import { useMemo } from "react";
import type { Transcript } from "@/api/types";

type Sentence = Transcript["paragraphs"][number]["sentences"][number];

interface TranscriptPlaybackState {
  /** The sentence currently being spoken, or null if between sentences */
  activeSentence: Sentence | null;
  /** Index of the active word within the transcript's flat words array */
  activeWordIndex: number | null;
}

export function useTranscriptPlayback(
  transcript: Transcript | undefined,
  currentTime: number,
): TranscriptPlaybackState {
  return useMemo(() => {
    if (!transcript) return { activeSentence: null, activeWordIndex: null };

    // Find the active sentence across all paragraphs
    let activeSentence: Sentence | null = null;
    for (const paragraph of transcript.paragraphs) {
      const sentence = paragraph.sentences.find(
        (s) => currentTime >= s.start && currentTime <= s.end,
      );
      if (sentence) {
        activeSentence = sentence;
        break;
      }
    }

    // Binary search for active word
    const words = transcript.words;
    let activeWordIndex: number | null = null;
    let lo = 0;
    let hi = words.length - 1;
    while (lo <= hi) {
      const mid = (lo + hi) >>> 1;
      if (currentTime < words[mid].start) hi = mid - 1;
      else if (currentTime > words[mid].end) lo = mid + 1;
      else {
        activeWordIndex = mid;
        break;
      }
    }

    return { activeSentence, activeWordIndex };
  }, [transcript, currentTime]);
}