readPackedTwosComplement

    pub fn readPackedTwosComplement(
        x: *Mutable,
        buffer: []const u8,
        bit_offset: usize,
        bit_count: usize,
        endian: Endian,
        signedness: Signedness,
    ) void {
        if (bit_count == 0) {
            x.limbs[0] = 0;
            x.len = 1;
            x.positive = true;
            return;
        }

        // Check whether the input is negative
        var positive = true;
        if (signedness == .signed) {
            const total_bits = bit_offset + bit_count;
            const last_byte = switch (endian) {
                .little => ((total_bits + 7) / 8) - 1,
                .big => buffer.len - ((total_bits + 7) / 8),
            };

            const sign_bit = @as(u8, 1) << @as(u3, @intCast((total_bits - 1) % 8));
            positive = ((buffer[last_byte] & sign_bit) == 0);
        }

        // Copy all complete limbs
        var carry: u1 = 1;
        var limb_index: usize = 0;
        var bit_index: usize = 0;
        while (limb_index < bit_count / @bitSizeOf(Limb)) : (limb_index += 1) {
            // Read one Limb of bits
            var limb = mem.readPackedInt(Limb, buffer, bit_index + bit_offset, endian);
            bit_index += @bitSizeOf(Limb);

            // 2's complement (bitwise not, then add carry bit)
            if (!positive) {
                const ov = @addWithOverflow(~limb, carry);
                limb = ov[0];
                carry = ov[1];
            }
            x.limbs[limb_index] = limb;
        }

        // Copy the remaining bits
        if (bit_count != bit_index) {
            // Read all remaining bits
            var limb = switch (signedness) {
                .unsigned => mem.readVarPackedInt(Limb, buffer, bit_index + bit_offset, bit_count - bit_index, endian, .unsigned),
                .signed => b: {
                    const SLimb = @Int(.signed, @bitSizeOf(Limb));
                    const limb = mem.readVarPackedInt(SLimb, buffer, bit_index + bit_offset, bit_count - bit_index, endian, .signed);
                    break :b @as(Limb, @bitCast(limb));
                },
            };

            // 2's complement (bitwise not, then add carry bit)
            if (!positive) {
                const ov = @addWithOverflow(~limb, carry);
                assert(ov[1] == 0);
                limb = ov[0];
            }
            x.limbs[limb_index] = limb;

            limb_index += 1;
        }

        x.positive = positive;
        x.len = limb_index;
        x.normalize(x.len);
    }