From e5829c438669e8b12236aaccd5b0c7c0c51e800d Mon Sep 17 00:00:00 2001
From: Adam Spragg <adam@spra.gg>
Date: Tue, 20 Dec 2022 18:53:53 +0000
Subject: Solve puzzle 20 part 1

---
 20.c | 100 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 100 insertions(+)
 create mode 100644 20.c

(limited to '20.c')

diff --git a/20.c b/20.c
new file mode 100644
index 0000000..e738d9c
--- /dev/null
+++ b/20.c
@@ -0,0 +1,100 @@
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+
+struct num {
+	int val;
+	int ord;
+};
+
+
+// Proper mathematical modulo, which returns a value in the range 0 <= n < m
+// Even if n is negative.
+// Warning, evaluates `m` multiple times (but `n` only once)
+#define modulo(n, m) ((((n) % (m)) + (m)) % (m))
+
+
+int
+main()
+{
+	char buf[BUFSIZ];
+	struct num * nums = NULL;
+	int nnums = 0, numsize = 0, i, pos;
+
+	// Read file.
+	while (fgets(buf, sizeof(buf), stdin)) {
+		char * end;
+
+		if (nnums == numsize) {
+			void * p;
+
+			numsize += BUFSIZ;
+			if ((p = realloc(nums, numsize * sizeof(struct num))) == NULL) {
+				fprintf(stderr, "Bad realloc(%zu)\n", numsize * sizeof(struct num));
+				free(nums);
+				return -1;
+			}
+			nums = p;
+		}
+
+		if ((nums[nnums].val = strtol(buf, &end, 10)) == 0
+				&& end == buf)
+		{
+			fprintf(stderr, "Invalid entry: %s\n", buf);
+			free(nums);
+			return -1;
+		}
+		nums[nnums].ord = nnums;
+		++nnums;
+	}
+
+	// Mix file.
+	for (i = 0, pos = 0; i < nnums; ++i) {
+		int newpos;
+		struct num tmp;
+
+		// Look for the num whose ordinal is i, starting from the
+		// position of the last num we looked at.
+		while (nums[pos].ord != i)
+			pos = modulo(pos + 1, nnums);
+
+		if (nums[pos].val == 0)
+			continue;
+
+		// Move this.
+		tmp = nums[pos];
+
+		newpos = modulo(pos + nums[pos].val, nnums - 1); // Buffer of nums does not include moving num
+		if (newpos > pos)
+			memmove(nums + pos, nums + pos + 1, (newpos - pos) * sizeof(struct num));
+		else
+			memmove(nums + newpos + 1, nums + newpos, (pos - newpos) * sizeof(struct num));
+
+		nums[newpos] = tmp;
+	}
+
+	// Find zero.
+	for (i = 0; i < nnums; ++i)
+		if (nums[i].val == 0)
+			break;
+	if (i == nnums) {
+		fprintf(stderr, "No zero found!\n");
+		free(nums);
+		return -1;
+	}
+
+	printf("Coords: %d,%d,%d; sum %d\n",
+			nums[modulo(i + 1000, nnums)].val,
+			nums[modulo(i + 2000, nnums)].val,
+			nums[modulo(i + 3000, nnums)].val,
+			nums[modulo(i + 1000, nnums)].val
+			+ nums[modulo(i + 2000, nnums)].val
+			+ nums[modulo(i + 3000, nnums)].val);
+
+	free(nums);
+
+	return 0;
+}
+
-- 
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