git ssb

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module.exports = function detectSync (peerId, upto, toSend, peerHas, onSync) {
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  // HACK: createHistoryStream does not emit sync event, so we don't
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  // know when it switches to live. Do it manually!
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  var sync = false
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  var last = (upto.sequence || upto.seq || 0)
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  // check sync after 500ms, hopefully we have the info from the peer by then
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  setTimeout(function () {
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    if (peerHas[peerId] && peerHas[peerId][upto.id] != null) {
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      checkSync()
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    } else {
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      // if we get here, the peer hasn't yet asked for this feed, or is not responding
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      // we can assume it doesn't have the feed, so lets call sync
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      broadcastSync()
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    }
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  }, 500)
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  return function (msg) {
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    if (msg.sync) {
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      // surprise! This peer actually has a sync event!
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      broadcastSync()
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      return false
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    }
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    last = msg.sequence
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    checkSync()
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    return true
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  }
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  function checkSync () {
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    if (!sync) {
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      var availableSeq = peerHas[peerId] && peerHas[peerId][upto.id]
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      if (availableSeq === last || availableSeq < toSend[upto.id]) {
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        // we've reached the maximum sequence this server has told us it knows about
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        // or we don't need anything from this server
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        broadcastSync()
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      }
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    }
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  }
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  function broadcastSync () {
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    if (!sync) {
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      sync = true
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      onSync && onSync()
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    }
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  }
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}
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