georedis/lib/range.js

Super fast geo queries.

var geohash = require('ngeohash');

var rangeIndex = [
  0.6, // 52
  1, // 50
  2.19, // 48
  4.57, // 46
  9.34, // 44
  14.4, // 42
  33.18, // 40
  62.1, // 38
  128.55, // 36
  252.9, // 34
  510.02, // 32
  1015.8, // 30
  2236.5, // 28
  3866.9, // 26
  8749.7, // 24
  15664, // 22
  33163.5, // 20
  72226.3, // 18
  150350, // 16
  306600, // 14
  474640, // 12
  1099600, // 10
  2349600, // 8
  4849600, // 6
  10018863 // 4
];


function rangeDepth(radius) {
  var i;
  for (i = 0; i < rangeIndex.length - 1; i++) {
    if (radius - rangeIndex[i] < rangeIndex[i + 1] - radius) {
      return 52 - (i * 2);
    }
  }
  return 2;
}

function rangeDepthWide(radius) {
  var i;
  for (i = 0; i < rangeIndex.length - 1; i++) {
    if (rangeIndex[i] > radius) {
      return 52 - (i * 2);
    }
  }
  return 2;
}

function getQueryRangesFromRadius(lat, lon, radius, wide) {
  var rangeBitDepth = (wide === true) ? rangeDepthWide(radius) : rangeDepth(radius);
  return getQueryRangesFromBitDepth(lat, lon, rangeBitDepth, 52);
}

function getQueryRangesFromBitDepth(lat, lon, radiusBitDepth, bitDepth) {
  var hash = geohash.encode_int(lat, lon, radiusBitDepth);
  var neighbors = buildBoxSet(hash, radiusBitDepth);
  var ranges = rangesFromBoxSet(neighbors);

  increaseRangeBitDepth(ranges, radiusBitDepth, bitDepth);

  return ranges;
}

function buildBoxSet(hash, radiusBitDepth) {
  var neighbors = geohash.neighbors_int(hash, radiusBitDepth);

  neighbors.push(hash);
  neighbors.sort();

  if (radiusBitDepth <= 4) {
    neighbors = getUniqueInArray(neighbors);
  }

  return neighbors;
}

function rangesFromBoxSet(neighbors) {
  var ranges = [];
  var lowerRange = 0;
  var upperRange = 0;
  var i;

  for (i = 0; i < neighbors.length; i++) {
    lowerRange = neighbors[i];
    upperRange = lowerRange + 1;
    while (neighbors[i + 1] === upperRange) {
      neighbors.shift();
      upperRange = neighbors[i] + 1;
    }
    ranges.push([lowerRange, upperRange]);
  }

  return ranges;
}

function increaseRangeBitDepth(ranges, radiusBitDepth, bitDepth) {
  var bitDiff = bitDepth - radiusBitDepth;
  var range;
  var i;

  for (i = 0; i < ranges.length; i++) {
    range = ranges[i];
    range[0] = leftShift(range[0], bitDiff);
    range[1] = leftShift(range[1], bitDiff);
  }
}


// helper methods

function leftShift(integer, shft) {
  return integer * Math.pow(2, shft);
}

function getUniqueInArray(arr) {
  var u = {};
  var a = [];
  var i;

  for (i = 0; i < arr.length; ++i) {
    if (u[arr[i]] === 1) {
      continue;
    }
    a.push(arr[i]);
    u[arr[i]] = 1;
  }

  return a;
}


module.exports = exports = getQueryRangesFromRadius;