### Build and Query PtrHash Index

Source: https://github.com/ragnargrootkoerkamp/ptrhash/blob/master/readme.md

Demonstrates how to create a PtrHash index from a large set of keys and then query for the minimal index of a specific key. It also shows how to get a non-minimal index and iterate over all indices.

```rust
use ptr_hash::{PtrHash, PtrHashParams};

// Generate some random keys.
let n = 1_000_000_000;
let keys = ptr_hash::util::generate_keys(n);

// Build the datastructure.
let mphf = <PtrHash>::new(&keys, PtrHashParams::default());

// Get the minimal index of a key.
let key = 0;
let idx = mphf.index(&key);
assert!(idx < n);

// Get the non-minimal index of a key. Slightly faster, but can be >=n.
let _idx = mphf.index_no_remap(&key);

// An iterator over the indices of the keys.
// 32: number of iterations ahead to prefetch.
// true: remap to a minimal key in [0, n).
let indices = mphf.index_stream::<32, true, _>(&keys);
assert_eq!(indices.sum::<usize>(), (n * (n - 1)) / 2);

// Test that all items map to different indices
let mut taken = vec![false; n];
for key in keys {
    let idx = mphf.index(&key);
    assert!(!taken[idx]);
    taken[idx] = true;
}
```

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