Add binary-search exercise (#93)

config.json

@@ -202,6 +202,14 @@
         "prerequisites": [],
         "difficulty": 5
       },
+      {
+        "slug": "binary-search",
+        "name": "Binary Search",
+        "uuid": "ee094601-1e0a-44dc-b885-a2caf92491f9",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5
+      },
       {
         "slug": "luhn",
         "name": "Luhn",

exercises/practice/binary-search/.docs/instructions.md

@@ -0,0 +1,29 @@
+# Instructions
+
+Your task is to implement a binary search algorithm.
+
+A binary search algorithm finds an item in a list by repeatedly splitting it in half, only keeping the half which contains the item we're looking for.
+It allows us to quickly narrow down the possible locations of our item until we find it, or until we've eliminated all possible locations.
+
+~~~~exercism/caution
+Binary search only works when a list has been sorted.
+~~~~
+
+The algorithm looks like this:
+
+- Find the middle element of a _sorted_ list and compare it with the item we're looking for.
+- If the middle element is our item, then we're done!
+- If the middle element is greater than our item, we can eliminate that element and all the elements **after** it.
+- If the middle element is less than our item, we can eliminate that element and all the elements **before** it.
+- If every element of the list has been eliminated then the item is not in the list.
+- Otherwise, repeat the process on the part of the list that has not been eliminated.
+
+Here's an example:
+
+Let's say we're looking for the number 23 in the following sorted list: `[4, 8, 12, 16, 23, 28, 32]`.
+
+- We start by comparing 23 with the middle element, 16.
+- Since 23 is greater than 16, we can eliminate the left half of the list, leaving us with `[23, 28, 32]`.
+- We then compare 23 with the new middle element, 28.
+- Since 23 is less than 28, we can eliminate the right half of the list: `[23]`.
+- We've found our item.

exercises/practice/binary-search/.docs/introduction.md

@@ -0,0 +1,13 @@
+# Introduction
+
+You have stumbled upon a group of mathematicians who are also singer-songwriters.
+They have written a song for each of their favorite numbers, and, as you can imagine, they have a lot of favorite numbers (like [0][zero] or [73][seventy-three] or [6174][kaprekars-constant]).
+
+You are curious to hear the song for your favorite number, but with so many songs to wade through, finding the right song could take a while.
+Fortunately, they have organized their songs in a playlist sorted by the title — which is simply the number that the song is about.
+
+You realize that you can use a binary search algorithm to quickly find a song given the title.
+
+[zero]: https://en.wikipedia.org/wiki/0
+[seventy-three]: https://en.wikipedia.org/wiki/73_(number)
+[kaprekars-constant]: https://en.wikipedia.org/wiki/6174_(number)

exercises/practice/binary-search/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "keiravillekode"
+  ],
+  "files": {
+    "solution": [
+      "binary_search.s"
+    ],
+    "test": [
+      "binary_search_test.c"
+    ],
+    "example": [
+      ".meta/example.s"
+    ]
+  },
+  "blurb": "Implement a binary search algorithm.",
+  "source": "Wikipedia",
+  "source_url": "https://en.wikipedia.org/wiki/Binary_search_algorithm"
+}

exercises/practice/binary-search/.meta/example.s

@@ -0,0 +1,37 @@
+.text
+.globl find
+
+/* extern ptrdiff_t find(int16_t value, int16_t *array, size_t count); */
+find:
+        mov     x3, #0                  /* lower bound, as byte offset */
+        lsl     x4, x2, #1              /* upper bound, as byte offset */
+
+.search:
+        cmp     x3, x4
+        beq     .absent
+
+        add     x5, x3, x4
+        lsr     x5, x5, #2              /* midpoint, as index */
+        lsl     x5, x5, #1              /* midpoint, as byte offset */
+        ldrh    w6, [x1, x5]
+
+        cmp     w6, w0
+        bgt     .earlier
+
+        cmp     w6, w0
+        blt     .later
+
+        lsr     x0, x5, #1              /* midpoint, as index */
+        ret
+
+.absent:
+        mov     x0, #-1
+        ret
+
+.earlier:
+        mov     x4, x5                  /* update upper bound */
+        b       .search
+
+.later:
+        add     x3, x5, #2              /* update lower bound */
+        b       .search

exercises/practice/binary-search/.meta/tests.toml

@@ -0,0 +1,43 @@
+# 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.
+
+[b55c24a9-a98d-4379-a08c-2adcf8ebeee8]
+description = "finds a value in an array with one element"
+
+[73469346-b0a0-4011-89bf-989e443d503d]
+description = "finds a value in the middle of an array"
+
+[327bc482-ab85-424e-a724-fb4658e66ddb]
+description = "finds a value at the beginning of an array"
+
+[f9f94b16-fe5e-472c-85ea-c513804c7d59]
+description = "finds a value at the end of an array"
+
+[f0068905-26e3-4342-856d-ad153cadb338]
+description = "finds a value in an array of odd length"
+
+[fc316b12-c8b3-4f5e-9e89-532b3389de8c]
+description = "finds a value in an array of even length"
+
+[da7db20a-354f-49f7-a6a1-650a54998aa6]
+description = "identifies that a value is not included in the array"
+
+[95d869ff-3daf-4c79-b622-6e805c675f97]
+description = "a value smaller than the array's smallest value is not found"
+
+[8b24ef45-6e51-4a94-9eac-c2bf38fdb0ba]
+description = "a value larger than the array's largest value is not found"
+
+[f439a0fa-cf42-4262-8ad1-64bf41ce566a]
+description = "nothing is found in an empty array"
+
+[2c353967-b56d-40b8-acff-ce43115eed64]
+description = "nothing is found when the left and right bounds cross"

exercises/practice/binary-search/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/binary-search/binary_search.s

@@ -0,0 +1,5 @@
+.text
+.globl find
+
+find:
+    ret

exercises/practice/binary-search/binary_search_test.c

@@ -0,0 +1,101 @@
+#include "vendor/unity.h"
+
+#include <stddef.h>
+#include <stdint.h>
+
+#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
+
+extern ptrdiff_t find(int16_t value, int16_t *array, size_t count);
+
+void setUp(void) {
+}
+
+void tearDown(void) {
+}
+
+void test_finds_a_value_in_an_array_with_one_element(void) {
+    int16_t array[] = {6};
+    TEST_ASSERT_EQUAL_INT(0, find(6, array, ARRAY_SIZE(array)));
+}
+
+void test_finds_a_value_in_the_middle_of_an_array(void) {
+    TEST_IGNORE();
+    int16_t array[] = {1, 3, 4, 6, 8, 9, 11};
+    TEST_ASSERT_EQUAL_INT(3, find(6, array, ARRAY_SIZE(array)));
+}
+
+void test_finds_a_value_at_the_beginning_of_an_array(void) {
+    TEST_IGNORE();
+    int16_t array[] = {1, 3, 4, 6, 8, 9, 11};
+    TEST_ASSERT_EQUAL_INT(0, find(1, array, ARRAY_SIZE(array)));
+}
+
+void test_finds_a_value_at_the_end_of_an_array(void) {
+    TEST_IGNORE();
+    int16_t array[] = {1, 3, 4, 6, 8, 9, 11};
+    TEST_ASSERT_EQUAL_INT(6, find(11, array, ARRAY_SIZE(array)));
+}
+
+void test_finds_a_value_in_an_array_of_odd_length(void) {
+    TEST_IGNORE();
+    int16_t array[] = {1, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 634};
+    TEST_ASSERT_EQUAL_INT(9, find(144, array, ARRAY_SIZE(array)));
+}
+
+void test_finds_a_value_in_an_array_of_even_length(void) {
+    TEST_IGNORE();
+    int16_t array[] = {1, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377};
+    TEST_ASSERT_EQUAL_INT(5, find(21, array, ARRAY_SIZE(array)));
+}
+
+void test_identifies_that_a_value_is_not_included_in_the_array(void) {
+    TEST_IGNORE();
+    int16_t array[] = {1, 3, 4, 6, 8, 9, 11};
+    TEST_ASSERT_EQUAL_INT(-1, find(7, array, ARRAY_SIZE(array)));
+}
+
+void test_a_value_smaller_than_the_arrays_smallest_value_is_not_found(void) {
+    TEST_IGNORE();
+    int16_t array[] = {1, 3, 4, 6, 8, 9, 11};
+    TEST_ASSERT_EQUAL_INT(-1, find(0, array, ARRAY_SIZE(array)));
+}
+
+void test_a_value_larger_than_the_arrays_largest_value_is_not_found(void) {
+    TEST_IGNORE();
+    int16_t array[] = {1, 3, 4, 6, 8, 9, 11};
+    TEST_ASSERT_EQUAL_INT(-1, find(13, array, ARRAY_SIZE(array)));
+}
+
+void test_nothing_is_found_in_an_empty_array(void) {
+    TEST_IGNORE();
+    TEST_ASSERT_EQUAL_INT(-1, find(1, NULL, 0));
+}
+
+void test_nothing_is_found_when_the_left_and_right_bounds_cross(void) {
+    TEST_IGNORE();
+    int16_t array[] = {1, 2};
+    TEST_ASSERT_EQUAL_INT(-1, find(0, array, ARRAY_SIZE(array)));
+}
+
+void test_five_digits(void) {
+    TEST_IGNORE();
+    int16_t array[] = {11638, 18389, 21454, 29416, 32039};
+    TEST_ASSERT_EQUAL_INT(2, find(21454, array, ARRAY_SIZE(array)));
+}
+
+int main(void) {
+    UNITY_BEGIN();
+    RUN_TEST(test_finds_a_value_in_an_array_with_one_element);
+    RUN_TEST(test_finds_a_value_in_the_middle_of_an_array);
+    RUN_TEST(test_finds_a_value_at_the_beginning_of_an_array);
+    RUN_TEST(test_finds_a_value_at_the_end_of_an_array);
+    RUN_TEST(test_finds_a_value_in_an_array_of_odd_length);
+    RUN_TEST(test_finds_a_value_in_an_array_of_even_length);
+    RUN_TEST(test_identifies_that_a_value_is_not_included_in_the_array);
+    RUN_TEST(test_a_value_smaller_than_the_arrays_smallest_value_is_not_found);
+    RUN_TEST(test_a_value_larger_than_the_arrays_largest_value_is_not_found);
+    RUN_TEST(test_nothing_is_found_in_an_empty_array);
+    RUN_TEST(test_nothing_is_found_when_the_left_and_right_bounds_cross);
+    RUN_TEST(test_five_digits);
+    return UNITY_END();
+}