@loaders.gl/mvt/src/lib/mvt-pbf/write-mvt-to-pbf.ts

Loader for Mapbox Vector Tiles

// loaders.gl
// SPDX-License-Identifier: MIT
// Copyright vis.gl contributors

// Forked from https://github.com/mapbox/vt-pbf under MIT License Copyright (c) 2015 Anand Thakker

import Protobuf from 'pbf';
import type {MVTTile, MVTLayer} from './mvt-types';
import * as MVT from './mvt-constants';

type WriteContext = {
  layer: MVTLayer | any;
  keys: string[];
  values: unknown[];
  keycache: Record<string, number>;
  valuecache: Record<string, number>;
  feature?: any;
};

export function writeMVT(tile: MVTTile, pbf?: Protobuf): void {
  for (const key in tile.layers) {
    const context: WriteContext = {
      layer: tile.layers[key],
      keys: [],
      values: [],
      keycache: {},
      valuecache: {}
    };

    pbf!.writeMessage(MVT.TileInfo.layers, writeLayer, context);
  }
}

function writeLayer(context: WriteContext, pbf?: Protobuf): void {
  const {layer} = context;

  pbf!.writeVarintField(MVT.LayerInfo.version, layer.version || 1);
  pbf!.writeStringField(MVT.LayerInfo.name, layer.name || '');
  pbf!.writeVarintField(MVT.LayerInfo.extent, layer.extent || 4096);

  for (let i = 0; i < layer.length; i++) {
    context.feature = layer.feature(i);
    pbf!.writeMessage(MVT.LayerInfo.features, writeFeature, context);
  }

  const keys = context.keys;
  for (let i = 0; i < keys.length; i++) {
    pbf!.writeStringField(MVT.LayerInfo.keys, keys[i]);
  }

  const values = context.values;
  for (let i = 0; i < values.length; i++) {
    pbf!.writeMessage(MVT.LayerInfo.values, writeValue, values[i]);
  }
}

function writeFeature(context: WriteContext, pbf?: Protobuf): void {
  const feature = context.feature!;

  if (feature.id !== undefined) {
    pbf!.writeVarintField(MVT.FeatureInfo.id, feature.id);
  }

  pbf!.writeMessage(MVT.FeatureInfo.tags, writeProperties, context);
  pbf!.writeVarintField(MVT.FeatureInfo.type, feature.type);
  pbf!.writeMessage(MVT.FeatureInfo.geometry, writeGeometry, feature);
}

function writeProperties(context: WriteContext, pbf?: Protobuf): void {
  const feature = context.feature!;
  const {keys, values, keycache, valuecache} = context;

  for (const key in feature.properties) {
    let keyIndex = keycache[key];
    if (typeof keyIndex === 'undefined') {
      keys.push(key);
      keyIndex = keys.length - 1;
      keycache[key] = keyIndex;
    }
    pbf!.writeVarint(keyIndex);

    let value = feature.properties[key];
    const type = typeof value;
    if (type !== 'string' && type !== 'boolean' && type !== 'number') {
      value = JSON.stringify(value);
    }
    const valueKey = `${type}:${value}`;
    let valueIndex = valuecache[valueKey];
    if (typeof valueIndex === 'undefined') {
      values.push(value);
      valueIndex = values.length - 1;
      valuecache[valueKey] = valueIndex;
    }
    pbf!.writeVarint(valueIndex);
  }
}

function command(cmd, length) {
  return (length << 3) + (cmd & 0x7);
}

function zigzag(num) {
  return (num << 1) ^ (num >> 31);
}

function writeGeometry(feature, pbf?: Protobuf): void {
  const geometry = feature.loadGeometry();
  const type = feature.type;
  let x = 0;
  let y = 0;
  const rings = geometry.length;
  for (let r = 0; r < rings; r++) {
    const ring = geometry[r];
    let count = 1;
    if (type === 1) {
      count = ring.length;
    }
    pbf!.writeVarint(command(1, count)); // moveto
    // do not write polygon closing path as lineto
    const lineCount = type === 3 ? ring.length - 1 : ring.length;
    for (let i = 0; i < lineCount; i++) {
      if (i === 1 && type !== 1) {
        pbf!.writeVarint(command(2, lineCount - 1)); // lineto
      }
      const dx = ring[i].x - x;
      const dy = ring[i].y - y;
      pbf!.writeVarint(zigzag(dx));
      pbf!.writeVarint(zigzag(dy));
      x += dx;
      y += dy;
    }
    if (type === 3) {
      pbf!.writeVarint(command(7, 1)); // closepath
    }
  }
}

function writeValue(value: unknown, pbf?: Protobuf): void {
  switch (typeof value) {
    case 'string':
      pbf!.writeStringField(1, value);
      break;
    case 'boolean':
      pbf!.writeBooleanField(7, value);
      break;
    case 'number':
      if (value % 1 !== 0) {
        pbf!.writeDoubleField(3, value);
      } else if (value < 0) {
        pbf!.writeSVarintField(6, value);
      } else {
        pbf!.writeVarintField(5, value);
      }
      break;
    default:
    // ignore
  }
}