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* @license Apache-2.0
*
* Copyright (c) 2018 The Stdlib Authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
'use strict';
// MODULES //
var factory = require( '@stdlib/random/sample' ).factory;
var dcopy = require( '@stdlib/blas/base/dcopy' ).ndarray;
// MAIN //
/**
* Initializes centroids by randomly sampling from a data buffer.
*
* @private
* @param {ndarray} out - output centroids `kxd` matrix
* @param {ndarray} buffer - buffer from which to sample
* @param {*} seed - PRNG seed
* @returns {ndarray} centroids
*/
function sample( out, buffer, seed ) {
var rand;
var inds;
var obuf;
var buf;
var sb1;
var sb2;
var so2;
var oo;
var s;
var M;
var N;
var i;
M = out.shape[ 0 ];
N = out.shape[ 1 ];
obuf = out.data;
so2 = out.strides[ 1 ];
oo = out.offset;
buf = buffer.data;
sb1 = buffer.strides[ 0 ];
sb2 = buffer.strides[ 1 ];
// Generate an array of data vector indices...
inds = [];
for ( i = 0; i < buffer.shape[ 0 ]; i++ ) {
inds.push( i );
}
// Only randomly sample from the data buffer if the number of centroids is not equal to the number of data vectors...
if ( M === inds.length ) {
// Buffer already qualifies as a "random" sample:
s = inds;
} else {
// Create a seeded random sampler (without replacement):
rand = factory({
'seed': seed,
'size': M,
'mutate': false,
'replacement': false
});
// Generate a random sample:
s = rand( inds );
}
// Update the centroids...
for ( i = 0; i < M; i++ ) {
// Note: the following is likely to be an "out-of-order" copy...
dcopy( N, buf, sb2, sb1*s[i], obuf, so2, oo );
}
return out;
}
// EXPORTS //
module.exports = sample;
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