day 13

day-13/main.c

@@ -0,0 +1,94 @@
+#include "aoc.h"
+#include <limits.h>
+#include <ctype.h>
+
+typedef struct Vec2i {
+    i64 x;
+    i64 y;
+} Vec2i;
+
+static Vec2i
+parse_numbers(str line, Arena *arena) {
+    bool x_set = false;
+    Vec2i result = {0};
+    for (isize i = 0; i < line.len; i++) {
+        if (isdigit(line.data[i])) {
+            isize end = i + 1;
+            while (end < line.len && isdigit(line.data[end])) {
+                end++;
+            }
+            str num_str = str_sub(line, i, end);
+            if (!x_set) {
+                result.x = (i32) parse_i64(num_str, *arena);
+                x_set = true;
+            }
+            else {
+                result.y = (i32) parse_i64(num_str, *arena);
+                return result;
+            }
+            i = end;
+        }
+    }
+    NOT_REACHABLE();
+}
+
+static bool 
+solve(Vec2i a, Vec2i b, Vec2i p, Vec2i *solution) {
+    i64 det = a.x * b.y - a.y * b.x;
+    ASSERT(det != 0);
+
+    i64 numerator_a = p.x * b.y - p.y * b.x;
+    i64 numerator_b = p.y * a.x - p.x * a.y;
+    bool solved = false;
+    if ((numerator_a % det == 0) && (numerator_b % det == 0)) {
+        i64 n_a = numerator_a / det;
+        i64 n_b = numerator_b / det;
+        if (n_a >= 0 && n_b >= 0) {
+            solution->x = n_a;
+            solution->y = n_b;
+            solved = true;
+        }
+    }
+    else {
+        // no integer solution
+    }
+    return solved;
+}
+
+int main(int argc, char **argv) {
+    Arena *arena = make_arena(Megabytes(1));
+    str input = read_file(arena, argv[1]);
+
+    str line;
+    i64 part_1 = 0;
+    i64 part_2 = 0;
+    while (input.len > 0) {
+        str a_line = str_next_token(&input, STR("\n"));
+        str b_line = str_next_token(&input, STR("\n"));
+        str prize_line = str_next_token(&input, STR("\n"));
+        str empty_line = str_next_token(&input, STR("\n"));
+        ASSERT(str_trim(empty_line).len == 0);
+
+        Vec2i a = parse_numbers(a_line, arena);
+        Vec2i b = parse_numbers(b_line, arena);
+        Vec2i p = parse_numbers(prize_line, arena);
+        Vec2i p_ = { p.x + 10000000000000ull, p.y + 10000000000000ull };
+
+#if 0
+        printf("a: %ld %ld\n", a.x, a.y);
+        printf("b: %ld %ld\n", b.x, b.y);
+        printf("p: %ld %ld\n", p.x, p.y);
+        printf("\n");
+#endif
+        Vec2i n;
+        if (solve(a, b, p, &n)) {
+            part_1 += n.x * 3 + n.y;
+        }
+
+        if (solve(a, b, p_, &n)) {
+            part_2 += n.x * 3 + n.y;
+        }
+    }
+    printf("%ld\n", part_1);
+    printf("%ld\n", part_2);
+}

day-13/test.txt

@@ -0,0 +1,15 @@
+Button A: X+94, Y+34
+Button B: X+22, Y+67
+Prize: X=8400, Y=5400
+
+Button A: X+26, Y+66
+Button B: X+67, Y+21
+Prize: X=12748, Y=12176
+
+Button A: X+17, Y+86
+Button B: X+84, Y+37
+Prize: X=7870, Y=6450
+
+Button A: X+69, Y+23
+Button B: X+27, Y+71
+Prize: X=18641, Y=10279