# Exonio

This gem brings some useful Excel formulas to Ruby. For now, it just has financial formulas, but could have more (like statistical formulas) in the future.

## Installation

Add this line to your application's Gemfile:

`gem 'exonio'`

And then execute:

```
$ bundle
```

Or install it yourself as:

```
$ gem install exonio
```

## Usage

To use Exonio you just have to call the method you like to use. Example:

` Exonio.pmt(0.075 / 12, 12 * 15, 200_000) # ==> -1854.0247200054619`

## Available Formulas

### EFFECT

The Excel EFFECT function returns the effective annual interest rate, given a nominal interest rate and the number of compounding periods per year. Effective annual interest rate is the interest rate actually earned due to compounding. More about this function EFFECT

`Exonio.effect(0.05, 10 * 12) # ==> 0.05126014873337037`

### NOMINAL

The Excel NOMINAL function returns the nominal interest rate when given an effective annual interest rate and the number of compounding periods per year. The effective rate is the actual rate due to compounding. The nominal rate is typically the stated rate. More about this function NOMINAL

`Exonio.nominal(0.05, 12 / 6) # ==> 0.04939015319191986`

### FV

What is the future value after 10 years of saving $100 now, with an additional monthly savings of $100 with the interest rate at 5% (annually) compounded monthly?

`Exonio.fv(0.05 / 12, 10 * 12, -100, -100) # ==> 15692.928894335748`

By convention, the negative sign represents cash flow out (i.e. money not available today). Thus, saving $100 a month at 5% annual interest leads to $15,692.93 available to spend in 10 years.

### IRR

Suppose one invests 100 units and then makes the following withdrawals at regular (fixed) intervals: 39, 59, 55, 20. Assuming the ending value is 0, one's 100 unit investment yields 173 units; however, due to the combination of compounding and the periodic withdrawals, the "average" rate of return is neither simply 0.73/4 nor (1.73)^0.25-1.

`Exonio.irr([-100, 39, 59, 55, 20]) # ==> 0.28095`

So, the internal rate of return is 28.09%

### IPMT

What is the interest part of a payment in the 8th period (i.e., 8th month), having a $5,000 loan to be paid in 2 years at an annual interest rate of 7.5%?

`Exonio.ipmt(0.075 / 12, 8, 12 * 2, 5_000.00) # ==> -22.612926783996798`

So, in the 8th payment, $22.61 are the interest part.

### NPER

If you only had $150/month to pay towards the loan, how long would it take to pay-off a loan of $8,000 at 7% annual interest?

`Exonio.nper(0.07 / 12, -150, 8000) # ==> 64.07334877066185`

So, over 64 months would be required to pay off the loan.

### NPV

Calculates the Net Present Value of an investment

`Exonio.npv(0.281, [-100, 39, 59, 55, 29]) # ==> -0.00661872883563408`

### PMT

What is the monthly payment needed to pay off a $200,000 loan in 15 years at an annual interest rate of 7.5%?

`Exonio.pmt(0.075 / 12, 12 * 15, 200_000) # ==> -1854.0247200054619`

In order to pay-off (i.e., have a future-value of 0) the $200,000 obtained
today, a monthly payment of $1,854.02 would be required. Note that this
example illustrates usage of `fv`

(future value) having a default value of 0.

### PV

What is the present value (e.g., the initial investment) of an investment that needs to total $20,000.00 after 10 years of saving $100 every month? Assume the interest rate is 5% (annually) compounded monthly.

`Exonio.pv(0.05 / 12, 12 * 10, -100, 20_000) # ==> -2715.0857731569663`

By convention, the negative sign represents cash flow out (i.e., money not available today). Thus, to end up with $20,000.00 in 10 years saving $100 a month at 5% annual interest, an initial deposit of $2715,09 should be made.

### RATE

Suppose you take a loan of $50,000.00 to pay in 3 years with a monthly payment of $2,500.00. What is the rate applied to this loan?

`Exonio.rate(12 * 3, 2_500, -50_000) # ==> 0.036006853458478955`

So, the rate applied is 3.60%.

## Statistical formulas

### Sum

`Exonio.sum([1, 2, 3, 4, 5]) # ==> 15`

### Mean

`Exonio.mean([1, 2, 3, 4, 5]) # ==> 3.0`

### Median

`Exonio.median([1, 2, 3, 6, 5, 4]) # ==> 3.5`

## TODO

There's a lot of formulas to be implemented, including:

- ACCRINT
- ACCRINTM
- AMORDEGRC
- AMORLINC
- DB
- DDB
- MIRR
- PPMT
- SLN
- SYD
- VDB

So feel free to pick one of those and open a pull request \o/.

## Contributing

- Fork the repository
- Create a branch
- Hack hack hack...
- Create a spec
- Open a Pull Request ;)

## License

Exonio is released under the MIT License.

## Special Thanks

A special thanks goes to the python NumPy project, which was the source for most of the formulas.