2017 day 6

Author: ictrobot

crates/year2017/src/day06.rs

@@ -0,0 +1,84 @@
+use utils::prelude::*;
+
+/// Finding cycles.
+///
+/// See <https://en.wikipedia.org/wiki/Cycle_detection#Brent's_algorithm>, which avoids storing and
+/// hashing every visited state at the expense of calculating extra iterations.
+#[derive(Clone, Debug)]
+pub struct Day06 {
+    part1: u32,
+    part2: u32,
+}
+
+impl Day06 {
+    pub fn new(input: &str, _: InputType) -> Result<Self, InputError> {
+        let banks = parser::u32()
+            .with_suffix(b' '.or(b'\t').optional())
+            .parse_all(input)?;
+
+        let (mut power, mut lambda) = (1, 1);
+        let mut tortoise = banks.clone();
+        let mut hare = banks.clone();
+        Self::next(&mut hare);
+
+        while tortoise != hare {
+            if power == lambda {
+                tortoise.copy_from_slice(hare.as_slice());
+                power *= 2;
+                lambda = 0;
+            }
+            Self::next(&mut hare);
+            lambda += 1;
+        }
+
+        tortoise.copy_from_slice(banks.as_slice());
+        hare.copy_from_slice(banks.as_slice());
+        for _ in 0..lambda {
+            Self::next(&mut hare);
+        }
+
+        let mut mu = 0;
+        while tortoise != hare {
+            Self::next(&mut tortoise);
+            Self::next(&mut hare);
+            mu += 1;
+        }
+
+        Ok(Self {
+            part1: mu + lambda,
+            part2: lambda,
+        })
+    }
+
+    fn next(banks: &mut [u32]) {
+        let (mut idx, mut remaining) = banks
+            .iter()
+            .copied()
+            .enumerate()
+            .rev()
+            .max_by_key(|&(_, v)| v)
+            .unwrap();
+
+        banks[idx] = 0;
+
+        while remaining > 0 {
+            idx = (idx + 1) % banks.len();
+            banks[idx] += 1;
+            remaining -= 1;
+        }
+    }
+
+    #[must_use]
+    pub fn part1(&self) -> u32 {
+        self.part1
+    }
+
+    #[must_use]
+    pub fn part2(&self) -> u32 {
+        self.part2
+    }
+}
+
+examples!(Day06 -> (u32, u32) [
+    {input: "0\t2\t7\t0", part1: 5, part2: 4},
+]);

crates/year2017/src/lib.rs

@@ -7,4 +7,5 @@ utils::year!(2017 => year2017, ${
     3 => day03::Day03,
     4 => day04::Day04<'_>,
     5 => day05::Day05,
+    6 => day06::Day06,
 });