tpl/collections: Add collections.Complement

Fixes #5400

4 files changed, 181 insertions, 0 deletions

diff --git a/tpl/collections/complement.go b/tpl/collections/complement.go
new file mode 100644
index 000000000..d975fabac
--- /dev/null
+++ b/tpl/collections/complement.go
@@ -0,0 +1,72 @@
+// Copyright 2018 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package collections
+
+import (
+	"errors"
+	"fmt"
+	"reflect"
+)
+
+// Complement gives the elements in the last element of seqs that are not in
+// any of the others.
+// All elements of seqs must be slices or arrays of comparable types.
+//
+// The reasoning behind this rather clumsy API is so we can do this in the templates:
+//    {{ $c := .Pages | complement $last4 }}
+func (ns *Namespace) Complement(seqs ...interface{}) (interface{}, error) {
+	if len(seqs) < 2 {
+		return nil, errors.New("complement needs at least two arguments")
+	}
+
+	universe := seqs[len(seqs)-1]
+	as := seqs[:len(seqs)-1]
+
+	aset := make(map[interface{}]struct{})
+
+	for _, av := range as {
+		v := reflect.ValueOf(av)
+		switch v.Kind() {
+		case reflect.Array, reflect.Slice:
+			for i := 0; i < v.Len(); i++ {
+				ev, _ := indirectInterface(v.Index(i))
+				if !ev.Type().Comparable() {
+					return nil, errors.New("elements in complement must be comparable")
+				}
+
+				aset[normalize(ev)] = struct{}{}
+			}
+		default:
+			return nil, fmt.Errorf("arguments to complement must be slices or arrays")
+		}
+	}
+
+	v := reflect.ValueOf(universe)
+	switch v.Kind() {
+	case reflect.Array, reflect.Slice:
+		sl := reflect.MakeSlice(v.Type(), 0, 0)
+		for i := 0; i < v.Len(); i++ {
+			ev, _ := indirectInterface(v.Index(i))
+			if !ev.Type().Comparable() {
+				return nil, errors.New("elements in complement must be comparable")
+			}
+			if _, found := aset[normalize(ev)]; !found {
+				sl = reflect.Append(sl, ev)
+			}
+		}
+		return sl.Interface(), nil
+	default:
+		return nil, fmt.Errorf("arguments to complement must be slices or arrays")
+	}
+}
diff --git a/tpl/collections/complement_test.go b/tpl/collections/complement_test.go
new file mode 100644
index 000000000..4cae7556f
--- /dev/null
+++ b/tpl/collections/complement_test.go
@@ -0,0 +1,84 @@
+// Copyright 2018 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package collections
+
+import (
+	"fmt"
+	"reflect"
+	"testing"
+
+	"github.com/gohugoio/hugo/deps"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestComplement(t *testing.T) {
+	t.Parallel()
+
+	assert := require.New(t)
+
+	ns := New(&deps.Deps{})
+
+	s1 := []TstX{TstX{A: "a"}, TstX{A: "b"}, TstX{A: "d"}, TstX{A: "e"}}
+	s2 := []TstX{TstX{A: "b"}, TstX{A: "e"}}
+
+	xa, xd := &TstX{A: "a"}, &TstX{A: "d"}
+
+	sp1 := []*TstX{xa, &TstX{A: "b"}, xd, &TstX{A: "e"}}
+	sp2 := []*TstX{&TstX{A: "b"}, &TstX{A: "e"}}
+
+	for i, test := range []struct {
+		s        interface{}
+		t        []interface{}
+		expected interface{}
+	}{
+		{[]string{"a", "b", "c"}, []interface{}{[]string{"c", "d"}}, []string{"a", "b"}},
+		{[]string{"a", "b", "c"}, []interface{}{[]string{"c", "d"}, []string{"a", "b"}}, []string{}},
+		{[]interface{}{"a", "b", nil}, []interface{}{[]string{"a", "d"}}, []interface{}{"b", nil}},
+		{[]int{1, 2, 3, 4, 5}, []interface{}{[]int{1, 3}, []string{"a", "b"}, []int{1, 2}}, []int{4, 5}},
+		{[]int{1, 2, 3, 4, 5}, []interface{}{[]int64{1, 3}}, []int{2, 4, 5}},
+		{s1, []interface{}{s2}, []TstX{TstX{A: "a"}, TstX{A: "d"}}},
+		{sp1, []interface{}{sp2}, []*TstX{xa, xd}},
+
+		// Errors
+		{[]string{"a", "b", "c"}, []interface{}{"error"}, false},
+		{"error", []interface{}{[]string{"c", "d"}, []string{"a", "b"}}, false},
+		{[]string{"a", "b", "c"}, []interface{}{[][]string{[]string{"c", "d"}}}, false},
+		{[]interface{}{[][]string{[]string{"c", "d"}}}, []interface{}{[]string{"c", "d"}, []string{"a", "b"}}, false},
+	} {
+
+		errMsg := fmt.Sprintf("[%d]", i)
+
+		args := append(test.t, test.s)
+
+		result, err := ns.Complement(args...)
+
+		if b, ok := test.expected.(bool); ok && !b {
+			require.Error(t, err, errMsg)
+			continue
+		}
+
+		require.NoError(t, err, errMsg)
+
+		if !reflect.DeepEqual(test.expected, result) {
+			t.Fatalf("%s got\n%T: %v\nexpected\n%T: %v", errMsg, result, result, test.expected, test.expected)
+		}
+	}
+
+	_, err := ns.Complement()
+	assert.Error(err)
+	_, err = ns.Complement([]string{"a", "b"})
+	assert.Error(err)
+
+}
diff --git a/tpl/collections/init.go b/tpl/collections/init.go
index 879e4738c..569932c08 100644
--- a/tpl/collections/init.go
+++ b/tpl/collections/init.go
@@ -39,6 +39,13 @@ func init() {
 			[][2]string{},
 		)
 
+		ns.AddMethodMapping(ctx.Complement,
+			[]string{"complement"},
+			[][2]string{
+				{`{{ slice "a" "b" "c" "d" "e" "f" | complement (slice "b" "c") (slice "d" "e")  }}`, `[a f]`},
+			},
+		)
+
 		ns.AddMethodMapping(ctx.Delimit,
 			[]string{"delimit"},
 			[][2]string{
diff --git a/tpl/collections/reflect_helpers.go b/tpl/collections/reflect_helpers.go
index 643a0a7e5..074396479 100644
--- a/tpl/collections/reflect_helpers.go
+++ b/tpl/collections/reflect_helpers.go
@@ -41,6 +41,24 @@ func numberToFloat(v reflect.Value) (float64, error) {
 	}
 }
 
+// normalizes different numeric types to make them comparable.
+// If not, any pointer will be unwrapped.
+func normalize(v reflect.Value) interface{} {
+	k := v.Kind()
+
+	switch {
+	case isNumber(k):
+		f, err := numberToFloat(v)
+		if err == nil {
+			return f
+		}
+	case k == reflect.Ptr:
+		v = v.Elem()
+	}
+
+	return v.Interface()
+}
+
 // There are potential overflows in this function, but the downconversion of
 // int64 etc. into int8 etc. is coming from the synthetic unit tests for Union etc.
 // TODO(bep) We should consider normalizing the slices to int64 etc.