1 files changed, 202 insertions, 0 deletions
diff --git a/2020/11/part2.c b/2020/11/part2.c
new file mode 100644
index 0000000..2443ddd
--- /dev/null
+++ b/2020/11/part2.c
@@ -0,0 +1,202 @@
+#include <stdio.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <malloc.h>
+#include <string.h>
+
+#define MAX_INPUT_WIDTH 128
+#define MAX_INPUT_HEIGTH 128
+#define MAX_INPUT MAX_INPUT_WIDTH * MAX_INPUT_HEIGTH
+
+int count_char(const char *source, char search)
+{
+    int oc = 0;
+    do {
+        oc += *source == search;
+    } while (*++source);
+
+    return oc;
+}
+
+void replace_char(char *destination, const char *source, char original, char replace)
+{
+    do {
+        if (*source == original) {
+            *destination = replace;
+        }
+        else {
+            *destination = *source;
+        }
+    } while (++destination && *++source);
+    *destination = 0;
+}
+
+char get_char(const char *source, size_t width, size_t y, size_t x) {
+    return source[y * width + x];
+}
+
+int sur_oc_seats(const char *source, size_t width, size_t height, size_t index_y, size_t index_x)
+{
+    int num = 0;
+    // Check upwards
+    size_t y = index_y;
+    size_t x = index_x;
+    while (--y < height) {
+        char c = get_char(source, width, y, x);
+        if (c == '.') {
+            continue;
+        }
+
+        num += c == '#';
+        break;
+    }
+
+    // Check downwards
+    y = index_y;
+    while (++y < height) {
+        char c = get_char(source, width, y, x);
+        if (c == '.') {
+            continue;
+        }
+
+        num += c == '#';
+        break;
+    }
+
+    // Check left
+    y = index_y;
+    x = index_x;
+    while (--x < width) {
+        char c = get_char(source, width, y, x);
+        if (c == '.') {
+            continue;
+        }
+
+        num += c == '#';
+        break;
+    }
+
+    // Check right
+    x = index_x;
+    while (++x < width) {
+        char c = get_char(source, width, y, x);
+        if (c == '.') {
+            continue;
+        }
+
+        num += c == '#';
+        break;
+    }
+
+    // Check left upwards
+    y = index_y;
+    x = index_x;
+    while (--y < height && --x < width) {
+        char c = get_char(source, width, y, x);
+        if (c == '.') {
+            continue;
+        }
+
+        num += c == '#';
+        break;
+    }
+
+    // Check left downwards
+    y = index_y;
+    x = index_x;
+    while (++y < height && --x < width) {
+        char c = get_char(source, width, y, x);
+        if (c == '.') {
+            continue;
+        }
+
+        num += c == '#';
+        break;
+    }
+
+    // Check right upwards
+    y = index_y;
+    x = index_x;
+    while (--y < height && ++x < width) {
+        char c = get_char(source, width, y, x);
+        if (c == '.') {
+            continue;
+        }
+
+        num += c == '#';
+        break;
+    }
+
+    // Check right downwards
+    y = index_y;
+    x = index_x;
+    while (++y < height && ++x < width ) {
+        char c = get_char(source, width, y, x);
+        if (c == '.') {
+            continue;
+        }
+
+        num += c == '#';
+        break;
+    }
+
+    return num;
+}
+
+int solve(const char *filename)
+{
+    FILE *file = fopen(filename, "r");
+
+    // Include space for newline and string terminator
+    char buffer[128] = { 0 };
+
+    char input[MAX_INPUT] = { 0 };
+    size_t height = 1;
+    fgets(buffer, 128, file);
+    size_t width = strchr(buffer, '\n') - buffer;
+    replace_char(input, buffer, 'L', '#');
+
+    while (fgets(buffer, 128, file)) {
+        // Round 1
+        replace_char(&input[height++ * width], buffer, 'L', '#');
+    }
+
+    fclose(file);
+
+    size_t size = height * width;
+
+    // Zero terminate
+    input[size] = 0;
+
+    char input2[MAX_INPUT] = { 0 };
+    do {
+        for (size_t i = 0; i < size; i++) {
+            if (input[i] == '.') {
+                input2[i] = '.';
+            }
+            else if (input[i] == 'L') {
+                if (sur_oc_seats(input, width, height, i / width, i % width) == 0) {
+                    input2[i] = '#';
+                }
+                else {
+                    input2[i] = 'L';
+                }
+            }
+            else if (input[i] == '#') {
+                if (sur_oc_seats(input, width, height, i / width, i % width) >= 5) {
+                    input2[i] = 'L';
+                }
+                else {
+                    input2[i] = '#';
+                }
+            }
+        }
+    } while (memcmp(input, input2, size) && memcpy(input, input2, size));
+
+    return count_char(input, '#');
+}
+
+int main(int argc, char *argv[])
+{
+    printf("%i\n", solve(argv[argc - 1]));
+}