From e70ab44a91e6016d84bfaba5d297b02d830571bc Mon Sep 17 00:00:00 2001
From: Adam Spragg <adam@spra.gg>
Date: Tue, 6 Dec 2022 15:01:51 +0000
Subject: Solve puzzle 6 part 2

---
 6.c | 66 +++++++++++++++++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 55 insertions(+), 11 deletions(-)

diff --git a/6.c b/6.c
index f36c934..5a9bb1f 100644
--- a/6.c
+++ b/6.c
@@ -1,31 +1,75 @@
 
 #include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
 
 
 int
-main()
+main(int argc, char ** argv)
 {
-	char buf[4];
-	int c, n = 0;
+	char buf[32];
+	int len = 4, c, n = 0;
 
-	while ((c = fgetc(stdin)) != EOF) {
-		buf[n % 4] = c;
-		++n;
+	while ((c = getopt(argc, argv, "p:l:")) != -1) {
+		switch (c) {
+		case 'p':
+			if (optarg[0] == '1')
+				len = 4;
+			else if (optarg[0] == '2')
+				len = 14;
+			else {
+				fprintf(stderr, "Unexpected part %s\n", optarg);
+				return -1;
+			}
+			break;
 
-		if (n >= 4
-				&& buf[0] != buf[1] && buf[0] != buf[2] && buf[0] != buf[3]
-				&& buf[1] != buf[2] && buf[1] != buf[3] && buf[2] != buf[3])
-		{
+		case 'l':
+			len = atoi(optarg);
+			if (len < 2 || len > 32) {
+				fprintf(stderr, "Unexpected marker length: %d\n", len);
+				return -1;
+			}
 			break;
+
+		default:
+			return -1;
 		}
 	}
 
+	while ((c = fgetc(stdin)) != EOF
+			&& c >= 'a' && c <= 'z')
+	{
+		unsigned int t = 0;
+		int i;
+
+		buf[n % len] = c;
+		++n;
+
+		if (n < len)
+			continue;
+
+		// All input chars are between 'a' and 'z'.
+		// Check for repeats with a bit field.
+		for (i = 0; i < len; ++i) {
+			unsigned int b = 1u << (buf[i] - 'a');
+			if (t & b)
+				break;
+			t |= b;
+		}
+		if (i == len) // No repeats
+			break;
+	}
+
 	if (c == EOF) {
 		printf("No marker read in %d bytes\n", n);
 		return 1;
 	}
+	if (c < 'a' || c > 'z') {
+		printf("Unexpected input after %d bytes\n", n);
+		return 1;
+	}
 
-	printf("Marker read after %d bytes\n", n);
+	printf("Marker of length %d read after %d bytes\n", len, n);
 
 	return 0;
 }
-- 
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