Fixes bugs in findStopLessThanOrEqualTo algorithm

src/style-spec/expression/stops.js

@@ -11,19 +11,22 @@ export type Stops = Array<[number, Expression]>;
  * @private
  */
 export function findStopLessThanOrEqualTo(stops: Array<number>, input: number) {
-    const n = stops.length;
+    const lastIndex = stops.length - 1;
     let lowerIndex = 0;
-    let upperIndex = n - 1;
+    let upperIndex = lastIndex;
     let currentIndex = 0;
-    let currentValue, upperValue;
+    let currentValue, nextValue;
 
     while (lowerIndex <= upperIndex) {
         currentIndex = Math.floor((lowerIndex + upperIndex) / 2);
         currentValue = stops[currentIndex];
-        upperValue = stops[currentIndex + 1];
-        if (input === currentValue || input > currentValue && input < upperValue) { // Search complete
-            return currentIndex;
-        } else if (currentValue < input) {
+        nextValue = stops[currentIndex + 1];
+
+        if (currentValue <= input) {
+            if (currentIndex === lastIndex || input < nextValue) { // Search complete
+                return currentIndex;
+            }
+
             lowerIndex = currentIndex + 1;
         } else if (currentValue > input) {
             upperIndex = currentIndex - 1;
@@ -32,5 +35,5 @@ export function findStopLessThanOrEqualTo(stops: Array<number>, input: number) {
         }
     }
 
-    return Math.max(currentIndex - 1, 0);
+    return 0;
 }

src/style-spec/function/index.js

@@ -7,6 +7,7 @@ import * as interpolate from '../util/interpolate';
 import Interpolate from '../expression/definitions/interpolate';
 import Formatted from '../expression/types/formatted';
 import { supportsInterpolation } from '../util/properties';
+import { findStopLessThanOrEqualTo } from '../expression/stops';
 
 export function isFunction(value) {
     return typeof value === 'object' && value !== null && !Array.isArray(value);
@@ -145,7 +146,7 @@ function evaluateIntervalFunction(parameters, propertySpec, input) {
     if (input <= parameters.stops[0][0]) return parameters.stops[0][1];
     if (input >= parameters.stops[n - 1][0]) return parameters.stops[n - 1][1];
 
-    const index = findStopLessThanOrEqualTo(parameters.stops, input);
+    const index = findStopLessThanOrEqualTo(parameters.stops.map((stop) => stop[0]), input);
 
     return parameters.stops[index][1];
 }
@@ -160,7 +161,7 @@ function evaluateExponentialFunction(parameters, propertySpec, input) {
     if (input <= parameters.stops[0][0]) return parameters.stops[0][1];
     if (input >= parameters.stops[n - 1][0]) return parameters.stops[n - 1][1];
 
-    const index = findStopLessThanOrEqualTo(parameters.stops, input);
+    const index = findStopLessThanOrEqualTo(parameters.stops.map((stop) => stop[0]), input);
     const t = interpolationFactor(
         input, base,
         parameters.stops[index][0],
@@ -203,34 +204,6 @@ function evaluateIdentityFunction(parameters, propertySpec, input) {
     return coalesce(input, parameters.default, propertySpec.default);
 }
 
-/**
- * Returns the index of the last stop <= input, or 0 if it doesn't exist.
- *
- * @private
- */
-function findStopLessThanOrEqualTo(stops, input) {
-    const n = stops.length;
-    let lowerIndex = 0;
-    let upperIndex = n - 1;
-    let currentIndex = 0;
-    let currentValue, upperValue;
-
-    while (lowerIndex <= upperIndex) {
-        currentIndex = Math.floor((lowerIndex + upperIndex) / 2);
-        currentValue = stops[currentIndex][0];
-        upperValue = stops[currentIndex + 1][0];
-        if (input === currentValue || input > currentValue && input < upperValue) { // Search complete
-            return currentIndex;
-        } else if (currentValue < input) {
-            lowerIndex = currentIndex + 1;
-        } else if (currentValue > input) {
-            upperIndex = currentIndex - 1;
-        }
-    }
-
-    return Math.max(currentIndex - 1, 0);
-}
-
 /**
  * Returns a ratio that can be used to interpolate between exponential function
  * stops.

test/unit/style-spec/stops.test.js

@@ -0,0 +1,27 @@
+import { test } from 'mapbox-gl-js-test';
+import { findStopLessThanOrEqualTo } from '../../../src/style-spec/expression/stops';
+
+test('findStopLessThanOrEqualTo', (t) => {
+    test('When the input > all stops it returns the last stop.', (t) => {
+        const index = findStopLessThanOrEqualTo([0, 1, 2, 3, 4, 5, 6, 7], 8);
+        t.equal(index, 7);
+        t.end();
+    });
+
+    test('When more than one stop has the same value it always returns the last stop', (t) => {
+        let index;
+
+        index = findStopLessThanOrEqualTo([0.5, 0.5], 0.5);
+        t.equal(index, 1);
+
+        index = findStopLessThanOrEqualTo([0.5, 0.5, 0.5], 0.5);
+        t.equal(index, 2);
+
+        index = findStopLessThanOrEqualTo([0.4, 0.5, 0.5, 0.6, 0.7], 0.5);
+        t.equal(index, 2);
+
+        t.end();
+    });
+
+    t.end();
+});