Bitwise packing & unpacking in cairo

packing.cairo

%lang starknet
%builtins pedersen range_check bitwise

from starkware.cairo.common.cairo_builtins import BitwiseBuiltin, HashBuiltin
from starkware.cairo.common.bitwise import bitwise_and
from starkware.cairo.common.math import assert_nn_le

const WORD = 2 ** 16
const MASK = WORD - 1
const MASK2 = MASK * WORD

@view
func pack2{range_check_ptr}(x : felt, y : felt) -> (res : felt):
    assert_nn_le(x, WORD)  # Could be removed as an optimization
    assert_nn_le(y, WORD)  # Could be removed as an optimization
    return (x * WORD + y)
end

@view
func unpack1{bitwise_ptr : BitwiseBuiltin*}(packed : felt) -> (res : felt):
    let (masked) = bitwise_and(packed, MASK2)
    let res = masked / WORD
    return (res=res)
end

@view
func unpack2{bitwise_ptr : BitwiseBuiltin*}(packed : felt) -> (res : felt):
    let (res) = bitwise_and(packed, MASK)
    return (res=res)
end

test_feltpack.py

import os
import pytest

from starkware.starknet.testing.starknet import Starknet

CONTRACT_FILE = os.path.join(os.path.dirname(__file__), "packing.cairo")


@pytest.mark.asyncio
async def test_bitwise_packing():
    starknet = await Starknet.empty()

    contract = await starknet.deploy(
        source=CONTRACT_FILE,
    )

    a = 2 ** 13 + 1
    b = 7

    (packed,) = (await contract.pack2(a, b).call()).result

    assert (await contract.unpack1(packed).call()).result == (a,)
    assert (await contract.unpack2(packed).call()).result == (b,)