Added template function intersect.

Author: Ted Chen

README.md

@@ -123,6 +123,7 @@ Within those templates, the object emitted by docker-gen will have [this structu
 * *`groupByMulti $containers $fieldPath $sep`*: Like `groupBy`, but the string value specified by `$fieldPath` is first split by `$sep` into a list of strings. A container whose `$fieldPath` value contains a list of strings will show up in the map output under each of those strings.
 * *`hasPrefix $prefix $string`*: Returns whether `$prefix` is a prefix of `$string`.
 * *`hasSuffix $suffix $string`*: Returns whether `$suffix` is a suffix of `$string`.
+* *`intersect $slice1 $slice2`*: Returns the strings that exist in both string slices.
 * *`json $value`*: Returns the JSON representation of `$value` as a `string`.
 * *`keys $map`*: Returns the keys from `$map`. If `$map` is `nil`, a `nil` is returned. If `$map` is not a `map`, an error will be thrown.
 * *`last $array`*: Returns the last value of an array.

template.go

@@ -94,6 +94,24 @@ func keys(input interface{}) (interface{}, error) {
 	return k, nil
 }
 
+func intersect(l1, l2 []string) []string {
+	m := make(map[string]bool)
+	m2 := make(map[string]bool)
+	for _, v := range l2 {
+		m2[v] = true
+	}
+	for _, v := range l1 {
+		if m2[v] {
+			m[v] = true
+		}
+	}
+	keys := make([]string, 0, len(m))
+	for k := range m {
+		keys = append(keys, k)
+	}
+	return keys
+}
+
 func contains(item map[string]string, key string) bool {
 	if _, ok := item[key]; ok {
 		return true
@@ -214,6 +232,7 @@ func generateFile(config Config, containers Context) bool {
 		"hasPrefix":    hasPrefix,
 		"hasSuffix":    hasSuffix,
 		"json":         marshalJson,
+		"intersect":	intersect,
 		"keys":         keys,
 		"last":         arrayLast,
 		"replace":      strings.Replace,

template_test.go

@@ -74,6 +74,24 @@ func TestKeysNil(t *testing.T) {
 	}
 }
 
+func TestIntersect(t *testing.T) {
+	if len(intersect([]string{"foo.fo.com", "bar.com"}, []string{"foo.bar.com"})) != 0 {
+		t.Fatal("Expected no match")
+	}
+
+	if len(intersect([]string{"foo.fo.com", "bar.com"}, []string{"bar.com", "foo.com"})) != 1 {
+		t.Fatal("Expected only one match")
+	}
+
+	if len(intersect([]string{"foo.com"}, []string{"bar.com", "foo.com"})) != 1 {
+		t.Fatal("Expected only one match")
+	}
+
+	if len(intersect([]string{"foo.fo.com", "foo.com", "bar.com"}, []string{"bar.com", "foo.com"})) != 2 {
+		t.Fatal("Expected two matches")
+	}
+}
+
 func TestGroupByExistingKey(t *testing.T) {
 	containers := []*RuntimeContainer{
 		&RuntimeContainer{