From dc3cfa2318eed11b43746a159ec9b22e90218e48 Mon Sep 17 00:00:00 2001
From: Jo <joie.software@gmail.com>
Date: Mon, 16 Dec 2019 16:33:49 +0100
Subject: [PATCH] Edit convert functions to output linear interpolation (#9107)

* Edit convert functions to output linear interpolation

* Edit unit tests to reflect linear interpolation

* Refactor code to make it succint

* minor whitespace updates
---
 src/style-spec/function/convert.js   | 10 ++++++++--
 test/unit/style-spec/migrate.test.js |  4 ++--
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/src/style-spec/function/convert.js b/src/style-spec/function/convert.js
index e248b9f0e5a..a177604b953 100644
--- a/src/style-spec/function/convert.js
+++ b/src/style-spec/function/convert.js
@@ -153,7 +153,12 @@ function convertPropertyFunction(parameters, propertySpec, stops) {
         ];
     } else if (type === 'exponential') {
         const base = parameters.base !== undefined ? parameters.base : 1;
-        const expression = [getInterpolateOperator(parameters), ['exponential', base], ['number', get]];
+        const expression = [
+            getInterpolateOperator(parameters),
+            base === 1 ? ["linear"] : ["exponential", base],
+            ["number", get]
+        ];
+
         for (const stop of stops) {
             appendStopPair(expression, stop[0], stop[1], false);
         }
@@ -177,7 +182,8 @@ function convertZoomFunction(parameters, propertySpec, stops, input = ['zoom'])
         isStep = true;
     } else if (type === 'exponential') {
         const base = parameters.base !== undefined ? parameters.base : 1;
-        expression = [getInterpolateOperator(parameters), ['exponential', base], input];
+        expression = [getInterpolateOperator(parameters), base === 1 ? ["linear"] : ["exponential", base], input];
+
     } else {
         throw new Error(`Unknown zoom function type "${type}"`);
     }
diff --git a/test/unit/style-spec/migrate.test.js b/test/unit/style-spec/migrate.test.js
index 441e9037673..f6581dc4a18 100644
--- a/test/unit/style-spec/migrate.test.js
+++ b/test/unit/style-spec/migrate.test.js
@@ -69,7 +69,7 @@ test('converts stop functions to expressions', (t) => {
     }, spec.latest.$version);
     t.deepEqual(migrated.layers[0].paint['background-opacity'], [
         'interpolate',
-        ['exponential', 1],
+        ['linear'],
         ['zoom'],
         0,
         1,
@@ -78,7 +78,7 @@ test('converts stop functions to expressions', (t) => {
     ]);
     t.deepEqual(migrated.layers[1].paint['background-opacity'], [
         'interpolate',
-        ['exponential', 1],
+        ['linear'],
         ['zoom'],
         0,
         ['literal', [1, 2]],