cart_lin 0.2.0

Conversion between cartesian and linear indices
Documentation 
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A lightweight library for converting between linear and cartesian indices for any number of dimensions.

This library offers the following functions for conversion between linear
and cartesian indices for any number of dimensions:
- `cart_to_lin` and `cart_to_lin_unchecked`: Convert a cartesian index (e.g. `[1, 2, 5]`
for a three-dimensional matrix) into a linear index (i.e. the position in the
underlying contiguous memory).
- `lin_to_cart` and `lin_to_cart_unchecked`: Convert a linear index into a cartesian index.
- `lin_to_cart_dyn` and `lin_to_cart_dyn_unchecked`: These variations of `lin_to_cart`
write the calculated cartesian indices into a caller-provided slice buffer instead of
returning an index array.

Additionally, `CartesianIndices` provides an iterator over cartesian indices which can be seen
as the multidimensional equivalent of the [`Range`](<https://doc.rust-lang.org/std/ops/struct.Range.html>) iterator.

This library has no dependencies besides the Rust stdlib and is therefore very lightweight.

# Cartesian to linear conversion


Let's use the following 2x3 matrix (two rows, three columns) as an example:

```bash
0 1 2
3 4 5
```

The cartesian index of element `0` is `[0, 0]`, that of `1` is `[0, 1]`, that of `5` is `[1, 2]` and so on.
`cart_to_lin` (as well as all other functions of this library) uses row-major order
(= last index changes fastest).
```
use cart_lin::cart_to_lin;

// Rows, columns
let dim_size = [2, 3];

assert_eq!(cart_to_lin(&[0, 0], &dim_size).unwrap(), 0);
assert_eq!(cart_to_lin(&[0, 1], &dim_size).unwrap(), 1);
assert_eq!(cart_to_lin(&[0, 2], &dim_size).unwrap(), 2);
assert_eq!(cart_to_lin(&[1, 0], &dim_size).unwrap(), 3);
assert_eq!(cart_to_lin(&[1, 1], &dim_size).unwrap(), 4);
assert_eq!(cart_to_lin(&[1, 2], &dim_size).unwrap(), 5);

```

For higher-dimensional matrices, it works in the same way (using the example of a matrix
with 4 rows, 3 columns and 2 pages):
```
use cart_lin::cart_to_lin;

// Rows, columns, pages
let dim_size = [4, 3, 2];

assert_eq!(cart_to_lin(&[0, 0, 0], &dim_size).unwrap(), 0);
assert_eq!(cart_to_lin(&[0, 0, 1], &dim_size).unwrap(), 1);
assert_eq!(cart_to_lin(&[0, 1, 0], &dim_size).unwrap(), 2);
assert_eq!(cart_to_lin(&[0, 1, 1], &dim_size).unwrap(), 3);
assert_eq!(cart_to_lin(&[0, 2, 0], &dim_size).unwrap(), 4);
assert_eq!(cart_to_lin(&[0, 2, 1], &dim_size).unwrap(), 5);
assert_eq!(cart_to_lin(&[1, 0, 0], &dim_size).unwrap(), 6);
```
`cart_to_lin` checks whether the given cartesian index is valid for the specified number of dimensions.
In order to avoid this check, use `cart_to_lin_unchecked` (which is not unsafe, but might return
invalid indices).

# Linear to cartesian conversion


The inverse of `cart_to_lin` is `lin_to_cart`:
```
use cart_lin::lin_to_cart;

// Rows, columns
let dim_size = [2, 3];

assert_eq!(lin_to_cart(0, &dim_size).unwrap(), [0, 0]);
assert_eq!(lin_to_cart(1, &dim_size).unwrap(), [0, 1]);
assert_eq!(lin_to_cart(2, &dim_size).unwrap(), [0, 2]);
assert_eq!(lin_to_cart(3, &dim_size).unwrap(), [1, 0]);
assert_eq!(lin_to_cart(4, &dim_size).unwrap(), [1, 1]);
assert_eq!(lin_to_cart(5, &dim_size).unwrap(), [1, 2]);
```

# Iterate over cartesian indices


```
use cart_lin::CartesianIndices;

// Two dimensions (2 x 3 matrix)
let mut cartiter = CartesianIndices::new([2, 3]);
assert_eq!(cartiter.next(), Some([0, 0]));
assert_eq!(cartiter.next(), Some([0, 1]));
assert_eq!(cartiter.next(), Some([0, 2]));
assert_eq!(cartiter.next(), Some([1, 0]));
assert_eq!(cartiter.next(), Some([1, 1]));
assert_eq!(cartiter.next(), Some([1, 2]));
assert_eq!(cartiter.next(), None);

// Four dimensions (2 x 2 x 2 x 2 matrix)
let mut cartiter = CartesianIndices::new([2, 2, 2, 2]);
assert_eq!(cartiter.next(), Some([0, 0, 0, 0]));
assert_eq!(cartiter.next(), Some([0, 0, 0, 1]));
assert_eq!(cartiter.next(), Some([0, 0, 1, 0]));
assert_eq!(cartiter.next(), Some([0, 0, 1, 1]));
assert_eq!(cartiter.next(), Some([0, 1, 0, 0]));
assert_eq!(cartiter.next(), Some([0, 1, 0, 1]));
assert_eq!(cartiter.next(), Some([0, 1, 1, 0]));
assert_eq!(cartiter.next(), Some([0, 1, 1, 1]));
assert_eq!(cartiter.next(), Some([1, 0, 0, 0]));
// ...
```

`CartesianIndices` can also be constructed by defining lower and upper bounds for each axis.
The following example is functionally equivalent to the previous one:
```
use cart_lin::CartesianIndices;

let mut cartiter = CartesianIndices::from_bounds([[0, 2], [0, 3]]).expect("bounds must be strictly monotonic increasing");
assert_eq!(cartiter.next(), Some([0, 0]));
assert_eq!(cartiter.next(), Some([0, 1]));
assert_eq!(cartiter.next(), Some([0, 2]));
assert_eq!(cartiter.next(), Some([1, 0]));
assert_eq!(cartiter.next(), Some([1, 1]));
assert_eq!(cartiter.next(), Some([1, 2]));
assert_eq!(cartiter.next(), None);
```

But it is also possible to add offsets via the lower bounds:
```
use cart_lin::CartesianIndices;

let mut cartiter = CartesianIndices::from_bounds([[1, 3], [2, 5]]).expect("bounds must be strictly monotonic increasing");
assert_eq!(cartiter.next(), Some([1, 2]));
assert_eq!(cartiter.next(), Some([1, 3]));
assert_eq!(cartiter.next(), Some([1, 4]));
assert_eq!(cartiter.next(), Some([2, 2]));
assert_eq!(cartiter.next(), Some([2, 3]));
assert_eq!(cartiter.next(), Some([2, 4]));
assert_eq!(cartiter.next(), None);
```

# Usage with matrix libraries


The `tests` directory contains examples on how to use this library together with [nalgebra](https://crates.io/crates/nalgebra) and [ndarray](https://crates.io/crates/ndarray).
However, neither of those libraries is a dependency of `cart_lin`.
*/