DoxigAlpha
std › math › big › int.Mutable › mulNoAlias

mulNoAlias

rma = a * b

rma may not alias with a or b. a and b may alias with each other.

Asserts the result fits in rma. An upper bound on the number of limbs needed by rma is given by a.limbs.len + b.limbs.len.

If allocator is provided, it will be used for temporary storage to improve multiplication performance. error.OutOfMemory is handled with a fallback algorithm.

Function parameters

Parameters

#
rma:*Mutable
allocator:?Allocator

Used to indicate either limit of a 2s-complement integer.

Types

#
TwosCompIntLimit
Used to indicate either limit of a 2s-complement integer.
Mutable
A arbitrary-precision big integer, with a fixed set of mutable limbs.
Const
A arbitrary-precision big integer, with a fixed set of immutable limbs.
Managed
An arbitrary-precision big integer along with an allocator which manages the memory.

Returns the number of limbs needed to store `scalar`, which must be a

Functions

#
calcLimbLen
Returns the number of limbs needed to store `scalar`, which must be a
calcSetStringLimbCount
Assumes `string_len` doesn't account for minus signs if the number is negative.
calcNonZeroTwosCompLimbCount
Compute the number of limbs required to store a 2s-complement number of `bit_count` bits.
calcTwosCompLimbCount
Compute the number of limbs required to store a 2s-complement number of `bit_count` bits.
addMulLimbWithCarry
a + b * c + *carry, sets carry to the overflow bits
llcmp
Returns -1, 0, 1 if |a| < |b|, |a| == |b| or |a| > |b| respectively for limbs.

Source

Implementation

#
pub fn mulNoAlias(rma: *Mutable, a: Const, b: Const, allocator: ?Allocator) void {
    assert(rma.limbs.ptr != a.limbs.ptr); // illegal aliasing
    assert(rma.limbs.ptr != b.limbs.ptr); // illegal aliasing

    if (a.limbs.len == 1 and b.limbs.len == 1) {
        rma.limbs[0], const overflow_bit = @mulWithOverflow(a.limbs[0], b.limbs[0]);
        if (overflow_bit == 0) {
            rma.len = 1;
            rma.positive = (a.positive == b.positive) or rma.limbs[0] == 0;
            return;
        }
    }

    @memset(rma.limbs[0 .. a.limbs.len + b.limbs.len], 0);

    llmulacc(.add, allocator, rma.limbs, a.limbs, b.limbs);

    rma.normalize(a.limbs.len + b.limbs.len);
    rma.positive = (a.positive == b.positive);
}