From 4d08e60b5dffd4c0be3b8e2d3d16c63d2fffae00 Mon Sep 17 00:00:00 2001
From: Tomasz Kramkowski <tk@the-tk.com>
Date: Sat, 27 Nov 2021 17:13:03 +0000
Subject: 10: very fast C solution

---
 10/solution.c | 70 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)
 create mode 100644 10/solution.c

diff --git a/10/solution.c b/10/solution.c
new file mode 100644
index 0000000..1292490
--- /dev/null
+++ b/10/solution.c
@@ -0,0 +1,70 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+struct rle {
+	unsigned char count : 2;
+	unsigned char digit   : 2;
+};
+
+struct buf {
+	struct rle *buf;
+	size_t count;
+};
+
+size_t len(const struct buf *buf)
+{
+	size_t len = 0;
+	for (size_t i = 0; i < buf->count; i++)
+		len += buf->buf[i].count;
+	return len;
+}
+
+void append(struct buf *buf, unsigned char digit)
+{
+	if (buf->count == 0 || buf->buf[buf->count - 1].digit != digit) {
+		buf->buf[buf->count++] = (struct rle){ 1, digit };
+		return;
+	}
+	buf->buf[buf->count - 1].count++;
+}
+
+void solve(const struct buf *input, size_t *p1, size_t *p2) {
+	struct buf bufs[2];
+	bufs[0].buf = malloc(sizeof *bufs[0].buf * 0xa00000);
+	bufs[1].buf = malloc(sizeof *bufs[1].buf * 0xa00000);
+	int srcbuf = 0;
+	memcpy(bufs[srcbuf].buf, input->buf,
+	       sizeof *bufs[srcbuf].buf * input->count);
+	bufs[srcbuf].count = input->count;
+
+	for (int i = 0; i < 50; i++) {
+		if (i == 40) *p1 = len(&bufs[srcbuf]);
+		bufs[!srcbuf].count = 0;
+		for (int j = 0; j < bufs[srcbuf].count; j++) {
+			append(&bufs[!srcbuf], bufs[srcbuf].buf[j].count);
+			append(&bufs[!srcbuf], bufs[srcbuf].buf[j].digit);
+		}
+		srcbuf = !srcbuf;
+	}
+
+	*p2 = len(&bufs[srcbuf]);
+}
+
+int main(void)
+{
+	struct buf input = {
+		(struct rle []){
+			{3, 1},
+			{1, 3},
+			{3, 2},
+			{2, 1},
+			{1, 3},
+		},
+		5,
+	};
+	size_t p1, p2;
+
+	solve(&input, &p1, &p2);
+	printf("%zu\n%zu\n", p1, p2);
+}
-- 
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