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fix(ssz): ignore BitVector padding bits #329

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fix(ssz): ignore BitVector padding bits #329

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9929350
fix(ssz): ignore BitVector padding bits
lodekeeper-z Apr 28, 2026
fe4768a
refactor(ssz): mask only final BitVector byte
lodekeeper-z May 7, 2026
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44 changes: 43 additions & 1 deletion src/ssz/type/bit_vector.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -40,6 +40,13 @@ pub fn BitVector(comptime _length: comptime_int) type {
}
}

fn maskedFinalByte(self: *const @This()) u8 {
const remainder_bits = length % 8;
std.debug.assert(remainder_bits != 0);
const tail_mask: u8 = (@as(u8, 1) << @intCast(remainder_bits)) - 1;
return self.data[byte_len - 1] & tail_mask;
}

pub fn getTrueBitIndexes(self: *const @This(), out: []usize) !usize {
if (out.len < length) {
return error.InvalidSize;
Expand All @@ -48,6 +55,11 @@ pub fn BitVector(comptime _length: comptime_int) type {

for (0..byte_len) |i_byte| {
var b = self.data[i_byte];
if (length % 8 != 0) {
if (i_byte + 1 == byte_len) {
b = self.maskedFinalByte();
}
}

while (b != 0) {
const lsb: usize = @as(u8, @ctz(b));
Expand All @@ -66,6 +78,11 @@ pub fn BitVector(comptime _length: comptime_int) type {

for (0..byte_len) |i_byte| {
var b = self.data[i_byte];
if (length % 8 != 0) {
if (i_byte + 1 == byte_len) {
b = self.maskedFinalByte();
}
}

while (b != 0) {
if (found_index != null) {
Expand Down Expand Up @@ -132,10 +149,15 @@ pub fn BitVector(comptime _length: comptime_int) type {
allocator: std.mem.Allocator,
values: *const [length]T,
) !std.array_list.AlignedManaged(T, null) {
var indices = try std.array_list.AlignedManaged(T, null).initCapacity(allocator, byte_len * 8);
var indices = try std.array_list.AlignedManaged(T, null).initCapacity(allocator, length);

for (0..byte_len) |i_byte| {
var b = self.data[i_byte];
if (length % 8 != 0) {
if (i_byte + 1 == byte_len) {
b = self.maskedFinalByte();
}
}
// Kernighan's algorithm to count the set bits instead of going through 0..8 for every byte
while (b != 0) {
const lsb: usize = @as(u8, @ctz(b)); // Get the index of least significant bit
Expand Down Expand Up @@ -559,6 +581,26 @@ test "BitVectorType - tree.deserializeFromBytes 128 bits" {
}
}

test "BitVectorType helpers ignore padding bits" {
const allocator = std.testing.allocator;
const Bits = BitVectorType(4);

var value = Bits.Type.empty;
value.data[0] = 0b1111_0001;

var indexes: [Bits.length]usize = undefined;
const count = try value.getTrueBitIndexes(indexes[0..]);
try std.testing.expectEqual(@as(usize, 1), count);
try std.testing.expectEqualSlices(usize, &.{0}, indexes[0..count]);

try std.testing.expectEqual(@as(?usize, 0), value.getSingleTrueBit());

const input_values = [_]u8{ 10, 11, 12, 13 };
var intersected = try value.intersectValues(u8, allocator, &input_values);
defer intersected.deinit();
try std.testing.expectEqualSlices(u8, &.{10}, intersected.items);
}

const TypeTestCase = @import("test_utils.zig").TypeTestCase;
test "BitVectorType of 128 bits" {
const testCases = [_]TypeTestCase{
Expand Down
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