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Add strided interface to reverse a strided array in-place
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‎lib/node_modules/@stdlib/blas/ext/base/srev/README.md

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<!--
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@license Apache-2.0
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Copyright (c) 2020 The Stdlib Authors.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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-->
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# srev
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> Reverse a single-precision floating-point strided array in-place.
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<section class="usage">
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## Usage
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```javascript
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var srev = require( '@stdlib/blas/ext/base/srev' );
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```
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#### srev( N, x, stride )
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Reverses a single-precision floating-point strided array `x` in-place.
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```javascript
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var Float32Array = require( '@stdlib/array/float32' );
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var x = new Float32Array( [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ] );
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srev( x.length, x, 1 );
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// x => <Float32Array>[ -3.0, -1.0, 0.0, 4.0, -5.0, 3.0, 1.0, -2.0 ]
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```
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The function has the following parameters:
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- **N**: number of indexed elements.
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- **x**: input [`Float32Array`][@stdlib/array/float32].
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- **stride**: index increment.
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The `N` and `stride` parameters determine which elements in `x` are accessed at runtime. For example, to reverse every other element
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```javascript
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var Float32Array = require( '@stdlib/array/float32' );
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var floor = require( '@stdlib/math/base/special/floor' );
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var x = new Float32Array( [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ] );
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var N = floor( x.length / 2 );
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srev( N, x, 2 );
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// x => <Float32Array>[ -1.0, 1.0, 4.0, -5.0, 3.0, 0.0, -2.0, -3.0 ]
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```
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Note that indexing is relative to the first index. To introduce an offset, use [`typed array`][mdn-typed-array] views.
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```javascript
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var Float32Array = require( '@stdlib/array/float32' );
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var floor = require( '@stdlib/math/base/special/floor' );
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// Initial array...
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var x0 = new Float32Array( [ 1.0, -2.0, 3.0, -4.0, 5.0, -6.0 ] );
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// Create an offset view...
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var x1 = new Float32Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
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var N = floor( x0.length/2 );
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// Reverse every other element...
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srev( N, x1, 2 );
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// x0 => <Float32Array>[ 1.0, -6.0, 3.0, -4.0, 5.0, -2.0 ]
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```
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#### srev.ndarray( N, x, stride, offset )
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Reverses a single-precision floating-point strided array `x` in-place using alternative indexing semantics.
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```javascript
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var Float32Array = require( '@stdlib/array/float32' );
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var x = new Float32Array( [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ] );
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srev.ndarray( x.length, x, 1, 0 );
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// x => <Float32Array>[ -3.0, -1.0, 0.0, 4.0, -5.0, 3.0, 1.0, -2.0 ]
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```
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The function has the following additional parameters:
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- **offset**: starting index.
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While [`typed array`][mdn-typed-array] views mandate a view offset based on the underlying `buffer`, the `offset` parameter supports indexing semantics based on a starting index. For example, to access only the last three elements of `x`
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```javascript
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var Float32Array = require( '@stdlib/array/float32' );
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var x = new Float32Array( [ 1.0, -2.0, 3.0, -4.0, 5.0, -6.0 ] );
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srev.ndarray( 3, x, 1, x.length-3 );
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// x => <Float32Array>[ 1.0, -2.0, 3.0, -6.0, 5.0, -4.0 ]
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```
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</section>
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<!-- /.usage -->
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<section class="notes">
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## Notes
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- If `N <= 0`, both functions return `x` unchanged.
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- Where possible, one should "reverse" a strided array by negating its stride, which is an `O(1)` operation, in contrast to performing an in-place reversal, which is `O(N)`. However, in certain circumstances, this is not tenable, particularly when interfacing with libraries which assume and/or expect a specific memory layout (e.g., strided array elements arranged in memory in ascending order). In general, when working with strided arrays, only perform an in-place reversal when strictly necessary.
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</section>
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<!-- /.notes -->
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<section class="examples">
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## Examples
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<!-- eslint no-undef: "error" -->
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```javascript
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var round = require( '@stdlib/math/base/special/round' );
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var randu = require( '@stdlib/random/base/randu' );
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var Float32Array = require( '@stdlib/array/float32' );
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var srev = require( '@stdlib/blas/ext/base/srev' );
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var rand;
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var sign;
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var x;
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var i;
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x = new Float32Array( 10 );
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for ( i = 0; i < x.length; i++ ) {
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rand = round( randu()*100.0 );
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sign = randu();
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if ( sign < 0.5 ) {
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sign = -1.0;
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} else {
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sign = 1.0;
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}
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x[ i ] = sign * rand;
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}
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console.log( x );
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srev( x.length, 5.0, x, 1 );
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console.log( x );
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```
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</section>
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<!-- /.examples -->
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<section class="links">
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[@stdlib/array/float32]: https://github.com/stdlib-js/stdlib
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[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray
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</section>
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<!-- /.links -->

‎lib/node_modules/@stdlib/blas/ext/base/srev/benchmark/benchmark.js

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/**
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* @license Apache-2.0
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*
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* Copyright (c) 2020 The Stdlib Authors.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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'use strict';
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// MODULES //
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var bench = require( '@stdlib/bench' );
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var randu = require( '@stdlib/random/base/randu' );
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var isnanf = require( '@stdlib/math/base/assert/is-nanf' );
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var pow = require( '@stdlib/math/base/special/pow' );
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var Float32Array = require( '@stdlib/array/float32' );
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var pkg = require( './../package.json' ).name;
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var srev = require( './../lib/srev.js' );
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// FUNCTIONS //
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/**
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* Create a benchmark function.
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*
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* @private
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* @param {PositiveInteger} len - array length
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* @returns {Function} benchmark function
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*/
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function createBenchmark( len ) {
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var x;
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var i;
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x = new Float32Array( len );
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for ( i = 0; i < len; i++ ) {
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x[ i ] = ( randu()*20.0 ) - 10.0;
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}
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return benchmark;
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function benchmark( b ) {
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var y;
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var i;
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b.tic();
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for ( i = 0; i < b.iterations; i++ ) {
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y = srev( x.length, x, 1 );
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if ( isnanf( y[ i%x.length ] ) ) {
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b.fail( 'should not return NaN' );
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}
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}
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b.toc();
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if ( isnanf( y[ i%x.length ] ) ) {
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b.fail( 'should not return NaN' );
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}
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b.pass( 'benchmark finished' );
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b.end();
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}
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}
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// MAIN //
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function main() {
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var len;
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var min;
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var max;
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var f;
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var i;
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min = 1; // 10^min
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max = 6; // 10^max
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for ( i = min; i <= max; i++ ) {
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len = pow( 10, i );
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f = createBenchmark( len );
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bench( pkg+':len='+len, f );
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}
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}
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main();

‎lib/node_modules/@stdlib/blas/ext/base/srev/benchmark/benchmark.native.js

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/**
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* @license Apache-2.0
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*
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* Copyright (c) 2020 The Stdlib Authors.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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'use strict';
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// MODULES //
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var resolve = require( 'path' ).resolve;
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var bench = require( '@stdlib/bench' );
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var randu = require( '@stdlib/random/base/randu' );
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var isnanf = require( '@stdlib/math/base/assert/is-nanf' );
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var pow = require( '@stdlib/math/base/special/pow' );
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var Float32Array = require( '@stdlib/array/float32' );
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var tryRequire = require( '@stdlib/utils/try-require' );
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var pkg = require( './../package.json' ).name;
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// VARIABLES //
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var srev = tryRequire( resolve( __dirname, './../lib/native.js' ) );
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var opts = {
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'skip': ( srev instanceof Error )
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};
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// FUNCTIONS //
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/**
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* Create a benchmark function.
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*
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* @private
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* @param {PositiveInteger} len - array length
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* @returns {Function} benchmark function
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*/
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function createBenchmark( len ) {
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var x;
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var i;
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x = new Float32Array( len );
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for ( i = 0; i < len; i++ ) {
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x[ i ] = ( randu()*20.0 ) - 10.0;
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}
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return benchmark;
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function benchmark( b ) {
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var y;
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var i;
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b.tic();
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for ( i = 0; i < b.iterations; i++ ) {
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y = srev( x.length, x, 1 );
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if ( isnanf( y[ i%x.length ] ) ) {
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b.fail( 'should not return NaN' );
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}
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}
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b.toc();
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if ( isnanf( y[ i%x.length ] ) ) {
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b.fail( 'should not return NaN' );
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}
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b.pass( 'benchmark finished' );
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b.end();
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}
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}
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// MAIN //
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function main() {
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var len;
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var min;
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var max;
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var f;
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var i;
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min = 1; // 10^min
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max = 6; // 10^max
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for ( i = min; i <= max; i++ ) {
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len = pow( 10, i );
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f = createBenchmark( len );
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bench( pkg+'::native:len='+len, opts, f );
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}
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}
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main();

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