feat(for tag): Provide the counter 0 indexed to the context

Closes #121

CHANGELOG.md

@@ -2,6 +2,11 @@
 
 ## Master
 
+### Enhancements
+
+- Adds `counter0` to for loop context allowing you to get the current index of
+  the for loop 0 indexed.
+
 ### Bug Fixes
 
 - Fixes a potential crash when using the `{% for %}` template tag with the

Sources/ForTag.swift

@@ -114,6 +114,7 @@ class ForNode : NodeType {
           "first": index == 0,
           "last": index == (count - 1),
           "counter": index + 1,
+          "counter0": index,
         ]
 
         return try context.push(dictionary: ["forloop": forContext]) {

Tests/StencilTests/ForNodeSpec.swift

@@ -83,6 +83,12 @@ func testForNode() {
       try expect(try node.render(context)) == "112233"
     }
 
+    $0.it("renders the given nodes while providing item counter") {
+      let nodes: [NodeType] = [VariableNode(variable: "item"), VariableNode(variable: "forloop.counter0")]
+      let node = ForNode(resolvable: Variable("items"), loopVariables: ["item"], nodes: nodes, emptyNodes: [])
+      try expect(try node.render(context)) == "102132"
+    }
+
     $0.it("renders the given nodes while filtering items using where expression") {
         let nodes: [NodeType] = [VariableNode(variable: "item"), VariableNode(variable: "forloop.counter")]
         let `where` = try parseExpression(components: ["item", ">", "1"], tokenParser: TokenParser(tokens: [], environment: Environment()))

docs/builtins.rst

@@ -57,7 +57,8 @@ The for block sets a few variables available within the loop:
 
 - ``first`` - True if this is the first time through the loop
 - ``last`` - True if this is the last time through the loop
-- ``counter`` - The current iteration of the loop
+- ``counter`` - The current iteration of the loop (1 indexed)
+- ``counter0`` - The current iteration of the loop (0 indexed)
 
 For example: