(ffprobePath, "-v", "quiet", "-print_format", "json", "-show_format", sourcePath)
	output, err := cmd.Output()
	if err != nil {
		return musicMetadata{}, err
	}
	var payload struct {
		Format struct {
			Tags map[string]string `json:"tags"`
		} `json:"format"`
	}
	if err := json.Unmarshal(output, &payload); err != nil {
		return musicMetadata{}, err
	}
	tags := make(map[string]string, len(payload.Format.Tags))
	for key, value := range payload.Format.Tags {
		tags[strings.ToLower(strings.TrimSpace(key))] = strings.TrimSpace(value)
	}
	return musicMetadata{
		Title:     firstNonEmpty(tags["title"]),
		Artist:    firstNonEmpty(tags["artist"], tags["album_artist"], tags["composer"]),
		Album:     firstNonEmpty(tags["album"]),
		Date:      firstNonEmpty(tags["date"], tags["year"]),
		Comment:   firstNonEmpty(tags["comment"], tags["description"]),
		Copyright: firstNonEmpty(tags["copyright"]),
		License:   firstNonEmpty(tags["license"], tags["license_url"]),
	}, nil
}

func convertMusicSource(ffmpegPath, sourcePath, outputPath string) error {
	cmd := exec.Command(
		ffmpegPath, "-y",
		"-i", sourcePath,
		"-vn",
		"-map_metadata", "-1",
		"-ar", "44100",
		"-ac", "2",
		"-b:a", "192k",
		"-codec:a", "libmp3lame",
		"-write_xing", "0",
		"-f", "mp3",
		outputPath,
	)
	output, err := cmd.CombinedOutput()
	if err != nil {
		message := strings.TrimSpace(string(output))
		if message == "" {
			message = err.Error()
		}
		return fmt.Errorf("%s", message)
	}
	return nil
}

func isMusicAssetPath(rel string) bool {
	rel = filepath.ToSlash(strings.TrimSpace(rel))
	return rel == "envi/music" || strings.HasPrefix(rel, "envi/music/")
}

func musicPrefixForDir(p *project.Project, dir string) string {
	dir = trimSlashes(filepath.ToSlash(dir))
	bestPrefix := ""
	bestLen := -1
	for key, value := range p.Config.Music.Prefixes {
		normalizedKey := trimSlashes(filepath.ToSlash(key))
		if normalizedKey == "" {
			continue
		}
		if dir != normalizedKey && !strings.HasPrefix(dir, normalizedKey+"/") {
			continue
		}
		if len(normalizedKey) > bestLen {
			bestLen = len(normalizedKey)
			bestPrefix = sanitizePrefix(value)
		}
	}
	return bestPrefix
}

func sanitizePrefix(prefix string) string {
	prefix = strings.ToLower(strings.TrimSpace(prefix))
	var builder strings.Builder
	for _, r := range prefix {
		switch {
		case r >= 'a' && r <= 'z':
			builder.WriteRune(r)
		case r >= '0' && r <= '9':
			builder.WriteRune(r)
		case r == '_':
			builder.WriteRune(r)
		}
	}
	result := builder.String()
	if strings.TrimSpace(prefix) != "" && strings.HasSuffix(strings.TrimSpace(prefix), "_") && !strings.HasSuffix(result, "_") {
		result += "_"
	}
	return result
}

func trimSlashes(value string) string {
	return strings.Trim(strings.TrimSpace(value), "/")
}

func generateMusicStem(fileName, prefix string, used map[string]struct{}) (string, error) {
	maxCore := maxMusicStemLen - len(prefix)
	if maxCore < 3 {
		return "", fmt.Errorf("prefix too long: %q", prefix)
	}
	base := strings.TrimSuffix(fileName, filepath.Ext(fileName))
	words := slugWords(base)
	core := makeMusicCore(words, maxCore)
	if core == "" {
		sum := sha1.Sum([]byte(fileName))
		core = fmt.Sprintf("%x", sum[:])[:maxCore]
	}
	stem := strings.TrimRight((prefix + core)[:min(maxMusicStemLen, len(prefix)+len(core))], "_")
	if stem == "" {
		return "", fmt.Errorf("could not derive music stem for %s", fileName)
	}
	return uniqueMusicName(stem, used), nil
}

func slugWords(value string) []string {
	value = musicBracketRE.ReplaceAllString(value, " ")
	value = strings.ReplaceAll(value, "&", " and ")
	value = strings.ReplaceAll(value, "'", "")
	value = strings.ReplaceAll(value, "’", "")
	value = strings.ToLower(value)
	var ascii strings.Builder
	for _, r := range value {
		switch {
		case r >= 'a' && r <= 'z':
			ascii.WriteRune(r)
		case r >= '0' && r <= '9':
			ascii.WriteRune(r)
		default:
			ascii.WriteRune(' ')
		}
	}
	rawWords := musicSplitRE.Split(ascii.String(), -1)
	words := make([]string, 0, len(rawWords))
	for _, word := range rawWords {
		if word == "" {
			continue
		}
		if _, drop := musicDropWords[word]; drop {
			continue
		}
		if musicYearRE.MatchString(word) {
			continue
		}
		words = append(words, word)
	}
	return words
}

func makeMusicCore(words []string, maxCore int) string {
	core := ""
	for _, word := range words {
		candidate := word
		if core != "" {
			candidate = core + "_" + word
		}
		if len(candidate) <= maxCore {
			core = candidate
			continue
		}
		break
	}
	if core != "" {
		return core
	}
	abbrev := make([]string, 0, min(4, len(words)))
	for _, word := range words {
		if len(abbrev) == 4 {
			break
		}
		abbrev = append(abbrev, abbreviateMusicWord(word, 5))
	}
	for _, word := range abbrev {
		candidate := word
		if core != "" {
			candidate = core + "_" + word
		}
		if len(candidate) <= maxCore {
			core = candidate
			continue
		}
		break
	}
	if core != "" {
		return core
	}
	if len(words) > 0 {
		return strings.Trim(words[0][:min(maxCore, len(words[0]))], "_")
	}
	return ""
}

func abbreviateMusicWord(word string, maxLen int) string {
	if len(word) <= maxLen {
		return word
	}
	consonants := musicVowelRE.ReplaceAllString(word, "")
	candidate := word[:1]
	if len(consonants) > 1 {
		candidate += consonants[1:]
	}
	if len(candidate) >= 3 {
		return candidate[:min(maxLen, len(candidate))]
	}
	return word[:min(maxLen, len(word))]
}

func uniqueMusicName(stem string, used map[string]struct{}) string {
	if _, exists := used[stem]; !exists {
		used[stem] = struct{}{}
		return stem
	}
	for index := 1; ; index++ {
		suffix := fmt.Sprintf("_%d", index)
		prefixLen := min(len(stem), max(0, maxMusicStemLen-len(suffix)))
		candidate := strings.TrimRight(stem[:prefixLen], "_") + suffix
		if _, exists := used[candidate]; exists {
			continue
		}
		used[candidate] = struct{}{}
		return candidate
	}
}

func reserveMusicName(stem string, used map[string]struct{}) error {
	stem = strings.ToLower(strings.TrimSpace(stem))
	if stem == "" {
		return fmt.Errorf("output filename is empty")
	}
	if _, exists := used[stem]; exists {
		return fmt.Errorf("output filename %s collides with an existing asset", stem)
	}
	used[stem] = struct{}{}
	return nil
}

func validateManualOutputFile(name string) error {
	name = strings.ToLower(strings.TrimSpace(name))
	if !strings.HasSuffix(name, ".bmu") {
		return fmt.Errorf("output file must end with .bmu")
	}
	stem := strings.TrimSuffix(name, ".bmu")
	if stem == "" {
		return fmt.Errorf("output file stem is empty")
	}
	if len(stem) > maxMusicStemLen {
		return fmt.Errorf("output file stem %q exceeds %d characters", stem, maxMusicStemLen)
	}
	for _, r := range stem {
		switch {
		case r >= 'a' && r <= 'z':
		case r >= '0' && r <= '9':
		case r == '_':
		default:
			return fmt.Errorf("output file stem %q contains unsupported character %q", stem, string(r))
		}
	}
	return nil
}

func min(a, b int) int {
	if a < b {
		return a
	}
	return b
}

func max(a, b int) int {
	if a > b {
		return a
	}
	return b
}