Prevent step and interpolate expressions from having duplicate stops

src/style-spec/expression/definitions/interpolate.js

@@ -116,7 +116,7 @@ class Interpolate implements Expression {
                 return context.error('Input/output pairs for "interpolate" expressions must be defined using literal numeric values (not computed expressions) for the input values.', labelKey);
             }
 
-            if (stops.length && stops[stops.length - 1][0] > label) {
+            if (stops.length && stops[stops.length - 1][0] >= label) {
                 return context.error('Input/output pairs for "interpolate" expressions must be arranged with input values in strictly ascending order.', labelKey);
             }
 

src/style-spec/expression/definitions/step.js

@@ -64,7 +64,7 @@ class Step implements Expression {
                 return context.error('Input/output pairs for "step" expressions must be defined using literal numeric values (not computed expressions) for the input values.', labelKey);
             }
 
-            if (stops.length && stops[stops.length - 1][0] > label) {
+            if (stops.length && stops[stops.length - 1][0] >= label) {
                 return context.error('Input/output pairs for "step" expressions must be arranged with input values in strictly ascending order.', labelKey);
             }
 

src/style-spec/function/convert.js

@@ -220,6 +220,11 @@ function fixupDegenerateStepCurve(expression) {
 }
 
 function appendStopPair(curve, input, output, isStep) {
+    // Skip duplicate stop values. They were not validated for functions, but they are for expressions.
+    // https://github.com/mapbox/mapbox-gl-js/issues/4107
+    if (curve.length > 3 && input === curve[curve.length - 2]) {
+        return;
+    }
     // step curves don't get the first input value, as it is redundant.
     if (!(isStep && curve.length === 2)) {
         curve.push(input);

test/integration/expression-tests/interpolate/duplicate_stops/test.json

@@ -0,0 +1,15 @@
+{
+  "expression": ["interpolate", ["linear"], 0, 0, 1, 0, 2],
+  "inputs": [[{}, {}]],
+  "expected": {
+    "compiled": {
+      "result": "error",
+      "errors": [
+        {
+          "key": "[5]",
+          "error": "Input/output pairs for \"interpolate\" expressions must be arranged with input values in strictly ascending order."
+        }
+      ]
+    }
+  }
+}

test/integration/expression-tests/step/duplicate_stops/test.json

@@ -0,0 +1,15 @@
+{
+  "expression": ["step", 0, "a", 0, "b", 0, "c"],
+  "inputs": [[{}, {}]],
+  "expected": {
+    "compiled": {
+      "result": "error",
+      "errors": [
+        {
+          "key": "[5]",
+          "error": "Input/output pairs for \"step\" expressions must be arranged with input values in strictly ascending order."
+        }
+      ]
+    }
+  }
+}

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

@@ -42,5 +42,61 @@ test('convertFunction', (t) => {
         t.end();
     });
 
+    t.test('duplicate step function stops', (t) => {
+        const functionValue = {
+            stops: [
+                [0, 'a'],
+                [1, 'b'],
+                [1, 'c'],
+                [2, 'd']
+            ]
+        };
+
+        const expression = convertFunction(functionValue, {
+            type: 'string',
+            function: 'piecewise-constant'
+        }, 'text-field');
+        t.deepEqual(expression, [
+            'step',
+            ['zoom'],
+            'a',
+            1,
+            'b',
+            2,
+            'd'
+        ]);
+
+        t.end();
+    });
+
+    t.test('duplicate interpolate function stops', (t) => {
+        const functionValue = {
+            stops: [
+                [0, 'a'],
+                [1, 'b'],
+                [1, 'c'],
+                [2, 'd']
+            ]
+        };
+
+        const expression = convertFunction(functionValue, {
+            type: 'number',
+            function: 'interpolated'
+        }, 'text-size');
+        t.deepEqual(expression, [
+            'interpolate',
+            ['exponential', 1],
+            ['zoom'],
+            0,
+            'a',
+            1,
+            'b',
+            2,
+            'd'
+        ]);
+
+        t.end();
+    });
+
     t.end();
 });