Correct bug in array index logic

fixes #33

README.md

@@ -1,6 +1,6 @@
 Package form
 ============
-<img align="right" src="https://raw.githubusercontent.com/go-playground/form/master/logo.jpg">![Project status](https://img.shields.io/badge/version-3.1.1-green.svg)
+<img align="right" src="https://raw.githubusercontent.com/go-playground/form/master/logo.jpg">![Project status](https://img.shields.io/badge/version-3.1.2-green.svg)
 [![Build Status](https://semaphoreci.com/api/v1/joeybloggs/form/branches/master/badge.svg)](https://semaphoreci.com/joeybloggs/form)
 [![Coverage Status](https://coveralls.io/repos/github/go-playground/form/badge.svg?branch=master)](https://coveralls.io/github/go-playground/form?branch=master)
 [![Go Report Card](https://goreportcard.com/badge/github.com/go-playground/form)](https://goreportcard.com/report/github.com/go-playground/form)
@@ -23,7 +23,8 @@ It has the following features:
 
 Common Questions
 
--   Does it support encoding.TextUnmarshaler? No because TextUnmarshaler only accepts []byte but posted values can have multiple values, so is not suitable.
+- Does it support encoding.TextUnmarshaler? No because TextUnmarshaler only accepts []byte but posted values can have multiple values, so is not suitable.
+- Mixing `array/slice` with `array[idx]/slice[idx]`, in which order are they parsed? `array/slice` then `array[idx]/slice[idx]`
 
 Supported Types ( out of the box )
 ----------

decoder.go

@@ -464,8 +464,8 @@ func (d *decoder) setFieldByType(current reflect.Value, namespace []byte, idx in
 
 		if ok && len(arr) > 0 {
 			var varr reflect.Value
-
-			overCapacity := v.Len() < len(arr)
+			l := len(arr)
+			overCapacity := v.Len() < l
 			if overCapacity {
 				// more values than array capacity, ignore values over capacity as it's possible some would just want
 				// to grab the first x number of elements; in the future strict mode logic should return an error
@@ -474,10 +474,13 @@ func (d *decoder) setFieldByType(current reflect.Value, namespace []byte, idx in
 			varr = reflect.Indirect(reflect.New(reflect.ArrayOf(v.Len(), v.Type().Elem())))
 			reflect.Copy(varr, v)
 
-			for i := 0; i < v.Len(); i++ {
+			if v.Len() < len(arr) {
+				l = v.Len()
+			}
+			for i := 0; i < l; i++ {
 				newVal := reflect.New(v.Type().Elem()).Elem()
 
-				if d.setFieldByType(newVal, namespace, 0) {
+				if d.setFieldByType(newVal, namespace, i) {
 					set = true
 					varr.Index(i).Set(newVal)
 				}

decoder_test.go

@@ -1571,6 +1571,10 @@ func TestDecodeArrayBug(t *testing.T) {
 		A [2]string
 		B [2]string
 		C [2]string
+		D [3]string
+		E [3]string
+		F [3]string
+		G [3]string
 	}
 	decoder := NewDecoder()
 	err := decoder.Decode(&data, url.Values{
@@ -1583,11 +1587,31 @@ func TestDecodeArrayBug(t *testing.T) {
 		"B[2]": {"40"},
 		// invalid array index
 		"C[q]": {""},
+		// index and mix tests
+		"D":    {"10"},
+		"E":    {"10", "20"},
+		"F":    {"10", "", "20"},
+		"G":    {"10"},
+		"G[2]": {"20"},
 	})
 	NotEqual(t, err, nil)
 	Equal(t, err.Error(), "Field Namespace:C ERROR:invalid array index 'q'")
 	Equal(t, data.A[0], "10")
 	Equal(t, data.A[1], "20")
 	Equal(t, data.B[0], "10")
 	Equal(t, data.B[1], "31")
+	Equal(t, data.C[0], "")
+	Equal(t, data.C[1], "")
+	Equal(t, data.D[0], "10")
+	Equal(t, data.D[1], "")
+	Equal(t, data.D[2], "")
+	Equal(t, data.E[0], "10")
+	Equal(t, data.E[1], "20")
+	Equal(t, data.E[2], "")
+	Equal(t, data.F[0], "10")
+	Equal(t, data.F[1], "")
+	Equal(t, data.F[2], "20")
+	Equal(t, data.G[0], "10")
+	Equal(t, data.G[1], "")
+	Equal(t, data.G[2], "20")
 }

doc.go

@@ -31,6 +31,9 @@ Questions
     No because TextUnmarshaler only accepts []byte but posted values can have
     multiple values, so is not suitable.
 
+	Mixing array/slice with array[idx]/slice[idx], in which order are they parsed?
+	array/slice then array[idx]/slice[idx]
+
 Supported Types
 
 out of the box supported types