refactor: update rfftf

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---

Author: gunjjoshi

lib/node_modules/@stdlib/fft/base/fftpack/rfftf/lib/main.js

@@ -60,6 +60,9 @@
 
 // MODULES //
 
+var isNonNegativeInteger = require( '@stdlib/assert/is-nonnegative-integer' );
+var isFloat64Array = require( '@stdlib/assert/is-float64array' );
+var isInteger = require( '@stdlib/assert/is-integer' );
 var rfftf1 = require( './rfftf1.js' );
 
 
@@ -70,24 +73,35 @@ var rfftf1 = require( './rfftf1.js' );
 *
 * @param {NonNegativeInteger} N - length of the sequence to transform
 * @param {Float64Array} r - real-valued input array containing the sequence to be transformed
+* @param {integer} strideR - stride length for `r`
 * @param {NonNegativeInteger} offsetR - starting index for `r`
 * @param {Float64Array} workspace - workspace array containing pre-computed values
+* @param {integer} strideW - stride length for `workspace`
 * @param {NonNegativeInteger} offsetW - starting index for `workspace`
 * @returns {void}
 */
-function rfftf( N, r, offsetR, workspace, offsetW ) { // FIXME: add stride support
+function rfftf( N, r, strideR, offsetR, workspace, strideW, offsetW ) {
 	var offsetT;
 	var offsetF;
 
+	if ( !isNonNegativeInteger( N ) || !isFloat64Array( r ) ||
+		!isInteger( strideR ) || !isNonNegativeInteger( offsetR ) ||
+		!isFloat64Array( workspace ) || !isInteger( strideW ) ||
+		!isNonNegativeInteger( offsetW ) ||
+		workspace.length < offsetW + ( ( ( 2*N ) + 34 ) * strideW ) ) {
+		return;
+	}
+
 	// When a sub-sequence is a single data point, the FFT is the identity, so no transformation necessary...
 	if ( N === 1 ) {
 		return;
 	}
+
 	// Resolve the starting indices for storing twiddle factors and factorization results:
-	offsetT = offsetW + N; // index offset for twiddle factors
-	offsetF = offsetT + N; // index offset for factors describing the sub-transforms
+	offsetT = offsetW + (N * strideW); // index offset for twiddle factors
+	offsetF = offsetT + (N * strideW); // index offset for factors describing the sub-transforms
 
-	rfftf1( N, r, offsetR, workspace, offsetW, workspace, offsetT, workspace, offsetF ); // eslint-disable-line max-len
+	rfftf1( N, r, strideR, offsetR, workspace, strideW, offsetW, workspace, strideW, offsetT, workspace, strideW, offsetF ); // eslint-disable-line max-len
 }