1 files changed, 78 insertions, 0 deletions
diff --git a/src/net/http/mapping.go b/src/net/http/mapping.go
new file mode 100644
index 0000000..87e6d5a
--- /dev/null
+++ b/src/net/http/mapping.go
@@ -0,0 +1,78 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package http
+
+// A mapping is a collection of key-value pairs where the keys are unique.
+// A zero mapping is empty and ready to use.
+// A mapping tries to pick a representation that makes [mapping.find] most efficient.
+type mapping[K comparable, V any] struct {
+	s []entry[K, V] // for few pairs
+	m map[K]V       // for many pairs
+}
+
+type entry[K comparable, V any] struct {
+	key   K
+	value V
+}
+
+// maxSlice is the maximum number of pairs for which a slice is used.
+// It is a variable for benchmarking.
+var maxSlice int = 8
+
+// add adds a key-value pair to the mapping.
+func (h *mapping[K, V]) add(k K, v V) {
+	if h.m == nil && len(h.s) < maxSlice {
+		h.s = append(h.s, entry[K, V]{k, v})
+	} else {
+		if h.m == nil {
+			h.m = map[K]V{}
+			for _, e := range h.s {
+				h.m[e.key] = e.value
+			}
+			h.s = nil
+		}
+		h.m[k] = v
+	}
+}
+
+// find returns the value corresponding to the given key.
+// The second return value is false if there is no value
+// with that key.
+func (h *mapping[K, V]) find(k K) (v V, found bool) {
+	if h == nil {
+		return v, false
+	}
+	if h.m != nil {
+		v, found = h.m[k]
+		return v, found
+	}
+	for _, e := range h.s {
+		if e.key == k {
+			return e.value, true
+		}
+	}
+	return v, false
+}
+
+// eachPair calls f for each pair in the mapping.
+// If f returns false, pairs returns immediately.
+func (h *mapping[K, V]) eachPair(f func(k K, v V) bool) {
+	if h == nil {
+		return
+	}
+	if h.m != nil {
+		for k, v := range h.m {
+			if !f(k, v) {
+				return
+			}
+		}
+	} else {
+		for _, e := range h.s {
+			if !f(e.key, e.value) {
+				return
+			}
+		}
+	}
+}