Description

The _decimal_div function uses the is_le function to compare the number of decimals between the numerator and the denominator. The specification of the is_le function states that it returns 1 when the first argument is less than or equal to the second argument:

// Returns 1 if a <= b (or more precisely 0 <= b - a < RANGE_CHECK_BOUND).
// Returns 0 otherwise.
@known_ap_change
func is_le{range_check_ptr}(a, b) -> felt {
    return is_nn(b - a);
}

The implementation of _decimal_div assumes otherwise. The is_le function will return TRUE if b_decimals <= a_decimals or a_decimals >= b_decimals. This is different from the code below, which assumes that the two numbers can only be equal in the else branch.

let b_fewer_dec = is_le(b_decimals, a_decimals);
if (b_fewer_dec == TRUE) {
    // x > y
    a_to_shift = a_value;
    result_decimals = a_decimals;
    tempvar range_check_ptr = range_check_ptr;
} else {
    // x <= y

As a result, the case when the two numbers are equal is handled by the first if branch instead of the else branch as expected by the code.

Impact

The correctness of the _decimal_div function is not affected by the incorrect usage of the is_le function as the code for handling the first if branch and the equality leads to the same outcome.

However, this same mistake may show up in other places, and such assumptions should be carefully verified before using them in code.

Recommendations

Rearrange the if conditions so that the case of equality is handled by the if branch rather than the else branch.

Remediation

The issue was addressed in a later update.