vote-schulze

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Vote calculation with Schulze method (Condorcet voting)

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# vote-schulze [![ci](https://github.com/asaaki/vote-schulze/actions/workflows/ci.yml/badge.svg)](https://github.com/asaaki/vote-schulze/actions/workflows/ci.yml)

This gem is a Ruby implementation of the Schulze voting method (with help of the Floyd–Warshall algorithm), a type of the Condorcet voting methods.

**It's usable, but not production grade.**

Wikipedia:

* [Schulze method](http://en.wikipedia.org/wiki/Schulze_method) ([deutsch](http://de.wikipedia.org/wiki/Schulze-Methode))
* [Floyd–Warshall algorithm](http://en.wikipedia.org/wiki/Floyd%E2%80%93Warshall_algorithm)

## Install

``` bash
gem install vote-schulze
```

## Usage

``` ruby
require 'vote-schulze'
vs = Vote::Schulze::Basic.call(vote_list, candidate_count)
vs.ranks
vs.ranking_abc
```

Input:

* `vote_list`
  * Array of Arrays: votes of each voter as weights `[ [A,B,C,...],[A,B,C,...],[A,B,C,...] ]`
  * String: "A;B;C\nA;B;C\n;3=A;B;C..."
  * File: first line **must** be a single integer, following lines like vote_list type String (see vote lists under `examples` directory)
* `candidate_count` Integer: number of candidates
  * **required** for vote_list types of Array and String
  * _leave empty if vote_list is a File handle!_

### String/File format:

A typical voters line looks like this:

```
A;B;C;D;E;F
```

You also can say that _n_ voters have the same preferences:

```
n=F;E;D;C;B;A
```

where _n_ is an integer value for the count.

Also it's possible to say that a voter has candidates equally weighted:

```
A,B;C,D;E,F
```

which means, that A + B, C + D and E + F are on the same weight level.

Here only 3 weight levels are used: (A,B) = 3, (C,D) = 2, (E,F) = 1

### Why I must write the candidate count in the first line of the vote file?

_or: Why I must give a candidate count value for Array/String inputs?_

Very easy: The reason is, that voters can leave out candidates (they give no special preferences).

So, vote-schulze needs to know, how many real candidates are in the voting process.

Okay, for Array inputs it's currently a little bit overhead, because the voters array normally should have the size of the candidates count.
See it as an visual reminder while coding with this gem.

### Examples

#### Array

(Only weight values, no letters here! See section "_preference order to weight_ example")

``` ruby
require 'vote-schulze'
vote_list_array = [[3,2,1],[1,3,2],[3,1,2]]
vs = Vote::Schulze::Basic.call(vote_list_array, 3)
vs.ranking_abc #=> result
```

#### String

``` ruby
require 'vote-schulze'
vote_list_string = <<EOF
A;B;C
B;C;A
A;C;B
A,C,B
4=C;A;B
EOF
vs = Vote::Schulze::Basic.call(vote_list_string, 3)
vs.ranking_abc #=> result
```

#### File

``` ruby
require 'vote-schulze'
voting_list = File.open('path/to/vote.list')
vs = Vote::Schulze::Basic.call(voting_list)
vs.ranking_abc #=> result
```

### _preference order to weight_ example

```
voter  => A D C B

weight => 4,1,2,3

A is on first position = highest prio == 4
B is on last position                 == 1
C is on third position                == 2
D is on second position               == 3
```

Later versions will have an automatic Preference-to-Weight algorithm.
(Internally only integers are used for calculation of ranking.)

### It doesn't matter if you start counting at 0 (zero) or 1 (one).

Also it's not important, if you use jumps (like `1 3 5 9`).

Internally it will only check if candidate X > candidate Y

## Example

Reference calculation: [Schulze Methode | blog.cgiesel.de (german)](http://blog.cgiesel.de/schulze-methode/)

Example file under `examples/vote4.list`

Result should be:

``` ruby
voting_list = File.open('examples/vote4.list')
voting = Vote::Schulze::Basic.call(voting_list)
voting.ranking_abc
#=> ["C:1", "D:2", "B:3", "A:4"]
```

which is the same result of the reference above.

The result strings are always in format `Candidate:Position`, because it's possible that multiple candidates can be on the same rank.

## Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/asaaki/vote-schulze.
This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the [Contributor Covenant](http://contributor-covenant.org) code of conduct.

## Code of Conduct

Everyone interacting in the vote-schulze project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the [code of conduct](https://github.com/asaaki/vote-schulze/blob/master/CODE_OF_CONDUCT.md).

## License

The gem is available as open source under the terms of the [MIT License](https://opensource.org/licenses/MIT).

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