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Documentation Index

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Accompanist Lyrics Core separates parsing from rendering: the library gives you structured timing data, and your UI layer decides how to visualise it. SyncedLyrics provides efficient binary-search methods to find the active line at any playback position, while KaraokeLine and KaraokeSyllable expose progress() helpers that produce the 0.0–1.0 float values your animation system needs.
All time values throughout the library — start, end, duration, and every method parameter — are in milliseconds.

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Integration walkthrough

1
Get the current line index by playback time
2
SyncedLyrics exposes two methods for resolving the active line from a playback position:
3
// Returns the index of the single first-matching line — covers the vast majority of use cases.
val currentIndex = lyrics.getCurrentFirstHighlightLineIndexByTime(currentTimeMs)

// Returns indices of ALL lines active at this moment — needed for overlapping or duet tracks.
val activeIndices = lyrics.getCurrentAllHighlightLineIndicesByTime(currentTimeMs)
4
Both methods perform a binary search over the sorted lines list, making them safe to call on every rendered frame without measurable overhead even for very long tracks.
5
getCurrentFirstHighlightLineIndexByTime returns an index into lyrics.lines. Guard your access with getOrNull to handle the edge case where time falls before the first line or after the last.
6
Poll on each playback tick
7
Connect your media player’s position callback (or a timer at ~60 fps) to a function that queries the current index and updates your UI state:
8
// Called ~60 fps or whenever the playback position changes
fun onPlaybackTick(currentTimeMs: Int) {
    val currentIndex = lyrics.getCurrentFirstHighlightLineIndexByTime(currentTimeMs)
    val currentLine = lyrics.lines.getOrNull(currentIndex)
    // Update UI state to highlight currentLine
}
9
The index is all you need to drive a scroll-to or highlight in a list. The line object itself carries every piece of information required for more detailed rendering.
10
Animate karaoke syllable highlighting
11
When the current line is a KaraokeLine.MainKaraokeLine, you can drive per-syllable fill animations using the progress() method on both the line and each of its syllables:
12
if (currentLine is KaraokeLine.MainKaraokeLine) {
    // Overall progress of the line — useful for a background progress bar.
    val lineProgress = currentLine.progress(currentTimeMs) // 0.0f to 1.0f

    // True while currentTimeMs is within start..end of the line.
    val isFocused = currentLine.isFocused(currentTimeMs)

    for (syllable in currentLine.syllables) {
        // Per-syllable progress — drive your highlight clip or colour lerp here.
        val syllableProgress = syllable.progress(currentTimeMs) // 0.0f to 1.0f
        // Apply syllableProgress to your animation
    }
}
13
progress() is defined on both KaraokeLine (the sealed interface) and KaraokeSyllable, so the same call pattern works for line-level and syllable-level animation without branching.
14
progress() always clamps its return value to [0.0, 1.0], so you can pass it directly to animation APIs without additional clamping.
15
Handle duet and multi-voice lines
16
For tracks where two singers overlap, use getCurrentAllHighlightLineIndicesByTime and read the alignment property to position each singer’s line in your UI:
17
val activeIndices = lyrics.getCurrentAllHighlightLineIndicesByTime(currentTimeMs)

for (index in activeIndices) {
    val line = lyrics.lines[index]
    if (line is KaraokeLine) {
        when (line.alignment) {
            KaraokeAlignment.Start       -> { /* position at leading edge (e.g. left) */ }
            KaraokeAlignment.End         -> { /* position at trailing edge (e.g. right) */ }
            KaraokeAlignment.Unspecified -> { /* use default centre position */ }
        }
    }
}
18
Render background / accompaniment vocal lines
19
KaraokeLine.MainKaraokeLine can carry a list of accompanimentLines for background vocals that overlap with the main vocal. Render them separately to keep the foreground and background vocals visually distinct:
20
if (currentLine is KaraokeLine.MainKaraokeLine) {
    currentLine.accompanimentLines?.forEach { bgLine ->
        val bgProgress = bgLine.progress(currentTimeMs)
        // Render background vocal highlight using bgProgress
    }
}
21
AccompanimentKaraokeLine implements the full KaraokeLine interface, so progress(), isFocused(), syllables, alignment, and phonetic are all available on it too.

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Controlling line visibility with isFocused

isFocused(current: Int) returns true while the playback position falls within a line’s start..end range. Use it to decide when to bring a line into the visible portion of your scroll container and when to fade it out: 
val shouldShow = currentLine.isFocused(currentTimeMs)
Combine isFocused with a small lead-in offset (e.g. pre-scroll to the next line 300–500 ms before it starts) for a smooth lyrics-following experience.

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Handling simple synced lines

Not all content has karaoke syllable data. When a line is a SyncedLine rather than a KaraokeLine, you only have start, end, and duration — no progress() helper. A simple active/inactive highlight based on currentIndex is the right approach: 
val currentIndex = lyrics.getCurrentFirstHighlightLineIndexByTime(currentTimeMs)

lyrics.lines.forEachIndexed { index, line ->
    val isActive = index == currentIndex
    // Apply active/inactive style to each line in your list
}

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Quick reference

TaskAPI
Find the highlighted line indexlyrics.getCurrentFirstHighlightLineIndexByTime(ms)
Find all active line indices (duet)lyrics.getCurrentAllHighlightLineIndicesByTime(ms)
Line fill animation progresskaraokeLine.progress(ms) → Float
Syllable fill animation progresskaraokeSyllable.progress(ms) → Float
Is this line currently active?karaokeLine.isFocused(ms) → Boolean
Singer side / alignmentkaraokeLine.alignment → KaraokeAlignment
Background vocal linesmainKaraokeLine.accompanimentLines

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