Day 20: Add C solution

2 files changed, 220 insertions, 0 deletions

diff --git a/20/input.txt b/20/input.txt
new file mode 100644
index 0000000..c678ba5
--- /dev/null
+++ b/20/input.txt
@@ -0,0 +1,102 @@
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\ No newline at end of file
diff --git a/20/solution.c b/20/solution.c
new file mode 100644
index 0000000..f222fad
--- /dev/null
+++ b/20/solution.c
@@ -0,0 +1,118 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+static void pbm(char *front, size_t stride) {
+    printf("P1\n%lu %lu\n", stride, stride);
+
+    for (size_t i = 0; i < stride * stride; i++) {
+        putchar(front[i] ? '1' : '0');
+    }
+}
+
+int main(int argc, char **argv) {
+    int vis = 0, opt;
+    long long maxiter = 50;
+
+    while ((opt = getopt(argc, argv, "vi:")) != -1) {
+        switch (opt) {
+        case 'v':
+            vis = 1;
+            break;
+        case 'i':
+            maxiter = atoll(optarg);
+            break;
+        default:
+usage:
+            fprintf(stderr, "Usage: %s [-v] [-i iterations] input\n\n"
+                            "-v output is NetPBM, readable by FFmpeg\n",
+                    argv[0]);
+            return 1;
+        }
+    }
+
+    if (optind >= argc)
+        goto usage;
+
+    char *algo = NULL, *front = NULL, *back = NULL, *s;
+    size_t stride = 0, S = 0, G = 0;
+
+    FILE *fp = fopen(argv[optind], "r");
+    if(!fp) {
+        fprintf(stderr, "%s: Could not open file '%s': ",
+                argv[0], argv[optind]);
+        perror("");
+
+        return 1;
+    }
+
+
+    fscanf(fp, "%ms", &algo);
+
+    while (fscanf(fp, "%ms\n", &s) == 1) {
+        size_t len = strlen(s);
+        char *dst;
+
+        if (!stride) {
+            stride = len * 3;
+            front = calloc(stride * stride, 1);
+            dst = front + stride * len;
+        }
+
+        dst += len;
+        for (size_t c = 0; c < len; c++)
+            *dst++ = s[c] == '#';
+        dst += len;
+
+        free(s);
+    }
+
+    fclose(fp);
+
+    back = malloc(stride * stride);
+    memcpy(back, front, stride * stride);
+
+    for (long long iter = 0; iter < maxiter; iter++) {
+        if (vis && iter & 1) pbm(front, stride);
+
+        if (iter == 2) {
+            for (int p = 0; p < stride * stride; p++)
+                S += front[p];
+        }
+
+        #pragma omp parallel for
+        for (size_t y = 0; y < stride; y++) {
+            for (size_t x = 0; x < stride; x++) {
+                int v = 0;
+
+                for (int yy = -1; yy <= 1; yy++) {
+                    for (int xx = -1; xx <= 1; xx++) {
+                        v <<= 1;
+
+                        if (y + yy > 0 && y + yy < stride &&
+                            x + xx > 0 && x + xx < stride)
+                            v |= front[(y + yy) * stride + x + xx];
+                        else
+                            v |= iter & 1;
+
+                        back[y * stride + x] = algo[v] == '#';
+                    }
+                }
+            }
+        }
+
+        char *tmp=front;
+        front=back;
+        back=tmp;
+    }
+
+    for (size_t p = 0; p < stride * stride; p++)
+        G += front[p];
+
+    fprintf(stderr, "Silver: %lu\nGold: %lu\n", S, G);
+
+    free(front);
+    free(algo);
+    free(back);
+}