
YARER - The math expression Resolver
===========================

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Yarer (Yet Another Rust Expression Resolver) is a library for resolving mathematical expressions. Internally it uses the shunting yard algorithm.

Example of usage of the library:

```rust
      let session = Session::init();
      let mut resolver = session.process("1+2"); // or even "(cos(10+e)+3*sin(9/pi))^2" 

      println!("The result is {}", resolver.resolve());
```

All that's needed is to get a new instance of the 'resolver' from a Session and hand over the expression to be analysed.
The library just returns a variant natural number, or a decimal number if one exists in the expression (i.e '2.1+1') or is present a trigonometric function (i.e. 1/cos(x+1)).

Yarer handles variables and functions. Here an example:

```rust
      let session = Session::init();
      let mut resolver = session.process("1/cos(x^2)");

      session.set("x",1);
      println!("The result is {}", resolver.resolve());
```

and of course, the expression can be re-evaluated if the variable changes.

```rust
      //...
      session.set("x",-1);
      println!("The result is {}", resolver.resolve());

      session.set("x",0.001); 
      println!("The result is {}", resolver.resolve());
      //...
```

The result can be simply casted into a i32 or a f64 (if decimal) simply with

```rust
      let result: Number = resolver.resolve().unwrap();

      let int : i32 = i32::from(result);
      // or
      let float : f64 = f64::from(result);
```

Yarer can be used also from command line, and behaves in a very similar manner to GNU bc

```rust
      $ yarer
      Yarer v.0.1.1 - Yet Another Rust Expression Resolver.
      License MIT OR Apache-2.0
      > (1+9)*(8+2)+0!
      101
      > (1./2)+atan(10)
      1.1483608274590869
      > x=10
      > 3/sin(5*x^2)
      -6.41338354698791
      > ln(1)
      0
      > log(10)
      1
      > -2^-2
      0.25
      > 1/(log(10)+cos(0))^-2
      4
      > 4.5+7.9*2.2
      21.88
      > 9801/(2206*sqrt(2)) // approx of PI
      3.1415927300133055
      
```

From Yarer version 0.1.5 it's possible to share a single session, and therefore a single heap of variables, for multiple resolvers. The library is not intended to be thread-safe.

```rust
    let session = Session::init();
    
    let mut res = session.process("x ^ 2");
    let mut res2 = session.process("x! - (x-1)!");

    session.set("x", 10);
   
    if let (Ok(a), Ok(b)) = (res.resolve(),res2.resolve()) {
        println!("{} {}", a, b); // 100 3265920
    }
```

## Built-in Defined Functions

There are several math functions defined that you can use in your expression. More to come!
There are many examples of processed expressions in the [integration test file](https://github.com/davassi/yarer/blob/master/tests/integration_tests.rs).

```rust
    Sin
    Cos
    Tan
    ASin
    ACos
    ATan
    Ln
    Log
    Abs
    Sqrt
```

## Built-in Defined Constants

There are 2 predefined math constants at the moment:

```rust
    PI -> 3.14159265...
    e -> 2.7182818...
```

## Execute

To run it from cargo, just type:

```console
cargo run -q -- 
```

For logging debug just run with:

```console
env RUST_LOG=yarer=debug cargo run -q -- 
```

or to build and install a release from the code:

```console
cargo build --release
cargo install --path .
./target/release/yarer
```

## Internal Implementation

Each expression is the result of an evaluation by the following actors

Step1 - Parser: A string is "regexed" and converted into a token array.

Step 2 - RpnResolver: Using the Shunting Yard algorithm the token array is converted from infix to postfix notation.

Step 3 - RpnResolver: The resulting RPN (Reverse Polish Notation) expression is evaluated.

Worth to mention that the Session is responsible to store all variables (and constants) that are borrowed by all the RpnResolvers.

## Contribution

Besides being stable, Yarer is a work in progress. If you have suggestions for features (i.e. more math functions to implement), or if you find any issues in the code, design, interface, etc, please feel free to share them on our [GitHub](https://github.com/davassi/yarer/issues).

I appreciate very much your feedback!
