sort tiles by zoom ascending

core/src/main/java/com/onthegomap/flatmap/geo/TileCoord.java

@@ -36,8 +36,7 @@ public record TileCoord(int encoded, int x, int y, int z) implements Comparable<
   }
 
   public static int decodeZ(int key) {
-    int result = key >> 28;
-    return result < 0 ? 16 + result : result;
+    return (key >> 28) + 8;
   }
 
   public static int decodeX(int key) {
@@ -62,6 +61,15 @@ public record TileCoord(int encoded, int x, int y, int z) implements Comparable<
     if (y >= max) {
       y = max;
     }
+    // since most significant bit is treated as the sign bit, make:
+    // z0-7 get encoded from 8 (0b1000) to 15 (0b1111)
+    // z8-14 get encoded from 0 (0b0000) to 6 (0b0110)
+    // so that encoded tile coordinates are ordered by zoom level
+    if (z < 8) {
+      z += 8;
+    } else {
+      z -= 8;
+    }
     return (z << 28) | (x << 14) | y;
   }
 

core/src/main/java/com/onthegomap/flatmap/write/MbtilesWriter.java

@@ -146,12 +146,18 @@ public class MbtilesWriter {
       .setMaxzoom(config.maxzoom())
       .setJson(layerStats.getTileStats());
 
+    int currentZoom = Integer.MIN_VALUE;
     try (var batchedWriter = db.newBatchedTileWriter()) {
       Mbtiles.TileEntry tile;
       while ((tile = tiles.get()) != null) {
+        int z = tile.tile().z();
+        if (z > currentZoom) {
+          LOGGER.info("[mbtiles] Starting z" + z);
+          currentZoom = z;
+        }
         batchedWriter.write(tile.tile(), tile.bytes());
-        stats.wroteTile(tile.tile().z(), tile.bytes().length);
-        tilesByZoom[tile.tile().z()].inc();
+        stats.wroteTile(z, tile.bytes().length);
+        tilesByZoom[z].inc();
       }
     }
 

core/src/test/java/com/onthegomap/flatmap/geo/TileCoordTest.java

@@ -2,6 +2,7 @@ package com.onthegomap.flatmap.geo;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.fail;
 
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
@@ -11,22 +12,33 @@ public class TileCoordTest {
 
   @ParameterizedTest
   @CsvSource({
-    "0,0,0,0",
-    "0,0,1,268435456",
-    "0,1,1,268435457",
-    "1,1,1,268451841",
-    "100,100,14,-535232412",
+    "0,0,0",
+    "0,0,1",
+    "0,1,1",
+    "1,1,1",
+    "100,100,14",
   })
-  public void testTileCoord(int x, int y, int z, int encoded) {
+  public void testTileCoord(int x, int y, int z) {
     TileCoord coord1 = TileCoord.ofXYZ(x, y, z);
-    assertEquals(encoded, coord1.encoded());
-    TileCoord coord2 = TileCoord.decode(encoded);
+    TileCoord coord2 = TileCoord.decode(coord1.encoded());
     assertEquals(coord1.x(), coord2.x());
     assertEquals(coord1.y(), coord2.y());
     assertEquals(coord1.z(), coord2.z());
     assertEquals(coord1, coord2);
   }
 
+  @Test
+  public void testTileSortOrder() {
+    int last = Integer.MIN_VALUE;
+    for (int z = 0; z <= 14; z++) {
+      int encoded = TileCoord.ofXYZ(0, 0, z).encoded();
+      if (encoded < last) {
+        fail("encoded value for z" + (z - 1) + " (" + last + ") is not less than z" + z + " (" + encoded + ")");
+      }
+      last = encoded;
+    }
+  }
+
   @Test
   public void testThrowsPastZ14() {
     assertThrows(AssertionError.class, () -> TileCoord.ofXYZ(0, 0, 15));