Add luhn exercise (#269)

config.json

@@ -333,6 +333,17 @@
           "strings"
         ]
       },
+      {
+        "slug": "luhn",
+        "name": "Luhn",
+        "uuid": "18e982ac-3e1e-497b-bfa0-04209223bb30",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 4,
+        "topics": [
+          "strings"
+        ]
+      },
       {
         "slug": "pig-latin",
         "name": "Pig Latin",

exercises/practice/luhn/.docs/instructions.md

@@ -0,0 +1,64 @@
+# Instructions
+
+Given a number determine whether or not it is valid per the Luhn formula.
+
+The [Luhn algorithm][luhn] is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers and Canadian Social Insurance Numbers.
+
+The task is to check if a given string is valid.
+
+## Validating a Number
+
+Strings of length 1 or less are not valid.
+Spaces are allowed in the input, but they should be stripped before checking.
+All other non-digit characters are disallowed.
+
+### Example 1: valid credit card number
+
+```text
+4539 3195 0343 6467
+```
+
+The first step of the Luhn algorithm is to double every second digit, starting from the right.
+We will be doubling
+
+```text
+4_3_ 3_9_ 0_4_ 6_6_
+```
+
+If doubling the number results in a number greater than 9 then subtract 9 from the product.
+The results of our doubling:
+
+```text
+8569 6195 0383 3437
+```
+
+Then sum all of the digits:
+
+```text
+8+5+6+9+6+1+9+5+0+3+8+3+3+4+3+7 = 80
+```
+
+If the sum is evenly divisible by 10, then the number is valid.
+This number is valid!
+
+### Example 2: invalid credit card number
+
+```text
+8273 1232 7352 0569
+```
+
+Double the second digits, starting from the right
+
+```text
+7253 2262 5312 0539
+```
+
+Sum the digits
+
+```text
+7+2+5+3+2+2+6+2+5+3+1+2+0+5+3+9 = 57
+```
+
+57 is not evenly divisible by 10, so this number is not valid.
+
+[luhn]: https://en.wikipedia.org/wiki/Luhn_algorithm

exercises/practice/luhn/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "keiravillekode"
+  ],
+  "files": {
+    "solution": [
+      "luhn.sml"
+    ],
+    "test": [
+      "test.sml"
+    ],
+    "example": [
+      ".meta/example.sml"
+    ]
+  },
+  "blurb": "Given a number determine whether or not it is valid per the Luhn formula.",
+  "source": "The Luhn Algorithm on Wikipedia",
+  "source_url": "https://en.wikipedia.org/wiki/Luhn_algorithm"
+}

exercises/practice/luhn/.meta/example.sml

@@ -0,0 +1,21 @@
+local
+  fun contribution (digit: char, count: int): int =
+    let
+      val number = ord digit - ord #"0"
+    in
+      if count mod 2 = 0 then number
+      else if 2 * number > 9 then 2 * number - 9
+      else 2 * number
+    end
+
+  fun recurse (count: int) (total: int) (l: char list): bool =
+    case l of
+        nil => count > 1 andalso total mod 10 = 0
+      | #" " :: rest => recurse count total rest
+      | first :: rest =>
+          if Char.isDigit first then recurse (count + 1) (total + contribution (first, count)) rest
+          else false
+in
+  val valid: string -> bool =
+    recurse 0 0 o rev o explode
+end

exercises/practice/luhn/.meta/tests.toml

@@ -0,0 +1,76 @@
+# 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.
+
+[792a7082-feb7-48c7-b88b-bbfec160865e]
+description = "single digit strings can not be valid"
+
+[698a7924-64d4-4d89-8daa-32e1aadc271e]
+description = "a single zero is invalid"
+
+[73c2f62b-9b10-4c9f-9a04-83cee7367965]
+description = "a simple valid SIN that remains valid if reversed"
+
+[9369092e-b095-439f-948d-498bd076be11]
+description = "a simple valid SIN that becomes invalid if reversed"
+
+[8f9f2350-1faf-4008-ba84-85cbb93ffeca]
+description = "a valid Canadian SIN"
+
+[1cdcf269-6560-44fc-91f6-5819a7548737]
+description = "invalid Canadian SIN"
+
+[656c48c1-34e8-4e60-9a5a-aad8a367810a]
+description = "invalid credit card"
+
+[20e67fad-2121-43ed-99a8-14b5b856adb9]
+description = "invalid long number with an even remainder"
+
+[7e7c9fc1-d994-457c-811e-d390d52fba5e]
+description = "invalid long number with a remainder divisible by 5"
+
+[ad2a0c5f-84ed-4e5b-95da-6011d6f4f0aa]
+description = "valid number with an even number of digits"
+
+[ef081c06-a41f-4761-8492-385e13c8202d]
+description = "valid number with an odd number of spaces"
+
+[bef66f64-6100-4cbb-8f94-4c9713c5e5b2]
+description = "valid strings with a non-digit added at the end become invalid"
+
+[2177e225-9ce7-40f6-b55d-fa420e62938e]
+description = "valid strings with punctuation included become invalid"
+
+[ebf04f27-9698-45e1-9afe-7e0851d0fe8d]
+description = "valid strings with symbols included become invalid"
+
+[08195c5e-ce7f-422c-a5eb-3e45fece68ba]
+description = "single zero with space is invalid"
+
+[12e63a3c-f866-4a79-8c14-b359fc386091]
+description = "more than a single zero is valid"
+
+[ab56fa80-5de8-4735-8a4a-14dae588663e]
+description = "input digit 9 is correctly converted to output digit 9"
+
+[b9887ee8-8337-46c5-bc45-3bcab51bc36f]
+description = "very long input is valid"
+
+[8a7c0e24-85ea-4154-9cf1-c2db90eabc08]
+description = "valid luhn with an odd number of digits and non zero first digit"
+
+[39a06a5a-5bad-4e0f-b215-b042d46209b1]
+description = "using ascii value for non-doubled non-digit isn't allowed"
+
+[f94cf191-a62f-4868-bc72-7253114aa157]
+description = "using ascii value for doubled non-digit isn't allowed"
+
+[8b72ad26-c8be-49a2-b99c-bcc3bf631b33]
+description = "non-numeric, non-space char in the middle with a sum that's divisible by 10 isn't allowed"

exercises/practice/luhn/luhn.sml

@@ -0,0 +1,2 @@
+fun valid (value: string): bool =
+  raise Fail "'valid' is not implemented"

exercises/practice/luhn/test.sml

@@ -0,0 +1,78 @@
+(* version 1.0.0 *)
+
+use "testlib.sml";
+use "luhn.sml";
+
+infixr |>
+fun x |> f = f x
+
+val testsuite =
+  describe "luhn" [
+    test "single digit strings can not be valid"
+      (fn _ => valid "1" |> Expect.falsy),
+
+    test "a single zero is invalid"
+      (fn _ => valid "0" |> Expect.falsy),
+
+    test "a simple valid SIN that remains valid if reversed"
+      (fn _ => valid "059" |> Expect.truthy),
+
+    test "a simple valid SIN that becomes invalid if reversed"
+      (fn _ => valid "59" |> Expect.truthy),
+
+    test "a valid Canadian SIN"
+      (fn _ => valid "055 444 285" |> Expect.truthy),
+
+    test "invalid Canadian SIN"
+      (fn _ => valid "055 444 286" |> Expect.falsy),
+
+    test "invalid credit card"
+      (fn _ => valid "8273 1232 7352 0569" |> Expect.falsy),
+
+    test "invalid long number with an even remainder"
+      (fn _ => valid "1 2345 6789 1234 5678 9012" |> Expect.falsy),
+
+    test "invalid long number with a remainder divisible by 5"
+      (fn _ => valid "1 2345 6789 1234 5678 9013" |> Expect.falsy),
+
+    test "valid number with an even number of digits"
+      (fn _ => valid "095 245 88" |> Expect.truthy),
+
+    test "valid number with an odd number of spaces"
+      (fn _ => valid "234 567 891 234" |> Expect.truthy),
+
+    test "valid strings with a non-digit added at the end become invalid"
+      (fn _ => valid "059a" |> Expect.falsy),
+
+    test "valid strings with punctuation included become invalid"
+      (fn _ => valid "055-444-285" |> Expect.falsy),
+
+    test "valid strings with symbols included become invalid"
+      (fn _ => valid "055# 444$ 285" |> Expect.falsy),
+
+    test "single zero with space is invalid"
+      (fn _ => valid " 0" |> Expect.falsy),
+
+    test "more than a single zero is valid"
+      (fn _ => valid "0000 0" |> Expect.truthy),
+
+    test "input digit 9 is correctly converted to output digit 9"
+      (fn _ => valid "091" |> Expect.truthy),
+
+    test "very long input is valid"
+      (fn _ => valid "9999999999 9999999999 9999999999 9999999999" |> Expect.truthy),
+
+    test "valid luhn with an odd number of digits and non zero first digit"
+      (fn _ => valid "109" |> Expect.truthy),
+
+    test "using ascii value for non-doubled non-digit isn't allowed"
+      (fn _ => valid "055b 444 285" |> Expect.falsy),
+
+    test "using ascii value for doubled non-digit isn't allowed"
+      (fn _ => valid ":9" |> Expect.falsy),
+
+    test "non-numeric, non-space char in the middle with a sum that's divisible by 10 isn't allowed"
+      (fn _ => valid "59%59" |> Expect.falsy)
+  ]
+
+val _ = Test.run testsuite