POS terminal (JS solution)

pos-functional.js

// Helper function:
Object.prototype.objReduce = function (func,initial) { for(x in this)  if (this[x]|0==this[x]) initial = func(x,this[x],initial); return initial; };

var pricelist = { A: { price: 1.25, pack: 3, ppack: 3}, B: { price: 4.25 }, C: { price: 1, pack:6,ppack:5}, D: {price: 0.75} }

// Collect array of items into object with fields of items and values of number of selled items.
// Example: A A B => { A: 2, B: 1 }
var collectItems = function(items) {
   return items.reduce(function(y,x) { if (y[x]) { y[x]++} else {y[x]=1}; return y;},{});
};

// Calculate price for 'value' items 'name' using pricelist
var calcPrice = function(name, value, pricelist) {
   return pricelist[name].pack?
        // if we have volume discount
        (pricelist[name].ppack*(~~(value/pricelist[name].pack))+pricelist[name].price*(value%pricelist[name].pack))
        // without volume discount
        :(pricelist[name].price*value);
};

// collect items and calcualte prices for them
var POS = function(items,pricelist) { return collectItems(items).objReduce(function(n,v,i) {  return i+calcPrice(n,v,pricelist); },0);}

console.log(POS(['A','B','C','D','A','B','A'],pricelist)); // -> 13.25
console.log(POS(['A','B','C','D'],pricelist)); // -> 7.25
console.log(POS(['C','C','C','C','C','C','C'],pricelist)); // -> 6

pos.js

// Helper function:
Object.prototype.objReduce = function (func,initial) { for(x in this)  if (this[x]|0==this[x]) initial = func(x,this[x],initial); return initial; };

var pricelist = { A: { price: 1.25, pack: 3, ppack: 3}, B: { price: 4.25 }, C: { price: 1, pack:6,ppack:5}, D: {price: 0.75} }

var POS = function(items) { return items.reduce(function(y,x) { if (y[x]) { y[x]++} else {y[x]=1}; return y;},{}).objReduce(function(n,v,i) { return i+(pricelist[n].pack?(pricelist[n].ppack*(~~(v/pricelist[n].pack))+pricelist[n].price*(v%pricelist[n].pack)):(pricelist[n].price*v)); },0);}

console.log(POS(['A','B','C','D','A','B','A'])); // -> 13.25
console.log(POS(['A','B','C','D'])); // -> 7.25
console.log(POS(['C','C','C','C','C','C','C'])); // -> 6