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* @license Apache-2.0
*
* Copyright (c) 2026 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 isnan = require( '@stdlib/math/base/assert/is-nan' );
var floor = require( '@stdlib/math/base/special/floor' );
var gindexOfLessThan = require( '@stdlib/blas/ext/base/gindex-of-less-than' ).ndarray;
// MAIN //
/**
* Returns the index of the first element in a sorted double-precision floating-point strided array which is less than a specified search element using alternative indexing semantics.
*
* ## Notes
*
* - If unable to find an element which is less than the search element, the function returns `-1`.
*
* @param {PositiveInteger} N - number of indexed elements
* @param {number} searchElement - search element
* @param {Float64Array} x - sorted input array
* @param {integer} strideX - stride length
* @param {NonNegativeInteger} offsetX - starting index
* @returns {integer} index
*
* @example
* var Float64Array = require( '@stdlib/array/float64' );
*
* var x = new Float64Array( [ 1.0, 2.0, 2.0, 3.0 ] );
*
* var idx = dindexOfLessThanSorted( x.length, 2.0, x, 1, 0 );
* // returns 0
*/
function dindexOfLessThanSorted( N, searchElement, x, strideX, offsetX ) {
var idx;
var mid;
var ix;
var lo;
var hi;
var v1;
var v2;
var v;
if ( N <= 0 ) {
return -1;
}
v1 = x[ offsetX ];
v2 = x[ offsetX + ( ( N - 1 ) * strideX ) ];
// If either the first or last indexed element is NaN, we cannot determine the sort order, so fall back to a linear search...
if ( isnan( v1 ) || isnan( v2 ) ) {
return gindexOfLessThan( N, searchElement, x, strideX, offsetX );
}
// Ascending order: elements which are less than the search element form a prefix, so only the first indexed element needs to be checked...
if ( v1 <= v2 ) {
if ( v1 < searchElement ) {
return 0;
}
return -1;
}
// Descending order: perform a binary search for the first element which is less than the search element...
lo = 0;
hi = N;
while ( lo < hi ) {
mid = lo + floor( ( hi - lo ) / 2 );
v = x[ offsetX + ( mid * strideX ) ];
// If we encounter a NaN, fall back to a linear search...
if ( isnan( v ) ) {
ix = offsetX + ( lo * strideX );
idx = gindexOfLessThan( N-lo, searchElement, x, strideX, ix );
return ( idx < 0 ) ? idx : idx + lo;
}
if ( v < searchElement ) {
hi = mid;
} else {
lo = mid + 1;
}
}
if ( lo === N ) {
return -1;
}
return lo;
}
// EXPORTS //
module.exports = dindexOfLessThanSorted;
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