### 3 releases (breaking)

0.3.0 | Feb 13, 2022 |
---|---|

0.2.0 | Oct 28, 2021 |

0.1.0 | Sep 28, 2021 |

#**1748** in Data structures

**526** downloads per month

Used in **10** crates
(8 directly)

**MIT/Apache**

32KB

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# ndshape

Simple, fast linearization of 2D, 3D, and 4D coordinates.

The canonical choice of linearization function is row-major, i.e. stepping linearly through an N dimensional array would
step by X first, then Y, then Z, etc, assuming that

coordinates are provided as `[`T`;` N`]`

. More explicitly:`[`X`,` Y`,` Z`,` `...``]`

`linearize``(``[`x`,` y`,` z`,` `...``]``)` `=` x `+` `X_SIZE` `*` y `+` `X_SIZE` `*` `Y_SIZE` `*` z `+` `...`

To achieve a different layout, one only needs to choose a different permutation of coordinates. For example, column-major
layout would require coordinates specified as

. For a 3D layout where each Y level set is contiguous in
memory, either layout `[``...``,` Z`,` Y`,` X`]`

or `[`X`,` Z`,` Y`]`

would work.`[`Z`,` X`,` Y`]`

## Example: Indexing Multidimensional Arrays

`use` `ndshape``::``{`Shape`,` ConstShape3u32`,` ConstShape4u32`,` ConstPow2Shape3u32`,` RuntimeShape`}``;`
`//` An arbitrary shape.
`let` shape `=` `ConstShape3u32``::``<`5, 6, 7`>``;`
`let` index `=` shape`.``linearize``(``[``1``,` `2``,` `3``]``)``;`
`assert_eq!``(`index`,` `101``)``;`
`assert_eq!``(`shape`.``delinearize``(`index`)``,` `[``1``,` `2``,` `3``]``)``;`
`//` A shape with power-of-two dimensions
`//` This allows us to use bit shifting and masking for linearization.
`let` shape `=` `ConstPow2Shape3u32``::``<`1, 2, 3`>``;` `//` These are number of bits per dimension.
`let` index `=` shape`.``linearize``(``[``1``,` `2``,` `3``]``)``;`
`assert_eq!``(`index`,` `0b011_10_1``)``;`
`assert_eq!``(`shape`.``delinearize``(`index`)``,` `[``1``,` `2``,` `3``]``)``;`
`//` A runtime shape.
`let` shape `=` `RuntimeShape``::``<``u32`, 3`>``::`new`(``[``5``,` `6``,` `7``]``)``;`
`let` index `=` shape`.``linearize``(``[``1``,` `2``,` `3``]``)``;`
`assert_eq!``(`index`,` `101``)``;`
`assert_eq!``(`shape`.``delinearize``(`index`)``,` `[``1``,` `2``,` `3``]``)``;`
`//` Use a shape for indexing an array in 4D.
`//` Step X, then Y, then Z, since that results in monotonic increasing indices.
`//` (Believe it or not, Rust's N-dimensional array (e.g. `[[T; N]; M]`)
`//` indexing is significantly slower than this).
`let` shape `=` `ConstShape4u32``::``<`5, 6, 7, 8`>``;`
`let` data `=` `[``0``;` `5` `*` `6` `*` `7` `*` `8``]``;`
`for` w `in` `0``..``8` `{`
`for` z `in` `0``..``7` `{`
`for` y `in` `0``..``6` `{`
`for` x `in` `0``..``5` `{`
`let` i `=` shape`.``linearize``(``[`x`,` y`,` z`,` w`]``)``;`
`assert_eq!``(``0``,` data`[`i `as` `usize``]``)``;`
`}`
`}`
`}`
`}`

## Example: Negative Strides with Modular Arithmetic

It is often beneficial to linearize a negative vector that results in a negative linear "stride." But when using unsigned
linear indices, a negative stride would require a modular arithmetic representation, where e.g.

maps to `-``1`

.
This works fine with any `u32``::``MAX`

. You just need to be sure to use modular arithmetic with the resulting
linear strides, e.g. `Shape`

and `u32`` ::`wrapping_add

`u32``::`wrapping_mul

. Also, it is not
possible to delinearize a negative stride with modular arithmetic. For that, you must use signed integer coordinates.`use` `ndshape``::``{`Shape`,` ConstShape3u32`,` ConstShape3i32`}``;`
`let` shape `=` `ConstShape3u32``::``<`10, 10, 10`>``;`
`let` stride `=` shape`.``linearize``(``[``0``,` `-``1``i32` `as` `u32``,` `0``]``)``;`
`assert_eq!``(`stride`,` `-``10``i32` `as` `u32``)``;`
`//` Delinearize does not work with unsigned coordinates!
`assert_ne!``(`shape`.``delinearize``(`stride`)``,` `[``0``,` `-``1``i32` `as` `u32``,` `0``]``)``;`
`assert_eq!``(`shape`.``delinearize``(`stride`)``,` `[``6``,` `8``,` `42949672``]``)``;`
`let` shape `=` `ConstShape3i32``::``<`10, 10, 10`>``;`
`let` stride `=` shape`.``linearize``(``[``0``,` `-``1``,` `0``]``)``;`
`assert_eq!``(`stride`,` `-``10``)``;`
`//` Delinearize works with signed coordinates.
`assert_eq!``(`shape`.``delinearize``(`stride`)``,` `[``0``,` `-``1``,` `0``]``)``;`

License: MIT OR Apache-2.0

#### Dependencies

~42KB