15 files changed, 1093 insertions, 0 deletions
diff --git a/common/collections/collections.go b/common/collections/collections.go
new file mode 100644
index 000000000..854f705b3
--- /dev/null
+++ b/common/collections/collections.go
@@ -0,0 +1,28 @@
+// 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 contains common Hugo functionality related to collection
+// handling.
+package collections
+
+// Grouper defines a very generic way to group items by a given key.
+type Grouper interface {
+	Group(key interface{}, items interface{}) (interface{}, error)
+}
+
+// Slicer definse a very generic way to create a typed slice. This is used
+// in collections.Slice template func to get types such as Pages, PageGroups etc.
+// instead of the less useful []interface{}.
+type Slicer interface {
+	Slice(items []interface{}) (interface{}, error)
+}
diff --git a/common/errors/errors.go b/common/errors/errors.go
new file mode 100644
index 000000000..673cd23ff
--- /dev/null
+++ b/common/errors/errors.go
@@ -0,0 +1,25 @@
+// 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 errors contains common Hugo errors and error related utilities.
+package errors
+
+import (
+	"errors"
+)
+
+// ErrFeatureNotAvailable denotes that a feature is unavailable.
+//
+// We will, at least to begin with, make some Hugo features (SCSS with libsass) optional,
+// and this error is used to signal those situations.
+var ErrFeatureNotAvailable = errors.New("this feature is not available in your current Hugo version")
diff --git a/common/hugio/readers.go b/common/hugio/readers.go
new file mode 100644
index 000000000..ba55e2d08
--- /dev/null
+++ b/common/hugio/readers.go
@@ -0,0 +1,53 @@
+// 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 hugio
+
+import (
+	"io"
+	"strings"
+)
+
+// ReadSeeker wraps io.Reader and io.Seeker.
+type ReadSeeker interface {
+	io.Reader
+	io.Seeker
+}
+
+// ReadSeekCloser is implemented by afero.File. We use this as the common type for
+// content in Resource objects, even for strings.
+type ReadSeekCloser interface {
+	ReadSeeker
+	io.Closer
+}
+
+// ReadSeekerNoOpCloser implements ReadSeekCloser by doing nothing in Close.
+type ReadSeekerNoOpCloser struct {
+	ReadSeeker
+}
+
+// Close does nothing.
+func (r ReadSeekerNoOpCloser) Close() error {
+	return nil
+}
+
+// NewReadSeekerNoOpCloser creates a new ReadSeekerNoOpCloser with the given ReadSeeker.
+func NewReadSeekerNoOpCloser(r ReadSeeker) ReadSeekerNoOpCloser {
+	return ReadSeekerNoOpCloser{r}
+}
+
+// NewReadSeekerNoOpCloserFromString uses strings.NewReader to create a new ReadSeekerNoOpCloser
+// from the given string.
+func NewReadSeekerNoOpCloserFromString(content string) ReadSeekerNoOpCloser {
+	return ReadSeekerNoOpCloser{strings.NewReader(content)}
+}
diff --git a/common/hugio/writers.go b/common/hugio/writers.go
new file mode 100644
index 000000000..2766146e1
--- /dev/null
+++ b/common/hugio/writers.go
@@ -0,0 +1,43 @@
+// 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 hugio
+
+import (
+	"io"
+)
+
+type multiWriteCloser struct {
+	io.Writer
+	closers []io.WriteCloser
+}
+
+func (m multiWriteCloser) Close() error {
+	var err error
+	for _, c := range m.closers {
+		if closeErr := c.Close(); err != nil {
+			err = closeErr
+		}
+	}
+	return err
+}
+
+// NewMultiWriteCloser creates a new io.WriteCloser that duplicates its writes to all the
+// provided writers.
+func NewMultiWriteCloser(writeClosers ...io.WriteCloser) io.WriteCloser {
+	writers := make([]io.Writer, len(writeClosers))
+	for i, w := range writeClosers {
+		writers[i] = w
+	}
+	return multiWriteCloser{Writer: io.MultiWriter(writers...), closers: writeClosers}
+}
diff --git a/common/loggers/loggers.go b/common/loggers/loggers.go
new file mode 100644
index 000000000..2f7f36b34
--- /dev/null
+++ b/common/loggers/loggers.go
@@ -0,0 +1,37 @@
+// 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 loggers
+
+import (
+	"io/ioutil"
+	"log"
+	"os"
+
+	jww "github.com/spf13/jwalterweatherman"
+)
+
+// NewDebugLogger is a convenience function to create a debug logger.
+func NewDebugLogger() *jww.Notepad {
+	return jww.NewNotepad(jww.LevelDebug, jww.LevelError, os.Stdout, ioutil.Discard, "", log.Ldate|log.Ltime)
+}
+
+// NewWarningLogger is a convenience function to create a warning logger.
+func NewWarningLogger() *jww.Notepad {
+	return jww.NewNotepad(jww.LevelWarn, jww.LevelError, os.Stdout, ioutil.Discard, "", log.Ldate|log.Ltime)
+}
+
+// NewErrorLogger is a convenience function to create an error logger.
+func NewErrorLogger() *jww.Notepad {
+	return jww.NewNotepad(jww.LevelError, jww.LevelError, os.Stdout, ioutil.Discard, "", log.Ldate|log.Ltime)
+}
diff --git a/common/maps/maps.go b/common/maps/maps.go
new file mode 100644
index 000000000..a114b557c
--- /dev/null
+++ b/common/maps/maps.go
@@ -0,0 +1,44 @@
+// 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 maps
+
+import (
+	"strings"
+
+	"github.com/spf13/cast"
+)
+
+// ToLower makes all the keys in the given map lower cased and will do so
+// recursively.
+// Notes:
+// * This will modify the map given.
+// * Any nested map[interface{}]interface{} will be converted to map[string]interface{}.
+func ToLower(m map[string]interface{}) {
+	for k, v := range m {
+		switch v.(type) {
+		case map[interface{}]interface{}:
+			v = cast.ToStringMap(v)
+			ToLower(v.(map[string]interface{}))
+		case map[string]interface{}:
+			ToLower(v.(map[string]interface{}))
+		}
+
+		lKey := strings.ToLower(k)
+		if k != lKey {
+			delete(m, k)
+			m[lKey] = v
+		}
+
+	}
+}
diff --git a/common/maps/maps_test.go b/common/maps/maps_test.go
new file mode 100644
index 000000000..37add5dc5
--- /dev/null
+++ b/common/maps/maps_test.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 maps
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestToLower(t *testing.T) {
+
+	tests := []struct {
+		input    map[string]interface{}
+		expected map[string]interface{}
+	}{
+		{
+			map[string]interface{}{
+				"abC": 32,
+			},
+			map[string]interface{}{
+				"abc": 32,
+			},
+		},
+		{
+			map[string]interface{}{
+				"abC": 32,
+				"deF": map[interface{}]interface{}{
+					23: "A value",
+					24: map[string]interface{}{
+						"AbCDe": "A value",
+						"eFgHi": "Another value",
+					},
+				},
+				"gHi": map[string]interface{}{
+					"J": 25,
+				},
+			},
+			map[string]interface{}{
+				"abc": 32,
+				"def": map[string]interface{}{
+					"23": "A value",
+					"24": map[string]interface{}{
+						"abcde": "A value",
+						"efghi": "Another value",
+					},
+				},
+				"ghi": map[string]interface{}{
+					"j": 25,
+				},
+			},
+		},
+	}
+
+	for i, test := range tests {
+		// ToLower modifies input.
+		ToLower(test.input)
+		if !reflect.DeepEqual(test.expected, test.input) {
+			t.Errorf("[%d] Expected\n%#v, got\n%#v\n", i, test.expected, test.input)
+		}
+	}
+}
diff --git a/common/maps/scratch.go b/common/maps/scratch.go
new file mode 100644
index 000000000..4b062d139
--- /dev/null
+++ b/common/maps/scratch.go
@@ -0,0 +1,136 @@
+// 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 maps
+
+import (
+	"reflect"
+	"sort"
+	"sync"
+
+	"github.com/gohugoio/hugo/common/math"
+)
+
+// Scratch is a writable context used for stateful operations in Page/Node rendering.
+type Scratch struct {
+	values map[string]interface{}
+	mu     sync.RWMutex
+}
+
+// Add will, for single values, add (using the + operator) the addend to the existing addend (if found).
+// Supports numeric values and strings.
+//
+// If the first add for a key is an array or slice, then the next value(s) will be appended.
+func (c *Scratch) Add(key string, newAddend interface{}) (string, error) {
+
+	var newVal interface{}
+	c.mu.RLock()
+	existingAddend, found := c.values[key]
+	c.mu.RUnlock()
+	if found {
+		var err error
+
+		addendV := reflect.ValueOf(existingAddend)
+
+		if addendV.Kind() == reflect.Slice || addendV.Kind() == reflect.Array {
+			nav := reflect.ValueOf(newAddend)
+			if nav.Kind() == reflect.Slice || nav.Kind() == reflect.Array {
+				newVal = reflect.AppendSlice(addendV, nav).Interface()
+			} else {
+				newVal = reflect.Append(addendV, nav).Interface()
+			}
+		} else {
+			newVal, err = math.DoArithmetic(existingAddend, newAddend, '+')
+			if err != nil {
+				return "", err
+			}
+		}
+	} else {
+		newVal = newAddend
+	}
+	c.mu.Lock()
+	c.values[key] = newVal
+	c.mu.Unlock()
+	return "", nil // have to return something to make it work with the Go templates
+}
+
+// Set stores a value with the given key in the Node context.
+// This value can later be retrieved with Get.
+func (c *Scratch) Set(key string, value interface{}) string {
+	c.mu.Lock()
+	c.values[key] = value
+	c.mu.Unlock()
+	return ""
+}
+
+// Delete deletes the given key.
+func (c *Scratch) Delete(key string) string {
+	c.mu.Lock()
+	delete(c.values, key)
+	c.mu.Unlock()
+	return ""
+}
+
+// Get returns a value previously set by Add or Set.
+func (c *Scratch) Get(key string) interface{} {
+	c.mu.RLock()
+	val := c.values[key]
+	c.mu.RUnlock()
+
+	return val
+}
+
+// SetInMap stores a value to a map with the given key in the Node context.
+// This map can later be retrieved with GetSortedMapValues.
+func (c *Scratch) SetInMap(key string, mapKey string, value interface{}) string {
+	c.mu.Lock()
+	_, found := c.values[key]
+	if !found {
+		c.values[key] = make(map[string]interface{})
+	}
+
+	c.values[key].(map[string]interface{})[mapKey] = value
+	c.mu.Unlock()
+	return ""
+}
+
+// GetSortedMapValues returns a sorted map previously filled with SetInMap.
+func (c *Scratch) GetSortedMapValues(key string) interface{} {
+	c.mu.RLock()
+
+	if c.values[key] == nil {
+		c.mu.RUnlock()
+		return nil
+	}
+
+	unsortedMap := c.values[key].(map[string]interface{})
+	c.mu.RUnlock()
+	var keys []string
+	for mapKey := range unsortedMap {
+		keys = append(keys, mapKey)
+	}
+
+	sort.Strings(keys)
+
+	sortedArray := make([]interface{}, len(unsortedMap))
+	for i, mapKey := range keys {
+		sortedArray[i] = unsortedMap[mapKey]
+	}
+
+	return sortedArray
+}
+
+// NewScratch returns a new instance Scratch.
+func NewScratch() *Scratch {
+	return &Scratch{values: make(map[string]interface{})}
+}
diff --git a/common/maps/scratch_test.go b/common/maps/scratch_test.go
new file mode 100644
index 000000000..8397ba830
--- /dev/null
+++ b/common/maps/scratch_test.go
@@ -0,0 +1,170 @@
+// 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 maps
+
+import (
+	"reflect"
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestScratchAdd(t *testing.T) {
+	t.Parallel()
+	scratch := NewScratch()
+	scratch.Add("int1", 10)
+	scratch.Add("int1", 20)
+	scratch.Add("int2", 20)
+
+	assert.Equal(t, int64(30), scratch.Get("int1"))
+	assert.Equal(t, 20, scratch.Get("int2"))
+
+	scratch.Add("float1", float64(10.5))
+	scratch.Add("float1", float64(20.1))
+
+	assert.Equal(t, float64(30.6), scratch.Get("float1"))
+
+	scratch.Add("string1", "Hello ")
+	scratch.Add("string1", "big ")
+	scratch.Add("string1", "World!")
+
+	assert.Equal(t, "Hello big World!", scratch.Get("string1"))
+
+	scratch.Add("scratch", scratch)
+	_, err := scratch.Add("scratch", scratch)
+
+	if err == nil {
+		t.Errorf("Expected error from invalid arithmetic")
+	}
+
+}
+
+func TestScratchAddSlice(t *testing.T) {
+	t.Parallel()
+	scratch := NewScratch()
+
+	_, err := scratch.Add("intSlice", []int{1, 2})
+	assert.Nil(t, err)
+	_, err = scratch.Add("intSlice", 3)
+	assert.Nil(t, err)
+
+	sl := scratch.Get("intSlice")
+	expected := []int{1, 2, 3}
+
+	if !reflect.DeepEqual(expected, sl) {
+		t.Errorf("Slice difference, go %q expected %q", sl, expected)
+	}
+
+	_, err = scratch.Add("intSlice", []int{4, 5})
+
+	assert.Nil(t, err)
+
+	sl = scratch.Get("intSlice")
+	expected = []int{1, 2, 3, 4, 5}
+
+	if !reflect.DeepEqual(expected, sl) {
+		t.Errorf("Slice difference, go %q expected %q", sl, expected)
+	}
+
+}
+
+func TestScratchSet(t *testing.T) {
+	t.Parallel()
+	scratch := NewScratch()
+	scratch.Set("key", "val")
+	assert.Equal(t, "val", scratch.Get("key"))
+}
+
+func TestScratchDelete(t *testing.T) {
+	t.Parallel()
+	scratch := NewScratch()
+	scratch.Set("key", "val")
+	scratch.Delete("key")
+	scratch.Add("key", "Lucy Parsons")
+	assert.Equal(t, "Lucy Parsons", scratch.Get("key"))
+}
+
+// Issue #2005
+func TestScratchInParallel(t *testing.T) {
+	var wg sync.WaitGroup
+	scratch := NewScratch()
+	key := "counter"
+	scratch.Set(key, int64(1))
+	for i := 1; i <= 10; i++ {
+		wg.Add(1)
+		go func(j int) {
+			for k := 0; k < 10; k++ {
+				newVal := int64(k + j)
+
+				_, err := scratch.Add(key, newVal)
+				if err != nil {
+					t.Errorf("Got err %s", err)
+				}
+
+				scratch.Set(key, newVal)
+
+				val := scratch.Get(key)
+
+				if counter, ok := val.(int64); ok {
+					if counter < 1 {
+						t.Errorf("Got %d", counter)
+					}
+				} else {
+					t.Errorf("Got %T", val)
+				}
+			}
+			wg.Done()
+		}(i)
+	}
+	wg.Wait()
+}
+
+func TestScratchGet(t *testing.T) {
+	t.Parallel()
+	scratch := NewScratch()
+	nothing := scratch.Get("nothing")
+	if nothing != nil {
+		t.Errorf("Should not return anything, but got %v", nothing)
+	}
+}
+
+func TestScratchSetInMap(t *testing.T) {
+	t.Parallel()
+	scratch := NewScratch()
+	scratch.SetInMap("key", "lux", "Lux")
+	scratch.SetInMap("key", "abc", "Abc")
+	scratch.SetInMap("key", "zyx", "Zyx")
+	scratch.SetInMap("key", "abc", "Abc (updated)")
+	scratch.SetInMap("key", "def", "Def")
+	assert.Equal(t, []interface{}{0: "Abc (updated)", 1: "Def", 2: "Lux", 3: "Zyx"}, scratch.GetSortedMapValues("key"))
+}
+
+func TestScratchGetSortedMapValues(t *testing.T) {
+	t.Parallel()
+	scratch := NewScratch()
+	nothing := scratch.GetSortedMapValues("nothing")
+	if nothing != nil {
+		t.Errorf("Should not return anything, but got %v", nothing)
+	}
+}
+
+func BenchmarkScratchGet(b *testing.B) {
+	scratch := NewScratch()
+	scratch.Add("A", 1)
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		scratch.Get("A")
+	}
+}
diff --git a/common/math/math.go b/common/math/math.go
new file mode 100644
index 000000000..3c5ef1f9d
--- /dev/null
+++ b/common/math/math.go
@@ -0,0 +1,135 @@
+// 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 math
+
+import (
+	"errors"
+	"reflect"
+)
+
+// DoArithmetic performs arithmetic operations (+,-,*,/) using reflection to
+// determine the type of the two terms.
+func DoArithmetic(a, b interface{}, op rune) (interface{}, error) {
+	av := reflect.ValueOf(a)
+	bv := reflect.ValueOf(b)
+	var ai, bi int64
+	var af, bf float64
+	var au, bu uint64
+	switch av.Kind() {
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		ai = av.Int()
+		switch bv.Kind() {
+		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+			bi = bv.Int()
+		case reflect.Float32, reflect.Float64:
+			af = float64(ai) // may overflow
+			ai = 0
+			bf = bv.Float()
+		case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
+			bu = bv.Uint()
+			if ai >= 0 {
+				au = uint64(ai)
+				ai = 0
+			} else {
+				bi = int64(bu) // may overflow
+				bu = 0
+			}
+		default:
+			return nil, errors.New("Can't apply the operator to the values")
+		}
+	case reflect.Float32, reflect.Float64:
+		af = av.Float()
+		switch bv.Kind() {
+		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+			bf = float64(bv.Int()) // may overflow
+		case reflect.Float32, reflect.Float64:
+			bf = bv.Float()
+		case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
+			bf = float64(bv.Uint()) // may overflow
+		default:
+			return nil, errors.New("Can't apply the operator to the values")
+		}
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
+		au = av.Uint()
+		switch bv.Kind() {
+		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+			bi = bv.Int()
+			if bi >= 0 {
+				bu = uint64(bi)
+				bi = 0
+			} else {
+				ai = int64(au) // may overflow
+				au = 0
+			}
+		case reflect.Float32, reflect.Float64:
+			af = float64(au) // may overflow
+			au = 0
+			bf = bv.Float()
+		case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
+			bu = bv.Uint()
+		default:
+			return nil, errors.New("Can't apply the operator to the values")
+		}
+	case reflect.String:
+		as := av.String()
+		if bv.Kind() == reflect.String && op == '+' {
+			bs := bv.String()
+			return as + bs, nil
+		}
+		return nil, errors.New("Can't apply the operator to the values")
+	default:
+		return nil, errors.New("Can't apply the operator to the values")
+	}
+
+	switch op {
+	case '+':
+		if ai != 0 || bi != 0 {
+			return ai + bi, nil
+		} else if af != 0 || bf != 0 {
+			return af + bf, nil
+		} else if au != 0 || bu != 0 {
+			return au + bu, nil
+		}
+		return 0, nil
+	case '-':
+		if ai != 0 || bi != 0 {
+			return ai - bi, nil
+		} else if af != 0 || bf != 0 {
+			return af - bf, nil
+		} else if au != 0 || bu != 0 {
+			return au - bu, nil
+		}
+		return 0, nil
+	case '*':
+		if ai != 0 || bi != 0 {
+			return ai * bi, nil
+		} else if af != 0 || bf != 0 {
+			return af * bf, nil
+		} else if au != 0 || bu != 0 {
+			return au * bu, nil
+		}
+		return 0, nil
+	case '/':
+		if bi != 0 {
+			return ai / bi, nil
+		} else if bf != 0 {
+			return af / bf, nil
+		} else if bu != 0 {
+			return au / bu, nil
+		}
+		return nil, errors.New("Can't divide the value by 0")
+	default:
+		return nil, errors.New("There is no such an operation")
+	}
+}
diff --git a/common/math/math_test.go b/common/math/math_test.go
new file mode 100644
index 000000000..613ac3073
--- /dev/null
+++ b/common/math/math_test.go
@@ -0,0 +1,109 @@
+// 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 Lice