From 22475eefc455ccb9b4b1bf23acf2acb9888f57ed Mon Sep 17 00:00:00 2001
From: Andreas Hocevar <andreas.hocevar@gmail.com>
Date: Fri, 29 Mar 2024 14:47:23 +0100
Subject: [PATCH] Properly handle fractional layer minzoom and maxzoom

---
 src/apply.js       | 13 ++++++++-----
 src/util.js        | 13 ++++++-------
 test/apply.test.js | 28 ++++++++++++++++++++++++++--
 3 files changed, 40 insertions(+), 14 deletions(-)

diff --git a/src/apply.js b/src/apply.js
index df593edd..3a2131d7 100644
--- a/src/apply.js
+++ b/src/apply.js
@@ -32,11 +32,11 @@ import {bbox as bboxStrategy} from 'ol/loadingstrategy.js';
 import {createXYZ} from 'ol/tilegrid.js';
 import {
   defaultResolutions,
-  defaultTileGrid,
   fetchResource,
   getFilterCache,
   getFunctionCache,
   getGlStyle,
+  getResolutionForZoom,
   getStyleFunctionKey,
   getTileJson,
   getZoomForResolution,
@@ -317,7 +317,10 @@ export function applyStyle(
                 tileGrid.getMinZoom() > 0
               ) {
                 layer.setMaxResolution(
-                  tileGrid.getResolution(tileGrid.getMinZoom()) + 1e-15,
+                  getResolutionForZoom(
+                    tileGrid.getMinZoom(),
+                    defaultResolutions,
+                  ) + 1e-15,
                 );
               }
             });
@@ -1233,21 +1236,21 @@ export function finalizeLayer(
           if (minZoom > 0 || sourceMinZoom > 0) {
             layer.setMaxResolution(
               Math.min(
-                defaultTileGrid.getResolution(minZoom),
+                getResolutionForZoom(minZoom, defaultResolutions),
                 tileGrid.getResolution(sourceMinZoom),
               ) + 1e-15,
             );
           }
           if (maxZoom < 24) {
             layer.setMinResolution(
-              defaultTileGrid.getResolution(maxZoom) + 1e-15,
+              getResolutionForZoom(maxZoom, defaultResolutions),
             );
           }
         }
       } else {
         if (minZoom > 0) {
           layer.setMaxResolution(
-            defaultTileGrid.getResolution(minZoom) + 1e-15,
+            getResolutionForZoom(minZoom, defaultResolutions) + 1e-15,
           );
         }
       }
diff --git a/src/util.js b/src/util.js
index 4a10bdd3..0e0efef4 100644
--- a/src/util.js
+++ b/src/util.js
@@ -1,8 +1,6 @@
-import TileGrid from 'ol/tilegrid/TileGrid.js';
 import TileState from 'ol/TileState.js';
 import {VectorTile} from 'ol';
 import {expandUrl} from 'ol/tileurlfunction.js';
-import {get as getProjection} from 'ol/proj.js';
 import {getUid} from 'ol/util.js';
 import {normalizeSourceUrl, normalizeStyleUrl} from './mapbox.js';
 import {toPromise} from 'ol/functions.js';
@@ -75,11 +73,6 @@ export const defaultResolutions = (function () {
   return resolutions;
 })();
 
-export const defaultTileGrid = new TileGrid({
-  extent: getProjection('EPSG:3857').getExtent(),
-  resolutions: defaultResolutions,
-});
-
 /**
  * @param {number} width Width of the canvas.
  * @param {number} height Height of the canvas.
@@ -108,6 +101,12 @@ export function getZoomForResolution(resolution, resolutions) {
   return ii - 1;
 }
 
+export function getResolutionForZoom(zoom, resolutions) {
+  const base = Math.floor(zoom);
+  const factor = Math.pow(2, zoom - base);
+  return resolutions[base] / factor;
+}
+
 const pendingRequests = {};
 /**
  * @param {ResourceType} resourceType Type of resource to load.
diff --git a/test/apply.test.js b/test/apply.test.js
index b76535f5..962ba5aa 100644
--- a/test/apply.test.js
+++ b/test/apply.test.js
@@ -718,7 +718,31 @@ describe('ol-mapbox-style', function () {
               defaultResolutions[10] + 1e-15,
             );
             should(map.getLayers().item(0).getMinResolution()).eql(
-              defaultResolutions[12] + 1e-15,
+              defaultResolutions[12],
+            );
+            done();
+          })
+          .catch(function (err) {
+            done(err);
+          });
+      });
+
+      it('respects fractional layer minzoom and maxzoom', function (done) {
+        context.layers[0].minzoom = 10.5;
+        context.layers[0].maxzoom = 12.5;
+        apply(target, context)
+          .then(function (map) {
+            should(map.getLayers().item(0).getMaxResolution()).greaterThan(
+              defaultResolutions[11],
+            );
+            should(map.getLayers().item(0).getMaxResolution()).lessThan(
+              defaultResolutions[10],
+            );
+            should(map.getLayers().item(0).getMinResolution()).greaterThan(
+              defaultResolutions[13],
+            );
+            should(map.getLayers().item(0).getMinResolution()).lessThan(
+              defaultResolutions[12],
             );
             done();
           })
@@ -893,7 +917,7 @@ describe('ol-mapbox-style', function () {
               defaultResolutions[7] + 1e-15,
             );
             should(map.getLayers().item(0).getMinResolution()).eql(
-              defaultResolutions[23] + 1e-15,
+              defaultResolutions[23],
             );
             done();
           })