tile-stencil/src/filter.js

Load a MapLibre style document and parse it into Javascript functions

export function buildFeatureFilter(filterObj) {
  // filterObj is a filter definition following the 'deprecated' syntax:
  // https://maplibre.org/maplibre-gl-js-docs/style-spec/other/#other-filter
  if (!filterObj) return () => true;
  const [type, ...vals] = filterObj;

  // If this is a combined filter, the vals are themselves filter definitions
  switch (type) {
    case "all": {
      const filters = vals.map(buildFeatureFilter);  // Iteratively recursive!
      return (d) => filters.every( filt => filt(d) );
    }
    case "any": {
      const filters = vals.map(buildFeatureFilter);
      return (d) => filters.some( filt => filt(d) );
    }
    case "none": {
      const filters = vals.map(buildFeatureFilter);
      return (d) => filters.every( filt => !filt(d) );
    }
    default:
      return getSimpleFilter(filterObj);
  }
}

function getSimpleFilter(filterObj) {
  const [type, key, ...vals] = filterObj;
  const getVal = initFeatureValGetter(key);

  switch (type) {
    // Existential Filters
    case "has":
      return d => !!getVal(d); // !! forces a Boolean return
    case "!has":
      return d => !getVal(d);

    // Comparison Filters
    case "==":
      return d => getVal(d) === vals[0];
    case "!=":
      return d => getVal(d) !== vals[0];
    case ">":
      return d => getVal(d) > vals[0];
    case ">=":
      return d => getVal(d) >= vals[0];
    case "<":
      return d => getVal(d) < vals[0];
    case "<=":
      return d => getVal(d) <= vals[0];

    // Set Membership Filters
    case "in" :
      return d => vals.includes( getVal(d) );
    case "!in" :
      return d => !vals.includes( getVal(d) );
    default:
      console.log("prepFilter: unknown filter type = " + filterObj[0]);
  }
  // No recognizable filter criteria. Return a filter that is always true
  return () => true;
}

function initFeatureValGetter(key) {
  switch (key) {
    case "$type":
      // NOTE: data includes MultiLineString, MultiPolygon, etc-NOT IN SPEC
      return f => {
        const t = f.geometry.type;
        if (t === "MultiPoint") return "Point";
        if (t === "MultiLineString") return "LineString";
        if (t === "MultiPolygon") return "Polygon";
        return t;
      };
    case "$id":
      return f => f.id;
    default:
      return f => f.properties[key];
  }
}