Finally implement 2022 day 17 part 2

2022/src/day17.rs

@@ -45,23 +45,13 @@ fn collides(shape: &[&[bool]], cavern: &IndexSet, x: usize, y: usize) -> bool {
     false
 }
 
-pub fn part1(input: &[u8]) -> Result<String> {
-    // Poor man's trim()
-    let input = if input[input.len() - 1] == b'\n' {
-        &input[..input.len() - 1]
-    } else {
-        input
-    };
-
-    let mut cavern = IndexSet::default();
-    let mut max_height = 0;
-
-    let mut gusts = input
-        .iter()
-        .cycle()
-        .map(|&b| if b == b'<' { -1 } else { 1 });
-
-    for &shape in SHAPES.iter().cycle().take(2022) {
+fn simulate(
+    mut cavern: IndexSet,
+    shapes: impl Iterator<Item = &'static &'static [&'static [bool]]>,
+    mut gusts: impl Iterator<Item = isize>,
+    mut max_height: usize,
+) -> usize {
+    for &shape in shapes {
         let mut x = 2usize;
         let mut y = max_height + shape.len() + 2;
 
@@ -95,11 +85,140 @@ pub fn part1(input: &[u8]) -> Result<String> {
         }
     }
 
-    Ok(max_height.to_string())
+    max_height
 }
 
-pub fn part2(_input: &[u8]) -> Result<String> {
-    anyhow::bail!("not implemented")
+pub fn part1(input: &[u8]) -> Result<String> {
+    // Poor man's trim()
+    let input = if input[input.len() - 1] == b'\n' {
+        &input[..input.len() - 1]
+    } else {
+        input
+    };
+
+    let gusts = input
+        .iter()
+        .cycle()
+        .map(|&b| if b == b'<' { -1 } else { 1 });
+
+    Ok(simulate(
+        IndexSet::default(),
+        SHAPES.iter().cycle().take(2022),
+        gusts,
+        0,
+    )
+    .to_string())
+}
+
+#[derive(Clone, Copy, Debug)]
+struct HeightMap([usize; 7]);
+
+impl HeightMap {
+    fn max(&self) -> &usize {
+        self.0.iter().max().unwrap()
+    }
+}
+
+impl PartialEq for HeightMap {
+    fn eq(&self, other: &Self) -> bool {
+        let self_min = *self.0.iter().min().unwrap();
+        let other_min = *other.0.iter().min().unwrap();
+
+        self.0
+            .iter()
+            .zip(&other.0)
+            .all(|(&a, &b)| a - self_min == b - other_min)
+    }
+}
+
+pub fn part2(input: &[u8]) -> Result<String> {
+    // Poor man's trim()
+    let input = if input[input.len() - 1] == b'\n' {
+        &input[..input.len() - 1]
+    } else {
+        input
+    };
+
+    let mut cavern = IndexSet::default();
+
+    let mut height_map = HeightMap([0; 7]);
+
+    let mut shapes = SHAPES.iter().enumerate().cycle();
+    let mut gusts = input
+        .iter()
+        .map(|&b| if b == b'<' { -1 } else { 1 })
+        .enumerate()
+        .cycle();
+
+    let mut tortoise = (0, 0, height_map);
+    let mut tortoise_time = 0;
+    let mut last_gust = 0;
+
+    const TARGET: usize = 1000000000000;
+
+    for (it, (shape_id, &shape)) in shapes.by_ref().enumerate() {
+        let mut x = 2usize;
+        let mut y = height_map.max() + shape.len() + 2;
+
+        // Acquire gust of wind
+        for (gust_id, offset) in gusts.by_ref() {
+            last_gust = gust_id;
+            if let Some(nx) = x.checked_add_signed(offset) {
+                if !collides(shape, &cavern, nx, y) {
+                    x = nx;
+                }
+            } else {
+                // Hit the left wall
+            }
+
+            // Move down if possible
+            if y >= shape.len() && !collides(shape, &cavern, x, y - 1) {
+                y -= 1;
+            } else {
+                break;
+            }
+        }
+
+        // If we get here we've successfully stopped falling
+        for (row, line) in shape.iter().enumerate() {
+            for (col, &on) in line.iter().enumerate() {
+                if on {
+                    height_map.0[x + col] = height_map.0[x + col].max(y - row + 1);
+                    cavern.insert((y - row) * WIDTH + x + col);
+                }
+            }
+        }
+
+        // See if we found a circle
+        let hare_time = it + 1;
+        let hare = (shape_id, last_gust, height_map);
+
+        if hare == tortoise {
+            let cycle_len = hare_time - tortoise_time;
+            let remaining = TARGET - hare_time;
+            let to_skip = remaining / cycle_len;
+            let remainder = remaining % cycle_len;
+
+            let current_max = *height_map.max();
+
+            // All of them rose by the same amount so we just need to compare the first one
+            let delta = height_map.0[0] - tortoise.2 .0[0];
+
+            let result = simulate(
+                cavern,
+                shapes.map(|(_, shape)| shape).take(remainder),
+                gusts.map(|(_, offset)| offset),
+                current_max,
+            ) + delta * to_skip;
+
+            return Ok(result.to_string());
+        } else if it.count_ones() == 1 {
+            tortoise = hare;
+            tortoise_time = hare_time;
+        }
+    }
+
+    Ok(height_map.max().to_string())
 }
 
 #[cfg(test)]
@@ -112,4 +231,9 @@ mod tests {
     fn sample_part1() {
         assert_eq!(part1(SAMPLE).unwrap(), "3068");
     }
+
+    #[test]
+    fn sample_part2() {
+        assert_eq!(part2(SAMPLE).unwrap(), "1514285714288");
+    }
 }