llsquareBasecase

fn llsquareBasecase(r: []Limb, x: []const Limb) void {
    const x_norm = x;
    assert(r.len >= 2 * x_norm.len + 1);
    assert(!slicesOverlap(r, x));

    // Compute the square of a N-limb bigint with only (N^2 + N)/2
    // multiplications by exploiting the symmetry of the coefficients around the
    // diagonal:
    //
    //           a   b   c *
    //           a   b   c =
    // -------------------
    //          ca  cb  cc +
    //      ba  bb  bc     +
    //  aa  ab  ac
    //
    // Note that:
    //  - Each mixed-product term appears twice for each column,
    //  - Squares are always in the 2k (0 <= k < N) column

    for (x_norm, 0..) |v, i| {
        // Accumulate all the x[i]*x[j] (with x!=j) products
        const overflow = llmulLimb(.add, r[2 * i + 1 ..], x_norm[i + 1 ..], v);
        assert(!overflow);
    }

    // Each product appears twice, multiply by 2
    _ = llshl(r, r[0 .. 2 * x_norm.len], 1);

    for (x_norm, 0..) |v, i| {
        // Compute and add the squares
        const overflow = llmulLimb(.add, r[2 * i ..], x[i..][0..1], v);
        assert(!overflow);
    }
}