Add octal exercise

config.json

@@ -167,6 +167,14 @@
         "prerequisites": [],
         "difficulty": 2
       },
+      {
+        "slug": "octal",
+        "name": "Octal",
+        "uuid": "98f71971-66cd-48b0-8f54-dab6b1505086",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 2
+      },
       {
         "slug": "proverb",
         "name": "Proverb",

exercises/practice/octal/.docs/instructions.md

@@ -0,0 +1,45 @@
+# Instructions
+
+Convert an octal number, represented as a string (e.g. '1735263'), to its decimal equivalent using first principles (i.e. no, you may not use built-in or external libraries to accomplish the conversion).
+
+Implement octal to decimal conversion.
+Given an octal input string, your program should produce a decimal output.
+
+## Note
+
+- Implement the conversion yourself.
+  Do not use something else to perform the conversion for you.
+- Treat invalid input as octal 0.
+
+## About Octal (Base-8)
+
+Decimal is a base-10 system.
+
+A number 233 in base 10 notation can be understood
+as a linear combination of powers of 10:
+
+- The rightmost digit gets multiplied by 10^0 = 1
+- The next number gets multiplied by 10^1 = 10
+- ...
+- The nth number gets multiplied by 10^_(n-1)_.
+- All these values are summed.
+
+So:
+
+```text
+   233 # decimal
+ = 2*10^2 + 3*10^1 + 3*10^0
+ = 2*100  + 3*10   + 3*1
+```
+
+Octal is similar, but uses powers of 8 rather than powers of 10.
+
+So:
+
+```text
+   233 # octal
+ = 2*8^2 + 3*8^1 + 3*8^0
+ = 2*64  + 3*8   + 3*1
+ = 128   + 24    + 3
+ = 155
+```

exercises/practice/octal/.meta/Example.roc

@@ -0,0 +1,21 @@
+module [parse]
+
+parseOctalDigit = \char ->
+    when char is
+        c if c >= '0' && c <= '7' -> Ok (c - '0' |> Num.toU64)
+        _ -> Err InvalidNumStr
+
+parse : Str -> Result U64 _
+parse = \string ->
+    if string == "" then
+        Err InvalidNumStr
+        else
+
+    string
+        |> Str.toUtf8
+        |> List.walkTry 0 \number, char ->
+            octalDigit = parseOctalDigit? char
+            if number > 0x1fffffffffffffff then
+                Err InvalidNumStr
+            else
+                number |> Num.shiftLeftBy 3 |> Num.add octalDigit |> Ok

exercises/practice/octal/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "ageron"
+  ],
+  "files": {
+    "solution": [
+      "Octal.roc"
+    ],
+    "test": [
+      "octal-test.roc"
+    ],
+    "example": [
+      ".meta/Example.roc"
+    ]
+  },
+  "blurb": "Convert a octal number, represented as a string (e.g. '1735263'), to its decimal equivalent using first principles (i.e. no, you may not use built-in or external libraries to accomplish the conversion).",
+  "source": "All of Computer Science",
+  "source_url": "https://www.wolframalpha.com/examples/mathematics/numbers/base-conversions"
+}

exercises/practice/octal/Octal.roc

@@ -0,0 +1,5 @@
+module [parse]
+
+parse : Str -> Result U64 _
+parse = \string ->
+    crash "Please implement the 'parse' function"

exercises/practice/octal/octal-test.roc

@@ -0,0 +1,219 @@
+app [main] {
+    pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br",
+}
+
+main =
+    Task.ok {}
+
+import Octal exposing [parse]
+
+# Parse "0"
+expect
+    result = parse "0"
+    result == Ok 0
+
+# Parse "1"
+expect
+    result = parse "1"
+    result == Ok 1
+
+# Parse "2"
+expect
+    result = parse "2"
+    result == Ok 2
+
+# Parse "3"
+expect
+    result = parse "3"
+    result == Ok 3
+
+# Parse "4"
+expect
+    result = parse "4"
+    result == Ok 4
+
+# Parse "5"
+expect
+    result = parse "5"
+    result == Ok 5
+
+# Parse "6"
+expect
+    result = parse "6"
+    result == Ok 6
+
+# Parse "7"
+expect
+    result = parse "7"
+    result == Ok 7
+
+# Parse "10"
+expect
+    result = parse "10"
+    result == Ok 8
+
+# Parse "11"
+expect
+    result = parse "11"
+    result == Ok 9
+
+# Parse "12"
+expect
+    result = parse "12"
+    result == Ok 10
+
+# Parse "13"
+expect
+    result = parse "13"
+    result == Ok 11
+
+# Parse "14"
+expect
+    result = parse "14"
+    result == Ok 12
+
+# Parse "15"
+expect
+    result = parse "15"
+    result == Ok 13
+
+# Parse "16"
+expect
+    result = parse "16"
+    result == Ok 14
+
+# Parse "17"
+expect
+    result = parse "17"
+    result == Ok 15
+
+# Parse "20"
+expect
+    result = parse "20"
+    result == Ok 16
+
+# Parse "21"
+expect
+    result = parse "21"
+    result == Ok 17
+
+# Parse "22"
+expect
+    result = parse "22"
+    result == Ok 18
+
+# Parse "77"
+expect
+    result = parse "77"
+    result == Ok 63
+
+# Parse "100"
+expect
+    result = parse "100"
+    result == Ok 64
+
+# Parse "1234567654321"
+expect
+    result = parse "1234567654321"
+    result == Ok 89755965649
+
+# Parse "1777777777777777777777", the largest U64 value
+expect
+    result = parse "1777777777777777777777"
+    result == Ok 18446744073709551615
+
+# Ignore leading zeros in "00000"
+expect
+    result = parse "00000"
+    result == Ok 0
+
+# Ignore leading zeros in "00001"
+expect
+    result = parse "00001"
+    result == Ok 1
+
+# Ignore leading zeros in "00007"
+expect
+    result = parse "00007"
+    result == Ok 7
+
+# Ignore leading zeros in "000010"
+expect
+    result = parse "000010"
+    result == Ok 8
+
+# Ignore leading zeros in "000077"
+expect
+    result = parse "000077"
+    result == Ok 63
+
+# Ignore leading zeros in "000070000"
+expect
+    result = parse "000070000"
+    result == Ok 28672
+
+# Ignore leading zeros even before the largest U64 value
+expect
+    result = parse "00001777777777777777777777"
+    result == Ok 18446744073709551615
+
+# Empty strings are invalid
+expect
+    result = parse ""
+    result |> Result.isErr
+
+# A string with only spaces is invalid
+expect
+    result = parse "   "
+    result |> Result.isErr
+
+# Leading spaces are invalid
+expect
+    result = parse " 1234567"
+    result |> Result.isErr
+
+# Trailing spaces are invalid
+expect
+    result = parse "1234567 "
+    result |> Result.isErr
+
+# Spaces anywhere are invalid
+expect
+    result = parse "123 4567"
+    result |> Result.isErr
+
+# Invalid character in "1*234"
+expect
+    result = parse "1*234"
+    result |> Result.isErr
+
+# Invalid character in "12abc"
+expect
+    result = parse "12abc"
+    result |> Result.isErr
+
+# Invalid character in "12/34"
+expect
+    result = parse "12/34"
+    result |> Result.isErr
+
+# Invalid character in "1234\n"
+expect
+    result = parse "1234\n"
+    result |> Result.isErr
+
+# Invalid character in "-1234"
+expect
+    result = parse "-1234"
+    result |> Result.isErr
+
+# Invalid character in "+1234"
+expect
+    result = parse "+1234"
+    result |> Result.isErr
+
+# For numbers that don't fit in an U64, `parse` should return an error
+# instead of crashing
+expect
+    result = parse "2000000000000000000000"
+    result |> Result.isErr