# [−][src]Crate calc_graph

Use this crate to split a calculation into related sub-calculations, known as nodes.

You can push information from outside into one or more source nodes, and you can read results from one or more output nodes. Values are only calculated as they're needed, and cached as long as their inputs don't change. This means that recalculations are efficient when you only change some of the inputs, and if you don't request the value from an output node, its value is never calculated.

# Example

```let graph = Graph::new();                        // create a Graph object
let mut source = graph.source(42);               // define one or more nodes for your inputs
let mut output = source.clone().map(|x| x + 1);  // build one or more nodes for your outputs

source.set(99);                                  // push new values to the input nodes...
assert_eq!(100, output.get_mut());               // ...and read the output nodes```

# Sharing

Func nodes (created by `Node::map`, `Node::zip` and related methods) own their inputs (precedent nodes). When you have a node that acts as an input to two or more func nodes, you need to use `shared()` first. You can then use this shared node multiple times via `clone()`:

```let input_node = calc_graph::const_(42).shared();
let mut output1_node = input_node.clone().map(|x| x + 1);
let mut output2_node = input_node.map(|x| x * x);
assert_eq!(43, output1_node.get_mut());
assert_eq!(1764, output2_node.get_mut());```

You can have multiple `Graph` objects in the same program, but when you define a new node, its precedents must come from the same graph.

# Boxing

A `Node` object remembers the full type information of its precedent nodes as well as the closure used to calculate its value. This means that the name of the `Node` type can be very long, or even impossible to write in the source code. In this situation you can use:

```let func_node: Node<Func1<_, i32, _>> = input_node.map(|x| x + 1);
let output_node: BoxNode<i32> = func_node.boxed();```

A call to `boxed()` is also needed if you want a variable that can hold either one or another node; these nodes can have different concrete types, and calling `boxed()` on each of them gives you a pair of nodes that have the same type.

`Node<Source>`, `SharedNode` and `BoxedNode` objects are `Send` and `Sync`, meaning they can be passed between threads. Calculations are performed on the thread that calls `node.get()`. Calculations are not parallelised automatically, although you can read separate output nodes from separate threads, even if they share parts of the same graph as inputs.

```let graph = Graph::new();
let input_node = graph.source(41);
let output_node = input_node.clone().map(|x| x * x).shared();
assert_eq!(1681, output_node.get());

let input_node = input_node.clone();
let output_node = output_node.clone();
move || {
input_node.update(|n| {
*n += 1;
true
});

output_node.get()
}
});

assert_eq!(1764, t.join().unwrap());

input_node.update(|n| {
*n += 1;
true
});

assert_eq!(1849, output_node.get());```

# `zip`, `zip_update` and others

Use `zip()`, `map2()` and related functions to create a new node that calculates its value from a `FnMut` provided by you and the values from one or more other nodes. For large objects, recalculating these nodes can be inefficient, as your `FnMut` returns a fresh object every time, which is cloned wherever it is needed.

For more efficiency you can use `zip_update()`, `map2_update()` and related functions. These work the same as their non-`update` equivalents, except that:

1. You provide the initial value of the new node when you create it
2. Your `FnMut` takes a `&mut T` as its first parameter. You update this value in place.
3. Your `FnMut` returns `true` if it changed value in the `&mut T`, or `false` otherwise. In turn, this determines whether dependent nodes are recalculated.

A useful technique for large objects is to put an `Arc<T>` in the node. When you recalculate the node, use `Arc::make_mut` to update the object in place where possible and avoid allocating a new `Arc`.

```let input_node = graph.source(42);

let mut output_node = input_node.clone().map_update(Arc::new([0; 1024]), |big_array, x| {
let new_value = x * x;
let big_array_ref = Arc::make_mut(big_array);
if big_array_ref[0] == new_value {
false
} else {
big_array_ref[0] = new_value;
true
}
});

assert_eq!(1764, output_node.get_mut()[0]);

input_node.update(|n| {
*n += 1;
true
});

assert_eq!(1849, output_node.get_mut()[0]);```

## Structs

 Const Calculates a node's value by returning the same value every time. Func1 Calculates a value from another node. Func2 Calculates a value from 2 nodes. Func3 Calculates a value from 3 nodes. Func4 Calculates a value from 4 nodes. Func5 Calculates a value from 5 nodes. Func6 Calculates a value from 6 nodes. Func7 Calculates a value from 7 nodes. Func8 Calculates a value from 8 nodes. Graph Returns new `Node` objects, which act as inputs to the rest of the graph. Inspect Provides the opportunity to inspect a node's value without changing it. Lazy Calculates a node's value by calling a function on demand and caching the result. Node Represents a value within the graph. Source Holds a value that can be updated directly from outside the graph. Update1 Calculates a value from the value currently in this node and another node. Update2 Calculates a value from the value currently in this node and 2 nodes. Update3 Calculates a value from the value currently in this node and 3 nodes. Update4 Calculates a value from the value currently in this node and 4 nodes. Update5 Calculates a value from the value currently in this node and 5 nodes. Update6 Calculates a value from the value currently in this node and 6 nodes. Update7 Calculates a value from the value currently in this node and 7 nodes. Update8 Calculates a value from the value currently in this node and 8 nodes.

## Traits

 Calc Calculates a node's value.

## Functions

 const_ Returns a node whose value never changes. lazy Returns a node whose value is calculated once on demand and cached.

## Type Definitions

 BoxNode A node returned by `Node::boxed`. SharedNode A node returned by `Node::shared`.