[board] Implement a TrailingBitScanner

This bit scanner iterates populated bits in a BitBoard from the trailing (least-significant) end.

Rename BitScanner → LeadingBitScanner.
Factor implementation of these two into a macro.
Clean up iterator returned by BitBoard::occupied_squares
Implement BitBoard::occupied_squares_trailing

board/src/bitboard/bit_scanner.rs

@@ -1,17 +1,24 @@
 // Eryn Wells <eryn@erynwells.me>
 
-pub(crate) struct BitScanner {
-    bits: u64,
-    shift: usize,
+macro_rules! bit_scanner {
+    ($name:ident) => {
+        pub(crate) struct $name {
+            bits: u64,
+            shift: usize,
+        }
+
+        impl $name {
+            pub(crate) fn new(bits: u64) -> Self {
+                Self { bits, shift: 0 }
+            }
+        }
+    };
 }
 
-impl BitScanner {
-    pub(crate) fn new(bits: u64) -> BitScanner {
-        BitScanner { bits, shift: 0 }
-    }
-}
+bit_scanner!(LeadingBitScanner);
+bit_scanner!(TrailingBitScanner);
 
-impl Iterator for BitScanner {
+impl Iterator for LeadingBitScanner {
     type Item = usize;
 
     fn next(&mut self) -> Option<Self::Item> {
@@ -37,30 +44,79 @@ impl Iterator for BitScanner {
     }
 }
 
+impl Iterator for TrailingBitScanner {
+    type Item = usize;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let u64bits = u64::BITS as usize;
+
+        if self.shift == u64bits {
+            return None;
+        }
+
+        let shifted_bits = self.bits >> self.shift;
+        let trailing_zeros = shifted_bits.trailing_zeros() as usize;
+
+        if trailing_zeros == u64bits {
+            self.shift = trailing_zeros;
+            return None;
+        }
+
+        let position = self.shift + trailing_zeros;
+        // Shift 1 additional place to account for the 1 that `leading_zeros` found.
+        self.shift += trailing_zeros + 1;
+
+        Some(position)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
     #[test]
-    fn zero() {
-        let mut scanner = BitScanner::new(0);
+    fn leading_zero() {
+        let mut scanner = LeadingBitScanner::new(0);
         assert_eq!(scanner.next(), None);
     }
 
     #[test]
-    fn one() {
-        let mut scanner = BitScanner::new(1);
+    fn leading_one() {
+        let mut scanner = LeadingBitScanner::new(1);
         assert_eq!(scanner.next(), Some(0));
         assert_eq!(scanner.next(), None);
     }
 
     #[test]
-    fn complex() {
-        let mut scanner = BitScanner::new(0b11000101);
+    fn leading_complex() {
+        let mut scanner = LeadingBitScanner::new(0b11000101);
         assert_eq!(scanner.next(), Some(7));
         assert_eq!(scanner.next(), Some(6));
         assert_eq!(scanner.next(), Some(2));
         assert_eq!(scanner.next(), Some(0));
         assert_eq!(scanner.next(), None);
     }
+
+    #[test]
+    fn trailing_zero() {
+        let mut scanner = TrailingBitScanner::new(0);
+        assert_eq!(scanner.next(), None);
+    }
+
+    #[test]
+    fn trailing_one() {
+        let mut scanner = TrailingBitScanner::new(1);
+        assert_eq!(scanner.next(), Some(0));
+        assert_eq!(scanner.next(), None);
+    }
+
+    #[test]
+    fn trailing_complex() {
+        let mut scanner = TrailingBitScanner::new(0b11000101);
+        assert_eq!(scanner.next(), Some(0));
+        assert_eq!(scanner.next(), Some(2));
+        assert_eq!(scanner.next(), Some(6));
+        assert_eq!(scanner.next(), Some(7));
+        assert_eq!(scanner.next(), None);
+    }
 }

board/src/bitboard/bitboard.rs

@@ -1,8 +1,8 @@
 // Eryn Wells <eryn@erynwells.me>
 
 use super::library::{library, FILES, RANKS};
-use super::BitScanner;
-use crate::Square;
+use super::{LeadingBitScanner, TrailingBitScanner};
+use crate::{square::Direction, Square};
 use std::fmt;
 use std::ops::{BitAnd, BitAndAssign, BitOr, BitOrAssign, Not};
 
@@ -62,11 +62,16 @@ impl BitBoard {
 }
 
 impl BitBoard {
+    /// Return an Iterator over the occupied squares, starting from the leading
+    /// (most-significant bit) end of the field.
     pub(crate) fn occupied_squares(&self) -> impl Iterator<Item = Square> {
-        BitScanner::new(self.0)
-            .map(|x| u8::try_from(x))
-            .take_while(|x| x.is_ok())
-            .map(|x| Square::from_index(x.unwrap() as usize))
+        LeadingBitScanner::new(self.0).map(Square::from_index)
+    }
+
+    /// Return an Iterator over the occupied squares, starting from the trailing
+    /// (least-significant bit) end of the field.
+    pub(crate) fn occupied_squares_trailing(&self) -> impl Iterator<Item = Square> {
+        LeadingBitScanner::new(self.0).map(Square::from_index)
     }
 }
 

board/src/bitboard/mod.rs

@@ -3,5 +3,5 @@ mod bitboard;
 mod library;
 mod shifts;
 
-pub(crate) use self::bit_scanner::BitScanner;
+pub(crate) use self::bit_scanner::{LeadingBitScanner, TrailingBitScanner};
 pub(crate) use self::bitboard::BitBoard;