2017 day 16

Author: ictrobot

crates/utils/src/parser/number.rs

@@ -128,6 +128,7 @@ impl<I: Integer + Parseable> Parser for NumberRange<I> {
 ///     Ok((123u8, &b", 120"[..]))
 /// );
 /// ```
+#[inline]
 #[must_use]
 pub fn number_range<I: Integer + Parseable>(range: RangeInclusive<I>) -> NumberRange<I> {
     let min = *range.start();

crates/utils/src/parser/simple.rs

@@ -74,6 +74,7 @@ impl Parser for ByteRange {
 ///     Ok((b'h', &b"ello world"[..]))
 /// );
 /// ```
+#[inline]
 #[must_use]
 pub fn byte_range(range: RangeInclusive<u8>) -> ByteRange {
     let min = *range.start();

crates/year2017/src/day16.rs

@@ -0,0 +1,130 @@
+use std::array;
+use std::fmt::{Display, Formatter, Write};
+use std::ops::MulAssign;
+use utils::prelude::*;
+
+/// Composing permutations.
+///
+/// The key optimization is that all the index-based permutations (Spin and Exchange) can be
+/// combined into one permutation, and all the value-based permutations (Partner) can be combined
+/// into another.
+#[derive(Clone, Debug)]
+pub struct Day16 {
+    dance: Dance,
+}
+
+#[derive(Clone, Debug)]
+enum DanceMove {
+    Spin(u8),
+    Exchange(u8, u8),
+    Partner(u8, u8),
+}
+
+#[derive(Copy, Clone, Debug)]
+struct Dance {
+    index_perm: [usize; 16],
+    value_perm: [usize; 16],
+}
+
+impl Day16 {
+    pub fn new(input: &str, _: InputType) -> Result<Self, InputError> {
+        let program = parser::byte_range(b'a'..=b'p').map(|b| b - b'a');
+        let position = parser::number_range(0..=15);
+
+        Ok(Self {
+            dance: parser::one_of((
+                position.with_prefix(b's').map(DanceMove::Spin),
+                position
+                    .with_prefix(b'x')
+                    .then(position.with_prefix(b'/'))
+                    .map(|(a, b)| DanceMove::Exchange(a, b)),
+                program
+                    .with_prefix(b'p')
+                    .then(program.with_prefix(b'/'))
+                    .map(|(a, b)| DanceMove::Partner(a, b)),
+            ))
+            .with_suffix(b','.or(parser::eof()))
+            .parse_iterator(input)
+            .collect::<Result<Dance, InputError>>()?,
+        })
+    }
+
+    #[must_use]
+    pub fn part1(&self) -> String {
+        self.dance.to_string()
+    }
+
+    #[must_use]
+    pub fn part2(&self) -> String {
+        // Exponentiation by squaring, but for dance permutations
+        let mut result = Dance::default();
+        let mut base = self.dance;
+        let mut exponent = 1_000_000_000;
+
+        while exponent > 0 {
+            if exponent % 2 == 1 {
+                result *= base;
+            }
+            exponent >>= 1;
+            base *= base;
+        }
+
+        result.to_string()
+    }
+}
+
+impl Default for Dance {
+    fn default() -> Self {
+        Dance {
+            index_perm: array::from_fn(|i| i),
+            value_perm: array::from_fn(|i| i),
+        }
+    }
+}
+
+impl FromIterator<DanceMove> for Dance {
+    #[inline]
+    fn from_iter<T: IntoIterator<Item = DanceMove>>(iter: T) -> Self {
+        let mut dance = Dance::default();
+        let mut value_positions: [usize; 16] = array::from_fn(|i| i);
+        let mut rotation = 0; // Rotate once at the end as it is expensive
+
+        for dance_move in iter {
+            match dance_move {
+                DanceMove::Spin(r) => rotation += 16 - r as usize,
+                DanceMove::Exchange(a, b) => {
+                    let a_idx = (a as usize + rotation) % 16;
+                    let b_idx = (b as usize + rotation) % 16;
+                    dance.index_perm.swap(a_idx, b_idx);
+                }
+                DanceMove::Partner(a, b) => {
+                    value_positions.swap(a as usize, b as usize);
+                    let a_idx = value_positions[a as usize];
+                    let b_idx = value_positions[b as usize];
+                    dance.value_perm.swap(a_idx, b_idx);
+                }
+            }
+        }
+        dance.index_perm.rotate_left(rotation % 16);
+
+        dance
+    }
+}
+
+impl Display for Dance {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        for &i in &self.index_perm {
+            f.write_char((self.value_perm[i] as u8 + b'a') as char)?;
+        }
+        Ok(())
+    }
+}
+
+impl MulAssign for Dance {
+    fn mul_assign(&mut self, rhs: Self) {
+        self.index_perm = self.index_perm.map(|i| rhs.index_perm[i]);
+        self.value_perm = self.value_perm.map(|i| rhs.value_perm[i]);
+    }
+}
+
+examples!(Day16 -> (&'static str, &'static str) []);

crates/year2017/src/lib.rs

@@ -19,4 +19,5 @@ utils::year!(2017 => year2017, ${
     13 => day13::Day13,
     14 => day14::Day14,
     15 => day15::Day15,
+    16 => day16::Day16,
 });