index.html

<script>
  const video = document.getElementById('videoPlayer');
  const speedRange = document.getElementById('speedRange');
  const speedInput = document.getElementById('speedInput');
  const volumeRange = document.getElementById('volumeRange');
  const volumeInput = document.getElementById('volumeInput');
  const loopCheckbox = document.getElementById('loopCheckbox');
  const audioWarning = document.getElementById('audioWarning');

  let audioContext = null;
  let source = null;
  let gainNode = null;
  let audioInitialized = false;

  function setupAudio() {
    try {
      audioContext = new (window.AudioContext || window.webkitAudioContext)();
      source = audioContext.createMediaElementSource(video);
      gainNode = audioContext.createGain();

      source.connect(gainNode);
      gainNode.connect(audioContext.destination);

      updateVolume(volumeRange.value);
      audioInitialized = true;
      audioWarning.style.display = 'none';
    } catch (e) {
      const msg = "音量増幅機能が使用できません: " + e.message;
      audioWarning.textContent = msg;
      audioWarning.style.display = 'block';
      alert(msg);
    }
  }

  function updatePlaybackRate(value) {
    const speed = parseFloat(value);
    if (isNaN(speed) || speed <= 0) {
      const msg = "再生速度が不正です: " + value;
      alert(msg);
      return;
    }
    speedInput.value = speed;
    speedRange.value = speed;
    video.playbackRate = speed;
  }

  function updateVolume(value) {
    const volume = parseFloat(value);
    if (isNaN(volume) || volume < 0) {
      const msg = "音量の値が不正です: " + value;
      alert(msg);
      return;
    }
    volumeInput.value = volume;
    volumeRange.value = volume;

    if (audioInitialized && gainNode) {
      try {
        gainNode.gain.value = volume / 100;
      } catch (e) {
        const msg = "音量設定中にエラーが発生しました: " + e.message;
        alert(msg);
      }
    } else {
      try {
        video.volume = Math.min(volume / 100, 1);
      } catch (e) {
        const msg = "標準音量設定に失敗しました: " + e.message;
        alert(msg);
      }
    }
  }

  ['input', 'change', 'mouseup'].forEach(eventName => {
    speedRange.addEventListener(eventName, () => {
      updatePlaybackRate(speedRange.value);
    });
    volumeRange.addEventListener(eventName, () => {
      updateVolume(volumeRange.value);
    });
  });

  speedInput.addEventListener('input', () => {
    updatePlaybackRate(speedInput.value);
  });

  volumeInput.addEventListener('input', () => {
    updateVolume(volumeInput.value);
  });

  loopCheckbox.addEventListener('change', () => {
    video.loop = loopCheckbox.checked;
  });

  function goFullscreen() {
    try {
      if (video.requestFullscreen) {
        video.requestFullscreen();
      } else if (video.webkitRequestFullscreen) {
        video.webkitRequestFullscreen();
      } else if (video.msRequestFullscreen) {
        video.msRequestFullscreen();
      } else {
        alert("全画面表示はこのブラウザではサポートされていません。");
      }
    } catch (e) {
      alert("全画面モードに失敗しました: " + e.message);
    }
  }

  video.addEventListener('play', () => {
    if (!audioInitialized) {
      setupAudio();
    } else if (audioContext && audioContext.state === 'suspended') {
      audioContext.resume().catch(e => {
        const msg = "AudioContextの再開に失敗しました: " + e.message;
        alert(msg);
      });
    }
  });

  video.addEventListener('loadedmetadata', () => {
    try {
      updatePlaybackRate(speedRange.value);
      updateVolume(volumeRange.value);
      video.loop = loopCheckbox.checked;
    } catch (e) {
      alert("動画のメタデータ読み込み処理でエラーが発生しました: " + e.message);
    }
  });

  // 初期警告表示
  audioWarning.style.display = 'block';
  alert("音量増幅機能を使用するには、動画を再生してください");
</script>