git ssb

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function isOriginCompatible(a, b) {
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  return !a || a === '*' || a.slice(0, b.length) === b
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}
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module.exports = function (win, origin) {
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  var cbs = [], queue = [], _read
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  if(origin && origin[0] === '/') {
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    origin = location.origin + origin
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  }
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  function gotWin(_win) {
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    win = _win
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    if(_read) doRead()
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  }
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  function onMessage(e) {
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    if(isOriginCompatible(origin, e.origin)) {
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      if(!win) gotWin(e.source)
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      if(!origin) origin = e.origin
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      if(cbs.length) cbs.shift()(null, e.data)
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      else queue.push(e.data)
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    }
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  }
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  var sinkEnded, sourceEnded
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  window.addEventListener('message', onMessage, false)
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  return {
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    source: function (end, cb) {
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      if(sourceEnded = end) {
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        window.removeEventListener('message', onMessage, false)
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        return cb(null)
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      }
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      if(queue.length) cb(null, queue.shift())
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      else cbs.push(cb)
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    },
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    sink: function (read) {
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      _read = read
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      if(win) doRead()
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    }
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  }
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  function doRead() {
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    _read(null, function next(end, data) {
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      if(sinkEnded = end) return
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      win.postMessage(data, origin || '*')
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      _read(null, next)
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    })
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  }
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}
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