106 lines
3.5 KiB
C
106 lines
3.5 KiB
C
/**
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* @license Apache-2.0
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*
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* Copyright (c) 2018 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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#include "stdlib/ndarray/base/sub2ind.h"
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#include "stdlib/ndarray/index_modes.h"
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#include <stdint.h>
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/**
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* Converts subscripts to a linear index.
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*
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* ## Notes
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*
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* - When provided fewer modes than dimensions, the function recycles modes using modulo arithmetic.
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* - When provided a stride array containing negative strides, if an `offset` is greater than `0`, the function treats subscripts as mapping to a linear index in an underlying data buffer for the array, thus returning a linear index from the perspective of that buffer. If an `offset` is equal to `0`, the function treats subscripts as mapping to a linear index in an array view, thus returning a linear index from the perspective of that view. In short, from the perspective of a view, view data is always ordered.
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* - In "error" mode, the function returns `-1` if a subscript is out-of-bounds.
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*
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* @param ndims number of dimensions
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* @param shape array shape (dimensions)
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* @param strides array strides
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* @param offset location of the first indexed value **based** on the stride array
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* @param sub subscripts
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* @param nmodes number of modes
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* @param modes specifies how to handle subscripts which exceed array dimensions
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* @return linear index
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*
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* @example
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* #include "stdlib/ndarray/index_modes.h"
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* #include "stdlib/ndarray/base/sub2ind.h"
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*
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* int64_t ndims = 3;
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* int64_t shape[] = { 3, 3, 3 };
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* int64_t strides[] = { 9, 3, 1 };
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* int64_t offset = 0;
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*
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* int64_t nmodes = 1;
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* int8_t modes[] = { STDLIB_NDARRAY_INDEX_ERROR };
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*
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* int64_t sub[] = { 1, 2, 2 };
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*
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* int64_t idx = stdlib_ndarray_sub2ind( ndims, shape, strides, offset, sub, nmodes, modes );
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* // returns 17
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*/
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int64_t stdlib_ndarray_sub2ind( int64_t ndims, int64_t *shape, int64_t *strides, int64_t offset, int64_t *sub, int64_t nmodes, int8_t *modes ) {
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enum STDLIB_NDARRAY_INDEX_MODE mode;
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int64_t idx;
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int64_t s;
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int64_t m;
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int64_t j;
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int64_t i;
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idx = offset;
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for ( i = 0; i < ndims; i++ ) {
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m = shape[ i ];
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j = sub[ i ];
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mode = modes[ i%nmodes ];
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if ( mode == STDLIB_NDARRAY_INDEX_CLAMP ) {
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if ( j < 0 ) {
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j = 0;
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} else if ( j >= m ) {
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j = m - 1;
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}
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} else if ( mode == STDLIB_NDARRAY_INDEX_WRAP ) {
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if ( j < 0 ) {
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j += m; // slight optimization to avoid modulo arithmetic when |j| <= m
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if ( j < 0 ) {
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j -= m*( (int64_t)( j/m ) ); // this is equivalent to `j mod m`, where the result has same sign as dividend (i.e., `j`); cannot use `%` as the sign of the result is implementation defined in C
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if ( j != 0 ) {
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j += m;
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}
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}
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} else if ( j >= m ) {
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j -= m; // slight optimization to avoid modulo arithmetic when m < j <= 2m
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if ( j >= m ) {
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j %= m;
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}
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}
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} else if ( j < 0 || j >= m ) { // mode == 'error'
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return -1;
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}
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s = strides[ i ];
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// Check if array view...
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if ( s < 0 && offset == 0 ) {
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idx -= j * s; // increments idx
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} else {
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idx += j * s; // may increment or decrement idx
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}
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}
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return idx;
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}
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