2020 Day 23 solution in C

Author: Programator2

2020/23b.c

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+#include <stdio.h>
+
+struct node {
+  int n;
+  struct node *next;
+  struct node *prev;
+};
+
+struct node c[1000001];
+
+struct node *remove_three(struct node *start_node) {
+  struct node *left = start_node->prev;
+  struct node *last = start_node->next->next;
+  struct node *right = last->next;
+
+  left->next = right;
+  start_node->prev = NULL;
+
+  right->prev = left;
+  last->next = NULL;
+  return start_node;
+}
+
+void insert_three(struct node *start_node, struct node *where) {
+  struct node *left = where;
+  struct node *right = where->next;
+
+  left->next = start_node;
+  start_node->prev = left;
+
+  struct node *last = start_node->next->next;
+  last->next = right;
+  right->prev = last;
+}
+
+void insert_before(struct node *node, struct node *where) {
+  struct node *left = where->prev;
+
+  left->next = node;
+  node->prev = left;
+
+  node->next = where;
+  where->prev = node;
+}
+
+int main() {
+  int input[] = {3, 6, 8, 1, 9, 5, 7, 4, 2};
+  for (int i=1; i < 1000001; i++)
+    c[i].n = i;
+  for (int i=10; i < 1000001; i++) {
+    c[i].next = c + i + 1;
+    c[i].prev = c + i - 1;
+  }
+  c[10].prev = c + 1000000;
+  c[1000000].next = c + 10;
+  for (int i = 0; i < 9; i++) {
+    insert_before(&c[input[i]], c + 10);
+  }
+
+  struct node *current_cup = &c[3];
+  for (int i = 0; i < 10000000; i++) {
+    int destination = current_cup->n - 1;
+    if (destination < 1)
+      destination = 1000000;
+    struct node *three = remove_three(current_cup->next);
+    while (destination == three->n ||
+	   destination == three->next->n ||
+	   destination == three->next->next->n ||
+	   destination == current_cup->n) {
+      destination--;
+      if (destination < 1)
+	destination = 1000000;
+    }
+    insert_three(three, c + destination);
+    current_cup = current_cup->next;
+  }
+
+  printf("%lld\n", ((long long) c[1].next->n)*c[1].next->next->n);
+}

2020/Makefile

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+23b.exe: 23b.c
+	gcc -o 23b.exe -Wall -Wextra -std=c11 -g 23b.c