Implement day 19 part 1

By brute force and lots of it

2021/src/common.rs

@@ -82,8 +82,9 @@ pub fn ordered<O: PartialOrd>(a: O, b: O) -> (O, O) {
     }
 }
 
-pub fn read_input<P, O>(input: &mut dyn Read, parser: P) -> O
+pub fn read_input<I, P, O>(mut input: I, parser: P) -> O
 where
+    I: Read,
     P: for<'a> FnOnce(&'a [u8]) -> IResult<&'a [u8], O>,
 {
     let mut buffer = Vec::new();

2021/src/day19.rs

@@ -1,9 +1,261 @@
+use std::collections::HashSet;
 use std::io::Read;
+use std::ops::Add;
+use std::ops::Sub;
 
-pub fn part1(_input: &mut dyn Read) -> String {
-    todo!()
+use nom::bytes::complete::tag;
+use nom::character::complete::newline;
+use nom::combinator::map;
+use nom::multi::many1;
+use nom::multi::separated_list1;
+use nom::sequence::delimited;
+use nom::sequence::preceded;
+use nom::sequence::terminated;
+use nom::sequence::tuple;
+use nom::IResult;
+
+use crate::common::read_input;
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, Default)]
+struct Point3(pub [i32; 3]);
+
+impl Sub for Point3 {
+    type Output = Self;
+
+    fn sub(self, rhs: Self) -> Self::Output {
+        Point3([
+            self.0[0] - rhs.0[0],
+            self.0[1] - rhs.0[1],
+            self.0[2] - rhs.0[2],
+        ])
+    }
+}
+
+impl Add for Point3 {
+    type Output = Self;
+
+    fn add(self, rhs: Self) -> Self::Output {
+        Point3([
+            self.0[0] + rhs.0[0],
+            self.0[1] + rhs.0[1],
+            self.0[2] + rhs.0[2],
+        ])
+    }
+}
+
+struct Rotations<'a> {
+    points: &'a [Point3],
+    axes: [usize; 3],
+    rotation_index: usize,
+}
+
+impl<'a> Rotations<'a> {
+    const ROTATIONS: [[i32; 3]; 8] = [
+        [1, 1, 1],
+        [1, 1, -1],
+        [1, -1, 1],
+        [1, -1, -1],
+        [-1, 1, 1],
+        [-1, 1, -1],
+        [-1, -1, 1],
+        [-1, -1, -1],
+    ];
+
+    pub fn new(points: &'a [Point3]) -> Self {
+        Self {
+            points,
+            axes: [0, 1, 2],
+            rotation_index: 0,
+        }
+    }
+}
+
+impl Iterator for Rotations<'_> {
+    type Item = Vec<Point3>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.rotation_index >= Self::ROTATIONS.len() {
+            if !next_permutation(&mut self.axes) {
+                return None;
+            }
+
+            self.rotation_index = 0;
+        }
+
+        let axes = &self.axes;
+        let rot = &Self::ROTATIONS[self.rotation_index];
+
+        let result = self
+            .points
+            .iter()
+            .map(|Point3(coords)| {
+                Point3([
+                    coords[axes[0]] * rot[0],
+                    coords[axes[1]] * rot[1],
+                    coords[axes[2]] * rot[2],
+                ])
+            })
+            .collect();
+
+        self.rotation_index += 1;
+
+        Some(result)
+    }
+}
+
+fn parse_point(input: &[u8]) -> IResult<&[u8], Point3> {
+    use nom::character::complete::char;
+    use nom::character::complete::i32;
+
+    map(
+        tuple((i32, preceded(char(','), i32), preceded(char(','), i32))),
+        |(x, y, z)| Point3([x, y, z]),
+    )(input)
+}
+
+fn parse_input(input: &[u8]) -> IResult<&[u8], Vec<Vec<Point3>>> {
+    use nom::character::complete::i32;
+    let parse_header = delimited(tag("--- scanner "), i32, tag(" ---\n"));
+
+    let parse_scanner = preceded(parse_header, many1(terminated(parse_point, newline)));
+    separated_list1(newline, parse_scanner)(input)
+}
+
+fn try_overlap(correct: &[(Point3, HashSet<Point3>)], candidate: &[Point3]) -> Option<Vec<Point3>> {
+    let mut relative = HashSet::new();
+    for rot in Rotations::new(candidate) {
+        for &start in &rot {
+            relative.clear();
+
+            relative.extend(rot.iter().map(|&other| other - start));
+
+            if let Some((base, _)) = correct.iter().find(|(_, correct_relative)| {
+                correct_relative.intersection(&relative).count() >= 12
+            }) {
+                // Found a solution, build the correct output
+                let translated = relative.drain().map(|point| point + *base).collect();
+
+                return Some(translated);
+            }
+        }
+    }
+
+    None
+}
+
+pub fn part1(input: &mut dyn Read) -> String {
+    let mut scanners = read_input(input, parse_input);
+
+    let mut points: HashSet<_> = scanners[0].iter().copied().collect();
+
+    let mut todo = vec![std::mem::take(&mut scanners[0])];
+    println!("Scanners: {}", scanners.len());
+
+    while let Some(matched) = todo.pop() {
+        if scanners.iter().all(Vec::is_empty) {
+            break;
+        }
+
+        let relative: Vec<(Point3, HashSet<Point3>)> = matched
+            .iter()
+            .map(|&base| (base, matched.iter().map(|&other| (other - base)).collect()))
+            .collect();
+
+        for candidate in &mut scanners {
+            if candidate.is_empty() {
+                continue;
+            }
+
+            if let Some(result) = try_overlap(&relative, candidate) {
+                points.extend(result.iter().copied());
+                todo.push(result);
+                candidate.clear();
+            }
+        }
+    }
+
+    scanners.retain(|s| !s.is_empty());
+
+    points.len().to_string()
 }
 
 pub fn part2(_input: &mut dyn Read) -> String {
     todo!()
 }
+
+pub fn next_permutation<T: Ord>(list: &mut [T]) -> bool {
+    // Based on: https://en.cppreference.com/w/cpp/algorithm/next_permutation
+    if list.len() <= 1 {
+        return false;
+    }
+
+    if let Some((i, val1)) = list
+        .windows(2)
+        .enumerate()
+        .rev()
+        .find_map(|(i, window)| (window[0] < window[1]).then(|| (i, &window[0])))
+    {
+        let it2 = list
+            .iter()
+            .enumerate()
+            .skip(i + 1)
+            .rev()
+            .find_map(|(idx, val2)| (val1 < val2).then(|| idx))
+            .expect("Unreachable, ascending pair exists");
+
+        list.swap(i, it2);
+        list[(i + 1)..].reverse();
+        true
+    } else {
+        list.reverse();
+        false
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::test_implementation;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/19.txt");
+
+    #[test]
+    fn test_next_permutation() {
+        let mut list = [1, 2, 3];
+
+        assert!(next_permutation(&mut list));
+        assert_eq!(list, [1, 3, 2]);
+
+        assert!(next_permutation(&mut list));
+        assert_eq!(list, [2, 1, 3]);
+
+        assert!(next_permutation(&mut list));
+        assert_eq!(list, [2, 3, 1]);
+
+        assert!(next_permutation(&mut list));
+        assert_eq!(list, [3, 1, 2]);
+
+        assert!(next_permutation(&mut list));
+        assert_eq!(list, [3, 2, 1]);
+
+        // Note the negation!
+        assert!(!next_permutation(&mut list));
+        assert_eq!(list, [1, 2, 3]);
+
+        // 24 is a bit too much to write out, but we can check the number
+        let mut list2 = [1, 2, 3, 4];
+        for _ in 1..24 {
+            assert!(next_permutation(&mut list2));
+        }
+
+        // Should be back to the start
+        assert!(!next_permutation(&mut list2));
+        assert_eq!(list2, [1, 2, 3, 4]);
+    }
+
+    #[test]
+    fn sample_part1() {
+        test_implementation(part1, SAMPLE, 79);
+    }
+}