refactor: to use zero-based indexing

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---

Author: kgryte

lib/node_modules/@stdlib/fft/base/fftpack/lib/radb2.js

@@ -154,7 +154,7 @@ function optr( i, k, j, L, M, stride, offset ) {
 * @returns {void}
 */
 function radb2( M, L, cc, oc, out, oo, twiddles, ot ) { // FIXME: support strides
-	var idp2;
+	var MP1;
 	var tr2;
 	var ti2;
 	var ip1;
@@ -196,37 +196,35 @@ function radb2( M, L, cc, oc, out, oo, twiddles, ot ) { // FIXME: support stride
 		return;
 	}
 
-	// FIXME: everything below needs updating as `iptr` and `optr` have been converted to zero-based indexing
-
 	if ( M !== 2 ) {
-		idp2 = M + 2;
+		MP1 = M + 1;
 
 		// FIXME: describe what is happening in the loops below
 		for ( k = 0; k < L; k++ ) {
-			for ( i = 3; i <= M; i += 2 ) {
-				ic = idp2 - i;
+			for ( i = 2; i < M; i += 2 ) {
+				ic = MP1 - i;
 
-				it1 = i - 2 + ot;
-				it2 = i - 1 + ot;
+				it1 = i - 1 + ot;
+				it2 = i + ot;
 
-				ip1 = iptr( i-1, 1, k, M, oc );
-				ip2 = iptr( ic-1, 2, k, M, oc );
+				ip1 = iptr( i, 0, k, M, sc, oc );
+				ip2 = iptr( ic-1, 1, k, M, sc, oc );
 				tr2 = cc[ ip1 ] - cc[ ip2 ];
 
-				io = optr( i-1, k, 1, L, M, oo );
+				io = optr( i, k, 0, L, M, so, oo );
 				out[ io ] = cc[ ip1 ] + cc[ ip2 ];
 
-				ip1 = iptr( i, 1, k, M, oc );
-				ip2 = iptr( ic, 2, k, M, oc );
+				ip1 = iptr( i+1, 0, k, M, sc, oc );
+				ip2 = iptr( ic, 1, k, M, sc, oc );
 				ti2 = cc[ ip1 ] + cc[ ip2 ];
 
-				io = optr( i, k, 1, L, M, oo );
+				io = optr( i+1, k, 0, L, M, so, oo );
 				out[ io ] = cc[ ip1 ] - cc[ ip2 ];
 
-				io = optr( i-1, k, 2, L, M, oo );
+				io = optr( i, k, 1, L, M, so, oo );
 				out[ io ] = ( twiddles[ it1 ] * tr2 ) - ( twiddles[ it2 ] * ti2 );
 
-				io = optr( i, k, 2, L, M, oo );
+				io = optr( i+1, k, 1, L, M, so, oo );
 				out[ io ] = ( twiddles[ it1 ] * ti2 ) + ( twiddles[ it2 ] * tr2 );
 			}
 		}
@@ -237,12 +235,12 @@ function radb2( M, L, cc, oc, out, oo, twiddles, ot ) { // FIXME: support stride
 	}
 	// FIXME: describe what is happening here
 	for ( k = 0; k < L; k++ ) {
-		ip1 = iptr( M, 1, k, M, oc );
-		io = optr( M, k, 1, L, M, oo );
+		ip1 = iptr( M-1, 0, k, M, sc, oc );
+		io = optr( M-1, k, 0, L, M, so, oo );
 		out[ io ] = cc[ ip1 ] + cc[ ip1 ];
 
-		ip1 = iptr( 1, 2, k, M, oc );
-		io = optr( M, k, 2, L, M, oo );
+		ip1 = iptr( 0, 1, k, M, sc, oc );
+		io = optr( M-1, k, 1, L, M, so, oo );
 		out[ io ] = -( cc[ ip1 ] + cc[ ip1 ] );
 	}
 }