evOo-local/packages/adapters/src/media/video-edit-plan.ts

import type { EditVideoStep, MediaFit } from "@rakazo/contracts";
import { MEDIA_PRESETS } from "@rakazo/contracts";
import type { LoudnessMeasurement, MediaProbe } from "./ffmpeg.js";
import { ffmpegInputArgs, ffmpegSeconds, loudnormFilter } from "./ffmpeg.js";

/**
 * Turns an ordered list of edit steps into one ffmpeg invocation with a single filter_complex
 * pass. Pure: the caller probes the inputs and runs the arguments.
 */

export interface PlanInput {
  path: string;
  mimeType: string;
  name: string;
  probe: MediaProbe;
}

export interface VideoEditPlanRequest {
  source: PlanInput;
  steps: readonly EditVideoStep[];
  /** Videos named by concat steps, by artifact id. */
  clips?: ReadonlyMap<string, PlanInput>;
  /** Images named by overlay steps, by artifact id. */
  images?: ReadonlyMap<string, PlanInput>;
  /** Subtitle files named by subtitles steps, by artifact id: safe names relative to ffmpeg's cwd. */
  subtitleFiles?: ReadonlyMap<string, string>;
  /**
   * What each remove_silence step keeps, on its own input timeline: speech segments, or null
   * when nothing is silent. Detected beforehand by an analysis plan (see silence.ts).
   */
  speech?: ReadonlyMap<EditVideoStep, readonly { start: number; end: number }[] | null>;
  /**
   * Analysis instead of an encode: the audio left by the steps runs through these filters
   * into a null output, so they can report on it (silencedetect writing a file, say).
   */
  analyzeAudio?: readonly string[];
  /** A platform's limits, applied after the steps. */
  target?: ExportTarget;
  outputPath: string;
}

/** The frame, frame rate cap, bitrate cap and loudness a platform export is rendered to. */
export interface ExportTarget {
  width: number;
  height: number;
  fit: MediaFit;
  maxFps: number;
  maxBitrateKbps: number;
  integratedLufs: number;
  truePeakDb: number;
}

export interface VideoEditPlan {
  args: string[];
  kind: "video" | "audio";
  durationMs: number;
  width?: number;
  height?: number;
  posterAtMs?: number;
}

export const VIDEO_ENCODE_ARGS = [
  "-c:v",
  "libx264",
  "-preset",
  "veryfast",
  "-crf",
  "21",
  "-pix_fmt",
  "yuv420p",
  "-movflags",
  "+faststart",
];
export const AUDIO_BITRATE_KBPS = 160;
const AUDIO_ENCODE_ARGS = ["-c:a", "aac", "-b:a", `${AUDIO_BITRATE_KBPS}k`];
const AUDIO_ONLY_ENCODE_ARGS = ["-c:a", "aac", "-b:a", "192k", "-movflags", "+faststart"];
/** Location and other camera metadata stay private; chapters do not survive cuts anyway. */
const STRIP_METADATA_ARGS = ["-map_metadata", "-1", "-map_chapters", "-1"];
const AUDIO_FORMAT = "aformat=sample_fmts=fltp:sample_rates=48000:channel_layouts=stereo";
const SAFE_FILE_NAME = /^[A-Za-z0-9][A-Za-z0-9._-]*$/;
const MIN_SEGMENT_SECONDS = 0.1;
const FALLBACK_FPS = 30;

interface Track {
  label: string;
  filters: string[];
}

interface Timeline {
  /** Null when the result is audio only: video work is skipped. */
  video: Track | null;
  audio: Track | null;
  width: number;
  height: number;
  fps: number;
  seconds: number;
  /** How appended clips fill the frame: the last reframe's fit, otherwise bars. */
  fit: MediaFit;
}

class FilterGraph {
  private readonly chains: string[] = [];
  private count = 0;

  label(kind: "v" | "a"): string {
    this.count += 1;
    return `${kind}${this.count}`;
  }

  add(inputs: readonly string[], filters: readonly string[], outputs: readonly string[]) {
    const pads = (labels: readonly string[]) => labels.map((label) => `[${label}]`).join("");
    this.chains.push(`${pads(inputs)}${filters.join(",")}${pads(outputs)}`);
  }

  /** Ends a track's pending filters in a labelled pad other chains can consume. */
  flush(track: Track, kind: "v" | "a"): string {
    if (track.filters.length === 0) return track.label;
    const output = this.label(kind);
    this.add([track.label], track.filters, [output]);
    track.label = output;
    track.filters = [];
    return output;
  }

  toString(): string {
    return this.chains.join(";");
  }
}

export function planVideoEdit(request: VideoEditPlanRequest): VideoEditPlan {
  const { source } = request;
  const video = source.probe.video;
  if (!video) throw new Error(`${source.name} has no video.`);
  const sourceSeconds = source.probe.durationMs / 1000;
  const audioOnly =
    request.steps.at(-1)?.op === "extract_audio" || request.analyzeAudio !== undefined;
  const graph = new FilterGraph();
  const inputs: string[][] = [];
  const addInput = (input: PlanInput, seek?: { start: number; end: number }) => {
    inputs.push(
      ffmpegInputArgs(input.path, {
        mimeType: input.mimeType,
        ...(seek ? { startSeconds: seek.start, durationSeconds: seek.end - seek.start } : {}),
      }),
    );
    return inputs.length - 1;
  };

  let steps = request.steps;
  // A leading cut seeks the input instead of decoding everything before it.
  const first = steps[0];
  let segments: Segment[] | undefined;
  if (first?.op === "trim" || first?.op === "keep") {
    segments = normalizeSegments(
      first.op === "trim" ? [first] : first.segments,
      sourceSeconds,
      first.op,
    );
    steps = steps.slice(1);
  }
  const seek = segments && { start: segments[0]!.start, end: segments.at(-1)!.end };
  const index = addInput(source, seek);
  const size = displaySize(video);
  const timeline: Timeline = {
    video: audioOnly ? null : { label: `${index}:v:0`, filters: sourceFilters(video) },
    audio: source.probe.hasAudio ? { label: `${index}:a:0`, filters: [] } : null,
    width: size.width,
    height: size.height,
    fps: video.fps,
    seconds: seek ? seek.end - seek.start : sourceSeconds,
    fit: "pad",
  };
  if (segments && seek && segments.length > 1) {
    keepSegments(
      graph,
      timeline,
      segments.map((segment) => ({
        start: segment.start - seek.start,
        end: segment.end - seek.start,
      })),
    );
  }

  let posterAt: number | undefined;
  for (const step of steps) {
    switch (step.op) {
      case "trim":
      case "keep":
        keepSegments(
          graph,
          timeline,
          normalizeSegments(step.op === "trim" ? [step] : step.segments, timeline.seconds, step.op),
        );
        break;
      case "concat":
        addClips(
          graph,
          timeline,
          step.artifact_ids.map((id) => {
            const clip = request.clips?.get(id);
            if (!clip) throw new Error(`Video ${id} is not available.`);
            return { clip, index: addInput(clip) };
          }),
          step.at,
        );
        break;
      case "reframe": {
        const preset = MEDIA_PRESETS[step.preset];
        if (timeline.video) {
          timeline.video = fitFrame(
            graph,
            timeline.video,
            timeline,
            { width: preset.width, height: preset.height },
            step.fit,
          );
        }
        timeline.width = preset.width;
        timeline.height = preset.height;
        timeline.fit = step.fit;
        break;
      }
      case "speed":
        timeline.video?.filters.push(
          `setpts=PTS/${number(step.factor)}`,
          ...(timeline.fps ? [`fps=${number(timeline.fps)}`] : []),
        );
        timeline.audio?.filters.push(...atempoFilters(step.factor));
        timeline.seconds /= step.factor;
        break;
      case "volume":
        timeline.audio?.filters.push(`volume=${number(step.gain)}`);
        break;
      case "mute":
        timeline.audio = null;
        break;
      case "subtitles": {
        const file = request.subtitleFiles?.get(step.artifact_id);
        if (!file) throw new Error(`Subtitles ${step.artifact_id} are not available.`);
        // Only generated names reach the filter graph, so nothing here needs escaping.
        if (!SAFE_FILE_NAME.test(file)) throw new Error("Unsafe subtitle file name.");
        timeline.video?.filters.push(
          `subtitles=filename=${file}:force_style='${subtitleStyle(timeline.width, timeline.height)}'`,
        );
        break;
      }
      case "extract_audio":
        break;
      case "poster":
        posterAt = step.at;
        break;
      case "remove_silence": {
        const speech = request.speech?.get(step);
        if (speech === undefined) throw new Error("Silence was not detected before planning.");
        if (speech) keepSegments(graph, timeline, speech);
        break;
      }
      case "overlay": {
        if (!timeline.video) break;
        const image = request.images?.get(step.artifact_id);
        if (!image) throw new Error(`Image ${step.artifact_id} is not available.`);
        if (step.start !== undefined && step.start >= timeline.seconds) {
          throw new Error(
            `overlay from ${seconds(step.start)} s starts after the end of the ${seconds(timeline.seconds)} s video.`,
          );
        }
        overlayImage(graph, timeline, timeline.video, step, addInput(image));
        break;
      }
    }
  }

  if (posterAt !== undefined && posterAt >= timeline.seconds) {
    throw new Error(
      `poster at ${seconds(posterAt)} s is past the end of the ${seconds(timeline.seconds)} s result.`,
    );
  }
  const { target } = request;
  if (target && !audioOnly) applyTarget(graph, timeline, target);
  const durationMs = Math.round(timeline.seconds * 1000);
  const output = `file:${request.outputPath}`;
  if (audioOnly || !timeline.video) {
    if (!timeline.audio) throw new Error(`${source.name} has no audio.`);
    if (request.analyzeAudio) {
      timeline.audio.filters.push(...request.analyzeAudio);
      const audio = graph.flush(timeline.audio, "a");
      return {
        args: [
          ...inputs.flat(),
          "-filter_complex",
          graph.toString(),
          "-map",
          `[${audio}]`,
          "-f",
          "null",
          "-",
        ],
        kind: "audio",
        durationMs,
      };
    }
    timeline.audio.filters.push(AUDIO_FORMAT);
    const audio = graph.flush(timeline.audio, "a");
    return {
      args: [
        ...inputs.flat(),
        "-filter_complex",
        graph.toString(),
        "-map",
        `[${audio}]`,
        ...AUDIO_ONLY_ENCODE_ARGS,
        ...STRIP_METADATA_ARGS,
        "-f",
        "mp4",
        output,
      ],
      kind: "audio",
      durationMs,
    };
  }
  timeline.video.filters.push("format=yuv420p");
  const videoOut = graph.flush(timeline.video, "v");
  let audioOut: string | undefined;
  if (timeline.audio) {
    timeline.audio.filters.push(AUDIO_FORMAT);
    audioOut = graph.flush(timeline.audio, "a");
  }
  return {
    args: [
      ...inputs.flat(),
      "-filter_complex",
      graph.toString(),
      "-map",
      `[${videoOut}]`,
      ...(audioOut ? ["-map", `[${audioOut}]`, ...AUDIO_ENCODE_ARGS] : []),
      ...VIDEO_ENCODE_ARGS,
      ...(target && !audioOnly ? bitrateCapArgs(target.maxBitrateKbps) : []),
      ...STRIP_METADATA_ARGS,
      "-f",
      "mp4",
      output,
    ],
    kind: "video",
    durationMs,
    width: timeline.width,
    height: timeline.height,
    ...(posterAt !== undefined ? { posterAtMs: Math.round(posterAt * 1000) } : {}),
  };
}

/**
 * Fits the result to a platform: frame rate cap, frame size and loudness. Loudness is one
 * dynamic loudnorm pass, which lands within about 1 LU of the target on most material without
 * decoding the source twice; the export measures the result and corrects it when it misses.
 */
function applyTarget(graph: FilterGraph, timeline: Timeline, target: ExportTarget) {
  if (timeline.video) {
    if (!timeline.fps || timeline.fps > target.maxFps + 0.01) {
      timeline.video.filters.push(`fps=${number(target.maxFps)}`);
      timeline.fps = target.maxFps;
    }
    timeline.video = fitFrame(graph, timeline.video, timeline, target, target.fit);
  }
  timeline.width = target.width;
  timeline.height = target.height;
  // loudnorm pads its last frame; trim the audio back to the video's length.
  timeline.audio?.filters.push(
    loudnormFilter(target),
    `atrim=end=${ffmpegSeconds(timeline.seconds)}`,
  );
}

/** CRF quality, capped so no stretch of the video runs above the platform's bitrate. */
function bitrateCapArgs(maxBitrateKbps: number): string[] {
  const kbps = Math.max(1, Math.round(maxBitrateKbps));
  return ["-maxrate", `${kbps}k`, "-bufsize", `${kbps * 2}k`];
}

/**
 * Second loudness pass over a rendered MP4: linear loudnorm with the measured values on the
 * audio, the video copied as it is.
 */
export function planLoudnessFix(request: {
  inputPath: string;
  outputPath: string;
  durationMs: number;
  target: { integratedLufs: number; truePeakDb: number };
  measured: LoudnessMeasurement;
}): string[] {
  const filters = [
    loudnormFilter(request.target, request.measured),
    `atrim=end=${ffmpegSeconds(request.durationMs / 1000)}`,
    AUDIO_FORMAT,
  ];
  return [
    ...ffmpegInputArgs(request.inputPath, { mimeType: "video/mp4" }),
    "-filter_complex",
    `[0:a:0]${filters.join(",")}[a1]`,
    ...["-map", "0:v:0", "-map", "[a1]", "-c:v", "copy"],
    ...AUDIO_ENCODE_ARGS,
    ...["-movflags", "+faststart"],
    ...STRIP_METADATA_ARGS,
    "-f",
    "mp4",
    `file:${request.outputPath}`,
  ];
}

interface Segment {
  start: number;
  end: number;
}

/** Validates ranges against the timeline, clamps ends to it, sorts and merges overlaps. */
function normalizeSegments(
  ranges: readonly Segment[],
  duration: number,
  op: "trim" | "keep",
): Segment[] {
  const clamped = ranges.map((range) => {
    if (range.start >= duration) {
      throw new Error(
        `${op} ${seconds(range.start)}–${seconds(range.end)} s is outside the ${seconds(duration)} s video.`,
      );
    }
    const end = Math.min(range.end, duration);
    if (end - range.start < MIN_SEGMENT_SECONDS) {
      throw new Error(
        `${op} ${seconds(range.start)}–${seconds(end)} s is shorter than ${MIN_SEGMENT_SECONDS} s.`,
      );
    }
    return { start: range.start, end };
  });
  clamped.sort((a, b) => a.start - b.start);
  const merged: Segment[] = [];
  for (const segment of clamped) {
    const last = merged.at(-1);
    if (last && segment.start <= last.end) last.end = Math.max(last.end, segment.end);
    else merged.push({ ...segment });
  }
  return merged;
}

/** Keeps sorted, disjoint segments of the timeline: one trim, or split + trims + concat. */
function keepSegments(graph: FilterGraph, timeline: Timeline, segments: readonly Segment[]) {
  const [only] = segments;
  if (segments.length === 1 && only) {
    const range = `start=${ffmpegSeconds(only.start)}:end=${ffmpegSeconds(only.end)}`;
    timeline.video?.filters.push(`trim=${range}`, "setpts=PTS-STARTPTS");
    timeline.audio?.filters.push(`atrim=${range}`, "asetpts=PTS-STARTPTS");
    timeline.seconds = only.end - only.start;
    return;
  }
  const count = segments.length;
  const branch = (track: Track | null, kind: "v" | "a"): string[] => {
    if (!track) return [];
    const input = graph.flush(track, kind);
    const splits = segments.map(() => graph.label(kind));
    graph.add([input], [`${kind === "v" ? "split" : "asplit"}=${count}`], splits);
    return segments.map((segment, index) => {
      const output = graph.label(kind);
      const range = `start=${ffmpegSeconds(segment.start)}:end=${ffmpegSeconds(segment.end)}`;
      graph.add(
        [splits[index]!],
        kind === "v"
          ? [`trim=${range}`, "setpts=PTS-STARTPTS"]
          : [`atrim=${range}`, "asetpts=PTS-STARTPTS"],
        [output],
      );
      return output;
    });
  };
  const videoParts = branch(timeline.video, "v");
  const audioParts = branch(timeline.audio, "a");
  concatParts(graph, timeline, videoParts, audioParts, count);
  timeline.seconds = segments.reduce((total, segment) => total + segment.end - segment.start, 0);
}

/**
 * Adds clips at the end or the start of the timeline, normalized to it: frame size, fit, fps
 * and 48 kHz stereo audio.
 */
function addClips(
  graph: FilterGraph,
  timeline: Timeline,
  clips: readonly { clip: PlanInput; index: number }[],
  at: "end" | "start",
) {
  const fps = timeline.fps || FALLBACK_FPS;
  const needsAudio = timeline.audio !== null || clips.some(({ clip }) => clip.probe.hasAudio);
  const videoParts: string[] = [];
  const audioParts: string[] = [];
  let mainVideo: string | undefined;
  let mainAudio: string | undefined;
  if (timeline.video) {
    timeline.video.filters.push("setsar=1", "format=yuv420p");
    mainVideo = graph.flush(timeline.video, "v");
  }
  if (needsAudio) mainAudio = audioPart(graph, timeline.audio, timeline.seconds);
  let seconds = timeline.seconds;
  for (const { clip, index } of clips) {
    const video = clip.probe.video;
    if (!video) throw new Error(`${clip.name} has no video.`);
    const clipSeconds = clip.probe.durationMs / 1000;
    if (clipSeconds <= 0) throw new Error(`Could not read the length of ${clip.name}.`);
    if (timeline.video) {
      const track = fitFrame(
        graph,
        { label: `${index}:v:0`, filters: sourceFilters(video) },
        displaySize(video),
        timeline,
        timeline.fit,
      );
      track.filters.push(`fps=${number(fps)}`, "format=yuv420p");
      videoParts.push(graph.flush(track, "v"));
    }
    if (needsAudio) {
      const track = clip.probe.hasAudio ? { label: `${index}:a:0`, filters: [] } : null;
      audioParts.push(audioPart(graph, track, clipSeconds));
    }
    seconds += clipSeconds;
  }
  const add = at === "start" ? "push" : "unshift";
  if (mainVideo) videoParts[add](mainVideo);
  if (mainAudio) audioParts[add](mainAudio);
  concatParts(graph, timeline, videoParts, audioParts, clips.length + 1);
  timeline.seconds = seconds;
}

/**
 * Scales an image to its share of the frame width, within the margins, and lays it over the
 * video at a corner or the center. Margins are wider on tall frames, where 9:16 apps put their
 * own buttons and caption, so the top corners stay visible there.
 */
function overlayImage(
  graph: FilterGraph,
  timeline: Timeline,
  video: Track,
  step: Extract<EditVideoStep, { op: "overlay" }>,
  index: number,
) {
  const { width, height } = timeline;
  const tall = height >= width * 1.5;
  const marginX = Math.round(width * (tall ? 0.06 : 0.04));
  const top = Math.round(height * (tall ? 0.15 : 0.04));
  const bottom = Math.round(height * (tall ? 0.2 : 0.04));
  const image = graph.label("v");
  graph.add(
    [`${index}:v:0`],
    [
      `scale=w=${even(width * step.width)}:h=${even(height - top - bottom)}:force_original_aspect_ratio=decrease:force_divisible_by=2`,
      "format=rgba",
      ...(step.opacity < 1 ? [`colorchannelmixer=aa=${number(step.opacity)}`] : []),
    ],
    [image],
  );
  const x = step.position.endsWith("left")
    ? `${marginX}`
    : step.position.endsWith("right")
      ? `W-w-${marginX}`
      : "(W-w)/2";
  const y = step.position.startsWith("top")
    ? `${top}`
    : step.position.startsWith("bottom")
      ? `H-h-${bottom}`
      : "(H-h)/2";
  // A still image ends after its one frame; overlay repeats it for as long as the video runs.
  const timed = step.start !== undefined || step.end !== undefined;
  const enable = timed
    ? `:enable='between(t,${ffmpegSeconds(step.start ?? 0)},${ffmpegSeconds(step.end ?? timeline.seconds)})'`
    : "";
  const output = graph.label("v");
  graph.add([graph.flush(video, "v"), image], [`overlay=x=${x}:y=${y}${enable}`], [output]);
  timeline.video = { label: output, filters: [] };
}

/** A clip's audio in the common format, or silence as long as the clip when it has none. */
function audioPart(graph: FilterGraph, track: Track | null, seconds: number): string {
  if (track) {
    track.filters.push(AUDIO_FORMAT);
    return graph.flush(track, "a");
  }
  const output = graph.label("a");
  graph.add(
    [],
    [
      "anullsrc=channel_layout=stereo:sample_rate=48000",
      `atrim=end=${ffmpegSeconds(seconds)}`,
      AUDIO_FORMAT,
    ],
    [output],
  );
  return output;
}

function concatParts(
  graph: FilterGraph,
  timeline: Timeline,
  videoParts: readonly string[],
  audioParts: readonly string[],
  count: number,
) {
  const inputs = Array.from({ length: count }, (_, index) => [
    ...(videoParts[index] ? [videoParts[index]] : []),
    ...(audioParts[index] ? [audioParts[index]] : []),
  ]).flat();
  const video = videoParts.length > 0 ? graph.label("v") : undefined;
  const audio = audioParts.length > 0 ? graph.label("a") : undefined;
  graph.add(
    inputs,
    [`concat=n=${count}:v=${video ? 1 : 0}:a=${audio ? 1 : 0}`],
    [...(video ? [video] : []), ...(audio ? [audio] : [])],
  );
  timeline.video = video ? { label: video, filters: [] } : null;
  timeline.audio = audio ? { label: audio, filters: [] } : null;
}

interface Size {
  width: number;
  height: number;
}

/** Scales a frame of size `from` into `to`: center crop, bars, or a blurred copy behind it. */
function fitFrame(graph: FilterGraph, track: Track, from: Size, to: Size, fit: MediaFit): Track {
  const { width, height } = to;
  if (Math.abs(from.width / from.height - width / height) < 0.01) {
    track.filters.push(`scale=${width}:${height}`, "setsar=1");
    return track;
  }
  const contain = `scale=${width}:${height}:force_original_aspect_ratio=decrease:force_divisible_by=2`;
  if (fit === "crop") {
    track.filters.push(
      `scale=${width}:${height}:force_original_aspect_ratio=increase`,
      `crop=${width}:${height}`,
      "setsar=1",
    );
    return track;
  }
  if (fit === "pad") {
    track.filters.push(
      contain,
      `pad=${width}:${height}:(ow-iw)/2:(oh-ih)/2:color=black`,
      "setsar=1",
    );
    return track;
  }
  // Blur a small copy and scale it up: as smooth as a large blur at a fraction of the cost.
  const input = graph.flush(track, "v");
  const [background, foreground, blurred, contained, output] = [
    graph.label("v"),
    graph.label("v"),
    graph.label("v"),
    graph.label("v"),
    graph.label("v"),
  ];
  const smallWidth = even(width / 4);
  const smallHeight = even(height / 4);
  const radius = Math.max(2, Math.floor(Math.min(smallWidth, smallHeight) / 10));
  graph.add([input], ["split=2"], [background, foreground]);
  graph.add(
    [background],
    [
      `scale=${smallWidth}:${smallHeight}:force_original_aspect_ratio=increase`,
      `crop=${smallWidth}:${smallHeight}`,
      `boxblur=${radius}:2`,
      `scale=${width}:${height}`,
      "setsar=1",
    ],
    [blurred],
  );
  graph.add([foreground], [contain, "setsar=1"], [contained]);
  graph.add([blurred, contained], ["overlay=(W-w)/2:(H-h)/2"], [output]);
  return { label: output, filters: [] };
}

/** Square pixels and even dimensions, which yuv420p H.264 requires. */
function sourceFilters(video: NonNullable<MediaProbe["video"]>): string[] {
  const size = displaySize(video);
  if (Math.abs(video.sampleAspectRatio - 1) > 0.01) {
    return [`scale=${size.width}:${size.height}`, "setsar=1"];
  }
  if (size.width !== video.width || size.height !== video.height) {
    return [`crop=${size.width}:${size.height}:0:0`];
  }
  return [];
}

function displaySize(video: NonNullable<MediaProbe["video"]>): Size {
  if (Math.abs(video.sampleAspectRatio - 1) > 0.01) {
    return {
      width: even(Math.round(video.width * video.sampleAspectRatio)),
      height: even(video.height),
    };
  }
  return { width: even(video.width), height: even(video.height) };
}

/** atempo takes 0.5–2 per instance on older ffmpeg builds, so larger changes chain it. */
export function atempoFilters(factor: number): string[] {
  const filters: string[] = [];
  let rest = factor;
  while (rest > 2) {
    filters.push("atempo=2");
    rest /= 2;
  }
  while (rest < 0.5) {
    filters.push("atempo=0.5");
    rest /= 0.5;
  }
  filters.push(`atempo=${number(rest)}`);
  return filters;
}

/**
 * Readable burned-in subtitles: bold white text with a dark outline, sized to the frame, and
 * lifted above the bottom fifth of tall frames where 9:16 apps place their own controls.
 */
export function subtitleStyle(width: number, height: number): string {
  const tall = height >= width * 1.5;
  const fontSize = Math.round(Math.min(width, height) * 0.06);
  const marginX = Math.round(width * (tall ? 0.1 : 0.05));
  const marginY = Math.round(height * (tall ? 0.2 : 0.06));
  return [
    `PlayResX=${width}`,
    `PlayResY=${height}`,
    "FontName=DejaVu Sans",
    `FontSize=${fontSize}`,
    "Bold=1",
    "PrimaryColour=&H00FFFFFF",
    "OutlineColour=&H00000000",
    "BackColour=&H80000000",
    "BorderStyle=1",
    `Outline=${Math.max(2, Math.round(fontSize / 16))}`,
    "Shadow=1",
    "Alignment=2",
    `MarginL=${marginX}`,
    `MarginR=${marginX}`,
    `MarginV=${marginY}`,
  ].join(",");
}

function even(value: number): number {
  return Math.max(2, 2 * Math.floor(value / 2));
}

function number(value: number): string {
  return String(Number(value.toFixed(6)));
}

function seconds(value: number): string {
  return String(Number(value.toFixed(2)));
}