git ssb

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var u = require('./util')
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/*
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  this uses localStorage, so it doesn't need async,
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  but i used async api anyway,
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  so it will be easy to switch to indexeddb.
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*/
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module.exports = function (prefix, storage) {
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  //pass in non-local storage, to make testing easy.
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  storage = storage || localStorage
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  var log
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  function _append (data, cb) {
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    var log = u.parse(storage[prefix]) || []
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    log.unshift(data)
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    storage[prefix] = JSON.stringify(log)
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    cb(null, data)
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  }
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  function filtered (log) {
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    var revert = null
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    var output = []
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    for(var i = 0; i < log.length; i++) {
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      var item = log[i]
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      if(revert && revert <= item.ts) //this op was reverted.
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        ;
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      else if(item.revert)
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        revert = item.revert
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      else
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        output.push(item)
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    }
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    return output
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  }
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  function getLog() {
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    return u.parse(storage[prefix]) || []
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  }
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  return log = {
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    head: function (cb) {
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      cb(null, filtered(getLog())[0])
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    },
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    filtered: function (cb) {
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      cb(null, filtered(getLog()))
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    },
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    unfiltered: function (cb) {
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      cb(null, getLog())
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    },
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    append: function (data, cb) {
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      _append({value: data, ts: Date.now()}, cb)
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    },
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    revert: function (ts, cb) {
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      if(!ts) return cb(new Error('log.revert: must provide ts to revert to'))
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      _append({revert: ts, ts: Date.now()}, function (err) {
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        if(err) cb(err)
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        else cb(null, filtered(getLog())[0])
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      })
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    }
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  }
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}
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