# paulfioravanti's solution

## to Change in the Ruby Track

Published at May 27 2019 · 0 comments
Instructions
Test suite
Solution

Correctly determine the fewest number of coins to be given to a customer such that the sum of the coins' value would equal the correct amount of change.

## For example

• An input of 15 with [1, 5, 10, 25, 100] should return one nickel (5) and one dime (10) or [5, 10]
• An input of 40 with [1, 5, 10, 25, 100] should return one nickel (5) and one dime (10) and one quarter (25) or [5, 10, 25]

## Edge cases

• Does your algorithm work for any given set of coins?
• Can you ask for negative change?
• Can you ask for a change value smaller than the smallest coin value?

For installation and learning resources, refer to the Ruby resources page.

For running the tests provided, you will need the Minitest gem. Open a terminal window and run the following command to install minitest:

``````gem install minitest
``````

If you would like color output, you can `require 'minitest/pride'` in the test file, or note the alternative instruction, below, for running the test file.

Run the tests from the exercise directory using the following command:

``````ruby change_test.rb
``````

To include color from the command line:

``````ruby -r minitest/pride change_test.rb
``````

## Source

Software Craftsmanship - Coin Change Kata https://web.archive.org/web/20130115115225/http://craftsmanship.sv.cmu.edu:80/exercises/coin-change-kata

## Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

### change_test.rb

``````require 'minitest/autorun'
require_relative 'change'

# Common test data version: 1.3.0 258c807
class ChangeTest < Minitest::Test
def test_single_coin_change
# skip
assert_equal [25], Change.generate([1, 5, 10, 25, 100], 25)
end

def test_multiple_coin_change
skip
assert_equal [5, 10], Change.generate([1, 5, 10, 25, 100], 15)
end

def test_change_with_lilliputian_coins
skip
assert_equal [4, 4, 15], Change.generate([1, 4, 15, 20, 50], 23)
end

def test_change_with_lower_elbonia_coins
skip
assert_equal [21, 21, 21], Change.generate([1, 5, 10, 21, 25], 63)
end

def test_large_target_values
skip
assert_equal [2, 2, 5, 20, 20, 50, 100, 100, 100, 100, 100, 100, 100, 100, 100], Change.generate([1, 2, 5, 10, 20, 50, 100], 999)
end

def test_possible_change_without_unit_coins_available
skip
assert_equal [2, 2, 2, 5, 10], Change.generate([2, 5, 10, 20, 50], 21)
end

def test_another_possible_change_without_unit_coins_available
skip
assert_equal [4, 4, 4, 5, 5, 5], Change.generate([4, 5], 27)
end

def test_no_coins_make_0_change
skip
assert_equal [], Change.generate([1, 5, 10, 21, 25], 0)
end

def test_error_testing_for_change_smaller_than_the_smallest_of_coins
skip
assert_raises(Change::ImpossibleCombinationError) do
Change.generate([5, 10], 3)
end
end

skip
assert_raises(Change::ImpossibleCombinationError) do
Change.generate([5, 10], 94)
end
end

def test_cannot_find_negative_change_values
skip
assert_raises(Change::NegativeTargetError) do
Change.generate([1, 2, 5], -5)
end
end
end``````
``````module Change
class ImpossibleCombinationError < StandardError; end
class NegativeTargetError < StandardError; end

GREATER = 1
private_constant :GREATER
EQUAL = 0
private_constant :EQUAL

module_function

def generate(coins, target)
return [] if target.zero?
raise NegativeTargetError if target.negative?
raise ImpossibleCombinationError if target < coins.min

denominations = generate_denominations(coins, target)
change_candidates = generate_change_candidates(denominations, target)
raise ImpossibleCombinationError if change_candidates.empty?

change_candidates.min_by(&:length)
end

def generate_denominations(coins, target)
coins
.select { |coin| coin <= target }
.sort
.reverse
end
private_class_method :generate_denominations

def generate_change_candidates(denominations, target)
coin_types = denominations.dup.freeze

denominations
.each
.with_object([denominations, coin_types, target])
.then(&method(:generate_change))
.first
end
private_class_method :generate_change_candidates

# rubocop:disable Metrics/MethodLength
def generate_change(denomination_enumerator)
denomination_enumerator
.each_with_object([[], []]) \
do |(coin, (denominations, coin_types, target)), (candidates, acc)|
acc.prepend(coin)
case acc.sum <=> target
when GREATER
try_next_smallest_coin(coin, denominations, coin_types, acc)
when EQUAL
store_candidate(denominations, coin_types, candidates, acc)
else # SMALLER
redo
end
next
end
end
private_class_method :generate_change
# rubocop:enable Metrics/MethodLength

def try_next_smallest_coin(coin, denominations, coin_types, acc)
acc.shift
return unless smallest_coin?(coin, coin_types)

acc.delete(coin)
return if acc.empty?

append_coin_type_subset(denominations, coin_types, acc.first)
acc.shift
end
private_class_method :try_next_smallest_coin

def smallest_coin?(coin, coin_types)
coin == coin_types.last
end
private_class_method :smallest_coin?

def store_candidate(denominations, coin_types, candidates, acc)
append_coin_type_subset(denominations, coin_types, acc.last)
candidates << acc.dup
acc.clear
end
private_class_method :store_candidate

def append_coin_type_subset(denominations, coin_types, coin)
next_smallest_coin_type_index = coin_types.index(coin) + 1
remaining_coin_types = coin_types[next_smallest_coin_type_index..-1]
denominations.append(*remaining_coin_types)
end
private_class_method :append_coin_type_subset
end``````