refactor: reduce arithmetic operations

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---

Author: Shabareesh Shetty

lib/node_modules/@stdlib/blas/base/dtbmv/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`
 *
@@ -57,14 +57,16 @@ var min = require( '@stdlib/math/base/special/min' );
 function dtbmv( uplo, trans, diag, N, K, A, strideA1, strideA2, offsetA, x, strideX, offsetX ) { // eslint-disable-line max-params, max-len
 	var nonunit;
 	var isrm;
-	var idx;
+	var ida;
 	var tmp;
 	var sa0;
 	var sa1;
 	var ix0;
 	var ix1;
+	var oa2;
 	var i0;
 	var i1;
+	var ia;
 	var oa;
 	var ox;
 
@@ -92,12 +94,14 @@ function dtbmv( uplo, trans, diag, N, K, A, strideA1, strideA2, offsetA, x, stri
 			oa = offsetA + ( sa1 * i1 );
 			tmp = x[ ix1 ];
 			if ( nonunit ) {
-				tmp = A[ oa + ( sa0 * K ) ] * x[ ix1 ];
+				oa2 = oa + ( sa0 * K );
+				tmp = A[ oa2 ] * x[ ix1 ];
 			}
 			for ( i0 = i1 + 1; i0 <= min( N - 1, i1 + K ); i0++ ) {
 				ix0 = ox + ( i0 * strideX );
-				idx = oa + ( sa0 * ( K + i0 - i1 ) );
-				tmp += A[ idx ] * x[ ix0 ];
+				ia = sa0 * ( K + i0 - i1 );
+				ida = oa + ia;
+				tmp += A[ ida ] * x[ ix0 ];
 			}
 			x[ ix1 ] = tmp;
 			ix1 += strideX;
@@ -113,12 +117,14 @@ function dtbmv( uplo, trans, diag, N, K, A, strideA1, strideA2, offsetA, x, stri
 			oa = offsetA;
 			tmp = x[ ix1 ];
 			if ( nonunit ) {
-				tmp = A[ oa + ( sa1 * i1 ) ] * x[ ix1 ];
+				oa2 = oa + ( sa1 * K );
+				tmp = A[ oa2 ] * x[ ix1 ];
 			}
 			for ( i0 = max( 0, i1 - K ); i0 < i1; i0++ ) {
-				idx = oa + ( sa1 * i0 ) + ( sa0 * ( i1 - i0 ) );
+				ia = sa0 * ( i1 - i0 );
+				ida = oa + ( sa1 * i0 ) + ia;
 				ix0 = ox + ( i0 * strideX );
-				tmp += A[ idx ] * x[ ix0 ];
+				tmp += A[ ida ] * x[ ix0 ];
 			}
 			x[ ix1 ] = tmp;
 			ix1 -= strideX;
@@ -134,12 +140,14 @@ function dtbmv( uplo, trans, diag, N, K, A, strideA1, strideA2, offsetA, x, stri
 			oa = offsetA + ( sa1 * i1 );
 			tmp = x[ ix1 ];
 			if ( nonunit ) {
-				tmp = A[ oa + ( sa0 * K ) ] * x[ ix1 ];
+				oa2 = oa + ( sa0 * K );
+				tmp = A[ oa2 ] * x[ ix1 ];
 			}
 			for ( i0 = max( 0, i1 - K ); i0 < i1; i0++ ) {
-				idx = oa + ( sa0 * ( K + i0 - i1 ) );
+				ia = sa0 * ( K + i0 - i1 );
+				ida = oa + ia;
 				ix0 = ox + ( i0 * strideX );
-				tmp += A[ idx ] * x[ ix0 ];
+				tmp += A[ ida ] * x[ ix0 ];
 			}
 			x[ ix1 ] = tmp;
 			ix1 -= strideX;
@@ -156,8 +164,9 @@ function dtbmv( uplo, trans, diag, N, K, A, strideA1, strideA2, offsetA, x, stri
 		}
 		for ( i0 = i1 + 1; i0 <= min( N - 1, i1 + K ); i0++ ) {
 			ix0 = ox + ( i0 * strideX );
-			idx = oa + ( sa0 * ( i0 - i1 ) );
-			tmp += A[ idx ] * x[ ix0 ];
+			ia = sa0 * ( i0 - i1 );
+			ida = oa + ia;
+			tmp += A[ ida ] * x[ ix0 ];
 		}
 		x[ ix1 ] = tmp;
 		ix1 += strideX;