diff --git a/inplace.go b/inplace.go
new file mode 100644
index 0000000..aa648ba
--- /dev/null
+++ b/inplace.go
@@ -0,0 +1,105 @@
+package pmap
+
+type InplaceMap interface {
+	Map
+	SetInplace(Key, Value) InplaceMap
+}
+
+func (p pmap) SetInplace(k Key, v Value) InplaceMap {
+	root, added := inplace_insert(p.root, k, v, p.hash)
+	p.root = root
+	if added {
+		p.len++
+	}
+	return p
+}
+
+func inplace_insert(n interface{}, key Key, val Value, hashFn HashFunc) (newNode interface{}, added bool) {
+	if n == nil {
+		return leaf{key, val}, true
+	}
+	hash := hashFn(key)
+	var insert func(n interface{}, shift uint32) interface{}
+	insert = func(n interface{}, shift uint32) interface{} {
+		// insert returns a new node (if the node should be replaced)
+		// or nil (if the node is unchanged or modified in place)
+
+		//fmt.Printf("insert %v %x %#v\n", shift, hash, n)
+		switch n := n.(type) {
+		case leaf:
+			if n.k == key {
+				// replace existing entry
+				added = false
+				return leaf{key, val}
+			} else if h := hashFn(n.k); h == hash {
+				// collision
+				added = true
+				return &collision{hash, []leaf{{key, val}, n}}
+			} else {
+				if h>>shift == hash>>shift {
+					panic("pmap: infinite loop in insert")
+				}
+				// not a collision, so we must still have some hash bits left
+				// split the trie
+				x := newnode(n, h, shift)
+				if x0 := insert(x, shift); x0 != nil {
+					return x0
+				}
+				return x
+			}
+		case *node:
+			c := n.getNode(shift, hash, key)
+			if c == nil {
+				// new node
+				c = leaf{key, val}
+				added = true
+
+				m := bitmask(hash >> shift)
+				n.bitmap |= m
+				i := n.index(m)
+				n.child = append(n.child, nil)   // allocate space
+				copy(n.child[i+1:], n.child[i:]) // shift array up
+				n.child[i] = c                   // mutate node
+			} else {
+				c = insert(c, shift+nodeShift)
+				if c == nil {
+					return nil
+				}
+				m := bitmask(hash >> shift)
+				i := n.index(m)
+				n.child[i] = c // mutate node
+			}
+			n.check()
+			return nil
+		case *collision:
+			if n.hash != hash {
+				// not a collision, so we must still have some hash bits left
+				// split the trie
+				x := newnode(n, n.hash, shift)
+				if x0 := insert(x, shift); x0 != nil {
+					return x0
+				}
+				return x
+			}
+			for i := range n.leaf {
+				if key == n.leaf[i].k {
+					// replace existing entry
+					n.leaf[i] = leaf{key, val} // mutate
+					added = false
+					return nil
+				}
+			}
+			// new collision
+			added = true
+			n.leaf = append(n.leaf, leaf{key, val}) // mutate
+			return nil
+		default:
+			panic("pmap: unhandled case in insert")
+		}
+	}
+	newNode = insert(n, 0)
+	if newNode == nil {
+		newNode = n
+	}
+	return
+}
diff --git a/inplace_test.go b/inplace_test.go
new file mode 100644
index 0000000..269b282
--- /dev/null
+++ b/inplace_test.go
@@ -0,0 +1,49 @@
+package pmap
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestInplace(t *testing.T) {
+	p := New(hash).(InplaceMap)
+	const numElems = 100
+	for i := range iter(numElems) {
+		p = p.SetInplace(i, i)
+	}
+	if p.Len() != numElems {
+		t.Fatalf("Len() = %v, want %v", p.Len(), numElems)
+	}
+	for i := range iter(numElems) {
+		v, ok := p.Get(i)
+		if v != i || !ok {
+			t.Errorf("Get(%d) = %v %v, want %v %v", i, v, ok, i, true)
+		}
+	}
+	fmt.Print(p.(pmap).stats())
+}
+
+func BenchmarkSetInplace(b *testing.B) {
+	b.Run("size=10", func(b *testing.B) { benchmarkSetInplace(b, 10) })
+	b.Run("size=100", func(b *testing.B) { benchmarkSetInplace(b, 100) })
+	b.Run("size=1000", func(b *testing.B) { benchmarkSetInplace(b, 1000) })
+	b.Run("size=10000", func(b *testing.B) { benchmarkSetInplace(b, 10000) })
+	if !testing.Short() {
+		b.Run("size=100000", func(b *testing.B) { benchmarkSetInplace(b, 100000) })
+	}
+}
+
+func benchmarkSetInplace(b *testing.B, numElems int) {
+	p := New(hash).(InplaceMap)
+	for i := range iter(numElems) {
+		p = p.SetInplace(i, i)
+	}
+	if p.Len() != numElems {
+		b.Fatalf("Len() = %v, want %v", p.Len(), numElems)
+	}
+	b.ResetTimer()
+	for i := range iter(b.N) {
+		k := numElems
+		p = p.SetInplace(k, i)
+	}
+}