Make cascade front matter order deterministic

Fixes #12594

common/maps/ordered.go

@@ -0,0 +1,133 @@
+// Copyright 2024 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 (
+	"github.com/gohugoio/hugo/common/hashing"
+)
+
+// Ordered is a map that can be iterated in the order of insertion.
+// Note that insertion order is not affected if a key is re-inserted into the map.
+// In a nil map, all operations are no-ops.
+// This is not thread safe.
+type Ordered[K comparable, T any] struct {
+	// The keys in the order they were added.
+	keys []K
+	// The values.
+	values map[K]T
+}
+
+// NewOrdered creates a new Ordered map.
+func NewOrdered[K comparable, T any]() *Ordered[K, T] {
+	return &Ordered[K, T]{values: make(map[K]T)}
+}
+
+// Set sets the value for the given key.
+// Note that insertion order is not affected if a key is re-inserted into the map.
+func (m *Ordered[K, T]) Set(key K, value T) {
+	if m == nil {
+		return
+	}
+	// Check if key already exists.
+	if _, found := m.values[key]; !found {
+		m.keys = append(m.keys, key)
+	}
+	m.values[key] = value
+}
+
+// Get gets the value for the given key.
+func (m *Ordered[K, T]) Get(key K) (T, bool) {
+	if m == nil {
+		var v T
+		return v, false
+	}
+	value, found := m.values[key]
+	return value, found
+}
+
+// Delete deletes the value for the given key.
+func (m *Ordered[K, T]) Delete(key K) {
+	if m == nil {
+		return
+	}
+	delete(m.values, key)
+	for i, k := range m.keys {
+		if k == key {
+			m.keys = append(m.keys[:i], m.keys[i+1:]...)
+			break
+		}
+	}
+}
+
+// Clone creates a shallow copy of the map.
+func (m *Ordered[K, T]) Clone() *Ordered[K, T] {
+	if m == nil {
+		return nil
+	}
+	clone := NewOrdered[K, T]()
+	for _, k := range m.keys {
+		clone.Set(k, m.values[k])
+	}
+	return clone
+}
+
+// Keys returns the keys in the order they were added.
+func (m *Ordered[K, T]) Keys() []K {
+	if m == nil {
+		return nil
+	}
+	return m.keys
+}
+
+// Values returns the values in the order they were added.
+func (m *Ordered[K, T]) Values() []T {
+	if m == nil {
+		return nil
+	}
+	var values []T
+	for _, k := range m.keys {
+		values = append(values, m.values[k])
+	}
+	return values
+}
+
+// Len returns the number of items in the map.
+func (m *Ordered[K, T]) Len() int {
+	if m == nil {
+		return 0
+	}
+	return len(m.keys)
+}
+
+// Range calls f sequentially for each key and value present in the map.
+// If f returns false, range stops the iteration.
+// TODO(bep) replace with iter.Seq2 when we bump go Go 1.24.
+func (m *Ordered[K, T]) Range(f func(key K, value T) bool) {
+	if m == nil {
+		return
+	}
+	for _, k := range m.keys {
+		if !f(k, m.values[k]) {
+			return
+		}
+	}
+}
+
+// Hash calculates a hash from the values.
+func (m *Ordered[K, T]) Hash() (uint64, error) {
+	if m == nil {
+		return 0, nil
+	}
+	return hashing.Hash(m.values)
+}