1 files changed, 109 insertions, 0 deletions
diff --git a/2020/08/part2.c b/2020/08/part2.c
new file mode 100644
index 0000000..44ebdec
--- /dev/null
+++ b/2020/08/part2.c
@@ -0,0 +1,109 @@
+#include <stdio.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+#define INPUT_LEN 650
+
+struct Operation
+{
+    char opcode;
+    int arg;
+    char has_been_executed;
+};
+
+int execute(struct Operation *ops, size_t opcount)
+{
+    struct Operation *cur_op = ops;
+    int acc = 0;
+    do {
+        if (cur_op->has_been_executed) {
+            return -1;
+        }
+
+        cur_op->has_been_executed = 1;
+        switch (cur_op->opcode)
+        {
+            case 0:
+                cur_op++;
+                break;
+            case 1:
+                acc += cur_op->arg;
+                cur_op++;
+                break;
+            case 2:
+                cur_op += cur_op->arg;
+                break;
+        }
+    } while (cur_op < ops + opcount);
+
+    return acc;
+}
+
+int exe_program(const char *filename)
+{
+    FILE *file = fopen(filename, "r");
+
+    // Include space for newline and string terminator
+    char buffer[16] = { 0 };
+
+    struct Operation ops[INPUT_LEN];
+    size_t opcount = 0;
+
+    while (fgets(buffer, 16, file)) {
+        struct Operation *op = &ops[opcount++];
+        op->has_been_executed = 0;
+        switch (buffer[0])
+        {
+            // nop
+            case 'n':
+                op->opcode = 0;
+                break;
+            // acc
+            case 'a':
+                op->opcode = 1;
+                break;
+            // jmp
+            case 'j':
+                op->opcode = 2;
+                break;
+        }
+
+        op->arg = atoi(&buffer[4]);
+    }
+
+    fclose(file);
+
+    struct Operation *cur_op = ops;
+    int flip_previous = 0;
+    do {
+        if (flip_previous) {
+            (cur_op - 1)->opcode ^= 2;
+            flip_previous = 0;
+        }
+
+        if (cur_op->opcode == 1
+            || (cur_op->opcode == 0 && cur_op->arg <= 0)
+            || (cur_op->opcode == 2 && cur_op->arg >= 0)) {
+            continue;
+        }
+
+        cur_op->opcode ^= 2;
+        flip_previous = 1;
+
+        for (size_t i = 0; i < opcount; i++) {
+            ops[i].has_been_executed = 0;
+        }
+
+        int acc = execute(ops, opcount);
+        if (acc != -1) {
+            return acc;
+        }
+    } while (++cur_op < ops + opcount);
+
+    return -1;
+}
+
+int main(int argc, char *argv[])
+{
+    printf("%i\n", exe_program(argv[argc - 1]));
+}