Add IndexOf/IndexOfT function (#79)

This includes the IndexOf function and IndexOfT, as well as a small
example of how to use the function to update items in the array.

example_test.go

@@ -725,6 +725,47 @@ func ExampleQuery_GroupJoin() {
 	// [{a [apple apricot]} {b [banana]} {c [cherry clementine]}]
 }
 
+// The following code example demonstrates how to use IndexOf
+// to retrieve the position of an item in the array and then
+// update that item.
+func ExampleQuery_IndexOf() {
+	type Item struct {
+		ID   uint64
+		Name string
+	}
+	items := []Item{
+		{
+			ID:   1,
+			Name: "Joe",
+		},
+		{
+			ID:   2,
+			Name: "Bob",
+		},
+		{
+			ID:   3,
+			Name: "Rickster",
+		},
+		{
+			ID:   4,
+			Name: "Jim",
+		},
+	}
+
+	index := From(items).IndexOf(func(i interface{}) bool {
+		item, ok := i.(Item)
+		return ok && item.Name == "Rickster"
+	})
+
+	if index >= 0 {
+		// We found the item in the array. Change the name using the index.
+		items[index].Name = "Joshua"
+		fmt.Println("Item found at:", index, "new name:", items[index].Name)
+	}
+	// Output:
+	// Item found at: 2 new name: Joshua
+}
+
 // The following code example demonstrates how to use Join
 // to perform an inner join of two slices based on a common key.
 func ExampleQuery_Join() {

index.go

@@ -0,0 +1,40 @@
+package linq
+
+// IndexOf searches for an element that matches the conditions defined by a specified predicate
+// and returns the zero-based index of the first occurrence within the collection. This method
+// returns -1 if an item that matches the conditions is not found.
+func (q Query) IndexOf(predicate func(interface{}) bool) int {
+	index := 0
+	next := q.Iterate()
+
+	for item, ok := next(); ok; item, ok = next() {
+		if predicate(item) {
+			return index
+		}
+		index++
+	}
+
+	return -1
+}
+
+// IndexOfT is the typed version of IndexOf.
+//
+//   - predicateFn is of type "func(int,TSource)bool"
+//
+// NOTE: IndexOf has better performance than IndexOfT.
+func (q Query) IndexOfT(predicateFn interface{}) int {
+
+	predicateGenericFunc, err := newGenericFunc(
+		"IndexOfT", "predicateFn", predicateFn,
+		simpleParamValidator(newElemTypeSlice(new(genericType)), newElemTypeSlice(new(bool))),
+	)
+	if err != nil {
+		panic(err)
+	}
+
+	predicateFunc := func(item interface{}) bool {
+		return predicateGenericFunc.Call(item).(bool)
+	}
+
+	return q.IndexOf(predicateFunc)
+}

index_test.go

@@ -0,0 +1,53 @@
+package linq
+
+import (
+	"testing"
+)
+
+func TestIndexOf(t *testing.T) {
+	tests := []struct {
+		input     interface{}
+		predicate func(interface{}) bool
+		expected  int
+	}{
+		{
+			input: [9]int{1, 2, 3, 4, 5, 6, 7, 8, 9},
+			predicate: func(i interface{}) bool {
+				return i.(int) == 3
+			},
+			expected: 2,
+		},
+		{
+			input: "sstr",
+			predicate: func(i interface{}) bool {
+				return i.(rune) == 'r'
+			},
+			expected: 3,
+		},
+		{
+			input: "gadsgsadgsda",
+			predicate: func(i interface{}) bool {
+				return i.(rune) == 'z'
+			},
+			expected: -1,
+		},
+	}
+
+	for _, test := range tests {
+		index := From(test.input).IndexOf(test.predicate)
+		if index != test.expected {
+			t.Errorf("From(%v).IndexOf() expected %v received %v", test.input, test.expected, index)
+		}
+
+		index = From(test.input).IndexOfT(test.predicate)
+		if index != test.expected {
+			t.Errorf("From(%v).IndexOfT() expected %v received %v", test.input, test.expected, index)
+		}
+	}
+}
+
+func TestIndexOfT_PanicWhenPredicateFnIsInvalid(t *testing.T) {
+	mustPanicWithError(t, "IndexOfT: parameter [predicateFn] has a invalid function signature. Expected: 'func(T)bool', actual: 'func(int)int'", func() {
+		From([]int{1, 1, 1, 2, 1, 2, 3, 4, 2}).IndexOfT(func(item int) int { return item + 2 })
+	})
+}