code quality: downstacking.rs

fish/src/eval/downstacking.rs

@@ -21,9 +21,10 @@ pub fn mystery_mdse(init_mat: &Mat) -> i32 {
         debug_assert!(!res_ys.is_empty(), "{res_ys:?}");
 
         let res = &mut mat[res_ys];
+        // determine pieces needed to fill residue by filling them
         if let Some((_x0, _y0, area)) = flood_fill(res) {
-            // estimate pieces needed to fill residue, and then fill them
-            let res_pc = (area + 3) / 4; // = ceil(area/4)
+            // min number of pieces = ceil(area/4)
+            let res_pc = (area + 3) / 4;
 
             // add to total count, but dampen by iteration count; residue cleared later on
             // most likely has blocks contributed to it by attempts to clear in earlier
@@ -54,39 +55,42 @@ pub fn mystery_mdse(init_mat: &Mat) -> i32 {
 ///     1 xxxxxxxx.x
 ///     0 xx..xxxxxx
 ///
-/// Given this stack, `residue` returns the range `2..=3`. The hole found is in row 1
+/// Given this stack, `residue` returns the range `2..4`. The hole found is in row 1
 /// column 8, and there are two rows of residue above it. Note that the hole in row 0
 /// columns 2-3 is also a valid hole, but row 1 is found first because it is on a higher
 /// row.
 fn residue(mat: &Mat) -> Option<Range<i16>> {
-    let mut y = mat.rows();
-    let mut prev_row = EMPTY_ROW; // = mat[y]
+    // find hole that is covered
+    let mut mask;
 
-    // find garbage row
-    let mask = loop {
-        if y == 0 {
+    let mut y0 = mat.rows();
+    let mut prev_row = EMPTY_ROW; // = mat[y]
+    loop {
+        if y0 == 0 {
             // hit the bottom
             return None;
         }
-        let curr_row = mat[y - 1];
-        let mask = prev_row & !curr_row;
+        let curr_row = mat[y0 - 1];
+
         // for some bit i, prev_row[i] == 1 && curr_row[i] == 0
+        mask = prev_row & !curr_row;
         if mask != 0 {
-            break mask;
+            break;
         }
-        y -= 1;
+
+        y0 -= 1;
         prev_row = curr_row;
-    };
+    }
 
     // find top of residue
-    let y0 = y;
-    let y1 = loop {
-        let curr_row = mat[y];
+    let mut y1 = y0;
+    loop {
+        let curr_row = mat[y1];
         if curr_row & mask == 0 {
-            break y;
+            break;
         }
-        y += 1;
-    };
+        y1 += 1;
+    }
 
     Some(y0..y1)
 }
@@ -98,6 +102,9 @@ fn residue(mat: &Mat) -> Option<Range<i16>> {
 fn flood_fill(rows: &mut [u16]) -> Option<(i16, i16, u32)> {
     fn init(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
+            // type).
             let x = row.trailing_ones();
             if x < 16 {
                 return Some((x as i16, y as i16));
@@ -107,10 +114,12 @@ fn flood_fill(rows: &mut [u16]) -> Option<(i16, i16, u32)> {
     }
 
     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 {
@@ -128,7 +137,6 @@ fn flood_fill(rows: &mut [u16]) -> Option<(i16, i16, u32)> {
     }
 
     let (x0, y0) = init(rows)?;
-
     Some((x0, y0, flood(rows, x0, y0)))
 }