From f7f03278c811fa2a013a1b64150a184b59021692 Mon Sep 17 00:00:00 2001
From: Luke Hubmayer-Werner <mongoose41@gmail.com>
Date: Wed, 7 Dec 2022 17:08:58 +1030
Subject: [PATCH] port Rust to C++

---
 2022/day6.cpp | 70 +++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 65 insertions(+), 5 deletions(-)

diff --git a/2022/day6.cpp b/2022/day6.cpp
index 8861e0d..b977ed3 100644
--- a/2022/day6.cpp
+++ b/2022/day6.cpp
@@ -1,3 +1,4 @@
+#include <bit>
 #include <cinttypes>
 #include <fstream>
 #include <iostream>
@@ -24,6 +25,61 @@ size_t find_first_unique_run(std::vector<uint8_t> *s, size_t num, size_t skip) {
 	return 0;
 }
 
+
+std::pair<size_t, size_t> find_first_unique_runs(std::vector<uint8_t> *s) {
+	const size_t BATCH_SIZE = 8;
+	const size_t BATCH_NUM = 4096/BATCH_SIZE;
+	size_t four = 0;
+	size_t fourteen = 0;
+	uint32_t masks[4096];
+	uint32_t scratch_masks[BATCH_SIZE];
+	uint8_t scratch_masks_bits[BATCH_SIZE];
+	for (int i=0; i<4096; i++) {
+		masks[i] = 1 << ((*s)[i]-'a');
+	}
+	// Turn masks into masks2. Indices are now +1.
+	for (int i=0; i<4095; i++) {
+		masks[i] |= masks[i+1];
+	}
+	for (int batch=1; batch<BATCH_NUM; batch++) {
+		for (int i=0; i<BATCH_SIZE; i++) {
+			int idx = batch*BATCH_SIZE + i;
+			scratch_masks[i] = masks[idx] | masks[idx-2];
+			scratch_masks_bits[i] = std::popcount(scratch_masks[i]);
+		}
+		for (int i=0; i<BATCH_SIZE; i++) {
+			if (scratch_masks_bits[i] == 4) {
+				four = batch*BATCH_SIZE + i;
+				goto four_found;
+			}
+		}
+	}
+four_found:
+	// Turn masks2 into masks4. Indices are now +3.
+	for (int i=four; i<4093; i++) {
+		masks[i] |= masks[i+2];
+	}
+	// Turn masks4 into masks8. Indices are now +7.
+	for (int i=fourteen; i<4089; i++) {
+		masks[i] |= masks[i+4];
+	}
+	for (int batch=(four/BATCH_SIZE); batch<BATCH_NUM; batch++) {
+		for (int i=0; i<BATCH_SIZE; i++) {
+			int idx = batch*BATCH_SIZE + i;
+			scratch_masks[i] = masks[idx] | masks[idx-6];
+			scratch_masks_bits[i] = std::popcount(scratch_masks[i]);
+		}
+		for (int i=0; i<BATCH_SIZE; i++) {
+			if (scratch_masks_bits[i] == 14) {
+				fourteen = batch*BATCH_SIZE + i;
+				goto fourteen_found;
+			}
+		}
+	}
+fourteen_found:
+	return std::pair<size_t, size_t>(four+2, fourteen+8);
+}
+
 void run_once(std::vector<uint8_t> *s) {
 	size_t four = find_first_unique_run(s, 4, 4);
 	size_t fourteen = find_first_unique_run(s, 14, four);
@@ -32,11 +88,15 @@ void run_once(std::vector<uint8_t> *s) {
 }
 
 void run_many(std::vector<uint8_t> *s, size_t iterations) {
-	size_t four = 0;
-	size_t fourteen = 0;
+	size_t volatile four = 0;  // Don't you dare optimize out the iterations!
+	size_t volatile fourteen = 0;
+	std::cout << "Running " << iterations << " iterations:\n";
 	for (size_t i = 0; i<iterations; i++) {
-		four = find_first_unique_run(s, 4, 4);
-		fourteen = find_first_unique_run(s, 14, four);
+		// four = find_first_unique_run(s, 4, 4);
+		// fourteen = find_first_unique_run(s, 14, four);
+		std::pair<size_t, size_t> result = find_first_unique_runs(s);
+		four = result.first;
+		fourteen = result.second;
 	}
 	std::cout << "Part 1: " << four << '\n';
 	std::cout << "Part 2: " << fourteen << '\n';
@@ -51,5 +111,5 @@ int main() {
 	input_file.read(reinterpret_cast<char*>(input.data()), eof_position);
 
 	// run_once(&input);
-	run_many(&input, 1000000);
+	run_many(&input, 10000000);
 }