cached 0.5.0

/*!
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> Caching structures and simplified function memoization

`cached` provides implementations of several caching structures as well as a handy macro
for defining memoized functions.


## Defining memoized functions using `cached!`

`cached!` defined functions will have their results cached using the function's arguments as a key
(or a specific expression when using `cached_key!`).
When a `cached!` defined function is called, the function's cache is first checked for an already
computed (and still valid) value before evaluating the function body.

Due to the requirements of storing arguments and return values in a global cache:

- Function return types must be owned and implement `Clone`
- Function arguments must either be owned and implement `Hash + Eq + Clone` OR the `cached_key!`
  macro must be used to convert arguments into an owned + `Hash + Eq + Clone` type.
- Arguments and return values will be `cloned` in the process of insertion and retrieval.
- `cached!` functions should not be used to produce side-effectual results!

**NOTE**: Any custom cache that implements `cached::Cached` can be used with the `cached` macros in place of the built-ins.

See [`examples`](https://github.com/jaemk/cached/tree/master/examples) for basic usage and
an example of implementing a custom cache-store.


### `cached!` and `cached_key!` Usage & Options:

There are several options depending on how explicit you want to be. See below for a full syntax breakdown.


1.) Using the shorthand will use an unbounded cache.


```rust,no_run
#[macro_use] extern crate cached;
#[macro_use] extern crate lazy_static;

cached!{
    FIB;
    fn fib(n: u64) -> u64 = {
        if n == 0 || n == 1 { return n }
        fib(n-1) + fib(n-2)
    }
}
# pub fn main() { }
```


2.) Using the full syntax requires specifying the full cache type and providing
    an instance of the cache to use. Note that the cache's key-type is a tuple
    of the function argument types. If you would like fine grained control over
    the key, you can use the `cached_key!` macro.
    For example, a `SizedCache` (LRU):

```rust,no_run
#[macro_use] extern crate cached;
#[macro_use] extern crate lazy_static;

use std::thread::sleep;
use std::time::Duration;
use cached::SizedCache;

cached!{
    FIB: SizedCache<(u64, u64), u64> = SizedCache::with_capacity(50);
    fn fib(a: u64, b: u64) -> u64 = {
        sleep(Duration::new(2, 0));
        return a * b;
    }
}
# pub fn main() { }
```


3.) The `cached_key` macro functions identically, but allows you define the
    cache key as an expression.

```rust,no_run
#[macro_use] extern crate cached;
#[macro_use] extern crate lazy_static;

use std::thread::sleep;
use std::time::Duration;
use cached::SizedCache;

cached_key!{
    FIB: SizedCache<String, usize> = SizedCache::with_capacity(50);
    Key = { format!("{}{}", a, b) };
    fn fib(a: &str, b: &str) -> usize = {
        let size = a.len() + b.len();
        sleep(Duration::new(size as u64, 0));
        size
    }
}
# pub fn main() { }
```


## Syntax

The complete macro syntax is:


```rust,ignore
cached_key!{
    CACHE_NAME: CacheType = CacheInstance;
    Key = KeyExpression;
    fn func_name(arg1: arg_type, arg2: arg_type) -> return_type = {
        // do stuff like normal
        return_type
    }
}
```

Where:

- `CACHE_NAME` is the unique name used to hold a `static ref` to the cache
- `CacheType` is the full type of the cache
- `CacheInstance` is any expression that yields an instance of `CacheType` to be used
  as the cache-store, followed by `;`
- When using the `cached_key!` macro, the "Key" line must be specified. This line must start with
  the literal tokens `Key = `, followed by an expression that evaluates to the key, followed by `;`
- `fn func_name(arg1: arg_type) -> return_type` is the same form as a regular function signature, with the exception
  that functions with no return value must be explicitly stated (e.g. `fn func_name(arg: arg_type) -> ()`)
- The expression following `=` is the function body assigned to `func_name`. Note, the function
  body can make recursive calls to its cached-self (`func_name`).

*/

pub mod macros;
pub mod stores;

pub use stores::{
    UnboundCache, SizedCache, TimedCache,
};


/// Cache operations
pub trait Cached<K, V> {
    /// Attempt to retrieve a cached value
    fn cache_get(&mut self, k: &K) -> Option<&V>;

    /// Insert a key, value pair
    fn cache_set(&mut self, k: K, v: V);

    /// Return the current cache size (number of elements)
    fn cache_size(&self) -> usize;

    /// Return the number of times a cached value was successfully retrieved
    fn cache_hits(&self) -> Option<u32> { None }

    /// Return the number of times a cached value was unable to be retrieved
    fn cache_misses(&self) -> Option<u32> { None }

    /// Return the cache capacity
    fn cache_capacity(&self) -> Option<usize> { None }

    /// Return the lifespan of cached values (time to eviction)
    fn cache_lifespan(&self) -> Option<u64> { None }
}