Rewrite left and right shift as single-pass algorithms

1 files changed, 28 insertions, 28 deletions

diff --git a/chilbert/detail/MultiBitVec.hpp b/chilbert/detail/MultiBitVec.hpp
index a184287..573deef 100644
--- a/chilbert/detail/MultiBitVec.hpp
+++ b/chilbert/detail/MultiBitVec.hpp
@@ -266,25 +266,25 @@ public:
 
 		const Index index{bits};
 
-		if (index.rack > 0) {
-			// Shift entire racks
+		if (index.bit == 0) {
+			// Simple rack-aligned shift
 			for (size_t i = num_racks() - 1; i >= index.rack; --i) {
 				rack(i) = rack(i - index.rack);
 			}
-			for (size_t i = 0; i < index.rack; ++i) {
-				rack(i) = 0;
+		} else {
+			// Rack + bit offset shift
+			const size_t right_shift = bits_per_rack - index.bit;
+			for (size_t i = num_racks() - index.rack - 1; i > 0; --i) {
+				rack(i + index.rack) =
+				  (rack(i) << index.bit) | (rack(i - 1) >> right_shift);
 			}
+
+			rack(index.rack) = rack(0) << index.bit;
 		}
 
-		if (index.bit > 0) {
-			// Shift bits within racks
-			size_t bi = bits_per_rack - index.bit;
-			size_t i;
-			for (i = num_racks() - 1; i >= index.rack + 1; --i) {
-				rack(i) <<= index.bit;
-				rack(i) |= rack(i - 1) >> bi;
-			}
-			rack(i) <<= index.bit;
+		// Zero least significant racks
+		for (size_t i = 0; i < index.rack; ++i) {
+			rack(i) = 0;
 		}
 
 		return *self();
@@ -301,26 +301,26 @@ public:
 
 		const Index index{bits};
 
-		if (index.rack > 0) {
-			// Shift entire racks
-			size_t i;
-			for (i = 0; i < num_racks() - index.rack; ++i) {
+		if (index.bit == 0) {
+			// Simple rack-aligned shift
+			for (size_t i = 0; i < num_racks() - index.rack; ++i) {
 				rack(i) = rack(i + index.rack);
 			}
-			for (; i < num_racks(); ++i) {
-				rack(i) = 0;
+		} else {
+			// Rack + bit offset shift
+			const size_t last       = num_racks() - 1;
+			const size_t left_shift = bits_per_rack - index.bit;
+			for (size_t i = index.rack; i < last; ++i) {
+				rack(i - index.rack) =
+				  (rack(i) >> index.bit) | (rack(i + 1) << left_shift);
 			}
+
+			rack(last - index.rack) = rack(last) >> index.bit;
 		}
 
-		if (index.bit > 0) {
-			// Shift bits within racks
-			size_t bi = bits_per_rack - index.bit;
-			size_t i;
-			for (i = 0; i < num_racks() - index.rack - 1; ++i) {
-				rack(i) >>= index.bit;
-				rack(i) |= rack(i + 1) << bi;
-			}
-			rack(i) >>= index.bit;
+		// Zero most significant racks
+		for (size_t i = num_racks() - index.rack; i < num_racks(); ++i) {
+			rack(i) = 0;
 		}
 
 		return *self();