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# Transient Index using B-Trees

`transient-btree-index` allows you to create a BTree index backed by temporary files.
This is helpful if you
 
- need to index large datasets (thus only working on disk) by inserting entries in unsorted order,
- want to query entries (get and range queries) while the index is still constructed, e.g. to check existence of a previous entry, and
- need support for all serde-serializable key and value types with varying key-size.
 
Because of its intended use case, it is therefore **not possible to**
 
- delete entries once they are inserted (you can use `Option` values and set them to `Option::None`, but this will not reclaim any used space),
- persist the index to a file (you can use other crates like [sstable](https://crates.io/crates/sstable) to create immutable maps), or
- load an existing index file (you might want to use an immutable map file and this index can act as an "overlay" for all changed entries).


## Example

```rust
use transient_btree_index::{BtreeConfig, BtreeIndex, Error};

fn main() -> std::result::Result<(), Error> {
    let mut b = BtreeIndex::<u16,u16>::with_capacity(BtreeConfig::default(), 10)?;
    b.insert(1,2)?;
    b.insert(200, 4)?;
    b.insert(20, 3)?;

    assert_eq!(true, b.contains_key(&200)?);
    assert_eq!(false, b.contains_key(&2)?);  

    assert_eq!(3, b.get(&20)?.unwrap());  

    for e in b.range(1..30)? {
        let (k, v) = e?;
        dbg!(k, v);
    }
    Ok(())
}
```