day 1

Makefile

@@ -0,0 +1,4 @@
+all: day1
+
+%: %.c
+	gcc-tree -o $@ -Wall -Werror -pedantic -D_GNU_SOURCE -std=gnu2y -O3 -g $<

day1.c

@@ -0,0 +1,93 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdcountof.h>
+
+char input[] = {
+#embed "day1_input"
+};
+
+char test[] = {
+#embed "day1_test"
+};
+
+unsigned grabnum(const char *s, const char **end) {
+    unsigned r = 0;
+    for (; *s >= '0' && *s <= '9'; s++)
+        r = r * 10 + (*s - '0');
+    *end = s;
+    return r;
+}
+
+unsigned long do_part1(size_t src_len, const char src[src_len]) {
+    const char *s = src;
+    unsigned long count = 0;
+    unsigned long pos = 1'000'000'000'050;
+    while (s < &src[src_len]) {
+        const char *ns = s + 1;
+        unsigned r = grabnum(ns, &ns);
+        if (*s == 'L') {
+            pos -= r;
+        } else {
+            pos += r;
+        }
+        count += pos % 100 == 0;
+        s = ns + 1;
+    }
+    return count;
+}
+
+unsigned do_part2(size_t src_len, const char src[src_len]) {
+    const char *s = src, *ns = s + 1;
+    unsigned count = 0;
+    unsigned pos = 50;
+    for (;s < &src[src_len]; s = ns + 1, ns = s + 1) {
+        unsigned r = grabnum(ns, &ns);
+        if (*s == 'L') {
+            if (pos == 0) pos = 100;
+            unsigned to_zero = pos;
+            if (r < to_zero) {
+                pos -= r;
+            } else {
+                unsigned over = r - to_zero;
+                unsigned rots = over / 100;
+                unsigned adjust = over % 100;
+                count += 1 + rots;
+                pos = (100 - adjust) % 100;
+            }
+        } else {
+            unsigned to_zero = 100 - pos;
+            if (r < to_zero) {
+                pos += r;
+            } else {
+                unsigned over = r - to_zero;
+                unsigned rots = over / 100;
+                unsigned adjust = over % 100;
+                count += 1 + rots;
+                pos = adjust;
+            }
+        }
+    }
+    return count;
+}
+
+int main() {
+    printf("PART 1 TEST: ");
+    if (unsigned long v = do_part1(countof(test), test); v != 3) {
+        printf("FAILED (got %lu, expected 3)\n", v);
+    } else {
+        printf("PASSED\n");
+    }
+
+    printf("PART 1 RESULT: %lu\n",
+        do_part1(countof(input), input));
+
+    printf("PART 2 TEST: ");
+    if (unsigned v = do_part2(countof(test), test); v != 6) {
+        printf("FAILED (got %u, expected 6)\n", v);
+    } else {
+        printf("PASSED\n");
+    }
+
+    printf("PART 2 RESULT: %u\n",
+        do_part2(countof(input), input));
+}