Add change exercise (#386)

config.json

@@ -888,6 +888,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 5
+      },
+      {
+        "slug": "change",
+        "name": "Change",
+        "uuid": "a625e5e9-43cf-4765-bd9a-e41135132258",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 6
       }
     ]
   },

exercises/practice/change/.docs/instructions.md

@@ -0,0 +1,8 @@
+# Instructions
+
+Determine the fewest number of coins to give a customer so that the sum of their values equals the correct amount of change.
+
+## Examples
+
+- An amount of 15 with available coin values [1, 5, 10, 25, 100] should return one coin of value 5 and one coin of value 10, or [5, 10].
+- An amount of 40 with available coin values [1, 5, 10, 25, 100] should return one coin of value 5, one coin of value 10, and one coin of value 25, or [5, 10, 25].

exercises/practice/change/.docs/introduction.md

@@ -0,0 +1,26 @@
+# Introduction
+
+In the mystical village of Coinholt, you stand behind the counter of your bakery, arranging a fresh batch of pastries.
+The door creaks open, and in walks Denara, a skilled merchant with a keen eye for quality goods.
+After a quick meal, she slides a shimmering coin across the counter, representing a value of 100 units.
+
+You smile, taking the coin, and glance at the total cost of the meal: 88 units.
+That means you need to return 12 units in change.
+
+Denara holds out her hand expectantly.
+"Just give me the fewest coins," she says with a smile.
+"My pouch is already full, and I don't want to risk losing them on the road."
+
+You know you have a few options.
+"We have Lumis (worth 10 units), Viras (worth 5 units), and Zenth (worth 2 units) available for change."
+
+You quickly calculate the possibilities in your head:
+
+- one Lumis (1 × 10 units) + one Zenth (1 × 2 units) = 2 coins total
+- two Viras (2 × 5 units) + one Zenth (1 × 2 units) = 3 coins total
+- six Zenth (6 × 2 units) = 6 coins total
+
+"The best choice is two coins: one Lumis and one Zenth," you say, handing her the change.
+
+Denara smiles, clearly impressed.
+"As always, you've got it right."

exercises/practice/change/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "erikschierboom"
+  ],
+  "files": {
+    "solution": [
+      "change.R"
+    ],
+    "test": [
+      "test_change.R"
+    ],
+    "example": [
+      ".meta/example.R"
+    ]
+  },
+  "blurb": "Correctly determine change to be given using the least number of coins.",
+  "source": "Software Craftsmanship - Coin Change Kata",
+  "source_url": "https://web.archive.org/web/20130115115225/http://craftsmanship.sv.cmu.edu:80/exercises/coin-change-kata"
+}

exercises/practice/change/.meta/example.R

@@ -0,0 +1,32 @@
+find_fewest_coins <- function(coins, target) {
+  if (target == 0) {
+    return(c())
+  }
+
+  stopifnot(target > 0)
+  stopifnot(min(coins) <= target)
+
+  min_amount_coins <- rep(NA, target) |> as.list()
+
+  for (amount in 1:target) {
+    for (coin in coins[coins <= amount]) {
+      if (amount - coin == 0) {
+        sub_amount_coins <- c()
+      } else {
+        sub_amount_coins <- min_amount_coins[[amount - coin]]
+      }
+      if (any(is.na(sub_amount_coins))) next
+
+      amount_coins <- min_amount_coins[[amount]]
+      if (!any(is.na(amount_coins)) &&
+          length(amount_coins) <= length(sub_amount_coins) + 1) {
+        next
+      }
+
+      min_amount_coins[[amount]] <- c(coin, sub_amount_coins)
+    }
+  }
+
+  stopifnot(!any(is.na(min_amount_coins[[target]])))
+  min_amount_coins[[target]]
+}

exercises/practice/change/.meta/tests.toml

@@ -0,0 +1,49 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[d0ebd0e1-9d27-4609-a654-df5c0ba1d83a]
+description = "change for 1 cent"
+
+[36887bea-7f92-4a9c-b0cc-c0e886b3ecc8]
+description = "single coin change"
+
+[cef21ccc-0811-4e6e-af44-f011e7eab6c6]
+description = "multiple coin change"
+
+[d60952bc-0c1a-4571-bf0c-41be72690cb3]
+description = "change with Lilliputian Coins"
+
+[408390b9-fafa-4bb9-b608-ffe6036edb6c]
+description = "change with Lower Elbonia Coins"
+
+[7421a4cb-1c48-4bf9-99c7-7f049689132f]
+description = "large target values"
+
+[f79d2e9b-0ae3-4d6a-bb58-dc978b0dba28]
+description = "possible change without unit coins available"
+
+[9a166411-d35d-4f7f-a007-6724ac266178]
+description = "another possible change without unit coins available"
+
+[ce0f80d5-51c3-469d-818c-3e69dbd25f75]
+description = "a greedy approach is not optimal"
+
+[bbbcc154-e9e9-4209-a4db-dd6d81ec26bb]
+description = "no coins make 0 change"
+
+[c8b81d5a-49bd-4b61-af73-8ee5383a2ce1]
+description = "error testing for change smaller than the smallest of coins"
+
+[3c43e3e4-63f9-46ac-9476-a67516e98f68]
+description = "error if no combination can add up to target"
+
+[8fe1f076-9b2d-4f44-89fe-8a6ccd63c8f3]
+description = "cannot find negative change values"

exercises/practice/change/change.R

@@ -0,0 +1,3 @@
+find_fewest_coins <- function(coins, target) {
+
+}

exercises/practice/change/test_change.R

@@ -0,0 +1,85 @@
+source("./change.R")
+library(testthat)
+
+test_that("Change for 1 cent", {
+  coins <- c(1, 5, 10, 25)
+  target <- 1
+  expected <- c(1)
+  expect_equal(find_fewest_coins(coins, target), expected)
+})
+
+test_that("Single coin change", {
+  coins <- c(1, 5, 10, 25, 100)
+  target <- 25
+  expected <- c(25)
+  expect_equal(find_fewest_coins(coins, target), expected)
+})
+
+test_that("Multiple coin change", {
+  coins <- c(1, 5, 10, 25, 100)
+  target <- 15
+  expected <- c(5, 10)
+  expect_equal(find_fewest_coins(coins, target), expected)
+})
+
+test_that("Change with Lilliputian Coins", {
+  coins <- c(1, 4, 15, 20, 50)
+  target <- 23
+  expected <- c(4, 4, 15)
+  expect_equal(find_fewest_coins(coins, target), expected)
+})
+
+test_that("Change with Lower Elbonia Coins", {
+  coins <- c(1, 5, 10, 21, 25)
+  target <- 63
+  expected <- c(21, 21, 21)
+  expect_equal(find_fewest_coins(coins, target), expected)
+})
+
+test_that("Large target values", {
+  coins <- c(1, 2, 5, 10, 20, 50, 100)
+  target <- 999
+  expected <- c(
+    2, 2, 5, 20, 20, 50, 100, 100, 100, 100, 100, 100, 100, 100, 100
+  )
+  expect_equal(find_fewest_coins(coins, target), expected)
+})
+
+test_that("Possible change without unit coins available", {
+  coins <- c(2, 5, 10, 20, 50)
+  target <- 21
+  expected <- c(2, 2, 2, 5, 10)
+  expect_equal(find_fewest_coins(coins, target), expected)
+})
+
+test_that("Another possible change without unit coins available", {
+  coins <- c(4, 5)
+  target <- 27
+  expected <- c(4, 4, 4, 5, 5, 5)
+  expect_equal(find_fewest_coins(coins, target), expected)
+})
+
+test_that("No coins make 0 change", {
+  coins <- c(1, 5, 10, 21, 25)
+  target <- 0
+  expected <- c()
+  expect_equal(find_fewest_coins(coins, target), expected)
+})
+
+test_that("Error testing for change smaller than the smallest of coins", {
+  coins <- c(5, 10)
+  target <- 3
+  expect_error(find_fewest_coins(coins, target))
+})
+
+test_that("Error if no combination can add up to target", {
+  coins <- c(5, 10)
+  target <- 94
+  expect_error(find_fewest_coins(coins, target))
+})
+
+test_that("Cannot find negative change values", {
+  coins <- c(1, 2, 5)
+  target <- -5
+  expect_error(find_fewest_coins(coins, target))
+})