vibesonic/internal/lyrics/lrc.go

// Package lyrics parses LRC and Enhanced LRC lyric files into a
// structured, timing-preserving representation, without inventing any
// data the source file doesn't actually contain: a plain LRC file
// produces line-level timing only; word-level cues are only populated
// when the file actually has inline word timestamps.
package lyrics

import (
	"regexp"
	"sort"
	"strconv"
	"strings"
)

// Word is one word/syllable-level timed cue within a Line, present only
// for Enhanced LRC input.
type Word struct {
	StartMs int
	Text    string
}

// Line is one line of lyrics, synced to a start time.
type Line struct {
	StartMs int
	Text    string
	Words   []Word // empty unless the source had inline word timing
}

// Parsed is the result of parsing one lyric file.
type Parsed struct {
	Lang     string // from an [la:xx] tag, "" if absent
	OffsetMs int    // from an [offset:+/-n] tag, 0 if absent
	Lines    []Line
	Enhanced bool // true if any line has word-level Words
}

var (
	lineTimeRE = regexp.MustCompile(`^\[(\d{1,3}):(\d{2})(?:[.:](\d{1,3}))?\]`)
	metaTagRE  = regexp.MustCompile(`^\[([a-zA-Z]+):(.*)\]$`)
	wordTimeRE = regexp.MustCompile(`<(\d{1,3}):(\d{2})(?:[.:](\d{1,3}))?>`)
)

// Parse parses LRC (and, transparently, Enhanced LRC) content. It never
// returns an error for merely-unrecognized lines — those are skipped,
// matching how real-world LRC files in the wild often carry stray
// metadata this parser doesn't know about; only genuinely empty input
// produces a Parsed with no lines.
func Parse(content string) *Parsed {
	p := &Parsed{}
	lines := strings.Split(strings.ReplaceAll(content, "\r\n", "\n"), "\n")

	for _, raw := range lines {
		line := strings.TrimSpace(raw)
		if line == "" {
			continue
		}

		if m := metaTagRE.FindStringSubmatch(line); m != nil && !lineTimeRE.MatchString(line) {
			applyMetaTag(p, strings.ToLower(m[1]), strings.TrimSpace(m[2]))
			continue
		}

		// A line may carry multiple leading [mm:ss.xx] timestamps
		// (repeated chorus lines sharing one piece of text).
		var times []int
		rest := line
		for {
			m := lineTimeRE.FindStringSubmatch(rest)
			if m == nil {
				break
			}
			times = append(times, parseTimeMs(m[1], m[2], m[3]))
			rest = rest[len(m[0]):]
		}
		if len(times) == 0 {
			continue // not a timed lyric line and not a recognized metadata tag
		}

		text, words := parseWords(rest)
		for _, t := range times {
			p.Lines = append(p.Lines, Line{StartMs: t, Text: text, Words: words})
			if len(words) > 0 {
				p.Enhanced = true
			}
		}
	}

	sortLines(p.Lines)
	return p
}

func applyMetaTag(p *Parsed, key, value string) {
	switch key {
	case "la":
		p.Lang = value
	case "offset":
		if n, err := strconv.Atoi(strings.TrimPrefix(value, "+")); err == nil {
			p.OffsetMs = n
		}
	}
}

// parseWords splits an Enhanced-LRC line body on inline <mm:ss.xx> word
// timestamps. If there are none, it returns the whole trimmed body as
// plain text with no words — a normal (non-enhanced) LRC line.
func parseWords(body string) (text string, words []Word) {
	matches := wordTimeRE.FindAllStringSubmatchIndex(body, -1)
	if len(matches) == 0 {
		return strings.TrimSpace(body), nil
	}

	for i, m := range matches {
		wordStart := m[1] // end of the <time> tag itself
		wordEnd := len(body)
		if i+1 < len(matches) {
			wordEnd = matches[i+1][0]
		}
		wordText := strings.TrimSpace(body[wordStart:wordEnd])
		startMs := parseTimeMs(body[m[2]:m[3]], body[m[4]:m[5]], subOrEmpty(body, m[6], m[7]))
		if wordText != "" {
			words = append(words, Word{StartMs: startMs, Text: wordText})
		}
	}
	// Plain text rendering: words joined with a single space, matching
	// how the line reads without timing markup.
	parts := make([]string, 0, len(words))
	for _, w := range words {
		parts = append(parts, w.Text)
	}
	return strings.Join(parts, " "), words
}

func subOrEmpty(s string, start, end int) string {
	if start < 0 || end < 0 {
		return ""
	}
	return s[start:end]
}

func parseTimeMs(minStr, secStr, fracStr string) int {
	min, _ := strconv.Atoi(minStr)
	sec, _ := strconv.Atoi(secStr)
	frac := 0
	if fracStr != "" {
		// Fractional part may be 1-3 digits (tenths, hundredths, or
		// milliseconds); normalize to milliseconds.
		f, _ := strconv.Atoi(fracStr)
		switch len(fracStr) {
		case 1:
			frac = f * 100
		case 2:
			frac = f * 10
		default:
			frac = f
		}
	}
	return min*60*1000 + sec*1000 + frac
}

func sortLines(lines []Line) {
	sort.SliceStable(lines, func(i, j int) bool { return lines[i].StartMs < lines[j].StartMs })
}