Add atbash-cipher exercise (#80)

config.json

@@ -122,6 +122,14 @@
         "prerequisites": [],
         "difficulty": 3
       },
+      {
+        "slug": "atbash-cipher",
+        "name": "Atbash Cipher",
+        "uuid": "309d7449-4f2e-40cb-b38b-faedcf2b88bb",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 4
+      },
       {
         "slug": "isbn-verifier",
         "name": "ISBN Verifier",

exercises/practice/atbash-cipher/.docs/instructions.md

@@ -0,0 +1,27 @@
+# Instructions
+
+Create an implementation of the atbash cipher, an ancient encryption system created in the Middle East.
+
+The Atbash cipher is a simple substitution cipher that relies on transposing all the letters in the alphabet such that the resulting alphabet is backwards.
+The first letter is replaced with the last letter, the second with the second-last, and so on.
+
+An Atbash cipher for the Latin alphabet would be as follows:
+
+```text
+Plain:  abcdefghijklmnopqrstuvwxyz
+Cipher: zyxwvutsrqponmlkjihgfedcba
+```
+
+It is a very weak cipher because it only has one possible key, and it is a simple mono-alphabetic substitution cipher.
+However, this may not have been an issue in the cipher's time.
+
+Ciphertext is written out in groups of fixed length, the traditional group size being 5 letters, leaving numbers unchanged, and punctuation is excluded.
+This is to make it harder to guess things based on word boundaries.
+All text will be encoded as lowercase letters.
+
+## Examples
+
+- Encoding `test` gives `gvhg`
+- Encoding `x123 yes` gives `c123b vh`
+- Decoding `gvhg` gives `test`
+- Decoding `gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt` gives `thequickbrownfoxjumpsoverthelazydog`

exercises/practice/atbash-cipher/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "keiravillekode"
+  ],
+  "files": {
+    "solution": [
+      "atbash_cipher.s"
+    ],
+    "test": [
+      "atbash_cipher_test.c"
+    ],
+    "example": [
+      ".meta/example.s"
+    ]
+  },
+  "blurb": "Create an implementation of the atbash cipher, an ancient encryption system created in the Middle East.",
+  "source": "Wikipedia",
+  "source_url": "https://en.wikipedia.org/wiki/Atbash"
+}

exercises/practice/atbash-cipher/.meta/example.s

@@ -0,0 +1,51 @@
+.text
+.globl encode
+.globl decode
+
+/* extern void encode(char *buffer, const char *phrase); */
+encode:
+        mov     x2, #5                  /* no groups */
+        b       process
+        ret
+
+/* extern void decode(char *buffer, const char *phrase); */
+decode:
+        mov     x2, #-1                 /* no groups */
+        b       process
+
+process:
+        mov     x3, x2                  /* remaining letters in group */
+        mov     w6, 32                  /* space */
+        mov     w7, #122                /* 'z' */
+
+.read:
+        ldrb    w4, [x1], #1            /* load byte, post-increment */
+        cbz     w4, .end
+
+        sub     w5, w4, #48             /* '0' */
+        cmp     w5, #10
+        blo     .accept                 /* unsigned < */
+
+        orr     w5, w4, #32             /* force lower case */
+        sub     w5, w5, 97              /* 'a' */
+        cmp     w5, #26
+        bhs     .read                   /* unsigned >= */
+
+        sub     w5, w7, w5              /* 'z' - letter index */
+        and     w4, w4, #32
+        orr     w4, w4, w5              /* rotated letter, with original case */
+
+.accept:
+        cbnz    x3, .write
+
+        strb    w6, [x0], #1            /* store space, post-increment */
+        mov     x3, x2                  /* remaining letters in group */
+
+.write:
+        strb    w4, [x0], #1            /* store byte, post-increment */
+        sub     x3, x3, 1               /* decrement remaining letters in group */
+        b       .read
+
+.end:
+        strb    w4, [x0]                /* store byte */
+        ret

exercises/practice/atbash-cipher/.meta/tests.toml

@@ -0,0 +1,52 @@
+# 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.
+
+[2f47ebe1-eab9-4d6b-b3c6-627562a31c77]
+description = "encode -> encode yes"
+
+[b4ffe781-ea81-4b74-b268-cc58ba21c739]
+description = "encode -> encode no"
+
+[10e48927-24ab-4c4d-9d3f-3067724ace00]
+description = "encode -> encode OMG"
+
+[d59b8bc3-509a-4a9a-834c-6f501b98750b]
+description = "encode -> encode spaces"
+
+[31d44b11-81b7-4a94-8b43-4af6a2449429]
+description = "encode -> encode mindblowingly"
+
+[d503361a-1433-48c0-aae0-d41b5baa33ff]
+description = "encode -> encode numbers"
+
+[79c8a2d5-0772-42d4-b41b-531d0b5da926]
+description = "encode -> encode deep thought"
+
+[9ca13d23-d32a-4967-a1fd-6100b8742bab]
+description = "encode -> encode all the letters"
+
+[bb50e087-7fdf-48e7-9223-284fe7e69851]
+description = "decode -> decode exercism"
+
+[ac021097-cd5d-4717-8907-b0814b9e292c]
+description = "decode -> decode a sentence"
+
+[18729de3-de74-49b8-b68c-025eaf77f851]
+description = "decode -> decode numbers"
+
+[0f30325f-f53b-415d-ad3e-a7a4f63de034]
+description = "decode -> decode all the letters"
+
+[39640287-30c6-4c8c-9bac-9d613d1a5674]
+description = "decode -> decode with too many spaces"
+
+[b34edf13-34c0-49b5-aa21-0768928000d5]
+description = "decode -> decode with no spaces"

exercises/practice/atbash-cipher/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/atbash-cipher/atbash_cipher.s

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

exercises/practice/atbash-cipher/atbash_cipher_test.c

@@ -0,0 +1,143 @@
+#include "vendor/unity.h"
+
+#define BUFFER_SIZE 80
+
+extern void encode(char *buffer, const char *phrase);
+
+extern void decode(char *buffer, const char *phrase);
+
+void setUp(void) {
+}
+
+void tearDown(void) {
+}
+
+void test_encode_yes(void) {
+    char buffer[BUFFER_SIZE];
+
+    encode(buffer, "yes");
+    TEST_ASSERT_EQUAL_STRING("bvh", buffer);
+}
+
+void test_encode_no(void) {
+    TEST_IGNORE();
+    char buffer[BUFFER_SIZE];
+
+    encode(buffer, "no");
+    TEST_ASSERT_EQUAL_STRING("ml", buffer);
+}
+
+void test_encode_omg(void) {
+    TEST_IGNORE();
+    char buffer[BUFFER_SIZE];
+
+    encode(buffer, "OMG");
+    TEST_ASSERT_EQUAL_STRING("lnt", buffer);
+}
+
+void test_encode_spaces(void) {
+    TEST_IGNORE();
+    char buffer[BUFFER_SIZE];
+
+    encode(buffer, "O M G");
+    TEST_ASSERT_EQUAL_STRING("lnt", buffer);
+}
+
+void test_encode_mindblowingly(void) {
+    TEST_IGNORE();
+    char buffer[BUFFER_SIZE];
+
+    encode(buffer, "mindblowingly");
+    TEST_ASSERT_EQUAL_STRING("nrmwy oldrm tob", buffer);
+}
+
+void test_encode_numbers(void) {
+    TEST_IGNORE();
+    char buffer[BUFFER_SIZE];
+
+    encode(buffer, "Testing,1 2 3, testing.");
+    TEST_ASSERT_EQUAL_STRING("gvhgr mt123 gvhgr mt", buffer);
+}
+
+void test_encode_deep_thought(void) {
+    TEST_IGNORE();
+    char buffer[BUFFER_SIZE];
+
+    encode(buffer, "Truth is fiction.");
+    TEST_ASSERT_EQUAL_STRING("gifgs rhurx grlm", buffer);
+}
+
+void test_encode_all_the_letters(void) {
+    TEST_IGNORE();
+    char buffer[BUFFER_SIZE];
+
+    encode(buffer, "The quick brown fox jumps over the lazy dog.");
+    TEST_ASSERT_EQUAL_STRING("gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt", buffer);
+}
+
+void test_decode_exercism(void) {
+    TEST_IGNORE();
+    char buffer[BUFFER_SIZE];
+
+    decode(buffer, "vcvix rhn");
+    TEST_ASSERT_EQUAL_STRING("exercism", buffer);
+}
+
+void test_decode_a_sentence(void) {
+    TEST_IGNORE();
+    char buffer[BUFFER_SIZE];
+
+    decode(buffer, "zmlyh gzxov rhlug vmzhg vkkrm thglm v");
+    TEST_ASSERT_EQUAL_STRING("anobstacleisoftenasteppingstone", buffer);
+}
+
+void test_decode_numbers(void) {
+    TEST_IGNORE();
+    char buffer[BUFFER_SIZE];
+
+    decode(buffer, "gvhgr mt123 gvhgr mt");
+    TEST_ASSERT_EQUAL_STRING("testing123testing", buffer);
+}
+
+void test_decode_all_the_letters(void) {
+    TEST_IGNORE();
+    char buffer[BUFFER_SIZE];
+
+    decode(buffer, "gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt");
+    TEST_ASSERT_EQUAL_STRING("thequickbrownfoxjumpsoverthelazydog", buffer);
+}
+
+void test_decode_with_too_many_spaces(void) {
+    TEST_IGNORE();
+    char buffer[BUFFER_SIZE];
+
+    decode(buffer, "vc vix    r hn");
+    TEST_ASSERT_EQUAL_STRING("exercism", buffer);
+}
+
+void test_decode_with_no_spaces(void) {
+    TEST_IGNORE();
+    char buffer[BUFFER_SIZE];
+
+    decode(buffer, "zmlyhgzxovrhlugvmzhgvkkrmthglmv");
+    TEST_ASSERT_EQUAL_STRING("anobstacleisoftenasteppingstone", buffer);
+}
+
+int main(void) {
+    UNITY_BEGIN();
+    RUN_TEST(test_encode_yes);
+    RUN_TEST(test_encode_no);
+    RUN_TEST(test_encode_omg);
+    RUN_TEST(test_encode_spaces);
+    RUN_TEST(test_encode_mindblowingly);
+    RUN_TEST(test_encode_numbers);
+    RUN_TEST(test_encode_deep_thought);
+    RUN_TEST(test_encode_all_the_letters);
+    RUN_TEST(test_decode_exercism);
+    RUN_TEST(test_decode_a_sentence);
+    RUN_TEST(test_decode_numbers);
+    RUN_TEST(test_decode_all_the_letters);
+    RUN_TEST(test_decode_with_too_many_spaces);
+    RUN_TEST(test_decode_with_no_spaces);
+    return UNITY_END();
+}