Fix bitpacking (#13)

Author: gatesn

src/bitpacking.zig

@@ -54,3 +54,23 @@ test "bitpack" {
     BP.decode(3, &packed_ints, &output);
     try std.testing.expectEqual(.{2} ** 1024, output);
 }
+
+test "bitpack range" {
+    const std = @import("std");
+    const fl = @import("./fastlanez.zig");
+    const BP = BitPacking(fl.FastLanez(u8));
+
+    const W = 3;
+
+    var ints: [1024]u8 = undefined;
+    for (0..1024) |i| {
+        ints[i] = @intCast(i % 7);
+    }
+
+    var packed_ints: [128 * W]u8 = undefined;
+    BP.encode(W, &ints, &packed_ints);
+
+    var output: [1024]u8 = undefined;
+    BP.decode(W, &packed_ints, &output);
+    try std.testing.expectEqual(ints, output);
+}

src/fastlanez.zig

@@ -123,10 +123,10 @@ pub fn FastLanez(comptime Element: type) type {
                 /// The position in the output that we're writing to. Will finish equal to Width.
                 out_idx: comptime_int = 0,
 
-                shift_bits: comptime_int = 0,
-                mask_bits: comptime_int = Width,
+                bit_idx: comptime_int = 0,
 
                 /// Invoke to store the next vector.
+                /// Called T times, and writes W times. bit_idx tracks how many bits have been written into the result.
                 pub inline fn pack(comptime self: *Self, out: *PackedBytes(Width), word: MM1024, state: MM1024) MM1024 {
                     var tmp: MM1024 = undefined;
                     if (self.t == 0) {
@@ -140,24 +140,23 @@ pub fn FastLanez(comptime Element: type) type {
                     }
                     self.t += 1;
 
-                    // If we didn't take all W bits last time, then we load the remainder
-                    if (self.mask_bits < Width) {
-                        tmp = or_(tmp, and_rshift(word, self.mask_bits, bitmask(self.shift_bits)));
-                    }
-
-                    // Update the number of mask bits
-                    self.mask_bits = @min(T - self.shift_bits, Width);
+                    const shift_bits = self.bit_idx % T;
+                    const mask_bits = @min(T - shift_bits, Width - (self.bit_idx % Width));
 
-                    // Pull the masked bits into the tmp register
-                    tmp = or_(tmp, and_lshift(word, self.shift_bits, bitmask(self.mask_bits)));
-                    self.shift_bits += Width;
+                    tmp = or_(tmp, and_lshift(word, shift_bits, bitmask(mask_bits)));
+                    self.bit_idx += mask_bits;
 
-                    if (self.shift_bits >= T) {
-                        // If we have a full 1024 bits, then store it and reset the tmp register
+                    if (self.bit_idx % T == 0) {
+                        // If we have a full T bits, then store it and reset the tmp register
                         store(out, self.out_idx, tmp);
                         tmp = @splat(0);
                         self.out_idx += 1;
-                        self.shift_bits -= T;
+
+                        // Put the remainder of the bits in the next word
+                        if (mask_bits < Width) {
+                            tmp = or_(tmp, and_rshift(word, mask_bits, bitmask(Width - mask_bits)));
+                            self.bit_idx += (Width - mask_bits);
+                        }
                     }
 
                     return tmp;
@@ -177,7 +176,7 @@ pub fn FastLanez(comptime Element: type) type {
                 t: comptime_int = 0,
 
                 input_idx: comptime_int = 0,
-                shift_bits: comptime_int = 0,
+                bit_idx: comptime_int = 0,
 
                 pub inline fn unpack(comptime self: *Self, input: *const PackedBytes(Width), state: MM1024) struct { MM1024, MM1024 } {
                     if (self.t > T) {
@@ -193,24 +192,19 @@ pub fn FastLanez(comptime Element: type) type {
                         tmp = state;
                     }
 
-                    const mask_bits = @min(T - self.shift_bits, Width);
+                    const shift_bits = self.bit_idx % T;
+                    const mask_bits = @min(T - shift_bits, Width - (self.bit_idx % Width));
 
-                    var next: MM1024 = undefined;
-                    if (self.shift_bits == T) {
-                        next = tmp;
-                    } else {
-                        next = and_rshift(tmp, self.shift_bits, bitmask(mask_bits));
-                    }
+                    var next: MM1024 = and_rshift(tmp, shift_bits, bitmask(mask_bits));
 
-                    if (mask_bits != Width) {
+                    if (mask_bits != Width and self.input_idx < Width) {
                         tmp = load(input, self.input_idx);
                         self.input_idx += 1;
 
                         next = or_(next, and_lshift(tmp, mask_bits, bitmask(Width - mask_bits)));
-
-                        self.shift_bits = Width - mask_bits;
+                        self.bit_idx += Width;
                     } else {
-                        self.shift_bits += Width;
+                        self.bit_idx += mask_bits;
                     }
 
                     return .{ next, tmp };