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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 isEven = require( '@stdlib/math/base/assert/is-even' );
var floor = require( '@stdlib/math/base/special/floor' );
var reinterpret = require( '@stdlib/strided/base/reinterpret-complex128' );
// MAIN //
/**
* Converts a real-valued double-precision floating-point array in FFTPACK half-complex format to a double-precision complex floating-point array.
*
* @param {NonNegativeInteger} N - length of the original real sequence
* @param {Float64Array} hc - input array in FFTPACK half-complex format
* @param {integer} strideHc - stride length for `hc`
* @param {NonNegativeInteger} offsetHc - starting index for `hc`
* @param {Complex128Array} out - output complex array
* @param {integer} strideOut - stride length for `out`
* @param {NonNegativeInteger} offsetOut - starting index for `out`
* @returns {Complex128Array} output complex array
*
* @example
* var Float64Array = require( '@stdlib/array/float64' );
* var Complex128Array = require( '@stdlib/array/complex128' );
* var floor = require( '@stdlib/math/base/special/floor' );
* var rffti = require( '@stdlib/fft/base/fftpack/float64/rffti' );
* var rfftf = require( '@stdlib/fft/base/fftpack/float64/rfftf' );
*
* var N = 4;
* var w = new Float64Array( ( 2*N ) + 34 );
* rffti( N, w, 1, 0 );
*
* var r = new Float64Array( [ 1.0, 2.0, 3.0, 4.0 ] );
* rfftf( N, r, 1, 0, w, 1, 0 );
* // r => <Float64Array>[ 10.0, -2.0, 2.0, -2.0 ]
*
* var out = new Complex128Array( floor( N/2 ) + 1 );
* hc2c( N, r, 1, 0, out, 1, 0 );
* // out => <Complex128Array>[ 10.0, 0.0, -2.0, 2.0, -2.0, 0.0 ]
*/
function hc2c( N, hc, strideHc, offsetHc, out, strideOut, offsetOut ) {
var view;
var so;
var io;
var M;
var k;
if ( N <= 0 ) {
return out;
}
// Reinterpret the complex output array as a real-valued array of interleaved real and imaginary components:
view = reinterpret( out, 0 );
// Adjust the stride and offset for the real-valued view:
so = strideOut * 2;
io = offsetOut * 2;
// Resolve the number of interior complex coefficients:
M = floor( ( N - 1 ) / 2 );
// DC component:
view[ io ] = hc[ offsetHc ];
view[ io + 1 ] = 0.0; // Always purely real
offsetHc += strideHc;
io += so;
// Complex coefficients for frequencies k = 1, ..., M:
for ( k = 0; k < M; k++ ) {
view[ io ] = hc[ offsetHc ]; // real part
view[ io + 1 ] = hc[ offsetHc + strideHc ]; // imaginary part
offsetHc += 2 * strideHc;
io += so;
}
// Nyquist component (only when N is even):
if ( isEven( N ) ) {
view[ io ] = hc[ offsetHc ];
view[ io + 1 ] = 0.0; // Always purely real
}
return out;
}
// EXPORTS //
module.exports = hc2c;
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