Add a newScratch template func

Fixes #4685

common/maps/scratch.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 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 ""
+}
+
+// Reset 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
+}
+
+func NewScratch() *Scratch {
+	return &Scratch{values: make(map[string]interface{})}
+}