From 58b1210a2cc8f0c0572fdc721ea1e13b95322a5c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Thorben=20G=C3=BCnther?= <admin@xenrox.net>
Date: Thu, 8 Apr 2021 01:48:00 +0200
Subject: [PATCH] Solve day 3 part 2

---
 src/days/day03.rs | 40 +++++++++++++++++++++++++++++++++-------
 1 file changed, 33 insertions(+), 7 deletions(-)

diff --git a/src/days/day03.rs b/src/days/day03.rs
index aaeb584..36e8911 100644
--- a/src/days/day03.rs
+++ b/src/days/day03.rs
@@ -8,7 +8,7 @@ fn parse_contents(contents: String) -> Vec<Vec<char>> {
     map
 }
 
-fn count_trees(map: &Vec<Vec<char>>) -> i32 {
+fn count_trees(map: &Vec<Vec<char>>, x_add: usize, y_add: usize) -> i32 {
     let mut x = 0;
     let mut y = 0;
     let mut count = 0;
@@ -16,9 +16,9 @@ fn count_trees(map: &Vec<Vec<char>>) -> i32 {
     let x_max = map[0].len();
     let y_max = map.len();
 
-    while y != y_max - 1 {
-        y += 1;
-        x = (x + 3) % x_max;
+    while y < y_max - y_add {
+        y += y_add;
+        x = (x + x_add) % x_max;
         if map[y][x] == '#' {
             count += 1;
         }
@@ -26,12 +26,30 @@ fn count_trees(map: &Vec<Vec<char>>) -> i32 {
     count
 }
 
+fn calc_solution2(map: &Vec<Vec<char>>) -> i64 {
+    // Using only i32 would cause on overflow
+    let mut product: i64 = 1;
+    let mut count = count_trees(map, 1, 1);
+    product *= count as i64;
+    count = count_trees(map, 3, 1);
+    product *= count as i64;
+    count = count_trees(map, 5, 1);
+    product *= count as i64;
+    count = count_trees(map, 7, 1);
+    product *= count as i64;
+    count = count_trees(map, 1, 2);
+    product *= count as i64;
+    product
+}
+
 pub fn get_solution() {
     let contents = fs::read_to_string("data/day03.dat").expect("Could not read data");
     let map = parse_contents(contents);
-    let count = count_trees(&map);
+    let count = count_trees(&map, 3, 1);
 
-    println!("Day 3, part 1: {}", count);
+    let product = calc_solution2(&map);
+
+    println!("Day 3, part 1: {}, part 2: {}", count, product);
 }
 
 #[cfg(test)]
@@ -42,7 +60,15 @@ mod tests {
     fn part1() {
         let contents = fs::read_to_string("data/day03_test.dat").expect("Could not read data");
         let map = parse_contents(contents);
-        let count = count_trees(&map);
+        let count = count_trees(&map, 3, 1);
         assert_eq!(count, 7);
     }
+
+    #[test]
+    fn part2() {
+        let contents = fs::read_to_string("data/day03_test.dat").expect("Could not read data");
+        let map = parse_contents(contents);
+        let product = calc_solution2(&map);
+        assert_eq!(product, 336);
+    }
 }
-- 
2.44.0