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Find if a number is divisible by 17 using bitwise operators

Result of division is a quotient and a remainder.
n/17 = floor(n/17) + (n%17)/17 (Assuming we are storing result in a float, not int)

Multiplying numerator and denominator on the right-hand-size by 16:

n/17 = 16n / 17*16
     = (17-1)n / 17*16
     = n/16 - n/17*16

Now, replacing n/16 with first definition: n/16 = floor(n/16) + (n%16)/16, we get:
     = floor(n/16) + (n%16)/16    -  (floor(n/16) + (n%16)/16)/17
     = floor(n/16)                -  (floor(n/16) - 17*(n%16)/16 + (n%16)/16)/17
     = floor(n/16)                -  (floor(n/16) - n%16)/17

The left-hand-side of this equation is n/17. That will be an integer only when the right-hand-side is an integer.
floor(n/16) has to be an integer by definition.
So the whole left-hand-side would be an integer if (floor(n/16) - n%16)/17 is also an integer.

This implies n is divisible by 17 if (floor(n/16) - n%16) is divisible by 17

Recursively, this can be written as:
boolean isDivBy17(int n)
    if (n == 0 || n == 17)
        return true;
    if (n < 17)
        return false;
    return isDivBy17((int)(n>>4) - (int)(n&15));

Note that this property holds true for any n of the form 2k + 1 like 5, 9, 17, 33 and so on
This is because all of them can be broken up using 2k

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Site Owner: Sachin Goyal