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

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/**
 * Multiplies two polynomials given in coefficient-array form.
 *
 *   out(x) = a(x) * b(x)
 *
 * Coefficient layout: index i is the coefficient of x^i (ascending). The
 * result has length `a_len + b_len - 1`. The first `out_len` entries of
 * `out` are written; entries beyond that are not touched.
 *
 * `out` must not alias `a` or `b` — the routine writes into `out` while it
 * still reads from the inputs.
 *
 * @param {number[]|Float64Array|Float32Array} out destination, length >= a_len + b_len - 1
 * @param {number[]|Float64Array|Float32Array} a
 * @param {number} a_len number of coefficients in a
 * @param {number[]|Float64Array|Float32Array} b
 * @param {number} b_len number of coefficients in b
 * @returns {number} number of coefficients written into `out` (a_len + b_len - 1)
 */
export function polynomial_multiply(out, a, a_len, b, b_len) {
    const out_len = a_len + b_len - 1;

    for (let i = 0; i < out_len; i++) {
        out[i] = 0;
    }

    for (let i = 0; i < a_len; i++) {
        const ai = a[i];

        if (ai === 0) {
            // short-circuit, 0*X == 0
            continue;
        }

        for (let j = 0; j < b_len; j++) {
            out[i + j] += ai * b[j];
        }
    }

    return out_len;
}