From 03b22627ca28abaa8ce34877efb425721c331e63 Mon Sep 17 00:00:00 2001
From: Jonni Liljamo <jonni@liljamo.com>
Date: Tue, 2 Dec 2025 12:43:27 +0200
Subject: [PATCH] feat: day1 part two

---
 crates/day1/src/main.rs | 102 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 102 insertions(+)

diff --git a/crates/day1/src/main.rs b/crates/day1/src/main.rs
index 947f601..9812bcd 100644
--- a/crates/day1/src/main.rs
+++ b/crates/day1/src/main.rs
@@ -47,6 +47,36 @@ fn one(input: &str) -> u16 {
     zeros
 }
 
+fn two(input: &str) -> u16 {
+    let mut zeros: u16 = 0;
+    let mut pos: i16 = 50;
+    for ins in input.split("\n") {
+        // Last entry is all unused bytes.
+        if ins.len() > 10 {
+            continue;
+        }
+
+        let (direction, amount_s) = ins.split_at(1);
+        let mut amount = amount_s.parse::<i16>().unwrap();
+
+        match direction {
+            "R" => {}
+            "L" => amount = -amount,
+            _ => unreachable!(),
+        }
+
+        pos += amount;
+        if pos <= 0 && amount != pos {
+            zeros += 1;
+        }
+        let overflows = pos.abs() / 100;
+        zeros += overflows as u16;
+        pos = pos.rem_euclid(100);
+    }
+
+    zeros
+}
+
 #[cfg(test)]
 mod test {
     use super::*;
@@ -65,5 +95,77 @@ L82"#;
     #[test]
     fn example() {
         assert_eq!(one(EXAMPLE_INPUT), 3);
+        assert_eq!(two(EXAMPLE_INPUT), 6);
+    }
+
+    #[test]
+    fn two_1000() {
+        assert_eq!(two("R1000"), 10);
+    }
+
+    #[test]
+    fn two_basics() {
+        // Move to zero (!), move one left, should be at 99.
+        assert_eq!(two("L50\nL1"), 1);
+        // Move to 99, move one right, should be at zero (!).
+        assert_eq!(two("R49\nR1"), 1);
+        // Move to 5, move ten left (!), should be at 95, move 5 right, should be at zero (!).
+        assert_eq!(two("L45\nL10\nR5"), 2);
+    }
+
+    #[test]
+    fn two_a() {
+        let input = r#"R50
+L1"#;
+
+        assert_eq!(two(input), 1);
+    }
+
+    #[test]
+    fn two_b() {
+        assert_eq!(two("L100"), 1);
+    }
+
+    #[test]
+    fn two_c() {
+        assert_eq!(two("L120"), 1);
+    }
+
+    #[test]
+    fn two_d() {
+        let input = r#"R250
+L1"#;
+
+        assert_eq!(two(input), 3);
+    }
+
+    #[test]
+    fn two_e() {
+        let input = r#"R400"#;
+
+        assert_eq!(two(input), 4);
+    }
+
+    #[test]
+    fn two_f() {
+        let input = r#"L250"#;
+
+        assert_eq!(two(input), 3);
+    }
+
+    #[test]
+    fn two_g() {
+        let input = r#"L50
+R200"#;
+
+        assert_eq!(two(input), 3);
+    }
+
+    #[test]
+    fn two_h() {
+        let input = r#"R38
+R17"#;
+
+        assert_eq!(two(input), 1);
     }
 }
-- 
2.44.1