byteSwap

    pub fn byteSwap(r: *Mutable, a: Const, signedness: Signedness, byte_count: usize) void {
        if (byte_count == 0) {
            r.limbs[0] = 0;
            r.len = 1;
            r.positive = true;
            return;
        }

        r.copy(a);
        const limbs_required = calcTwosCompLimbCount(8 * byte_count);

        if (!a.positive) {
            r.positive = true; // Negate.
            r.bitNotWrap(r.toConst(), .unsigned, 8 * byte_count); // Bitwise NOT.
            r.addScalar(r.toConst(), 1); // Add one.
        } else if (limbs_required > a.limbs.len) {
            // Zero-extend to our output length
            for (r.limbs[a.limbs.len..limbs_required]) |*limb| {
                limb.* = 0;
            }
            r.len = limbs_required;
        }

        // 0b0..01..1 with @log2(@sizeOf(Limb)) trailing ones
        const endian_mask: usize = @sizeOf(Limb) - 1;

        var bytes = std.mem.sliceAsBytes(r.limbs);
        assert(bytes.len >= byte_count);

        var k: usize = 0;
        while (k < (byte_count + 1) / 2) : (k += 1) {
            var i = k;
            var rev_i = byte_count - k - 1;

            // This "endian mask" remaps a low (LE) byte to the corresponding high
            // (BE) byte in the Limb, without changing which limbs we are indexing
            if (native_endian == .big) {
                i ^= endian_mask;
                rev_i ^= endian_mask;
            }

            const byte_i = bytes[i];
            const byte_rev_i = bytes[rev_i];
            bytes[rev_i] = byte_i;
            bytes[i] = byte_rev_i;
        }

        // Calculate signed-magnitude representation for output
        if (signedness == .signed) {
            const last_byte = switch (native_endian) {
                .little => bytes[byte_count - 1],
                .big => bytes[(byte_count - 1) ^ endian_mask],
            };

            if (last_byte & (1 << 7) != 0) { // Check sign bit of last byte
                r.bitNotWrap(r.toConst(), .unsigned, 8 * byte_count); // Bitwise NOT.
                r.addScalar(r.toConst(), 1); // Add one.
                r.positive = false; // Negate.
            }
        }
        r.normalize(r.len);
    }