refactor: reduce arithmetic operation

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---

Author: Shabareesh Shetty

lib/node_modules/@stdlib/blas/base/dtbsv/lib/base.js

@@ -41,7 +41,7 @@ var min = require( '@stdlib/math/base/special/min' );
 * @param {integer} strideA2 - stride of the second dimension of `A`
 * @param {NonNegativeInteger} offsetA - starting index for `A`
 * @param {Float64Array} x - input vector
-* @param {integer} strideX - `x` stride length
+* @param {integer} strideX - stride length for `x`
 * @param {NonNegativeInteger} offsetX - starting index for `x`
 * @returns {Float64Array} `x`
 *
@@ -61,6 +61,7 @@ function dtbsv( uplo, trans, diag, N, K, A, strideA1, strideA2, offsetA, x, stri
 	var tmp;
 	var sa0;
 	var sa1;
+	var sa2;
 	var ix0;
 	var ix1;
 	var i0;
@@ -94,7 +95,8 @@ function dtbsv( uplo, trans, diag, N, K, A, strideA1, strideA2, offsetA, x, stri
 				oa = offsetA + ( sa1 * i1 );
 				for ( i0 = i1 + 1; i0 <= min( N - 1, i1 + K ); i0++ ) {
 					ix0 = ox + ( i0 * strideX );
-					idx = oa + ( sa0 * ( K + i0 - i1 ) );
+					sa2 = ( K + i0 - i1 );
+					idx = oa + ( sa0 * sa2 );
 					tmp -= x[ ix0 ] * A[ idx ];
 				}
 				if ( nonunit ) {
@@ -117,7 +119,8 @@ function dtbsv( uplo, trans, diag, N, K, A, strideA1, strideA2, offsetA, x, stri
 				oa = offsetA + ( sa1 * i1 );
 				for ( i0 = max( 0, i1-K ); i0 < i1; i0++ ) {
 					ix0 = ox + ( i0 * strideX );
-					idx = offsetA + ( ( i0 * sa1 ) + ( (i1 - i0) * sa0 ) );
+					sa2 = ( (i1 - i0) * sa0 );
+					idx = offsetA + ( ( i0 * sa1 ) + sa2 );
 					tmp -= A[ idx ] * x[ ix0 ];
 				}
 				if ( nonunit ) {
@@ -139,7 +142,8 @@ function dtbsv( uplo, trans, diag, N, K, A, strideA1, strideA2, offsetA, x, stri
 			oa = offsetA + ( sa1*i1 );
 			for ( i0 = max( 0, i1 - K ); i0 < i1; i0++ ) {
 				ix0 = offsetX + ( i0 * strideX );
-				tmp -= x[ ix0 ] * A[ oa + ( sa0 * (K + i0 - i1 ) ) ];
+				sa2 = ( sa0 * (K + i0 - i1 ) );
+				tmp -= x[ ix0 ] * A[ oa + sa2 ];
 			}
 			if ( nonunit ) {
 				tmp /= A[ oa + ( sa0 * K ) ];
@@ -157,7 +161,8 @@ function dtbsv( uplo, trans, diag, N, K, A, strideA1, strideA2, offsetA, x, stri
 		oa = offsetA + (sa1 * i1);
 		for ( i0 = min( N - 1, i1 + K ); i0 > i1; i0-- ) {
 			ix0 = ox - ( ( N - 1 - i0 ) * strideX );
-			tmp -= x[ ix0 ] * A[ oa + ( sa0 * (i0 - i1) ) ];
+			sa2 = ( sa0 * (i0 - i1) );
+			tmp -= x[ ix0 ] * A[ oa + sa2 ];
 		}
 		if ( nonunit ) {
 			tmp /= A[ oa ];