Expressions / add utilities for using strings in GLSL & more type checking

It is now possible to specify a type hint when parsing an expression, which
helps determine the output value type.

When no single output type can be inferred, an error is thrown.

For strings, every literal value will be replaced by a number and a map of
these associations will be kept in the parsing context, which is passed
recursively.

test/spec/ol/style/expressions.test.js

@@ -1,8 +1,8 @@
 import {
   arrayToGlsl, colorToGlsl,
   expressionToGlsl,
-  getValueType,
-  numberToGlsl,
+  getValueType, isTypeUnique,
+  numberToGlsl, stringToGlsl,
   ValueTypes
 } from '../../../../src/ol/style/expressions.js';
 
@@ -53,6 +53,38 @@ describe('ol.style.expressions', function() {
     });
   });
 
+  describe('stringToGlsl', function() {
+    let context;
+    beforeEach(function() {
+      context = {
+        stringLiteralsMap: {}
+      };
+    });
+
+    it('maps input string to stable numbers', function() {
+      expect(stringToGlsl(context, 'abcd')).to.eql('0.0');
+      expect(stringToGlsl(context, 'defg')).to.eql('1.0');
+      expect(stringToGlsl(context, 'hijk')).to.eql('2.0');
+      expect(stringToGlsl(context, 'abcd')).to.eql('0.0');
+      expect(stringToGlsl(context, 'def')).to.eql('3.0');
+    });
+  });
+
+  describe('isTypeUnique', function() {
+    it('return true if only one value type', function() {
+      expect(isTypeUnique(ValueTypes.NUMBER)).to.eql(true);
+      expect(isTypeUnique(ValueTypes.STRING)).to.eql(true);
+      expect(isTypeUnique(ValueTypes.COLOR)).to.eql(true);
+    });
+    it('return false if several value types', function() {
+      expect(isTypeUnique(ValueTypes.NUMBER | ValueTypes.COLOR)).to.eql(false);
+      expect(isTypeUnique(ValueTypes.ANY)).to.eql(false);
+    });
+    it('return false if no value type', function() {
+      expect(isTypeUnique(ValueTypes.NUMBER & ValueTypes.COLOR)).to.eql(false);
+    });
+  });
+
   describe('getValueType', function() {
 
     it('correctly analyzes a literal value', function() {
@@ -116,7 +148,8 @@ describe('ol.style.expressions', function() {
     beforeEach(function() {
       context = {
         variables: [],
-        attributes: []
+        attributes: [],
+        stringLiteralsMap: {}
       };
     });
 
@@ -156,7 +189,7 @@ describe('ol.style.expressions', function() {
     });
 
     it('gives precedence to the string type unless asked otherwise', function() {
-      expect(expressionToGlsl(context, 'lightgreen')).to.eql('lightgreen');
+      expect(expressionToGlsl(context, 'lightgreen')).to.eql('0.0');
       expect(expressionToGlsl(context, 'lightgreen', ValueTypes.COLOR)).to.eql(
         'vec4(0.5647058823529412, 0.9333333333333333, 0.5647058823529412, 1.0)');
     });
@@ -225,7 +258,6 @@ describe('ol.style.expressions', function() {
       }
       expect(thrown).to.be(true);
     });
-
   });
 
 });