Initial commit

2020/day09/day09.c

@@ -0,0 +1,98 @@
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <limits.h>
+
+bool in_state(unsigned int m, unsigned int state[m][m], unsigned int x) {
+  for (int i = 0; i < m; ++i) {
+    for (int j = i + 1; j < m; ++j) {
+      if (state[i][j] == x) {
+        return true;
+      }
+    }
+  }
+  return false;
+}
+
+void print_state(unsigned int m, unsigned int state[m][m]) {
+  for (int i = 0; i < m; ++i) {
+    for (int j = 0; j < m; ++j) {
+      printf("%u ", state[i][j]);
+    }
+    printf("\n");
+  }
+}
+
+void update_state(unsigned int m, unsigned int state[m][m], unsigned int *input,
+                  unsigned int k) {
+  for (int i = k - m +1; i < k; ++i) {
+    state[k % m][i % m] = input[i] + input[k];
+    state[i % m][k % m] = input[i] + input[k];
+  }
+}
+
+int main(int argc, char *argv[]) {
+  if (argc < 3) {
+    printf("Usage: %s <input file> <preamble size>\n", argv[0]);
+    return 0;
+  }
+
+  FILE *fp = fopen(argv[1], "r");
+  if (fp == NULL) {
+    printf("Could not open file %s\n", argv[1]);
+    return 1;
+  }
+
+  unsigned int m;
+  sscanf(argv[2], "%d", &m);
+
+  unsigned int input[1024] = {0};
+  unsigned int n = 0;
+  unsigned int d;
+  while (fscanf(fp, "%d\n", &d) == 1) {
+    input[n] = d;
+    n++;
+  }
+  fclose(fp);
+
+  // Part 1
+  unsigned int state[m][m];
+  for (int i = 0; i < m; ++i) {
+    for (int j = 0; j < m; ++j) {
+      if (i != j) {
+	state[i][j] = input[i] + input[j];
+      } else {
+	state[i][j] = 0;
+      }
+    }
+  }
+
+  unsigned int k = m;
+  while (in_state(m, state, input[k])) {
+    update_state(m, state, input, k);
+    k++;
+  }
+
+  printf("%u\n", input[k]);
+
+  // Part 2
+  unsigned int invalid_number = input[k];
+  for (int width = 2; width < n; ++width) {
+    for (int i = 0; i < n - width; ++i) {
+      unsigned int sum = 0;
+      unsigned int min = UINT_MAX;
+      unsigned int max = 0;
+      for (int j = 0; j < width; ++j) {
+	sum += input[i + j];
+	min = (input[i + j] < min) ? input[i + j] : min;
+	max = (input[i + j] > max) ? input[i + j] : max;
+      }
+      if (sum == invalid_number) {
+	printf("%u\n", min + max);
+	return 0;
+      }
+    }
+  }
+
+  return 0;
+}