# Forth Kata

## Instructions

Implement an evaluator for a very simple subset of Forth.

[Forth](https://en.wikipedia.org/wiki/Forth_%28programming_language%29) is a stack-based
programming language. Implement a very basic evaluator for a small subset of Forth.

Your evaluator has to support the following words:

* `+`, `-`, `*`, `/` (integer arithmetic)
* `DUP`, `DROP`, `SWAP`, `OVER` (stack manipulation)

Your evaluator also has to support defining new words using the customary syntax:

```forth
: word-name definition ;
```

To keep things simple the only data type you need to support is signed integers of at least 16 bits size.

You should use the following rules for the syntax: a number is a sequence of one or more (ASCII) digits,
a word is a sequence of one or more letters, digits, symbols or punctuation that is not a number.
(Forth probably uses slightly different rules, but this is close enough.)

Words are case-insensitive.

## Reference

Forth runs on an imaginary machine having only a stack (virtually infinite) of values.
All instructions modify this stack in a way.
Some instructions require the stack to have at least a certain amount of values.

### Numbers

**Minimum required values on the stack: 0**

Numbers just push the given value on the stack.

```c
23   // Stack is [23]
42   // Stack is [42, 23]
1337 // Stack is [1337, 42, 23]
```

### `+`, `-`, `*`, `/`

**Minimum required values on the stack: 2**

The arithmetic instructions all take the top 2 values from the stack, calculate the result,
and then put the result on the stack.

Example:

```c
23 // Stack is [23]
42 // Stack is [42, 23]
+  // Stack is [65]
```

For `-` and `/` the order is relevant.

```c
23 // Stack is [23]
42 // Stack is [42, 23]
-  // Stack is [19]

4  // Stack is [4, 19]
64 // Stack is [64, 4, 19]
/  // Stack is [16, 19]
```

### `DUP` - Duplicate the top value on the stack

**Minimum required values on the stack: 1**

```c
1337 // Stack is [1337]
DUP  // Stack is [1337, 1337]
```

### `DROP` - Remove the top value from the stack

**Minimum required values on the stack: 1**

```c
1337 // Stack is [1337]
DROP // Stack is []
```

### `SWAP` - Swap the two top values on the stack

**Minimum required values on the stack: 2**

```c
23   // Stack is [23]
42   // Stack is [42, 23]
SWAP // Stack is [23, 42]
```

### `OVER` - Duplicate the second to top value on the stack

**Minimum required values on the stack: 2**

```c
23   // Stack is [23]
42   // Stack is [42, 23]
OVER // Stack is [23, 42, 23]
```