Add variable-length-quantity exercise (#401)

Author: frequenter-at-exercism

config.json

@@ -947,6 +947,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 6
+      },
+      {
+        "slug": "variable-length-quantity",
+        "name": "Variable Length Quantity",
+        "uuid": "4e1b9e0f-383b-4aea-9769-27882c98cffd",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 4
       }
     ]
   },

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/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "frequenter-at-exercism"
+  ],
+  "files": {
+    "solution": [
+      "variable-length-quantity.R"
+    ],
+    "test": [
+      "test_variable-length-quantity.R"
+    ],
+    "example": [
+      ".meta/example.R"
+    ]
+  },
+  "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/example.R

@@ -0,0 +1,31 @@
+# octicosahectimal?
+
+encode <- \(nn) {
+  nn |>
+    lapply(\(n) {
+      bytes <- n %% 128
+      while (n >= 128) {
+        n <- n %/% 128
+        bytes <- c(n %% 128 + 128, bytes)
+      }
+      bytes
+    }) |>
+    (\(seqs) do.call(c, seqs))()
+}
+
+decode <- \(allbytes) {
+  n <- length(allbytes)
+  ends <- which(allbytes < 128)
+  if (tail(ends, 1) != n) stop()
+  starts <- c(1, head(ends, -1) + 1)
+  1:length(ends) |>
+    sapply(\(i) {
+      bytes <- allbytes[starts[i]:ends[i]]
+      b128 <- bytes %% 128
+      val <- 0
+      for (d in b128) {
+        val <- 128 * val + d
+      }
+      val
+    })
+}

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

@@ -0,0 +1,103 @@
+# 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"
+
+[890bc344-cb80-45af-b316-6806a6971e81]
+description = "Encode a series of integers, producing a series of bytes. -> asymmetric 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"
+
+[4977d113-251b-4d10-a3ad-2f5a7756bb58]
+description = "Encode a series of integers, producing a series of bytes. -> asymmetric 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"
+
+[6731045f-1e00-4192-b5ae-98b22e17e9f7]
+description = "Encode a series of integers, producing a series of bytes. -> asymmetric 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"
+
+[b45ef770-cbba-48c2-bd3c-c6362679516e]
+description = "Encode a series of integers, producing a series of bytes. -> asymmetric 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"
+
+[9be46731-7cd5-415c-b960-48061cbc1154]
+description = "Encode a series of integers, producing a series of bytes. -> asymmetric 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/test_variable-length-quantity.R

@@ -0,0 +1,109 @@
+source("./variable-length-quantity.R")
+library(testthat)
+
+test_that("encode zero", {
+  expect_equal(encode(0), 0)
+})
+
+test_that("encode arbitrary single byte", {
+  expect_equal(encode(64), 64)
+})
+
+test_that("encode largest single byte", {
+  expect_equal(encode(127), 127)
+})
+
+test_that("encode smallest double byte", {
+  expect_equal(encode(128), c(129, 0))
+})
+
+test_that("encode arbitrary double byte", {
+  expect_equal(encode(8192), c(192, 0))
+})
+
+test_that("encode largest double byte", {
+  expect_equal(encode(16383), c(255, 127))
+})
+
+test_that("encode smallest triple byte", {
+  expect_equal(encode(16384), c(129, 128, 0))
+})
+
+test_that("encode arbitrary triple byte", {
+  expect_equal(encode(1048576), c(192, 128, 0))
+})
+
+test_that("encode largest triple byte", {
+  expect_equal(encode(2097151), c(255, 255, 127))
+})
+
+test_that("encode smallest quadruple byte", {
+  expect_equal(encode(2097152), c(129, 128, 128, 0))
+})
+
+test_that("encode arbitrary quadruple byte", {
+  expect_equal(encode(134217728), c(192, 128, 128, 0))
+})
+
+test_that("encode largest quadruple byte", {
+  expect_equal(encode(268435455), c(255, 255, 255, 127))
+})
+
+test_that("encode smallest quintuple byte", {
+  expect_equal(encode(268435456), c(129, 128, 128, 128, 0))
+})
+
+test_that("encode maximum 32-bit integer", {
+  expect_equal(encode(4294967295), c(143, 255, 255, 255, 127))
+})
+
+test_that("encode two single-byte values", {
+  expect_equal(encode(c(64, 127)), c(64, 127))
+})
+
+test_that("encode two multi-byte values", {
+  expect_equal(encode(c(16384, 1193046)), c(129, 128, 0, 200, 232, 86))
+})
+
+test_that("encode many multi-byte values", {
+  expect_equal(
+    encode(
+      c(8192, 1193046, 268435455, 0, 16383, 16384)),
+      c(192, 0, 200, 232, 86, 255, 255, 255, 127, 0, 255, 127, 129, 128, 0))
+})
+
+test_that("decode one byte", {
+  expect_equal(decode(127), 127)
+})
+
+test_that("decode two bytes", {
+  expect_equal(decode(c(192, 0)), 8192)
+})
+
+test_that("decode three bytes", {
+  expect_equal(decode(c(255, 255, 127)), 2097151)
+})
+
+test_that("decode four bytes", {
+  expect_equal(decode(c(129, 128, 128, 0)), 2097152)
+})
+
+test_that("decode maximum 32-bit integer", {
+  expect_equal(decode(c(143, 255, 255, 255, 127)), 4294967295)
+})
+
+test_that("decoding incomplete sequence causes error", {
+  expect_error(decode(c(255)))
+})
+
+test_that("decoding incomplete sequence causes error, even if value is zero", {
+  expect_error(decode(c(128)))
+})
+
+test_that("decode multiple values", {
+  expect_equal(
+    decode(
+      c(192, 0, 200, 232, 86, 255, 255, 255, 127, 0, 255, 127, 129, 128, 0)),
+      c(8192, 1193046, 268435455, 0, 16383, 16384))
+})
+

exercises/practice/variable-length-quantity/variable-length-quantity.R

@@ -0,0 +1,5 @@
+encode <- function(numbers) {
+}
+
+decode <- function(bytes) {
+}