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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 toUniqueNormalizedIndices = require( '@stdlib/ndarray/base/to-unique-normalized-indices' );
var getDType = require( '@stdlib/ndarray/base/dtype' );
var getShape = require( '@stdlib/ndarray/base/shape' );
var getStrides = require( '@stdlib/ndarray/base/strides' );
var getOffset = require( '@stdlib/ndarray/base/offset' );
var getOrder = require( '@stdlib/ndarray/base/order' );
var getData = require( '@stdlib/ndarray/base/data-buffer' );
var join = require( '@stdlib/array/base/join' );
var min = require( '@stdlib/math/base/special/min' );
var format = require( '@stdlib/string/format' );
// MAIN //
/**
* Returns a view of the diagonal of a matrix (or stack of matrices).
*
* ## Notes
*
* - The order of the dimension indices contained in `dims` matters. The first element specifies the row-like dimension. The second element specifies the column-like dimension.
* - Each provided dimension index must reside on the interval `[-ndims, ndims-1]`.
* - The diagonal offset `k` is interpreted as `column - row`. Accordingly, when `k = 0`, the function returns the main diagonal; when `k > 0`, the function returns the diagonal above the main diagonal; and when `k < 0`, the function returns the diagonal below the main diagonal.
* - The returned ndarray is a **view** of the input ndarray. Accordingly, writing to the original ndarray will **mutate** the returned ndarray and vice versa.
* - The `writable` parameter **only** applies to ndarray constructors supporting **read-only** instances.
*
* @param {ndarray} x - input array
* @param {IntegerArray} dims - dimension indices defining the plane from which to extract the diagonal
* @param {integer} k - diagonal offset
* @param {boolean} writable - boolean indicating whether the returned ndarray should be writable
* @throws {RangeError} must provide exactly two dimension indices
* @throws {RangeError} input ndarray must have at least two dimensions
* @throws {RangeError} must provide valid dimension indices
* @throws {Error} must provide unique dimension indices
* @returns {ndarray} ndarray view
*
* @example
* var array = require( '@stdlib/ndarray/array' );
*
* var x = array( [ [ 1.0, 2.0, 3.0 ], [ 4.0, 5.0, 6.0 ], [ 7.0, 8.0, 9.0 ] ] );
* // returns <ndarray>[ [ 1.0, 2.0, 3.0 ], [ 4.0, 5.0, 6.0 ], [ 7.0, 8.0, 9.0 ] ]
*
* var y = diagonal( x, [ 0, 1 ], 0, false );
* // returns <ndarray>[ 1.0, 5.0, 9.0 ]
*/
function diagonal( x, dims, k, writable ) {
var strides;
var offset;
var ndims;
var shape;
var rows;
var cols;
var sh;
var st;
var sr;
var sc;
var ro;
var co;
var d;
var L;
var i;
if ( dims.length !== 2 ) {
throw new RangeError( format( 'invalid argument. Must provide exactly two dimension indices. Value: `[%s]`.', join( dims, ', ' ) ) );
}
sh = getShape( x );
ndims = sh.length;
if ( ndims < 2 ) {
throw new RangeError( format( 'invalid argument. First argument must be an ndarray having at least two dimensions. Number of dimensions: %d.', ndims ) );
}
st = getStrides( x );
// Resolve dimension indices...
d = toUniqueNormalizedIndices( dims, ndims-1 );
if ( d === null ) {
throw new RangeError( format( 'invalid argument. Specified dimension index is out-of-bounds. Must be on the interval: [-%u, %u]. Value: `[%s]`.', ndims, ndims-1, join( dims, ', ' ) ) );
}
if ( d.length !== dims.length ) {
throw new Error( format( 'invalid argument. Must provide unique dimension indices. Value: `[%s]`.', join( dims, ', ' ) ) );
}
sr = st[ d[0] ];
sc = st[ d[1] ];
// Resolve the row and column offsets corresponding to the diagonal offset `k = column - row`:
co = ( k > 0 ) ? k : 0;
ro = co - k;
// Compute the length of the diagonal:
rows = sh[ d[0] ] - ro;
cols = sh[ d[1] ] - co;
L = min( rows, cols );
if ( L < 0 ) {
L = 0;
}
// Adjust the offset so that we point to the first element along the specified diagonal:
offset = getOffset( x );
if ( L > 0 ) {
offset += ( co*sc ) + ( ro*sr );
}
// Drop the specified dimensions and append the diagonal dimension:
shape = [];
strides = [];
for ( i = 0; i < ndims; i++ ) {
if ( i === d[0] || i === d[1] ) {
continue;
}
shape.push( sh[ i ] );
strides.push( st[ i ] );
}
shape.push( L );
strides.push( sr + sc );
return new x.constructor( getDType( x ), getData( x ), shape, strides, offset, getOrder( x ), { // eslint-disable-line max-len
'readonly': !writable
});
}
// EXPORTS //
module.exports = diagonal;
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