From 452f6e5f141bfa20b062d7c79b7dae363511cbf2 Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Tue, 21 Dec 2021 09:35:50 +0100
Subject: [PATCH] Implement fast day 21 part 1

---
 2021/src/day21.rs | 110 +++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 108 insertions(+), 2 deletions(-)

diff --git a/2021/src/day21.rs b/2021/src/day21.rs
index 113ba49..ff30d6f 100644
--- a/2021/src/day21.rs
+++ b/2021/src/day21.rs
@@ -1,9 +1,115 @@
 use std::io::Read;
 
-pub fn part1(_input: &mut dyn Read) -> String {
-    todo!()
+use crate::common::LineIter;
+
+fn read_input(input: &mut dyn Read) -> (i32, i32) {
+    let mut reader = LineIter::new(input);
+
+    let a = reader
+        .next()
+        .unwrap()
+        .split(' ')
+        .last()
+        .unwrap()
+        .parse()
+        .unwrap();
+
+    let b = reader
+        .next()
+        .unwrap()
+        .split(' ')
+        .last()
+        .unwrap()
+        .parse()
+        .unwrap();
+
+    (a, b)
+}
+
+#[inline]
+fn simulate(die: i32, pos: i32) -> i32 {
+    (pos + 3 * die + 3 - 1) % 10 + 1
+}
+
+fn find_repetition(mut pos: i32, mut die: i32) -> i32 {
+    let mut advance = 0;
+
+    for _ in 0..10 {
+        pos = simulate(die, pos);
+        advance += pos;
+        die = (die + 6) % 10;
+    }
+
+    advance
+}
+
+pub fn part1(input: &mut dyn Read) -> String {
+    const TARGET_SCORE: i32 = 1000;
+
+    let (a, b) = read_input(input);
+
+    let a10 = find_repetition(a, 1);
+    let b10 = find_repetition(b, 4);
+
+    let a_win = TARGET_SCORE / a10;
+    let b_win = TARGET_SCORE / b10;
+
+    let mut rolls = 3 * 20 * a_win.min(b_win);
+    let mut a_score = rolls / 3 / 20 * a10;
+    let mut b_score = rolls / 3 / 20 * b10;
+    let mut a_pos = a;
+    let mut b_pos = b;
+    let mut die = 1;
+
+    let (loser_score, rolls) = if a_win < b_win {
+        while a_score < TARGET_SCORE {
+            a_pos = simulate(die, a_pos);
+            a_score += a_pos;
+            rolls += 3;
+
+            if a_score < TARGET_SCORE {
+                b_pos = simulate(die + 3, b_pos);
+                b_score += b_pos;
+                rolls += 3;
+            }
+
+            die = (die + 6) % 10;
+        }
+
+        (b_score, rolls)
+    } else {
+        while b_score < TARGET_SCORE {
+            a_pos = simulate(die, a_pos);
+            a_score += a_pos;
+
+            b_pos = simulate(die + 3, b_pos);
+            b_score += b_pos;
+
+            rolls += 6;
+
+            die = (die + 6) % 10;
+        }
+
+        (a_score, rolls)
+    };
+
+    (loser_score * rolls).to_string()
 }
 
 pub fn part2(_input: &mut dyn Read) -> String {
     todo!()
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::test_implementation;
+
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/21.txt");
+
+    #[test]
+    fn sample_part1() {
+        test_implementation(part1, SAMPLE, 739785);
+    }
+}