Saint Lague / Schepers committee seat distribution

README.md

Webster/Sainte-Laguë/Schepers method

https://en.m.wikipedia.org/wiki/Webster/Sainte-Laguë_method

Calculates seat distribution of a committee, using the Webster/Sainte-Laguë/Schepers method for seat allocation.

It takes a number of seats in the committee and a set of parties with their number of seats in the actual parliament.

const committeeSize = 34;

const parties = {
  cdu: {
    seats: 123
  },
  spd: {
    seats: 120
  },
  ...
};
// parliament size is calculated by adding the parties' seats - duh :P

It returns an Object with the initial object's keys (preferably the parties' names) and the seats for that key in the committee.

=> {
  cdu: 17,
  spd: 11,
  ...
}

index.js

#!/usr/bin/env node

const _ = require('lodash');

const COMMITTEE_SIZE = 34;

const PARTIES = {
  cdu: {
    seats: 311
  },
  spd: {
    seats: 193
  },
  left: {
    seats: 64
  },
  greens: {
    seats: 63
  }
};

Object.defineProperty(PARTIES, 'seatsTotal', {
  get: () => {
    let seatsTotal = 0;
    for(let k in PARTIES) {
      seatsTotal += PARTIES[k].seats || 0;
    }
    return seatsTotal;
  }
});

((...args) => {

  let [committeeSize, parties] = args;
  /**
   * Generates Saint Lague/Schepers distribution
   * @return {[type]} [description]
   */
  const getSLS = (committeeSize, parties) => {
    console.log('Generating SLS distribution for a');
    console.log('committee with', committeeSize, 'seats.');
    console.log('Eligable parties: ', _.keys(parties));

    console.log('Calculting ranks...');
    // add ranks to parties
    let rankedParties = _.mapValues(parties, (party) => {
      party.ranks = calculateRanks(committeeSize, parties.seatsTotal, party);
      return party;
    });

    let ranks = _.reduce(rankedParties, (ranks, party, key) => {
      ranks[key] = party.ranks;
      return ranks;
    }, {});

    console.log('Comparing ranks...');
    let partyKeys = _.keys(ranks);
    let committeeSeats = _.zipObject(partyKeys, new Array(partyKeys.length).fill(0));
    for (let i = 0; i < committeeSize; i++) {
      let smallest = determineSmallestRank(ranks);
      ranks = smallest.ranks;
      committeeSeats[smallest.key]++;
    }

    console.log('Done.');
    console.log('Your comiittee\'s seat distribution is', committeeSeats);
  };

  const determineSmallestRank = (ranks) => {
    // find smallest and its key
    let smallest = Infinity;
    let smallestKey = '';

    for (let key in ranks) {
      let probe = ranks[key][0];
      if (probe < smallest) {
        smallest = probe;
        smallestKey = key;
      }
    }

    if (smallest < Infinity) {
      // remove smallest from array under key
      ranks[smallestKey].shift();
    }

    return { ranks: ranks, key: smallestKey };
  };

  const calculateRanks = (committeeSize, seatsTotal, party) => {
    let incrementalFactor = 0.5;
    let seatsFactor = seatsTotal / party.seats;
    // - number of ranks === total seats in committee, to be safe
    //   (although that scenario would mean a new dictator ^_^)
    // - first rank wants 0.5, so we 'increment after'
    // - example uses 3 decimals
    //   round(rank * 1000) / 1000
    //   round(12.1839384 * 1000 [=12183.9384]) [= 12184] / 1000 = 12.184
    return new Array(committeeSize).fill(0)
      .map(() => Math.round((seatsFactor * incrementalFactor++) * 1000) / 1000);
  };

  getSLS(committeeSize, parties);

})(COMMITTEE_SIZE, PARTIES);