Calculate change from available pool of coins simulating automatic cash register.

change.go

package main

import (
	"fmt"
	"log"
	"strings"
)

func main() {
	b := NewBank(map[Coin]int{
		Penny:  100,
		Nickle: 20,
		Dime:   10,
		Quater: 2,
	})

	fmt.Printf("Started with money in the bank: %v\n", b)
	fmt.Println("Start dispensing change")
	fmt.Println()

	for _, v := range []int{3, 5, 10, 17, 23, 25, 41, 91, 99, 50} {
		fmt.Printf("Requesting change of %d cents\n", v)
		fmt.Printf("Dispensed coins: %s\n", b.DispenseCoins(v))
		fmt.Printf("Money in the bank: %s\n", b)
		fmt.Println()

	}
}

type Coin int

const (
	Penny  Coin = 1
	Nickle      = 5
	Dime        = 10
	Quater      = 25
)

func (c Coin) singular() string {
	switch c {
	case Penny:
		return "penny"
	case Nickle:
		return "nickle"
	case Dime:
		return "dime"
	case Quater:
		return "quater"
	}
	return "unknown coin"
}

func (c Coin) plural() string {
	switch c {
	case Penny:
		return "pennies"
	default:
		return c.singular() + "s"
	}
}

type CoinStack struct {
	Type  Coin
	Count int
}

func (cs CoinStack) Value() int {
	return int(cs.Type) * cs.Count
}

func (cs *CoinStack) Get(amount int) (*CoinStack, int) {
	needCoins := amount / int(cs.Type)
	balance := amount % int(cs.Type)
	if needCoins > cs.Count {
		defficit := needCoins - cs.Count
		balance += defficit * int(cs.Type)
		needCoins, cs.Count = cs.Count, 0
	} else {
		cs.Count -= needCoins
	}

	if cs.Count == 0 {
		log.Printf("out of %s", cs.Type.plural())
	}

	return &CoinStack{Type: cs.Type, Count: needCoins}, balance
}

func (cs CoinStack) String() string {
	denom := cs.Type.plural
	if cs.Count == 1 {
		denom = cs.Type.singular
	}
	return fmt.Sprintf("%d %s", cs.Count, denom())
}

type Bank struct {
	Store [4]*CoinStack
}

func NewBank(money map[Coin]int) *Bank {
	bank := new(Bank)
	// Transfer coins into bank
	for i, c := range []Coin{Quater, Dime, Nickle, Penny} {
		if count, ok := money[c]; ok {
			bank.Store[i] = &CoinStack{Type: c, Count: count}

		}
	}
	return bank
}

func (b *Bank) DispenseCoins(amount int) string {
	res := make([]string, 0, 4)
	for _, cs := range b.Store[:] {
		if cs.Count == 0 {
			continue
		}

		var coins *CoinStack
		coins, amount = cs.Get(amount)

		if coins.Count > 0 {
			res = append(res, coins.String())
		}

		if amount == 0 {
			break
		}
	}

	if amount > 0 {
		log.Fatalf("Not enough coins to make change on %d cents balance", amount)
	}

	return strings.Join(res, ", ")
}

func (b Bank) String() string {
	res := make([]string, 0, 4)
	val := 0

	for _, cs := range b.Store[:] {
		val += cs.Value()
		res = append(res, cs.String())
	}

	return fmt.Sprintf("%.2f dollars [%v]", float64(val)/100, strings.Join(res, ", "))
}

Live code - https://play.golang.org/p/SrQkFRmRzgC

Output:

Started with money in the bank: 3.50 dollars [2 quaters, 10 dimes, 20 nickles, 100 pennies]
Start dispensing change

Requesting change of 3 cents
Dispensed coins: 3 pennies
Money in the bank: 3.47 dollars [2 quaters, 10 dimes, 20 nickles, 97 pennies]

Requesting change of 5 cents
Dispensed coins: 1 nickle
Money in the bank: 3.42 dollars [2 quaters, 10 dimes, 19 nickles, 97 pennies]

Requesting change of 10 cents
Dispensed coins: 1 dime
Money in the bank: 3.32 dollars [2 quaters, 9 dimes, 19 nickles, 97 pennies]

Requesting change of 17 cents
Dispensed coins: 1 dime, 1 nickle, 2 pennies
Money in the bank: 3.15 dollars [2 quaters, 8 dimes, 18 nickles, 95 pennies]

Requesting change of 23 cents
Dispensed coins: 2 dimes, 3 pennies
Money in the bank: 2.92 dollars [2 quaters, 6 dimes, 18 nickles, 92 pennies]

Requesting change of 25 cents
Dispensed coins: 1 quater
Money in the bank: 2.67 dollars [1 quater, 6 dimes, 18 nickles, 92 pennies]

Requesting change of 41 cents
2009/11/10 23:00:00 out of quaters
Dispensed coins: 1 quater, 1 dime, 1 nickle, 1 penny
Money in the bank: 2.26 dollars [0 quaters, 5 dimes, 17 nickles, 91 pennies]

Requesting change of 91 cents
2009/11/10 23:00:00 out of dimes
Dispensed coins: 5 dimes, 8 nickles, 1 penny
Money in the bank: 1.35 dollars [0 quaters, 0 dimes, 9 nickles, 90 pennies]

Requesting change of 99 cents
2009/11/10 23:00:00 out of nickles
Dispensed coins: 9 nickles, 54 pennies
Money in the bank: 0.36 dollars [0 quaters, 0 dimes, 0 nickles, 36 pennies]

Requesting change of 50 cents
2009/11/10 23:00:00 out of pennies
2009/11/10 23:00:00 Not enough coins to make change on 14 cents balance