refactor flood fill to use a more optimal algorithm

Cargo.lock

@@ -405,6 +405,7 @@ dependencies = [
  "criterion",
  "hashbrown 0.13.2",
  "mino",
+ "smallvec",
  "tracing",
 ]
 

fish/Cargo.toml

@@ -10,6 +10,7 @@ mino = { path = "../mino" }
 ahash = "0.8"
 bumpalo = "3.12"
 hashbrown = "0.13"
+smallvec = "1.10"
 tracing = { version = "0.1", default_features = false }
 
 [dev-dependencies]

fish/src/eval/downstacking.rs

@@ -1,8 +1,9 @@
 #![allow(dead_code)]
 
 use core::ops::Range;
-use mino::matrix::EMPTY_ROW;
+use mino::matrix::{EMPTY_ROW, FULL_ROW};
 use mino::{Mat, MatBuf};
+use smallvec::SmallVec;
 
 /// This is the algorithm that mystery and I developed. It computes the "minimum downstack
 /// estimate", which approximates the minimum pieces to downstack a given board state. It
@@ -100,7 +101,8 @@ fn residue(mat: &Mat) -> Option<Range<i16>> {
 /// `(x,y,area)` where `(x,y)` is first point in the region filled in, and `area` is the
 /// number of empty cells filled.
 fn flood_fill(rows: &mut [u16]) -> Option<(i16, i16, u32)> {
-    fn init(rows: &[u16]) -> Option<(i16, i16)> {
+    // find an empty cell in the matrix somewhere
+    fn find_empty(rows: &[u16]) -> Option<(i16, i16)> {
         for (y, row) in rows.iter().enumerate() {
             // trailing_ones() finds some unoccipied cell in `row`. if the row is full
             // then this return 16 (since there are 16 bits in the row representation
@@ -113,37 +115,67 @@ fn flood_fill(rows: &mut [u16]) -> Option<(i16, i16, u32)> {
         None
     }
 
-    fn flood(rows: &mut [u16], x: i16, y: i16) -> u32 {
-        // test if (x,y) is OOB
-        if x < 0 || y < 0 || (y as usize) >= rows.len() {
-            return 0;
-        }
-
-        // try to fill in (x,y)
-        let idx = y as usize;
-        let mask = 1 << x;
-        if rows[idx] & mask != 0 {
-            return 0;
-        }
-        rows[idx] |= mask;
-
-        // FIXME: improve this:
-        // - the max recursion depth is small (~400?) so we should use an explicit stack
-        // - scan filling could be more efficient than plain recursion
-        1 + flood(rows, x - 1, y)
-            + flood(rows, x + 1, y)
-            + flood(rows, x, y - 1)
-            + flood(rows, x, y + 1)
+    // given a column in the row that is empty, find the left and rightmost columns that
+    // are also empty and adjacent to to it.
+    fn find_span(row: u16, x: i16) -> (i16, i16) {
+        let left = row & !(FULL_ROW << x);
+        let right = row & !((1 << x) - 1);
+        let lx = 16 - left.leading_zeros() as i16;
+        let rx = right.trailing_zeros() as i16;
+        (lx, rx)
     }
 
-    let (x0, y0) = init(rows)?;
-    Some((x0, y0, flood(rows, x0, y0)))
+    // list the new columns that should be passed on the stack for the given row and range
+    fn scan(row: u16, lx: i16, rx: i16) -> impl Iterator<Item = i16> {
+        let mut added = false;
+        (lx..rx).filter_map(move |x| {
+            if row & (1 << x) != 0 {
+                added = false;
+                None
+            } else if added {
+                None
+            } else {
+                added = true;
+                Some(x)
+            }
+        })
+    }
+
+    let mut area = 0;
+    let (x0, y0) = find_empty(rows)?;
+
+    type Stack = SmallVec<[(i16, i16); 4]>;
+    let mut stack = Stack::new();
+    stack.push((x0, y0));
+
+    while let Some((x, y)) = stack.pop() {
+        let row = &mut rows[y as usize];
+
+        let (lx, rx) = find_span(*row, x);
+        area += (rx - lx) as u32;
+        *row |= (FULL_ROW << lx) & !(FULL_ROW << rx);
+
+        if y > 0 {
+            for x in scan(rows[(y - 1) as usize], lx, rx) {
+                stack.push((x, y - 1));
+            }
+        }
+
+        if ((y + 1) as usize) < rows.len() {
+            for x in scan(rows[(y + 1) as usize], lx, rx) {
+                stack.push((x, y + 1));
+            }
+        }
+    }
+
+    Some((x0, y0, area))
 }
 
 #[cfg(test)]
 mod test {
     use super::*;
-    use mino::{mat, matrix::EMPTY_ROW};
+
+    use mino::mat;
 
     #[test]
     fn test_flood_fill() {