Add ´is_even(value, rc_bound)´ function to ´math_utils.py´

[omarespejel]

Hey, looking to contribute and find this a necessity.

Description

Add a function that allows checking if a value is even.

Why?

Existing functions like merkle_update could benefit from such a check. Further functions (I am creating one to calculate the median value of an array) could benefit from distinguishing between even and odd values easily.

What

Created is_even(value, rc_bound) which allows:

• asserts value is an integer
• asserts value is within the (-rc_bound, rc_bound) range
• returns true if value is even

Who could review?

Thank you and please let me know if any change is required.

@glrn @l-henri @liorgold2

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This change is 

[liorgold2]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: all files reviewed, 5 unresolved discussions (waiting on @omarespejel)

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src/starkware/cairo/common/math_utils.py line 26 at r1 (raw file):

    assert abs(val) < rc_bound, f"value={val} is out of the valid range."
    return val > 0

And a second blank line between the two functions.

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src/starkware/cairo/common/math_utils.py line 27 at r1 (raw file):

    return val > 0

def is_even(value, rc_bound):

I wonder if this function is necessary. How do you intend to use it?

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src/starkware/cairo/common/math_utils.py line 29 at r1 (raw file):

def is_even(value, rc_bound):
    """
    Returns true if the input is even and within the range (-rc_bound, rc_bound).

Use the same phrasing as above:

Returns True if the lift of the given field element, as an integer in the range
(-rc_bound, rc_bound), is even.

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src/starkware/cairo/common/math_utils.py line 32 at r1 (raw file):

    Raises an exception if the element is not within the accepted range.
    """
    assert_integer(value)

You should use as_int as above to make sure that if value is, for example, PRIME - 1, it will be interpreted as -1 (otherwise, the function will fail on the "out of range" error instead of simply returning False)

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src/starkware/cairo/common/math_utils.py line 34 at r1 (raw file):

    assert_integer(value)
    assert abs(value) < rc_bound, f"value={value} is out of the valid range."
    return value % 2 == 0

End file with the new-line character.

[liorgold2]

Reviewable status: all files reviewed, 5 unresolved discussions (waiting on @omarespejel)

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src/starkware/cairo/common/math_utils.py line 27 at r1 (raw file):

Previously, liorgold2 wrote…

I wonder if this function is necessary. How do you intend to use it?

In particular, can you give a short code example taking advantage of this function?

[omarespejel]

Thank you for your kind review @liorgold2! I applied the feedback. I will add a couple of examples of use cases in my next commit.

Meanwhile, I convert the PR to draft.