class AudioManager {
  constructor(audioCtx) {
    this.list = [];
    this.audioCtx = audioCtx || new (window.AudioContext || window.webkitAudioContext)();
    
    this.pool = {};
  }
  
  async load(url) {
    let data = this.pool[url];
    if(!data) {
      /* global fetch */
      const buffer = await fetch(url).then(res => res.arrayBuffer());
      data = await this.audioCtx.decodeAudioData(buffer);
      this.pool[url] = data;
    }
    
    const source = this.audioCtx.createBufferSource();
    source.buffer = data;
    
    return source;
  }
  
  /*
  list[]: {
    url: 
    when:
    offset:
    duration:
    next[]: index
  }
  */
  async play(list) {
    this.stop();
    this.list = list;
    
    for(let index = 0; index < list.length; index += 1) {
      const item = list[index];
      // preload
      await this.load(item.url);
      
      item.start = async () => {
        const current = item;
        
        const source = await this.load(current.url);
        source.connect(this.audioCtx.destination);
        current.source = source;
        
        if(!current.stoped) {
          current.source.addEventListener('ended', (e) => {
            current.next.forEach((i) => {
              list[i].start();
            });
            
            current.source.disconnect(this.audioCtx.destination);
          });
          
          source.start(current.when, current.offset, current.duration);
        }
        
      };
    }
    
    list[0].start();
  }
  
  async stop() {
    this.list.forEach((item) => {
      item.stoped = true;
    });
    
    this.list.forEach((item) => {
      if(!item.ended && item.source) {
        item.ended = true;
        item.source.stop();
      }
    });
  }
}