timezone-encode.js

/*
  Node.js script to shrink the moment-timezone data file.
  
  Works by replacing chunks in the 'indices' data, and numbers in the 'untils' data, with indexes.
  
  Shrinks the latest data file from 903K to 377K.
*/

const fs = require('fs');
const largeFile = require('latest.json');

// String size when indexing 'indices'.

const I_LEN = 32;

// Get all 'untils' and 'incides' values, sorted by number of occurrences.

const uCounts = new Map();
const iCounts = new Map();

for (let i = 0; i < largeFile.zones.length; i++) {

  const arr = largeFile.zones[i].split('|');

  // Indices.

  if (arr[3]) {

    const indices = arr[3];

    for (let i = 0; i < indices.length; i += I_LEN) {
      const str = indices.substr(i, I_LEN);
      iCounts.set(str, (uCounts.get(str) || 0) + 1);
    }
  }

  // Untils.

  if (arr[4]) {

    const untils = arr[4].split(' ');

    for (let u of untils) {
      uCounts.set(u, (uCounts.get(u) || 0) + 1);
    }
  }
}

const uArr = [...uCounts.entries()].sort((a, b) => b[1] - a[1]).map(a => a[0]);
const iArr = [...iCounts.entries()].sort((a, b) => b[1] - a[1]).map(a => a[0]);

largeFile._z = uArr.join(' ');
largeFile._i = iArr.join(' ');

// Iterate through zones and replace 'untils' and 'indices' with base36 indexes
// pointing to the 'uArr' and 'iArr' respectively.

const uMap = new Map(uArr.map((str, i) => [str, i]));
const iMap = new Map(iArr.map((str, i) => [str, i]));

for (let i = 0; i < largeFile.zones.length; i++) {

  const arr = largeFile.zones[i].split('|');

  // Indices.

  if (arr[3]) {

    let iArr = [];

    const indices = arr[3];

    for (let i = 0; i < indices.length; i += I_LEN) {
      const str = indices.substr(i, I_LEN);
      iArr.push(iMap.get(str).toString(36));
    }

    arr[3] = iArr.join(' ');
  }

  // Untils.

  if (arr[4]) {

    const untils = arr[4].split(' ');

    for (let i = 0; i < untils.length; i++) {
      untils[i] = uMap.get(untils[i]).toString(36);
    }

    arr[4] = untils.join(' ');
  }

  largeFile.zones[i] = arr.join('|');
}

fs.writeFileSync('./latest.json', JSON.stringify(largeFile, null, 2));

/*
  Example of how to decode the output.
*/

function decode(data) {

  const zArr = data._z.split(' ');
  const iArr = data._i.split(' ');

  for (let i = 0; i < data.zones.length; i++) {

    const arr = data.zones[i].split('|');

    if (arr[3]) {
      arr[3] = arr[3].split(' ').map(a => iArr[parseInt(a, 36)]).join('');
    }

    if (arr[4]) {
      arr[4] = arr[4].split(' ').map(a => zArr[parseInt(a, 36)]).join(' ');
    }

    data.zones[i] = arr.join('|');
  }

  delete data._i;
  delete data._z;

  return data;
};