@woosh/meep-engine/src/core/math/linalg/givens/givens_apply_rows.js

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/**
 * Applies a Givens rotation (c, s) to two row vectors in place:
 *
 *     row_i' =  c * row_i + s * row_j
 *     row_j' = -s * row_i + c * row_j
 *
 * Each row is treated as a length-n vector and rotated element-by-element.
 *
 * Accepts plain arrays or typed arrays. The two row arguments can be
 * standalone arrays or subarray views into a larger flat matrix — useful
 * for row-major QR/Hessenberg maintenance where the matrix is a single
 * Float64Array and the caller hands in `M.subarray(i*n, (i+1)*n)` and
 * `M.subarray(j*n, (j+1)*n)`.
 *
 * @param {Float64Array|number[]} row_i length >= n
 * @param {Float64Array|number[]} row_j length >= n
 * @param {number} c
 * @param {number} s
 * @param {number} n number of elements to rotate
 */
export function givens_apply_rows(row_i, row_j, c, s, n) {
    for (let i = 0; i < n; ++i) {
        const a = row_i[i];
        const b = row_j[i];
        row_i[i] = c * a + s * b;
        row_j[i] = c * b - s * a;
    }
}