Day20

day20/shuffle.c

@@ -0,0 +1,104 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+// Finally a task that is favoring C
+#define BUFFER_SIZE 64
+#define MAX_LINES 8192
+// MIX_COUNT and DECRYPT_KEY is 1 for Part 1
+#define MIX_COUNT 10
+#define DECRYPT_KEY 811589153
+
+int main() {
+    char buf[BUFFER_SIZE], *p, c;
+    memset(buf, 0, BUFFER_SIZE);
+    p = buf;
+    long long lines[MAX_LINES], line_count = 0;
+
+    while ((c = getchar()) != EOF) {
+        *p++ = c;
+        if (c == '\n') {
+            sscanf(buf, "%lli", &lines[line_count]);
+            lines[line_count] *= DECRYPT_KEY;
+            line_count++;
+            memset(buf, 0, BUFFER_SIZE);
+            p = buf;
+        }
+    }
+
+    // Create an array of pointers to keep track of the original order
+    long long **order = (long long**)malloc(line_count * sizeof(long long*));
+    for (int i = 0; i < line_count; i++) {
+        order[i] = &lines[i];
+    };
+    long long *head = &lines[line_count-1];
+
+    // Shuffle
+    for (int ll = 0; ll < MIX_COUNT; ll++) {
+        for (int i = 0; i < line_count; i++) {
+            // Remove element from its original position
+            long long element = *order[i];
+            long long *q = order[i];
+            while (q != head) {
+                // Shift elements back
+                *q = *(q+1);
+                q++;
+            }
+            // Shift pointers
+            for (int j = 0; j < line_count; j++) {
+                if (order[j] > order[i]) {
+                    order[j]--;
+                }
+            }
+
+
+            // Move where we need to insert
+            order[i] += element;
+            // Wrap around at the end
+            // I don't get this wrapping logic that's in the example
+            // it doesn't make any sense that I can not shift an element to the
+            // last position because it already wraps to the first element
+            if (order[i] > head) {
+                order[i] = &lines[(order[i] - head) % (line_count-1)];
+            } else if (order[i] <= lines) {
+                order[i] = head - (lines - order[i]) % (line_count-1);
+            }
+
+            // Push every element back at insertion point
+            q = head;
+            while (q >= order[i] + 1) {
+                // Shift elements
+                *q = *(q-1);
+                q--;
+            }
+            // Shift pointers
+            for (int j = 0; j < line_count; j++) {
+                if (order[j] >= order[i] && i != j) {
+                    order[j]++;
+                }
+            }
+
+            // Insert element
+            *order[i] = element;
+        }
+    }
+
+    // Find 0
+    long long *q = lines;
+    while (*q != 0) { q++; }
+
+    long long *th[3];
+
+    th[0] = q + 1000;
+    th[1] = q + 2000;
+    th[2] = q + 3000;
+
+    for (int i = 0; i < 3; i++) {
+        if (th[i] > head) {
+            th[i] = lines + ((th[i] - head) % line_count) - 1;
+        }
+    }
+
+    printf("%lli\n", *(th[0])+*(th[1])+*(th[2]));
+    free(order);
+}