Day 2

Makefile

@@ -2,7 +2,7 @@ CC ?= clang
 CFLAGS = -O0 -g -Wall -Wextra
 
 .PHONY: all
-all: day_1
+all: day_1 day_2
 
 clean:
 	rm -rf *.o
@@ -14,3 +14,7 @@ clean:
 day_1: day_1.o
 	$(CC) -o $@ $^
 	./day_1 day_1.in
+
+day_2: day_2.o
+	$(CC) -o $@ $^
+	./day_2 day_2.in

day_2.c

@@ -0,0 +1,85 @@
+#include "stdlib.h"
+#include "stdio.h"
+#include "string.h"
+
+int numbers[2048*2048];
+int sizes[2048];
+
+int _is_safe(int n, int size, int* levels) {
+  if(size <= 0) return 0;
+
+  int current = -1;
+  int direction = 0;
+
+  for(int i = 0; i < size; i++) {
+    if(i == n) continue;
+    int next = levels[i];
+    if(current == -1) {
+      current = next;
+    } else if(direction == 0 && (next < current && next >= current - 3)) {
+      direction = 1;
+    } else if(direction == 0 && (next > current && next <= current + 3)) {
+      direction = 2;
+    } else if(direction == 0) {
+      return 0;
+    } else if(direction == 1 && (next >= current || next < current - 3)) {
+      return 0;
+    } else if(direction == 2 && (next <= current || next > current + 3)) {
+      return 0;
+    }
+
+    current = next;
+  }
+
+  return 1;
+}
+
+int is_safe(int size, int* levels) {
+  for(int i = -1; i < size; i++) {
+    if(_is_safe(i, size, levels)) return 1;
+  }
+  return 0;
+}
+
+int main(int argc, char* argv[]) {
+  if(argc < 2) {
+    fprintf(stderr, "Filename required\n");
+    return 1;
+  }
+
+  FILE* file = fopen(argv[1], "r");
+  if(file == NULL) {
+    fprintf(stderr, "Could not open %s\n", argv[1]);
+    return 2;
+  }
+  
+  memset(numbers, 0, sizeof(numbers));
+  char* line = NULL;
+  size_t line_size = 0;
+  int i = 0;
+  while(getline(&line, &line_size, file) > 0) {
+    char* save = NULL;
+    char* token = strtok_r(line, " ", &save);
+    if(token == NULL || token[0] == '\n' || token[0] == '\0') break;
+    int j = 0;
+    do {
+      numbers[i*2048 + j] = atoi(token);
+      j += 1;
+    } while((token = strtok_r(NULL, " ", &save)));
+    sizes[i] = j;
+    i += 1;
+  }
+  free(line);
+
+  int safe = 0;
+  for(i = 0; i < 2048; i++) {
+    if(sizes[i] == 0) continue;
+    if(is_safe(sizes[i], &numbers[2048*i])) {
+      safe += 1;
+    }
+  }
+
+  fprintf(stdout, "Safe: %d\n", safe);
+
+  return 0;
+}