gistfile1.txt

function reduceAhead(arr,fn,seed,nextSeed) {
  const l = arguments.length;
  seed = l < 3 ? arr.shift() : seed;
  let tmp = {
    out:seed,
    entry:arr.shift(),
    at: l < 3 ? 1 : 0
  };
  tmp = arr.reduce(function(tmp,next){
    tmp.out = fn.call(arr,tmp.out,tmp.entry,next,tmp.at,arr);
    tmp.entry = next;
    tmp.at = tmp.at + 1;
    return tmp;
  },tmp);
  return fn.call(arr,tmp.out,tmp.entry,nextSeed,tmp.at);
}

function reduceAround(arr,fn,seed,lastSeed,nextSeed) {
  const l = arguments.length;
  seed = l < 3 ? arr.shift() : seed;
  let tmp = {
    out:seed,
    last:lastSeed,
    entry:arr.shift(),
    at: l < 3 ? 1 : 0
  };
  tmp = arr.reduce(function(tmp,next){
    tmp.out = fn.call(arr,tmp.out,tmp.entry,tmp.last,next,tmp.at,arr);
    tmp.last = tmp.entry;
    tmp.entry = next;
    tmp.at = tmp.at + 1;
    return tmp;
  },tmp);
  return fn.call(arr,tmp.out,tmp.entry,tmp.last,nextSeed,tmp.at);
}

function reduceBehind(arr,fn,seed,lastSeed) {
  const l = arguments.length;
  seed = l < 3 ? arr.shift() : seed;
  let tmp = {
    out:seed,
    last:lastSeed,
    at: l < 3 ? 1 : 0
  };
  tmp = arr.reduce(function(tmp,entry){
    tmp.out = fn.call(arr,tmp.out,entry,tmp.last,tmp.at,arr);
    tmp.last = entry;
    tmp.at = tmp.at + 1;
    return tmp;
  },tmp);
  return tmp.out;
}