git ssb

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var fs = require('fs')
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function isFunction (f) {
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  return 'function' === typeof f
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}
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module.exports = function (filename, suffix, _codec) {
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  var codec = _codec || require('./codec')
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  suffix = suffix || '~'
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  var queue = []
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  var value
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  return {
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    get: function (cb) {
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      if(value) return cb(null, value)
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      else fs.readFile(filename, 'utf8', function (err, _value) {
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        if(err) return cb(err)
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        try {
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          value = codec.decode(_value)
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        } catch (err) {
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          return cb(err)
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        }
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        cb(null, value)
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      })
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    },
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    //only allow one update at a time.
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    set: function put (_value, cb) {
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      if(!isFunction(cb)) throw new Error('cb must be function')
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      if(queue.length) {
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        return queue.push(function retry () {
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          put(_value, cb)
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        })
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      }
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      queue.push(cb)
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      function done (err) {
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        var _queue = queue
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        queue = []
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        while(_queue.length) _queue.shift()(err)
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      }
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      fs.writeFile(filename+suffix, codec.encode(_value), function (err) {
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        if(err) return done(err)
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        fs.rename(filename+suffix, filename, function (err) {
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          if(err) done(err)
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          else done(null, value = _value)
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        })
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      })
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    }
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  }
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}
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