Fix corner case with dash-array 0-length collapsing (mapbox#9385)

A dash array with values [0,0] results in a full collapse of values since it does not represent
any dashes, fix the behavior with early return and add some more unit test coverage for other
corner cases

src/render/line_atlas.js

@@ -162,13 +162,15 @@ class LineAtlas {
  let length = 0;
  for (let i = 0; i < dasharray.length; i++) { length += dasharray[i]; }
 
- const stretch = this.width / length;
- const ranges = this.getDashRanges(dasharray, this.width, stretch);
-
- if (round) {
- this.addRoundDash(ranges, stretch, n);
- } else {
- this.addRegularDash(ranges);
+ if (length !== 0) {
+ const stretch = this.width / length;
+ const ranges = this.getDashRanges(dasharray, this.width, stretch);
+
+ if (round) {
+ this.addRoundDash(ranges, stretch, n);
+ } else {
+ this.addRegularDash(ranges);
+ }
  }
 
  const dashEntry = {

test/unit/render/line_atlas.test.js

@@ -0,0 +1,48 @@
+import {test} from '../../util/test';
+import LineAtlas from '../../../src/render/line_atlas';
+
+test('LineAtlas', (t) => {
+ const lineAtlas = new LineAtlas(64, 64);
+ t.test('round [0, 0]', (t) => {
+ const entry = lineAtlas.addDash([0, 0], true);
+ t.equal(entry.width, 0);
+ t.end();
+ });
+ t.test('round [1, 0]', (t) => {
+ const entry = lineAtlas.addDash([1, 0], true);
+ t.equal(entry.width, 1);
+ t.end();
+ });
+ t.test('round [0, 1]', (t) => {
+ const entry = lineAtlas.addDash([0, 1], true);
+ t.equal(entry.width, 1);
+ t.end();
+ });
+ t.test('odd round [1, 2, 1]', (t) => {
+ const entry = lineAtlas.addDash([1, 2, 1], true);
+ t.equal(entry.width, 4);
+ t.end();
+ });
+
+ t.test('regular [0, 0]', (t) => {
+ const entry = lineAtlas.addDash([0, 0], false);
+ t.equal(entry.width, 0);
+ t.end();
+ });
+ t.test('regular [1, 0]', (t) => {
+ const entry = lineAtlas.addDash([1, 0], false);
+ t.equal(entry.width, 1);
+ t.end();
+ });
+ t.test('regular [0, 1]', (t) => {
+ const entry = lineAtlas.addDash([0, 1], false);
+ t.equal(entry.width, 1);
+ t.end();
+ });
+ t.test('odd regular [1, 2, 1]', (t) => {
+ const entry = lineAtlas.addDash([1, 2, 1], false);
+ t.equal(entry.width, 4);
+ t.end();
+ });
+ t.end();
+});