From 4c928e06ac138929d43efde19485630b305bbcef Mon Sep 17 00:00:00 2001
From: Antoine Martin <antoine97.martin@gmail.com>
Date: Sun, 13 Dec 2020 17:28:29 +0100
Subject: [PATCH] 2020: day13: part 2

---
 aoc2020/src/day13.rs | 59 ++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 57 insertions(+), 2 deletions(-)

diff --git a/aoc2020/src/day13.rs b/aoc2020/src/day13.rs
index 020b7c9..2dc7b8e 100644
--- a/aoc2020/src/day13.rs
+++ b/aoc2020/src/day13.rs
@@ -3,11 +3,13 @@ use std::fmt::Write;
 use aoc::err;
 
 const INPUT: &str = include_str!("../input/day13.txt");
+const PROVIDED: &str = include_str!("../input/day13_provided.txt");
 
 pub fn run() -> aoc::Result<String> {
     let mut res = String::with_capacity(128);
 
     writeln!(res, "part 1: {}", part1(INPUT)?)?;
+    writeln!(res, "part 2: {}", part2(INPUT)?)?;
 
     Ok(res)
 }
@@ -46,12 +48,55 @@ fn part1(input: &str) -> aoc::Result<u64> {
     Ok(bus_id * (earliest_departure - earliest_timestamp))
 }
 
+fn part2(input: &str) -> aoc::Result<u64> {
+    let mut lines = input.lines();
+
+    // we don't need the first line anymore, skip it
+    lines.next().ok_or_else(|| err!("input was empty"))?;
+
+    let bus_ids: Vec<(u64, u64)> = lines
+        .next()
+        .ok_or_else(|| err!("no second line"))?
+        .split(',')
+        .enumerate()
+        .filter_map(|(idx, num)| {
+            if num == "x" {
+                None
+            } else {
+                Some((idx as u64, num.parse::<u64>().map_err(|e| err!("{}", e))))
+            }
+        })
+        .map(|(idx, res)| match res {
+            Ok(num) => Ok((idx, num)),
+            Err(e) => Err(e),
+        })
+        .collect::<aoc::Result<_>>()?;
+
+    // previous constraints is empty for now
+    let mut current_solution = 0;
+    let mut step = 1;
+
+    for constraint in bus_ids {
+        while !satisfies_constraint(current_solution, constraint) {
+            current_solution += step;
+        }
+
+        let (_, divisor) = constraint;
+
+        step *= divisor;
+    }
+
+    Ok(current_solution)
+}
+
+fn satisfies_constraint(solution: u64, (remainder, divisor): (u64, u64)) -> bool {
+    ((solution + remainder) % divisor) == 0
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
-    const PROVIDED: &str = include_str!("../input/day13_provided.txt");
-
     #[test]
     fn part1_provided() {
         assert_eq!(part1(PROVIDED).unwrap(), 295);
@@ -61,4 +106,14 @@ mod tests {
     fn part1_real() {
         assert_eq!(part1(INPUT).unwrap(), 3269);
     }
+
+    #[test]
+    fn part2_provided() {
+        assert_eq!(part2(PROVIDED).unwrap(), 1068781);
+    }
+
+    #[test]
+    fn part2_real() {
+        assert_eq!(part2(INPUT).unwrap(), 672754131923874);
+    }
 }