**crate feature**only.

`alloc`

## Expand description

Randomly sample exactly `amount`

distinct indices from `0..length`

, and
return them in random order (fully shuffled).

This method is used internally by the slice sampling methods, but it can sometimes be useful to have the indices themselves so this is provided as an alternative.

The implementation used is not specified; we automatically select the
fastest available algorithm for the `length`

and `amount`

parameters
(based on detailed profiling on an Intel Haswell CPU). Roughly speaking,
complexity is `O(amount)`

, except that when `amount`

is small, performance
is closer to `O(amount^2)`

, and when `length`

is close to `amount`

then
`O(length)`

.

Note that performance is significantly better over `u32`

indices than over
`u64`

indices. Because of this we hide the underlying type behind an
abstraction, `IndexVec`

.

If an allocation-free `no_std`

function is required, it is suggested
to adapt the internal `sample_floyd`

implementation.

Panics if `amount > length`

.