git ssb

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function fixJSON(key, value) {
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  if (value instanceof Error) return {
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    message: value.message,
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    stack: value.stack,
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    name: value.name,
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    code: value.code,
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  }
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  return value
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}
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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 _cb, _msg
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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 post(msg) {
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    if (!win) _msg = msg
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    else win.postMessage(JSON.stringify(msg, fixJSON), origin || '*')
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  }
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  function gotWin(_win) {
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    win = _win
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    if (_msg) {
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      var msg = _msg
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      _msg = null
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      post(msg)
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    }
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  }
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  return function (read) {
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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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        var msg = JSON.parse(e.data)
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        switch (msg[0]) {
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          case 'read': return onRead(msg[1])
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          case 'next': return onNext(msg[1], msg[2])
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        }
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      }
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    }
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    function onRead(end) {
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      read(end, function (end, data) {
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        post(['next', end, data])
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      })
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    }
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    function onNext(end, data) {
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      if (end) window.removeEventListener('message', onMessage, false)
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      if (!_cb) return console.error('unexpected data', end, data)
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      var cb = _cb
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      _cb = null
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      return cb(end, data)
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    }
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    window.addEventListener('message', onMessage, false)
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    return function (end, cb) {
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      if(_cb) return cb(new Error('read too soon'))
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      _cb = cb
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      post(['read', end])
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    }
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  }
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}
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