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"a * b mod c"

Is there any well-known, efficient and simple algorithm to implement it?

I only know it can be calculated sth. like this..

__int64 a, b, c

a = x1 * 2^32 + x0

b = y1 * 2^32 + y0

then a * b = x1y1 * 2^64 + (x0y1 + x1y0) * 2^32 + x0y0

but I dunno what to do in the next step

Note that this cannot be calculated using some classes like BigInt, gmp

Thank you very much!!!