Added solution for 24

1 files changed, 76 insertions, 0 deletions

diff --git a/2023/24/24a.c b/2023/24/24a.c
new file mode 100644
index 0000000..6c75c08
--- /dev/null
+++ b/2023/24/24a.c
@@ -0,0 +1,76 @@
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#define N    300
+#define XMIN 2e14
+#define XMAX 4e14
+#define eps  1e-6
+
+#define ABS(x) ((x)>0?(x):-(x))
+
+typedef struct { double x, y, z; } point_t;
+typedef struct { point_t p, v; } line_t;
+
+char *buf, line[N];
+int n, s;
+line_t l[N];
+
+bool isnum(char c) { return (c >= '0' && c <= '9') || c == '-'; }
+
+line_t readl(char *buf) {
+        double a[6];
+        for (int i = 0; i < 6; i++) {
+                a[i] = (double)atoll(buf);
+                while (isnum(*buf)) buf++;
+                while (*buf != '\n' && !isnum(*buf)) buf++;
+        }
+        return (line_t) {
+            .p = (point_t){.x = a[0], .y = a[1], .z = a[2]},
+            .v = (point_t){.x = a[3], .y = a[4], .z = a[5]}
+        };
+}
+
+double det(double a, double b, double c, double d) { return a*d-b*c; }
+
+bool parallel2d(line_t l1, line_t l2) {
+        double d = det(l1.v.x, l2.v.x, l1.v.y, l2.v.y);
+        return ABS(d) < eps;
+}
+
+point_t collide2d(line_t l1, line_t l2, double *t1, double *t2) {
+        double a1 = l1.v.x;
+        double b1 = -l2.v.x;
+        double c1 = l2.p.x - l1.p.x;
+        double a2 = l1.v.y;
+        double b2 = -l2.v.y;
+        double c2 = l2.p.y - l1.p.y;
+        
+        double d = det(a1, b1, a2, b2);
+        *t1 = det(c1, b1, c2, b2) / d;
+        *t2 = det(a1, c1, a2, c2) / d;
+
+        double x = l1.p.x + l1.v.x * (*t1);
+        double y = l1.p.y + l1.v.y * (*t1);
+
+        return (point_t) {.x = x, .y = y};
+}
+
+int main() {
+        for (n = 0; (buf = fgets(line, N, stdin)) != NULL; n++)
+                l[n] = readl(buf);
+
+        for (int i = 0; i < n; i++) {
+                for (int j = i+1; j < n; j++) {
+                        if (parallel2d(l[i], l[j])) continue;
+                        double t1, t2;
+                        point_t p = collide2d(l[i], l[j], &t1, &t2);
+                        if (t1 < -eps || t2 < -eps) continue;
+                        s += p.x > XMIN-eps && p.x < XMAX+eps &&
+                             p.y > XMIN-eps && p.y < XMAX+eps;
+                }
+        }
+
+        printf("%d\n", s);
+        return 0;
+}