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subMulLimbWithBorrow

a - b * c - *carry, sets carry to the overflow bits

int.subMulLimbWithBorrow

fn subMulLimbWithBorrow(a: Limb, b: Limb, c: Limb, carry: *Limb) Limb

File

lib/std/math/big/int.zig:149

Code

fn subMulLimbWithBorrow(a: Limb, b: Limb, c: Limb, carry: *Limb) Limb {
    // ov1[0] = a - *carry
    const ov1 = @subWithOverflow(a, carry.*);

    // r2 = b * c
    const bc = @as(DoubleLimb, std.math.mulWide(Limb, b, c));
    const r2 = @as(Limb, @truncate(bc));
    const c2 = @as(Limb, @truncate(bc >> limb_bits));

    // ov2[0] = ov1[0] - r2
    const ov2 = @subWithOverflow(ov1[0], r2);
    carry.* = ov1[1] + c2 + ov2[1];

    return ov2[0];
}