JS reverse engineering a bit value

Question

I'm playing around with handling stuff in bit values while on a side project for learning. I'm unsure if what I'm doing is correct.

I have an object, which contains a list of bit values:

const bitVal = {
   food_item_1: 0x04,
   food_item_2: 0x08,
   food_item_3: 0x40
}

and so on.

I have an operation that when a user selects a combination of food items, it will add these values together like so

bitValue.food_item_1 | bitValue.food_item_3

and then i'll save this in the database.

My issue is, when I then get the bit values back. How do i figure out what items make up the bit number?

So food item 1 and food item 2 = 68. Based on that, when i get, 68 back from the DB how would i figure out what two, or three (there can be more than two, example is just two) is used?

My first attempt, is a function that iterates over all the values in the bitValue object and computes each one and see if it equals the value in the DB.

Issue here is. If i have 10 or so values in that bit field, that's a lot of computation to figure it out.

Is there a better way of handling this?

Nina Scholz · Accepted Answer · 2018-07-22 18:56:10Z

1

You could iterate the values and perform a bitwise AND with the combined value for a truthy check.

const
    bitVal = { food_item_1: 0x04, food_item_2: 0x08, food_item_3: 0x40 },
    value = 68;
    
Object.entries(bitVal).forEach(([k, v]) => {
    if (v & value) {
        console.log(k);
    }
});

answered Jul 22, 2018 at 18:56

Nina Scholz

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