flume-js/reduce.js

a library for defining applications as a set of inputs and transformations

var maybeAsync = require('./core').maybeAsync
var resultState = require('./utils/resultState')

module.exports = function reduce(init, reducer) {
  if (arguments.length < 2) {
    reducer = init
    init = retNull
  }

  reducer = maybeAsync(parseReducer(reducer))

  return {
    init: init,
    transform: reduceFn
  }

  function reduceFn(state, v, opts) {
    return reducer(state, v, opts).then(resultState)
  }
}

function parseReducer(obj) {
  if (typeof obj == 'function') return obj
  if (Array.isArray(obj)) return caseSourceOfReducer(obj)
  throw new Error('Unsupported reducer type given to reduce(): ' + typeof obj)
}

function caseSourceOfReducer(pairs) {
  var defaultFn = identityReducer

  if (typeof pairs[pairs.length - 1] == 'function') {
    defaultFn = pairs[pairs.length - 1]
    pairs = pairs.slice(0, -1)
  }

  return function caseSourceOfFn(state, v, opts) {
    var source = opts.source
    var i = -1
    var n = pairs.length
    var pair

    while (++i < n) {
      pair = pairs[i]
      if (pair[0] === source) return pair[1](state, v, opts)
    }

    return defaultFn(state, v, opts)
  }
}

function retNull() {
  return null
}

function identityReducer(state, v) {
  return v
}