apy

Apy

Various helpers for calculating interest

TODO

• Amortization
• Update method signatures to only use day counts
• Comprehensive examples w/ tests
• Setup ci

Installation

Add this line to your application's Gemfile:

gem "apy"

And then execute:

$ bundle install

Or install it yourself as:

$ gem install apy

Usage

• As a module
  • Method: weighted_harmonic_mean
  • Method: compound
• Interest class
• Loan class

As a Module

Apy::Calculable is the base module with the most exposed utility. Simply include it in a class which needs the functionality.

weighted_harmonic_mean

Link

Given a series of invested amounts, this can be used to effectively calculate the average share price, or DCA (dollar cost average) price of a position.

Given the following:

Amount invested	Share price
1000	156.23
1000	156.30
1000	173.15
1000	188.72
1000	204.61
1000	178.23

Investing a total of 6000 with the above share prices results in 174.57 for the average price paid per share.

Given the following:

Amount invested	Share price
500	200.00
1000	100.00

Investing a total of 1500 with the above share prices results in 100.00 for the average price paid per share.

The module method accepts a matrix, with each array in the set having the following signature [invested_amount, share_price]:

dca = [
  [1000, 156.23],
  [1000, 156.30],
  [1000, 173.15],
  [1000, 188.72],
  [1000, 204.61],
  [1000, 178.23]
]
weighted_harmonic_mean(dca) == 174.5655590168175

compound

Link

Simple compound interest formula. Variable names correspond to the following:

• principal The base amount before interest
• rate The expected interest rate
• times The number of times interest is calculated per term
• terms The number of terms to allow the principal accrue interest

Example: 1200@10% over 1y, interest paid monthly

compound(1200, rate: 0.1, times: 12, terms: 1) == 1325.66

dca_compound

A variant of #compound, wherein an amount is continually invested at varying interest rates. Similar to weighted_harmonic_mean, this method also accepts a matrix. The size of the matrix corresponds to the number of terms all funds will accrue.

Example: 1200@10% over 2y, interest paid monthly

dca = [
  [1200, 0.1],
  [1200, 0.1]
]
dca_compound(dca, times: 12) == 2790.125

Interest

todo

Loan

todo

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and the created tag, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/fire-pls/apy.

License

The gem is available as open source under the terms of the MIT License.