Add variable-length-quantity exercise (#116)

config.json

@@ -370,6 +370,14 @@
         "prerequisites": [],
         "difficulty": 6
       },
+      {
+        "slug": "variable-length-quantity",
+        "name": "Variable Length Quantity",
+        "uuid": "52f28bf9-4da4-46bc-a293-2054106c6e55",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 6
+      },
       {
         "slug": "pythagorean-triplet",
         "name": "Pythagorean Triplet",

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": [
+    "keiravillekode"
+  ],
+  "files": {
+    "solution": [
+      "variable_length_quantity.s"
+    ],
+    "test": [
+      "variable_length_quantity_test.c"
+    ],
+    "example": [
+      ".meta/example.s"
+    ]
+  },
+  "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.s

@@ -0,0 +1,80 @@
+.text
+.globl encode
+.globl decode
+
+/* extern size_t encode(uint8_t *buffer, const uint32_t *integers, size_t integer_count); */
+encode:
+        lsl     x2, x2, #2              /* size of integers array, in bytes */
+        add     x2, x1, x2              /* end of integers */
+        mov     x3, x0                  /* start of output */
+
+.encode_loop:
+        cmp     x1, x2
+        beq     .encode_end
+
+        ldr     w4, [x1], #4
+        cmp     w4, #127
+        bls     .one
+
+        lsr     w5, w4, #7
+        cmp     w5, #127
+        bls     .two
+
+        lsr     w6, w5, #7
+        cmp     w6, #127
+        bls     .three
+
+        lsr     w7, w6, #7
+        cmp     w7, #127
+        bls     .four
+
+        lsr     w8, w7, #7
+        orr     w8, w8, #128
+        strb    w8, [x0], #1
+
+.four:
+        orr     w7, w7, #128
+        strb    w7, [x0], #1
+
+.three:
+        orr     w6, w6, #128
+        strb    w6, [x0], #1
+
+.two:
+        orr     w5, w5, #128
+        strb    w5, [x0], #1
+
+.one:
+        and     w4, w4, #127
+        strb    w4, [x0], #1
+        b       .encode_loop
+
+.encode_end:
+        sub     x0, x0, x3
+        ret
+
+/* extern size_t decode(uint32_t *buffer, const uint8_t *integers, size_t integer_count); */
+decode:
+        add     x2, x1, x2              /* end of integers array */
+        mov     x3, x0                  /* start of output */
+
+.loop:
+        mov     w4, wzr
+        cmp     x1, x2
+        beq     .end
+
+.read:
+        ldrb    w5, [x1], #1
+        lsl     w4, w4, #7
+        and     w6, w5, #127
+        orr     w4, w4, w6
+        tst     w5, #128
+        bne     .read
+
+        str     w4, [x0], #4
+        b       .loop
+
+.end:
+        sub     x0, x0, x3              /* length of output, in bytes */
+        lsr     x0, x0, #2              /* number of uint32_t values output */
+        ret

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

@@ -0,0 +1,90 @@
+# 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"
+include = false
+
+[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"
+include = false
+
+[a91e6f5a-c64a-48e3-8a75-ce1a81e0ebee]
+description = "Decode a series of bytes, producing a series of integers. -> multiple values"

exercises/practice/variable-length-quantity/Makefile

@@ -0,0 +1,36 @@
+AS = aarch64-linux-gnu-as
+CC = aarch64-linux-gnu-gcc
+
+CFLAGS = -g -Wall -Wextra -pedantic -Werror
+LDFLAGS =
+
+ALL_LDFLAGS = -pie -Wl,--fatal-warnings
+
+ALL_CFLAGS = -std=c99 -fPIE $(CFLAGS)
+ALL_LDFLAGS += $(LDFLAGS)
+
+C_OBJS = $(patsubst %.c,%.o,$(wildcard *.c))
+AS_OBJS = $(patsubst %.s,%.o,$(wildcard *.s))
+ALL_OBJS = $(filter-out example.o,$(C_OBJS) $(AS_OBJS) vendor/unity.o)
+
+CC_CMD = $(CC) $(ALL_CFLAGS) -c -o $@ $<
+
+all: tests
+	qemu-aarch64 -L /usr/aarch64-linux-gnu ./$<
+
+tests: $(ALL_OBJS)
+	@$(CC) $(ALL_CFLAGS) $(ALL_LDFLAGS) -o $@ $(ALL_OBJS)
+
+%.o: %.s
+	@$(AS) -o $@ $<
+
+%.o: %.c
+	@$(CC_CMD)
+
+vendor/unity.o: vendor/unity.c vendor/unity.h vendor/unity_internals.h
+	@$(CC_CMD)
+
+clean:
+	@rm -f *.o vendor/*.o tests
+
+.PHONY: all clean

exercises/practice/variable-length-quantity/variable_length_quantity.s

@@ -0,0 +1,9 @@
+.text
+.globl encode
+.globl decode
+
+encode:
+    ret
+
+decode:
+    ret