[board] Implement a BitScanner on u64

This struct implements an Iterator that returns the positions of the 1 bits in the u64.
BitBoard takes this iterator and maps it into Squares in the pieces() method.

This is laying the groundwork for an iterator over the pieces in a Position. 👀

board/src/bitboard.rs

@@ -1,11 +1,14 @@
 // Eryn Wells <eryn@erynwells.me>
 
+mod bit_scanner;
+
+use self::bit_scanner::BitScanner;
 use crate::square::Square;
 use std::fmt;
 use std::ops::{BitAnd, BitOr, Not};
 
 #[derive(Clone, Copy, Default, Eq, Hash, Ord, PartialEq, PartialOrd)]
-pub struct BitBoard(u64);
+pub(crate) struct BitBoard(u64);
 
 impl BitBoard {
     pub fn empty() -> BitBoard {
@@ -16,7 +19,7 @@ impl BitBoard {
         BitBoard(bits)
     }
 
-    fn is_empty(&self) -> bool {
+    pub fn is_empty(&self) -> bool {
         self.0 == 0
     }
 
@@ -31,20 +34,6 @@ impl BitBoard {
     fn remove_piece_at(&mut self, sq: &Square) {
         self.0 &= !(1 << sq.index)
     }
-
-    #[cfg(target_arch = "arm")]
-    fn _arm_count_leading_zeros(&self) -> u8 {
-        let mut number_of_leading_zeros: u8 = 0;
-        unsafe {
-            asm!(
-                "clz {count}, {bitfield}",
-                count = out(reg) number_of_leading_zeros,
-                bitfield = in(reg) self.0,
-            );
-        }
-
-        number_of_leading_zeros
-    }
 }
 
 impl BitBoard {
@@ -92,6 +81,15 @@ impl BitBoard {
     }
 }
 
+impl BitBoard {
+    fn pieces(&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_unsafe(x.unwrap()))
+    }
+}
+
 impl fmt::Debug for BitBoard {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
         let bits = self.0;
@@ -229,12 +227,15 @@ mod tests {
         );
     }
 
-    #[cfg(target_arch = "arm")]
     #[test]
-    fn arm_count_leading_zeros() {
-        assert_eq!(BitBoard(0)._arm_count_leading_zeros(), 0);
-        assert_eq!(BitBoard(0b10)._arm_count_leading_zeros(), 62);
-        assert_eq!(BitBoard(0b1010)._arm_count_leading_zeros(), 60);
-        assert_eq!(BitBoard(0x80000000)._arm_count_leading_zeros(), 0);
+    fn pieces() {
+        let bb = BitBoard(0x1001_1010); // e4
+
+        let mut pieces = bb.pieces();
+        assert_eq!(pieces.next(), Some(Square::from_index_unsafe(28)));
+        assert_eq!(pieces.next(), Some(Square::from_index_unsafe(16)));
+        assert_eq!(pieces.next(), Some(Square::from_index_unsafe(12)));
+        assert_eq!(pieces.next(), Some(Square::from_index_unsafe(4)));
+        assert_eq!(pieces.next(), None);
     }
 }

board/src/bitboard/bit_scanner.rs

@@ -0,0 +1,67 @@
+// Eryn Wells <eryn@erynwells.me>
+
+use super::BitBoard;
+use crate::square::Square;
+
+pub(crate) struct BitScanner {
+    bits: u64,
+    shift: u32,
+}
+
+impl BitScanner {
+    pub(crate) fn new(bits: u64) -> BitScanner {
+        BitScanner { bits, shift: 0 }
+    }
+}
+
+impl Iterator for BitScanner {
+    type Item = u32;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.shift == u64::BITS {
+            return None;
+        }
+
+        let shifted_bits = self.bits << self.shift;
+        let leading_zeros = shifted_bits.leading_zeros();
+
+        if leading_zeros == u64::BITS {
+            self.shift = leading_zeros;
+            return None;
+        }
+
+        let position = u64::BITS - (self.shift + leading_zeros + 1);
+        // Shift 1 additional place to account for the 1 that `leading_zeros` found.
+        self.shift += leading_zeros + 1;
+
+        Some(position)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn zero() {
+        let mut scanner = BitScanner::new(0);
+        assert_eq!(scanner.next(), None);
+    }
+
+    #[test]
+    fn one() {
+        let mut scanner = BitScanner::new(1);
+        assert_eq!(scanner.next(), Some(0));
+        assert_eq!(scanner.next(), None);
+    }
+
+    #[test]
+    fn complex() {
+        let mut scanner = BitScanner::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);
+    }
+}

board/src/square.rs

@@ -25,7 +25,7 @@ impl Square {
         Ok(Square::from_index_unsafe(index))
     }
 
-    fn from_index_unsafe(index: u8) -> Square {
+    pub(crate) fn from_index_unsafe(index: u8) -> Square {
         Square {
             rank: index / 8,
             file: index % 8,