@stdlib/math-base-tools-evalrationalf/lib/main.js

Evaluate a rational function using single-precision floating-point arithmetic.

/**
* @license Apache-2.0
*
* Copyright (c) 2024 The Stdlib Authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*    http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*
*
* ## Notice
*
* The original C++ code and copyright notice are from the [Boost library]{@link http://www.boost.org/doc/libs/1_60_0/boost/math/tools/rational.hpp}. The implementation has been modified for JavaScript.
*
* ```text
* (C) Copyright John Maddock 2006.
*
* Use, modification and distribution are subject to the
* Boost Software License, Version 1.0. (See accompanying file
* LICENSE or copy at http://www.boost.org/LICENSE_1_0.txt)
* ```
*/

'use strict';

// MODULES //

var float64ToFloat32 = require( '@stdlib/number-float64-base-to-float32' );
var absf = require( '@stdlib/math-base-special-absf' );


// MAIN //

/**
* Evaluates a rational function (i.e., the ratio of two polynomials described by the coefficients stored in \\(P\\) and \\(Q\\)) using single-precision floating-point arithmetic.
*
* ## Notes
*
* -   Coefficients should be sorted in ascending degree.
* -   The implementation uses [Horner's rule][horners-method] for efficient computation.
*
* [horners-method]: https://en.wikipedia.org/wiki/Horner%27s_method
*
* @param {NumericArray} P - numerator polynomial coefficients sorted in ascending degree
* @param {NumericArray} Q - denominator polynomial coefficients sorted in ascending degree
* @param {number} x - value at which to evaluate the rational function
* @returns {number} evaluated rational function
*
* @example
* var Float32Array = require( '@stdlib/array-float32' );
*
* var P = new Float32Array( [ -6.0, -5.0 ] );
* var Q = new Float32Array( [ 3.0, 0.5 ] );
*
* var v = evalrationalf( P, Q, 6.0 ); // => ( -6*6^0 - 5*6^1 ) / ( 3*6^0 + 0.5*6^1 ) = (-6-30)/(3+3)
* // returns -6.0
*
* @example
* var Float32Array = require( '@stdlib/array-float32' );
*
* // 2x^3 + 4x^2 - 5x^1 - 6x^0 => degree 4
* var P = new Float32Array( [ -6.0, -5.0, 4.0, 2.0 ] );
*
* // 0.5x^1 + 3x^0 => degree 2
* var Q = new Float32Array( [ 3.0, 0.5, 0.0, 0.0 ] ); // zero-padded
*
* var v = evalrationalf( P, Q, 6.0 ); // => ( -6*6^0 - 5*6^1 + 4*6^2 + 2*6^3 ) / ( 3*6^0 + 0.5*6^1 + 0*6^2 + 0*6^3 ) = (-6-30+144+432)/(3+3)
* // returns ~90.0
*/
function evalrationalf( P, Q, x ) {
	var len;
	var s1;
	var s2;
	var i;

	len = P.length;
	if ( len === 0 ) {
		return NaN;
	}
	if ( len !== Q.length ) {
		return NaN;
	}
	if ( x === 0.0 || len === 1 ) {
		return float64ToFloat32( P[ 0 ] / Q[ 0 ] );
	}
	// Use Horner's method...
	if ( absf( x ) <= 1.0 ) {
		s1 = P[ len-1 ];
		s2 = Q[ len-1 ];
		for ( i = len-2; i >= 0; --i ) {
			s1 = float64ToFloat32( s1 * x );
			s2 = float64ToFloat32( s2 * x );
			s1 = float64ToFloat32( s1 + P[ i ] );
			s2 = float64ToFloat32( s2 + Q[ i ] );
		}
	} else {
		x = float64ToFloat32( 1.0 / x ); // use inverse to avoid overflow
		s1 = P[ 0 ];
		s2 = Q[ 0 ];
		for ( i = 1; i < len; ++i ) {
			s1 = float64ToFloat32( s1 * x );
			s2 = float64ToFloat32( s2 * x );
			s1 = float64ToFloat32( s1 + P[ i ] );
			s2 = float64ToFloat32( s2 + Q[ i ] );
		}
	}
	return float64ToFloat32( s1 / s2 );
}


// EXPORTS //

module.exports = evalrationalf;