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llsubcarry

Knuth 4.3.1, Algorithm S.

int.llsubcarry

fn llsubcarry(r: []Limb, a: []const Limb, b: []const Limb) Limb

File

lib/std/math/big/int.zig:3977

Code

fn llsubcarry(r: []Limb, a: []const Limb, b: []const Limb) Limb {
    assert(a.len != 0 and b.len != 0);
    assert(a.len >= b.len);
    assert(r.len >= a.len);
    assert(!slicesOverlap(r, a) or @intFromPtr(r.ptr) <= @intFromPtr(a.ptr));
    assert(!slicesOverlap(r, b) or @intFromPtr(r.ptr) <= @intFromPtr(b.ptr));

    var i: usize = 0;
    var borrow: Limb = 0;

    while (i < b.len) : (i += 1) {
        const ov1 = @subWithOverflow(a[i], b[i]);
        r[i] = ov1[0];
        const ov2 = @subWithOverflow(r[i], borrow);
        r[i] = ov2[0];
        borrow = @as(Limb, ov1[1]) + ov2[1];
    }

    while (i < a.len) : (i += 1) {
        const ov = @subWithOverflow(a[i], borrow);
        r[i] = ov[0];
        borrow = ov[1];
    }

    return borrow;
}