Add variable-length-quantity exercise

config.json

@@ -595,6 +595,14 @@
         "prerequisites": [],
         "difficulty": 4
       },
+      {
+        "slug": "variable-length-quantity",
+        "name": "Variable Length Quantity",
+        "uuid": "3d6cb550-b77f-44f3-a617-1243a4a52dcb",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 4
+      },
       {
         "slug": "all-your-base",
         "name": "All Your Base",

exercises/practice/variable-length-quantity/.docs/instructions.md

@@ -0,0 +1,34 @@
+# Instructions
+
+Implement variable length quantity encoding and decoding.
+
+The goal of this exercise is to implement [VLQ][vlq] encoding/decoding.
+
+In short, the goal of this encoding is to encode integer values in a way that would save bytes.
+Only the first 7 bits of each byte are significant (right-justified; sort of like an ASCII byte).
+So, if you have a 32-bit value, you have to unpack it into a series of 7-bit bytes.
+Of course, you will have a variable number of bytes depending upon your integer.
+To indicate which is the last byte of the series, you leave bit #7 clear.
+In all of the preceding bytes, you set bit #7.
+
+So, if an integer is between `0-127`, it can be represented as one byte.
+Although VLQ can deal with numbers of arbitrary sizes, for this exercise we will restrict ourselves to only numbers that fit in a 32-bit unsigned integer.
+Here are examples of integers as 32-bit values, and the variable length quantities that they translate to:
+
+```text
+ NUMBER        VARIABLE QUANTITY
+00000000              00
+00000040              40
+0000007F              7F
+00000080             81 00
+00002000             C0 00
+00003FFF             FF 7F
+00004000           81 80 00
+00100000           C0 80 00
+001FFFFF           FF FF 7F
+00200000          81 80 80 00
+08000000          C0 80 80 00
+0FFFFFFF          FF FF FF 7F
+```
+
+[vlq]: https://en.wikipedia.org/wiki/Variable-length_quantity

exercises/practice/variable-length-quantity/.meta/Example.roc

@@ -0,0 +1,40 @@
+module [encode, decode]
+
+encode : List U32 -> List U8
+encode = \integers ->
+    integers |> List.joinMap encodeInteger
+
+encodeInteger : U32 -> List U8
+encodeInteger = \integer ->
+    help = \bytes, n ->
+        if n == 0 then
+            bytes
+        else
+            nextN = n // 128 # same as n |> Num.shiftRightZfBy 7
+            last7Bits = n % 128 |> Num.toU8 # same as n |> Num.bitwiseAnd 0b1111111 |> Num.toU8
+            byte =
+                if bytes == [] then
+                    last7Bits
+                else
+                    last7Bits + 128 # same as last7Bits |> Num.bitwiseOr 0b10000000
+            help (bytes |> List.append byte) nextN
+
+    if integer == 0 then [0] else help [] integer |> List.reverse
+
+decode : List U8 -> Result (List U32) [IncompleteSequence]
+decode = \bytes ->
+    when bytes is
+        [] -> Err IncompleteSequence
+        [.., last] if last >= 128 -> Err IncompleteSequence
+        _ ->
+            bytes
+            |> List.walk { integers: [], integer: 0 } \state, byte ->
+                last7Bits = byte % 128
+                integer = state.integer * 128 + Num.toU32 last7Bits
+                if byte >= 128 then
+                    { state & integer }
+                else
+                    { integers: state.integers |> List.append integer, integer: 0 }
+            |> .integers
+            |> Ok
+

exercises/practice/variable-length-quantity/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "ageron"
+  ],
+  "files": {
+    "solution": [
+      "VariableLengthQuantity.roc"
+    ],
+    "test": [
+      "variable-length-quantity-test.roc"
+    ],
+    "example": [
+      ".meta/Example.roc"
+    ]
+  },
+  "blurb": "Implement variable length quantity encoding and decoding.",
+  "source": "A poor Splice developer having to implement MIDI encoding/decoding.",
+  "source_url": "https://splice.com"
+}

exercises/practice/variable-length-quantity/.meta/template.j2

@@ -0,0 +1,33 @@
+{%- import "generator_macros.j2" as macros with context -%}
+{{ macros.canonical_ref() }}
+{{ macros.header() }}
+
+import {{ exercise | to_pascal }} exposing [encode, decode]
+
+{% for supercase in cases %}
+##
+## {{ supercase["description"] }}
+##
+
+{% for case in supercase["cases"] -%}
+# {{ case["description"] }}
+{%- if case["property"] == "encode" %}
+expect
+    integers = {{ case["input"]["integers"] | to_roc }} 
+    result = encode integers
+    expected = {{ case["expected"] | to_roc }} 
+    result == expected
+{%- else %}
+expect
+    bytes = {{ case["input"]["integers"] | to_roc }} 
+    result = decode bytes
+    {%- if case["expected"]["error"] %}
+    result |> Result.isErr
+    {%- else %}
+    expected = Ok {{ case["expected"] | to_roc }} 
+    result == expected
+    {%- endif %}
+{%- endif %}
+
+{% endfor %}
+{% endfor %}

exercises/practice/variable-length-quantity/.meta/tests.toml

@@ -0,0 +1,88 @@
+# 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.
+
+[35c9db2e-f781-4c52-b73b-8e76427defd0]
+description = "Encode a series of integers, producing a series of bytes. -> zero"
+
+[be44d299-a151-4604-a10e-d4b867f41540]
+description = "Encode a series of integers, producing a series of bytes. -> arbitrary single byte"
+
+[ea399615-d274-4af6-bbef-a1c23c9e1346]
+description = "Encode a series of integers, producing a series of bytes. -> largest single byte"
+
+[77b07086-bd3f-4882-8476-8dcafee79b1c]
+description = "Encode a series of integers, producing a series of bytes. -> smallest double byte"
+
+[63955a49-2690-4e22-a556-0040648d6b2d]
+description = "Encode a series of integers, producing a series of bytes. -> arbitrary double byte"
+
+[29da7031-0067-43d3-83a7-4f14b29ed97a]
+description = "Encode a series of integers, producing a series of bytes. -> largest double byte"
+
+[3345d2e3-79a9-4999-869e-d4856e3a8e01]
+description = "Encode a series of integers, producing a series of bytes. -> smallest triple byte"
+
+[5df0bc2d-2a57-4300-a653-a75ee4bd0bee]
+description = "Encode a series of integers, producing a series of bytes. -> arbitrary triple byte"
+
+[f51d8539-312d-4db1-945c-250222c6aa22]
+description = "Encode a series of integers, producing a series of bytes. -> largest triple byte"
+
+[da78228b-544f-47b7-8bfe-d16b35bbe570]
+description = "Encode a series of integers, producing a series of bytes. -> smallest quadruple byte"
+
+[11ed3469-a933-46f1-996f-2231e05d7bb6]
+description = "Encode a series of integers, producing a series of bytes. -> arbitrary quadruple byte"
+
+[d5f3f3c3-e0f1-4e7f-aad0-18a44f223d1c]
+description = "Encode a series of integers, producing a series of bytes. -> largest quadruple byte"
+
+[91a18b33-24e7-4bfb-bbca-eca78ff4fc47]
+description = "Encode a series of integers, producing a series of bytes. -> smallest quintuple byte"
+
+[5f34ff12-2952-4669-95fe-2d11b693d331]
+description = "Encode a series of integers, producing a series of bytes. -> arbitrary quintuple byte"
+
+[7489694b-88c3-4078-9864-6fe802411009]
+description = "Encode a series of integers, producing a series of bytes. -> maximum 32-bit integer input"
+
+[f9b91821-cada-4a73-9421-3c81d6ff3661]
+description = "Encode a series of integers, producing a series of bytes. -> two single-byte values"
+
+[68694449-25d2-4974-ba75-fa7bb36db212]
+description = "Encode a series of integers, producing a series of bytes. -> two multi-byte values"
+
+[51a06b5c-de1b-4487-9a50-9db1b8930d85]
+description = "Encode a series of integers, producing a series of bytes. -> many multi-byte values"
+
+[baa73993-4514-4915-bac0-f7f585e0e59a]
+description = "Decode a series of bytes, producing a series of integers. -> one byte"
+
+[72e94369-29f9-46f2-8c95-6c5b7a595aee]
+description = "Decode a series of bytes, producing a series of integers. -> two bytes"
+
+[df5a44c4-56f7-464e-a997-1db5f63ce691]
+description = "Decode a series of bytes, producing a series of integers. -> three bytes"
+
+[1bb58684-f2dc-450a-8406-1f3452aa1947]
+description = "Decode a series of bytes, producing a series of integers. -> four bytes"
+
+[cecd5233-49f1-4dd1-a41a-9840a40f09cd]
+description = "Decode a series of bytes, producing a series of integers. -> maximum 32-bit integer"
+
+[e7d74ba3-8b8e-4bcb-858d-d08302e15695]
+description = "Decode a series of bytes, producing a series of integers. -> incomplete sequence causes error"
+
+[aa378291-9043-4724-bc53-aca1b4a3fcb6]
+description = "Decode a series of bytes, producing a series of integers. -> incomplete sequence causes error, even if value is zero"
+
+[a91e6f5a-c64a-48e3-8a75-ce1a81e0ebee]
+description = "Decode a series of bytes, producing a series of integers. -> multiple values"

exercises/practice/variable-length-quantity/VariableLengthQuantity.roc

@@ -0,0 +1,9 @@
+module [encode, decode]
+
+encode : List U32 -> List U8
+encode = \integers ->
+    crash "Please implement the 'encode' function"
+
+decode : List U8 -> Result (List U32) _
+decode = \bytes ->
+    crash "Please implement the 'decode' function"