update tests and readme about minzoom [#120]

README.md

@@ -19,7 +19,7 @@ Convert an [MBTiles](https://github.com/mapbox/mbtiles-spec/tree/master/1.3) arc
     pmtiles extract INPUT.pmtiles OUTPUT.pmtiles --maxzoom=MAXZOOM --bucket=s3://BUCKET_NAME
 
 * `--region` a GeoJSON Polygon, Multipolygon, Feature, or FeatureCollection
-* `--maxzoom=13` extract only a subset of zoom levels
+* `--maxzoom=13`, `--minzoom=12` extract only a subset of zoom levels, see [docs for details](https://docs.protomaps.com/pmtiles/cli#extract)
 * `--download-threads` parallel requests to speed up downloads
 * `--overfetch` extra data to download to batch small requests: 0.05 is 5%
 

pmtiles/bitmap_test.go

@@ -36,3 +36,12 @@ func TestGeneralizeOr(t *testing.T) {
 	assert.True(t, b.Contains(ZxyToId(1, 0, 0)))
 	assert.True(t, b.Contains(ZxyToId(0, 0, 0)))
 }
+
+func TestGeneralizeOrMinZoom(t *testing.T) {
+	b := roaring64.New()
+	b.Add(ZxyToId(3, 0, 0))
+	generalizeOr(b, 2)
+	assert.Equal(t, uint64(2), b.GetCardinality())
+	assert.True(t, b.Contains(ZxyToId(2, 0, 0)))
+	assert.False(t, b.Contains(ZxyToId(1, 0, 0)))
+}