From e7c9108b95e39d7ea5a29ae06d619c4727f11027 Mon Sep 17 00:00:00 2001
From: Thomas Voss <thomasvoss@live.com>
Date: Fri, 29 Oct 2021 23:02:39 +0200
Subject: Initial commit

---
 2020/08/puzzles.c | 112 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 112 insertions(+)
 create mode 100644 2020/08/puzzles.c

(limited to '2020/08/puzzles.c')

diff --git a/2020/08/puzzles.c b/2020/08/puzzles.c
new file mode 100644
index 0000000..cfa9389
--- /dev/null
+++ b/2020/08/puzzles.c
@@ -0,0 +1,112 @@
+#include <err.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define LINECOUNT 653
+
+struct Inst {
+	char opp;
+	int val;
+};
+
+/* Check if the given instruction has been executed already */
+static bool
+repeati(int const * const lines, int const rip)
+{
+	for (int i = 0; i < LINECOUNT; i++) {
+		if (lines[i] == rip)
+			return true;
+	}
+	return false;
+}
+
+static int
+run_circuit(struct Inst *circuit)
+{
+	int i, acc, rip;
+	int lines[LINECOUNT] = {0};
+
+	i = acc = rip = 0;
+
+	/* Execute the circuit */
+	do {
+		lines[i++] = rip;
+		switch (circuit[rip].opp) {
+		case 'j':
+			rip += circuit[rip].val;
+			if (rip >= LINECOUNT)
+				return acc;
+			break;
+		case 'a':
+			acc += circuit[rip].val;
+			/* FALLTHROUGH */
+		case 'n':
+			rip++;
+			break;
+		}
+	} while (!repeati(lines, rip));
+
+#ifdef PART2
+	return -1;
+#else
+	return acc;
+#endif
+}
+
+int
+main(void)
+{
+	int i = 0;
+	char cl[10];
+	FILE *fp;
+	struct Inst circuit[LINECOUNT];
+
+	if (!(fp = fopen("input", "r")))
+		err(EXIT_FAILURE, "fopen");
+
+	/* Load the entire circuit */
+	while (fgets(cl, sizeof(cl), fp)) {
+		char *val;
+		/* struct Inst operation; */
+
+		val = strtok(cl, " ");
+		circuit[i++] = (struct Inst) {
+			.opp = cl[0],
+			.val = atoi(strtok(NULL, " "))
+		};
+	}
+	fclose(fp);
+
+	int result = -1;
+#ifdef PART2
+	int count, prev_count;
+	count = prev_count = 1;
+
+	/* Run circuit until it completes successfully */
+	while (result == -1 && count < LINECOUNT) {
+		for (i = 0; i < LINECOUNT; i++) {
+			if (circuit[i].opp == 'j' || circuit[i].opp == 'n')
+				count--;
+
+			/* Swap jmp and nop */
+			if (!count) {
+				circuit[i].opp = (circuit[i].opp == 'j') ? 'n' : 'j';
+				break;
+			}
+		}
+
+		count = ++prev_count;
+		result = run_circuit(circuit);
+
+		/* Return to original array */
+		circuit[i].opp = (circuit[i].opp == 'j') ? 'n' : 'j';
+	}
+#else
+	result = run_circuit(circuit);
+#endif
+
+	printf("%d\n", result);
+	return EXIT_SUCCESS;
+}
-- 
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