source files for faster directory fetches [#26]

js/pmtiles.ts

@@ -0,0 +1,48 @@
+export const shift = (n:number, shift:number) => {
+    return n * Math.pow(2, shift);
+}
+
+export const unshift = (n:number, shift:number) => {
+	return Math.floor(n / Math.pow(2, shift));
+}
+
+export const getUint24 = (view:DataView, pos:number) => {
+  return shift(view.getUint16(pos+1,true),8) + view.getUint8(pos)
+}
+
+export const getUint48 = (view:DataView, pos:number) => {
+    return shift(view.getUint32(pos+2,true),16) + view.getUint16(pos,true)
+}
+
+const compare = (tz:number,tx:number,ty:number,view:DataView,i:number) => {
+	if (tz != view.getUint8(i)) return tz - view.getUint8(i);
+	var x = getUint24(view,i+1);
+	if (tx != x) return tx - x;
+	var y = getUint24(view,i+4);
+	if (ty != y) return ty - y;
+	return 0;
+}
+
+export const getLeafdir = (view:DataView, z:number, x:number, y:number) => {
+	return getTile(view,z | 0x80,x,y);
+}
+
+export const getTile = (view:DataView, z:number, x:number, y:number) => {
+	var m = 0;
+	var n = view.byteLength / 17 - 1;	
+	while (m <= n) {
+		var k = (n + m) >> 1;
+		var cmp = compare(z,x,y,view,k*17);
+		if (cmp > 0) {
+			m = k + 1;
+		} else if (cmp < 0) {
+			n = k - 1;
+		} else {
+			return {
+				offset:getUint48(view,k*17+7),
+				length:view.getUint32(k*17+13,true)
+			}
+		}
+	}
+	return null;
+}