A proof of concept for efficient point encoding in vector tiles

convert.js

var tile = require('./points.json');

tile.layers.forEach(function (layer) {

    layer.id = [];
    layer.geometry = [];
    layer.tags = [];

    var x0 = 0;
    var y0 = 0;
    var id = 0;

    layer.features.sort(compareFeatures);

    for (var k = 0; k < layer.features.length; k++) {

        var f = layer.features[k];

        layer.id.push(f.id - id);
        id = f.id;

        var x = fromZigZag(f.geometry[1]);
        var y = fromZigZag(f.geometry[2]);

        layer.geometry.push(toZigZag(x - x0));
        layer.geometry.push(toZigZag(y - y0));

        var tags = [];
        for (var i = 0; i < f.tags.length; i += 2) {
            tags[i / 2] = f.tags[i + 1];
        }
        for (i = 0; i < layer.keys.length; i++) {
            layer.tags.push(tags[i] || 0);
        }

        x0 = x;
        y0 = y;
    }

    delete layer.features;
});

console.log(JSON.stringify(tile));

function compareFeatures(a, b) {
    // return zOrder(a) - zOrder(b);
    return a.id - b.id;
}

function fromZigZag(n) {
    return n % 2 === 0 ? n / 2 : -(n + 1) / 2;
}

function toZigZag(n) {
    return n >= 0 ? n * 2 : -n * 2 - 1;
}

function zOrder(f) {
    x = Math.round(1024 * (fromZigZag(f.geometry[1]) + 1024) / 6144);
    y = Math.round(1024 * (fromZigZag(f.geometry[2]) + 1024) / 6144);

    x = (x | (x << 8)) & 0x00FF00FF;
    x = (x | (x << 4)) & 0x0F0F0F0F;
    x = (x | (x << 2)) & 0x33333333;
    x = (x | (x << 1)) & 0x55555555;

    y = (y | (y << 8)) & 0x00FF00FF;
    y = (y | (y << 4)) & 0x0F0F0F0F;
    y = (y | (y << 2)) & 0x33333333;
    y = (y | (y << 1)) & 0x55555555;

    return x | (y << 1);
}