diff --git a/README.md b/README.md
index b652aba..7b9d92f 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,3 @@
 # keyedmutex
 
-Keyed mutex library.
\ No newline at end of file
+Keyed mutex library.
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..5cb6856
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module git.crumpington.com/lib/keyedmutex
+
+go 1.25.1
diff --git a/keyedmutex.go b/keyedmutex.go
new file mode 100644
index 0000000..7bc8494
--- /dev/null
+++ b/keyedmutex.go
@@ -0,0 +1,69 @@
+package keyedmutex
+
+import (
+	"sync"
+)
+
+type keyedLock struct {
+	lock  sync.Mutex
+	count int
+}
+
+type KeyedMutex[K comparable] struct {
+	mu    sync.Mutex
+	byKey map[K]*keyedLock
+}
+
+func New[K comparable]() *KeyedMutex[K] {
+	return &KeyedMutex[K]{
+		byKey: map[K]*keyedLock{},
+	}
+}
+
+func (m *KeyedMutex[K]) getLock(key K) *keyedLock {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	item, ok := m.byKey[key]
+	if !ok {
+		item = &keyedLock{}
+		m.byKey[key] = item
+	}
+	item.count++
+	return item
+}
+
+func (m *KeyedMutex[K]) release(key K, unlock bool) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	item, ok := m.byKey[key]
+	if !ok {
+		panic("unlock of unlocked mutex")
+	}
+
+	item.count--
+	if unlock {
+		item.lock.Unlock()
+	}
+
+	if item.count == 0 {
+		delete(m.byKey, key)
+	}
+}
+
+func (m *KeyedMutex[K]) Lock(key K) {
+	m.getLock(key).lock.Lock()
+}
+
+func (m *KeyedMutex[K]) TryLock(key K) bool {
+	if ok := m.getLock(key).lock.TryLock(); !ok {
+		m.release(key, false)
+		return false
+	}
+	return true
+}
+
+func (m *KeyedMutex[K]) Unlock(key K) {
+	m.release(key, true)
+}
diff --git a/keyedmutex_test.go b/keyedmutex_test.go
new file mode 100644
index 0000000..da0ee2f
--- /dev/null
+++ b/keyedmutex_test.go
@@ -0,0 +1,111 @@
+package keyedmutex
+
+import (
+	"sync"
+	"testing"
+)
+
+func TestLock_CleansUpMap(t *testing.T) {
+	m := New[string]()
+	m.Lock("a")
+	m.Unlock("a")
+	if len(m.byKey) != 0 {
+		t.Fatalf("expected empty byKey, got %v", m.byKey)
+	}
+}
+
+func TestTryLock_SucceedsOnFreeKey(t *testing.T) {
+	m := New[string]()
+	if !m.TryLock("a") {
+		t.Fatal("expected TryLock to succeed on free key")
+	}
+	m.Unlock("a")
+}
+
+func TestTryLock_FailsOnLockedKey(t *testing.T) {
+	m := New[string]()
+	m.Lock("a")
+	if m.TryLock("a") {
+		t.Fatal("expected TryLock to fail on locked key")
+	}
+	m.Unlock("a")
+}
+
+func TestTryLock_FailureDoesNotCorruptLock(t *testing.T) {
+	// A failed TryLock must not call Unlock on the key's mutex.
+	// The old bug did this unconditionally, so the Unlock below would
+	// double-unlock and panic.
+	m := New[string]()
+	m.Lock("a")
+	m.TryLock("a") // must return false and leave the lock intact
+	m.Unlock("a")  // must not panic
+}
+
+func TestTryLock_FailureDecrementsCount(t *testing.T) {
+	// A failed TryLock must undo its getLock increment so that the
+	// original holder's Unlock cleans up the map entry.
+	m := New[string]()
+	m.Lock("a")
+	m.TryLock("a") // fails; a leaked count would leave a stale map entry
+	m.Unlock("a")
+	if len(m.byKey) != 0 {
+		t.Fatalf("expected empty byKey after unlock, got %v", m.byKey)
+	}
+}
+
+func TestMultipleKeys_AreIndependent(t *testing.T) {
+	m := New[string]()
+	m.Lock("a")
+	if !m.TryLock("b") {
+		t.Fatal("expected TryLock on different key to succeed while 'a' is locked")
+	}
+	m.Unlock("b")
+	m.Unlock("a")
+}
+
+func TestConcurrentLock_MutualExclusion(t *testing.T) {
+	m := New[string]()
+	const N = 100
+	var wg sync.WaitGroup
+	var shared int // intentionally non-atomic: race detector catches improper access
+
+	for range N {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			m.Lock("a")
+			shared++
+			m.Unlock("a")
+		}()
+	}
+
+	wg.Wait()
+
+	if shared != N {
+		t.Fatalf("expected %d, got %d", N, shared)
+	}
+	if len(m.byKey) != 0 {
+		t.Fatalf("expected empty byKey after all goroutines done, got %v", m.byKey)
+	}
+}
+
+func TestKeyedMutex_unlockUnlocked(t *testing.T) {
+	defer func() {
+		if r := recover(); r == nil {
+			t.Fatal(r)
+		}
+	}()
+
+	m := New[string]()
+	m.Unlock("aldkfj")
+}
+
+func BenchmarkUncontendedMutex(b *testing.B) {
+	m := New[string]()
+	key := "xyz"
+
+	for b.Loop() {
+		m.Lock(key)
+		m.Unlock(key)
+	}
+}