1 files changed, 169 insertions, 0 deletions
diff --git a/Advent-of-Code-2021/AOC-13/main.c b/Advent-of-Code-2021/AOC-13/main.c
new file mode 100644
index 0000000..876dae7
--- /dev/null
+++ b/Advent-of-Code-2021/AOC-13/main.c
@@ -0,0 +1,169 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <assert.h>
+
+#if 0
+#define PART_1
+#else
+#define PART_2
+#endif
+
+#if 0
+#define FILE_PATH "example.txt"
+#define RULES  2
+int WIDTH  = 11;
+int HEIGHT = 15;
+#else
+#define FILE_PATH "input.txt"
+#define RULES  12
+int WIDTH  = 1311;
+int HEIGHT = 895;
+#endif
+
+#define BOARD_SZ ((WIDTH)*(HEIGHT))
+
+int *board = NULL;
+int rules_val[RULES] = {0};
+int rules_axis[RULES] = {0}; // 0 is x; 1 is y
+
+void parse()
+{
+    FILE *fp = fopen(FILE_PATH, "r");
+    if(!fp) {
+        fprintf(stderr, "Could not open file: %s", FILE_PATH);
+        return;
+    }
+
+    board = malloc(BOARD_SZ*sizeof(int));
+
+    char line[256];
+    char *tok;
+    int read_rules = -1;
+    while(fgets(line, sizeof(line), fp) != NULL)
+    {
+        if(line[0] == '\n') {
+            read_rules = 0;
+        } else if(read_rules < 0) {
+            tok = strtok(line, ",");
+            int x = atoi(tok);
+            tok = strtok(NULL, ",");
+            int y = atoi(tok);
+
+            assert(strtok(NULL, ",") == NULL);
+            assert(x >= 0 && x < WIDTH);
+            assert(y >= 0 && y < HEIGHT);
+            board[y * WIDTH + x] = 1;
+        } else {
+            tok = strtok(line, "=");
+            rules_axis[read_rules] = (tok[11] == 'y');
+            tok = strtok(NULL, "=");
+            rules_val[read_rules] = atoi(tok);
+            read_rules++;
+            assert(strtok(NULL, "=") == NULL);
+        }
+    }
+
+    fclose(fp);
+}
+
+void fold(int rules_i)
+{
+    int *board_cpy = malloc(BOARD_SZ*sizeof(int));
+
+    memcpy(board_cpy, board, BOARD_SZ*sizeof(int));
+    free(board);
+
+    int old_width = WIDTH;
+    if(rules_axis[rules_i] == 0) WIDTH = (WIDTH-1)/2;
+    else                         HEIGHT = (HEIGHT-1)/2;
+
+    board = malloc(BOARD_SZ*sizeof(int));
+
+    for(int i = 0; i < HEIGHT; i++)
+    {
+        for(int j = 0; j < WIDTH; j++)
+        {
+            int index = i * WIDTH + j;
+            int bc_index = i * old_width + j;
+            int mirror;
+
+            if(rules_axis[rules_i] == 0) {
+                mirror = i * old_width + (2*rules_val[rules_i] - j);
+            } else {
+                mirror = ((2*rules_val[rules_i] - i)
+                    * old_width) + j;
+            }
+
+            board[index] =
+                board_cpy[bc_index] +
+                board_cpy[mirror];
+        }
+    }
+
+    free(board_cpy);
+}
+
+void print_board()
+{
+    for(int i = 0; i < HEIGHT; i++)
+    {
+        for(int j = 0; j < WIDTH; j++)
+        {
+            if(board[i * WIDTH + j] == 0)
+                putc('.', stdout);
+            else
+                putc('#', stdout);
+        }
+        putc('\n', stdout);
+    }
+}
+
+void print_rules()
+{
+    for(int i = 0; i < RULES; i++)
+    {
+        char axis;
+        if(rules_axis[i] == 0)  axis = 'x';
+        else                    axis = 'y';
+        printf("fold along %c=%d\n", axis, rules_val[i]);
+    }
+}
+
+void part_1()
+{
+    parse();
+    fold(0);
+
+    int visible = 0;
+    for(int i = 0; i < BOARD_SZ; i++)
+    {
+        if(board[i] > 0) visible++;
+    }
+
+    printf("%d dots are visible\n", visible);
+}
+
+void part_2()
+{
+    parse();
+    for(int i = 0; i < RULES; i++)
+        fold(i);
+
+    print_board();
+}
+
+int main(void)
+{
+
+#ifdef PART_1
+    part_1();
+#endif
+
+#ifdef PART_2
+    part_2();
+#endif
+
+    free(board);
+    return 0;
+}