feat: add function to return the index offset in units of elements

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Author: kgryte

lib/node_modules/@stdlib/ndarray/ctor/README.md

@@ -1469,6 +1469,48 @@ The function accepts the following arguments:
 int64_t stdlib_ndarray_offset( const struct ndarray *arr );
 ```
 
+#### stdlib_ndarray_offset_elements( \*arr )
+
+Returns an ndarray index offset (in elements).
+
+```c
+#include "stdlib/ndarray/ctor.h"
+#include <stdint.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+// ...
+
+// Create an ndarray:
+struct ndarray *x = stdlib_ndarray_allocate( ... );
+if ( x == NULL ) {
+    fprintf( stderr, "Error allocating memory.\n" );
+    exit( 1 );
+}
+
+// ...
+
+// Retrieve the index offset:
+int64_t offset = stdlib_ndarray_offset_elements( x );
+
+// ...
+
+// Free allocated memory:
+stdlib_ndarray_free( x );
+```
+
+The function accepts the following arguments:
+
+-   **arr**: `[in] struct ndarray*` input ndarray.
+
+```c
+int64_t stdlib_ndarray_offset_elements( const struct ndarray *arr );
+```
+
+Notes:
+
+-   The function assumes that the ndarray is **aligned** (i.e., the index offset in bytes is a multiple of the number of bytes per ndarray element).
+
 #### stdlib_ndarray_order( \*arr )
 
 Returns the order of an ndarray.

lib/node_modules/@stdlib/ndarray/ctor/benchmark/c/benchmark.c

@@ -3263,6 +3263,51 @@ static double benchmark66( void ) {
 	return elapsed;
 }
 
+/**
+* Runs a benchmark.
+*
+* @return elapsed time in seconds
+*/
+static double benchmark67( void ) {
+	double elapsed;
+	int64_t v;
+	int64_t j;
+	double t;
+	int i;
+
+	uint8_t buffer[] = { 0, 0, 0, 0, 0, 0 };
+	int64_t ndims = 2;
+	int64_t shape[] = { 3, 2 };
+	int64_t strides[] = { -2, -1 };
+	int64_t offset = 5;
+	int64_t nsubmodes = 1;
+	int8_t submodes[] = { STDLIB_NDARRAY_INDEX_ERROR };
+
+	struct ndarray *arr = stdlib_ndarray_allocate( STDLIB_NDARRAY_UINT8, buffer, ndims, shape, strides, offset, STDLIB_NDARRAY_ROW_MAJOR, STDLIB_NDARRAY_INDEX_ERROR, nsubmodes, submodes );
+	if ( arr == NULL ) {
+		printf( "unable to allocate memory\n" );
+		exit( 1 );
+	}
+
+	t = tic();
+	for ( i = 0; i < ITERATIONS; i++ ) {
+		j = (int64_t)( rand_double()*ndims );
+		v = stdlib_ndarray_offset_elements( arr, j );
+		if ( v != offset ) {
+			printf( "unexpected result\n" );
+			break;
+		}
+	}
+	elapsed = tic() - t;
+
+	if ( v != offset ) {
+		printf( "unexpected result\n" );
+	}
+	stdlib_ndarray_free( arr );
+
+	return elapsed;
+}
+
 /**
 * Main execution sequence.
 */
@@ -3739,5 +3784,12 @@ int main( void ) {
 		print_results( elapsed );
 		printf( "ok %d benchmark finished\n", count );
 	}
+	for ( i = 0; i < REPEATS; i++ ) {
+		count += 1;
+		printf( "# c::native::%s::get:offset_elements\n", NAME );
+		elapsed = benchmark67();
+		print_results( elapsed );
+		printf( "ok %d benchmark finished\n", count );
+	}
 	print_summary( count, count );
 }

lib/node_modules/@stdlib/ndarray/ctor/include/stdlib/ndarray/ctor.h

@@ -113,6 +113,11 @@ int64_t stdlib_ndarray_ndims( const struct ndarray *arr );
 */
 int64_t stdlib_ndarray_offset( const struct ndarray *arr );
 
+/**
+* Returns an ndarray index offset (in elements).
+*/
+int64_t stdlib_ndarray_offset_elements( const struct ndarray *arr );
+
 /**
 * Returns the order of an ndarray.
 */

lib/node_modules/@stdlib/ndarray/ctor/src/main.c

@@ -368,12 +368,26 @@ int64_t stdlib_ndarray_nsubmodes( const struct ndarray *arr ) {
 * Returns an ndarray index offset (in bytes).
 *
 * @param arr  input ndarray
-* @return     array strides
+* @return     index offset
 */
 int64_t stdlib_ndarray_offset( const struct ndarray *arr ) {
 	return arr->offset;
 }
 
+/**
+* Returns an ndarray index offset (in elements).
+*
+* ## Notes
+*
+* -   The function assumes that an ndarray is aligned (i.e., offset is a multiple of the number of bytes per ndarray element).
+*
+* @param arr  input ndarray
+* @return     index offset
+*/
+int64_t stdlib_ndarray_offset_elements( const struct ndarray *arr ) {
+	return arr->offset / arr->BYTES_PER_ELEMENT;
+}
+
 /**
 * Returns the order of an ndarray.
 *