freezmo/lib/podcast-speech.ts

"use client";

/**
 * The podcast player's voice: one section at a time, with the spoken word
 * reported back so the transcript can highlight it (the Spotify-lyrics bit).
 *
 * Two clocks drive the highlight. Where the browser fires `onboundary`
 * (desktop Chrome and friends) that is exact — the character index maps onto
 * the word list. Where it does not (iOS Safari, several Android engines) a
 * timer estimates each word from its length and the playback rate, so the
 * highlight still moves at roughly speaking speed. The first boundary seen
 * retires the timer for the rest of the session.
 *
 * Pause/seek are implemented by cancelling and re-speaking from a word: the
 * platform `pause()` is unreliable across browsers, and re-speaking also makes
 * "jump to a chapter" and "resume where you left off" the same operation.
 */

import { speechAvailable } from "./speech";
import type { PodcastSection } from "./schemas";

export type PodcastVoice = { section: number; word: number };

export type PodcastSpeechEvents = {
  sections: PodcastSection[];
  locale: string;
  rate: number;
  /** The chosen voice (lib/podcast-voices); null lets the platform decide. */
  voice?: SpeechSynthesisVoice | null;
  /** Global word position changed (word = index inside its section). */
  onWord: (position: PodcastVoice) => void;
  /** Playback actually began (after the engine accepted it). */
  onPlay?: () => void;
  onPause?: () => void;
  onEnd: () => void;
  onError?: (message: string) => void;
};

export type Token = {
  section: number;
  /** Index of the word inside its section. */
  index: number;
  /** Index across the whole episode (the transcript's data-word). */
  global: number;
  start: number;
  end: number;
  text: string;
};

/** Split into words, keeping each one's character span inside its section. */
export function tokenizeSections(sections: PodcastSection[]): Token[] {
  const tokens: Token[] = [];
  sections.forEach((section, sectionIndex) => {
    const text = section.text;
    const pattern = /\S+/g;
    let index = 0;
    let match: RegExpExecArray | null;
    while ((match = pattern.exec(text)) !== null) {
      tokens.push({
        section: sectionIndex,
        index,
        global: tokens.length,
        start: match.index,
        end: match.index + match[0].length,
        text: match[0],
      });
      index += 1;
    }
  });
  return tokens;
}

/** Estimated spoken time of one word at rate 1 (ms). */
function wordDuration(text: string): number {
  return 95 + 42 * Math.max(1, text.replace(/[^\p{L}\p{N}]/gu, "").length);
}

export type PodcastSpeech = {
  play: (from?: PodcastVoice) => void;
  pause: () => void;
  stop: () => void;
  playing: () => boolean;
  position: () => PodcastVoice | null;
  words: Token[];
};

export function createPodcastSpeech(events: PodcastSpeechEvents): PodcastSpeech {
  const tokens = tokenizeSections(events.sections);
  let playing = false;
  let position: PodcastVoice | null = null;
  let boundariesWork: boolean | null = null;
  let timer: number | null = null;
  /** Cleared by cancel(): stops stale timers/events from a previous utterance. */
  let generation = 0;

  const clearTimer = () => {
    if (timer !== null) {
      window.clearTimeout(timer);
      timer = null;
    }
  };

  const finishPass = () => {
    playing = false;
    events.onPause?.();
  };

  const wordsIn = (section: number) => tokens.filter((token) => token.section === section);

  /** Speak the section from `word` on. Empty word list → the next section. */
  const speakFrom = (target: PodcastVoice) => {
    if (!speechAvailable()) {
      events.onError?.("unsupported");
      return;
    }
    const section = events.sections[target.section];
    if (!section) {
      position = null;
      finishPass();
      events.onEnd();
      return;
    }
    const sectionTokens = wordsIn(target.section);
    if (sectionTokens.length === 0) {
      speakFrom({ section: target.section + 1, word: 0 });
      return;
    }
    const word = Math.min(Math.max(0, target.word), sectionTokens.length - 1);
    position = { section: target.section, word };

    const from = sectionTokens[word].start;
    const text = section.text.slice(from);
    const current = ++generation;
    try {
      window.speechSynthesis.cancel();
      const utterance = new SpeechSynthesisUtterance(text);
      utterance.lang = events.locale;
      utterance.rate = events.rate;
      if (events.voice) utterance.voice = events.voice;

      const offsetFor = (charIndex: number) => {
        /* First token whose span ends after the reported character. */
        let found = word;
        for (let i = word; i < sectionTokens.length; i += 1) {
          const token = sectionTokens[i];
          if (token.end > from + charIndex) {
            found = i;
            break;
          }
          found = i;
        }
        return found;
      };

      utterance.onboundary = (event) => {
        if (generation !== current) return;
        if (event.name && event.name !== "word") return;
        boundariesWork = true;
        clearTimer();
        const index = offsetFor(event.charIndex);
        position = { section: target.section, word: index };
        events.onWord(position);
      };

      utterance.onstart = () => {
        if (generation !== current) return;
        playing = true;
        events.onPlay?.();
        events.onWord(position ?? { section: target.section, word });
        /* The fallback clock: only while this browser has never reported a
         * boundary. Each tick schedules the next word by its length. */
        if (boundariesWork !== true) {
          let index = word;
          const tick = () => {
            if (generation !== current) return;
            index += 1;
            if (index >= sectionTokens.length) {
              /* Let onend take over; a long word list can outlast the audio
               * by a hair, and stopping the tick avoids double-advancing. */
              timer = null;
              return;
            }
            position = { section: target.section, word: index };
            events.onWord(position);
            timer = window.setTimeout(tick, wordDuration(sectionTokens[index].text) / events.rate);
          };
          timer = window.setTimeout(tick, wordDuration(sectionTokens[word].text) / events.rate);
        }
      };

      utterance.onend = () => {
        if (generation !== current) return;
        clearTimer();
        /* An utterance that ended without a single boundary means the browser
           does not report them: the fallback clock was the right call. */
        if (boundariesWork === null) boundariesWork = false;
        if (target.section + 1 >= events.sections.length) {
          position = null;
          finishPass();
          events.onEnd();
          return;
        }
        speakFrom({ section: target.section + 1, word: 0 });
      };

      utterance.onerror = (event) => {
        if (generation !== current) return;
        clearTimer();
        playing = false;
        /* "interrupted"/"canceled" are our own stops, not failures. */
        const reason = (event as SpeechSynthesisErrorEvent).error;
        if (reason !== "interrupted" && reason !== "canceled") {
          events.onError?.(String(reason ?? "error"));
          events.onPause?.();
        }
      };

      window.speechSynthesis.speak(utterance);
    } catch {
      playing = false;
      events.onError?.("error");
    }
  };

  return {
    play(from) {
      speakFrom(from ?? position ?? { section: 0, word: 0 });
    },
    pause() {
      generation += 1;
      clearTimer();
      try {
        window.speechSynthesis.cancel();
      } catch {
        /* already gone */
      }
      playing = false;
      events.onPause?.();
    },
    stop() {
      this.pause();
      position = null;
    },
    playing: () => playing,
    position: () => position,
    words: tokens,
  };
}