How to multiply a BigInteger with a decimal?

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How to multiply a BigInteger with a decimal in C#?

var bi = new BigInteger(1000);
var d = 0.9m;
// HowTo:
var res = BigInteger.Multiply(bi, d); // res = 900

Of course, the result should be floored down to the previous full integer value.

There is a practical background, but in regards to the posted "duplicate", I'm also interested into an answer to the question from a theoretical point of view. I'm not looking for a workaround by using double instead.

biginteger .net c#
1 Answers

You can represent decimal as fraction

(BigInteger numerator, BigInteger denominator) Fraction(decimal d) {
    int[] bits = decimal.GetBits(d);
    BigInteger numerator = (1 - ((bits[3] >> 30) & 2)) *
                           unchecked(((BigInteger)(uint)bits[2] << 64) |
                                     ((BigInteger)(uint)bits[1] << 32) |
                                      (BigInteger)(uint)bits[0]);
    BigInteger denominator = BigInteger.Pow(10, (bits[3] >> 16) & 0xff);
    return (numerator, denominator);
}

Then you can multiply BigInteger to numerator and divide by denominator:

var bi = new BigInteger(1000);
var d = 0.9m;
var f = Fraction(d);
var res = bi * f.numerator / f.denominator;
user_40034070
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