From 4937f08971192cec1346ef85ef453d5864b68d20 Mon Sep 17 00:00:00 2001
From: ahocevar <andreas.hocevar@gmail.com>
Date: Thu, 3 Oct 2013 11:41:02 -0600
Subject: [PATCH] Adding counter expression function

This function can e.g. be used to get a symbolizer literal with
an ever increasing zIndex.
---
 src/ol/expr/expression.js            | 18 ++++++++++++++++++
 test/spec/ol/expr/expression.test.js |  9 +++++++++
 2 files changed, 27 insertions(+)

diff --git a/src/ol/expr/expression.js b/src/ol/expr/expression.js
index f1376a0d9b..65a6411147 100644
--- a/src/ol/expr/expression.js
+++ b/src/ol/expr/expression.js
@@ -96,6 +96,7 @@ ol.expr.lib = {};
  */
 ol.expr.functions = {
   CONCAT: 'concat',
+  COUNTER: 'counter',
   EXTENT: 'extent',
   FID: 'fid',
   GEOMETRY_TYPE: 'geometryType',
@@ -126,6 +127,23 @@ ol.expr.lib[ol.expr.functions.CONCAT] = function(var_args) {
 };
 
 
+/**
+ * Returns a counter which increases every time this function is called.
+ * @param {number=} opt_start Start. If not provided, the counter starts at 0.
+ * @return {number} Counter.
+ */
+ol.expr.lib[ol.expr.functions.COUNTER] = (function() {
+  var counter = 0;
+  return function(opt_start) {
+    var result = ++counter;
+    if (goog.isDef(opt_start)) {
+      result += opt_start;
+    }
+    return result;
+  };
+})();
+
+
 /**
  * Determine if a feature's extent intersects the provided extent.
  * @param {number} minX Minimum x-coordinate value.
diff --git a/test/spec/ol/expr/expression.test.js b/test/spec/ol/expr/expression.test.js
index 9157867f3e..22e8b68606 100644
--- a/test/spec/ol/expr/expression.test.js
+++ b/test/spec/ol/expr/expression.test.js
@@ -674,6 +674,15 @@ describe('ol.expr.lib', function() {
 
   });
 
+  describe('counter()', function() {
+    var counter = parse('counter()');
+    var counterWithStart = parse('counter(1000)');
+    var start = evaluate(counter);
+    expect(evaluate(counter).to.be(start + 1));
+    expect(evaluate(counter).to.be(start + 2));
+    expect(evaluate(counterWithStart).to.be(start + 3 + 1000));
+  });
+
   describe('extent()', function() {
 
     var nw = new ol.Feature({