11 files changed, 1176 insertions, 0 deletions
diff --git a/ref/store/memory/batch.go b/ref/store/memory/batch.go
new file mode 100644
index 00000000..0326e4f5
--- /dev/null
+++ b/ref/store/memory/batch.go
@@ -0,0 +1,206 @@
+package memory
+
+import (
+	"lindenii.org/go/furgit/object/id"
+	"lindenii.org/go/furgit/ref/store"
+)
+
+// Batch stages in-memory updates for one subset commit.
+type Batch struct {
+	store *Memory
+	ops   []queuedUpdate
+}
+
+var _ store.Batch = (*Batch)(nil)
+
+// BeginBatch creates one new in-memory batch.
+func (memory *Memory) BeginBatch() (store.Batch, error) {
+	return &Batch{
+		store: memory,
+		ops:   make([]queuedUpdate, 0, 8),
+	}, nil
+}
+
+// Create queues a direct reference creation.
+func (batch *Batch) Create(name string, newID id.ObjectID) error {
+	return batch.queue(queuedUpdate{name: name, kind: updateCreate, newID: newID})
+}
+
+// Update queues a direct reference update.
+func (batch *Batch) Update(name string, newID, oldID id.ObjectID) error {
+	return batch.queue(queuedUpdate{name: name, kind: updateReplace, newID: newID, oldID: oldID})
+}
+
+// Delete queues a direct reference deletion.
+func (batch *Batch) Delete(name string, oldID id.ObjectID) error {
+	return batch.queue(queuedUpdate{name: name, kind: updateDelete, oldID: oldID})
+}
+
+// Verify queues a direct reference verification.
+func (batch *Batch) Verify(name string, oldID id.ObjectID) error {
+	return batch.queue(queuedUpdate{name: name, kind: updateVerify, oldID: oldID})
+}
+
+// CreateSymbolic queues a symbolic reference creation.
+func (batch *Batch) CreateSymbolic(name, newTarget string) error {
+	return batch.queue(queuedUpdate{name: name, kind: updateCreateSymbolic, newTarget: newTarget})
+}
+
+// UpdateSymbolic queues a symbolic reference update.
+func (batch *Batch) UpdateSymbolic(name, newTarget, oldTarget string) error {
+	return batch.queue(queuedUpdate{name: name, kind: updateReplaceSymbolic, newTarget: newTarget, oldTarget: oldTarget})
+}
+
+// DeleteSymbolic queues a symbolic reference deletion.
+func (batch *Batch) DeleteSymbolic(name, oldTarget string) error {
+	return batch.queue(queuedUpdate{name: name, kind: updateDeleteSymbolic, oldTarget: oldTarget})
+}
+
+// VerifySymbolic queues a symbolic reference verification.
+func (batch *Batch) VerifySymbolic(name, oldTarget string) error {
+	return batch.queue(queuedUpdate{name: name, kind: updateVerifySymbolic, oldTarget: oldTarget})
+}
+
+// Apply validates queued operations,
+// drops rejected operations,
+// and applies the remaining compatible set.
+// Concurrent readers observe either the pre-Apply state
+// or the post-Apply state.
+func (batch *Batch) Apply() ([]store.BatchResult, error) {
+	results := make([]store.BatchResult, len(batch.ops))
+	remainingIdx := make([]int, 0, len(batch.ops))
+	remainingOps := make([]queuedUpdate, 0, len(batch.ops))
+	seenTargets := make(map[string]struct{}, len(batch.ops))
+
+	batch.store.mu.Lock()
+	defer batch.store.mu.Unlock()
+
+	for i, op := range batch.ops {
+		results[i].Name = op.name
+
+		target, err := resolveQueuedUpdateTarget(batch.store.refs, op)
+		if err != nil {
+			if isBatchRejected(err) {
+				results[i].Status = store.BatchStatusRejected
+				results[i].Error = err
+
+				continue
+			}
+
+			markFatal(results, batch.ops, i, err)
+
+			return results, err
+		}
+
+		if _, exists := seenTargets[target.name]; exists {
+			results[i].Status = store.BatchStatusRejected
+			results[i].Error = store.ErrDuplicateUpdate
+
+			continue
+		}
+
+		seenTargets[target.name] = struct{}{}
+
+		remainingIdx = append(remainingIdx, i)
+		remainingOps = append(remainingOps, op)
+	}
+
+	return batch.applyRemaining(results, remainingIdx, remainingOps)
+}
+
+// Abort abandons the batch.
+func (batch *Batch) Abort() error {
+	return nil
+}
+
+// applyRemaining repeatedly prepares the remaining operations,
+// dropping one rejected operation per round,
+// until either the whole set applies cleanly or a fatal failure occurs.
+func (batch *Batch) applyRemaining(results []store.BatchResult, remainingIdx []int, remainingOps []queuedUpdate) ([]store.BatchResult, error) {
+	for len(remainingOps) > 0 {
+		prepared, failedName, err := prepareUpdates(batch.store.refs, remainingOps)
+		if err == nil {
+			next := cloneRefs(batch.store.refs)
+			applyPreparedUpdates(next, prepared)
+			batch.store.refs = next
+
+			for _, idx := range remainingIdx {
+				results[idx].Status = store.BatchStatusApplied
+			}
+
+			return results, nil
+		}
+
+		if !isBatchRejected(err) {
+			markFatalRemaining(results, remainingIdx, remainingOps, failedName, err)
+
+			return results, err
+		}
+
+		rejectedAt := indexOfName(remainingOps, failedName)
+		if rejectedAt < 0 {
+			for _, idx := range remainingIdx {
+				results[idx].Status = store.BatchStatusNotAttempted
+				results[idx].Error = err
+			}
+
+			return results, err
+		}
+
+		results[remainingIdx[rejectedAt]].Status = store.BatchStatusRejected
+		results[remainingIdx[rejectedAt]].Error = err
+		remainingIdx = append(remainingIdx[:rejectedAt], remainingIdx[rejectedAt+1:]...)
+		remainingOps = append(remainingOps[:rejectedAt], remainingOps[rejectedAt+1:]...)
+	}
+
+	return results, nil
+}
+
+func (batch *Batch) queue(op queuedUpdate) error {
+	err := validateQueuedUpdate(batch.store.objectFormat, op)
+	if err != nil {
+		return err
+	}
+
+	batch.ops = append(batch.ops, op)
+
+	return nil
+}
+
+func markFatal(results []store.BatchResult, ops []queuedUpdate, at int, err error) {
+	results[at].Status = store.BatchStatusFatal
+	results[at].Error = err
+
+	for j := at + 1; j < len(results); j++ {
+		results[j].Name = ops[j].name
+		results[j].Status = store.BatchStatusNotAttempted
+		results[j].Error = err
+	}
+}
+
+func markFatalRemaining(results []store.BatchResult, remainingIdx []int, remainingOps []queuedUpdate, failedName string, err error) {
+	fatalMarked := false
+
+	for i, idx := range remainingIdx {
+		if !fatalMarked && failedName != "" && remainingOps[i].name == failedName {
+			results[idx].Status = store.BatchStatusFatal
+			results[idx].Error = err
+			fatalMarked = true
+
+			continue
+		}
+
+		results[idx].Status = store.BatchStatusNotAttempted
+		results[idx].Error = err
+	}
+}
+
+func indexOfName(ops []queuedUpdate, name string) int {
+	for i, op := range ops {
+		if op.name == name {
+			return i
+		}
+	}
+
+	return -1
+}
diff --git a/ref/store/memory/batch_test.go b/ref/store/memory/batch_test.go
new file mode 100644
index 00000000..518dc7b9
--- /dev/null
+++ b/ref/store/memory/batch_test.go
@@ -0,0 +1,115 @@
+package memory_test
+
+import (
+	"errors"
+	"testing"
+
+	"lindenii.org/go/furgit/object/id"
+	"lindenii.org/go/furgit/ref"
+	"lindenii.org/go/furgit/ref/store"
+	"lindenii.org/go/furgit/ref/store/memory"
+)
+
+func TestBatchRejectsDuplicateResolvedTargetAndAppliesRemainder(t *testing.T) {
+	t.Parallel()
+
+	for _, objectFormat := range id.SupportedObjectFormats() {
+		t.Run(objectFormat.String(), func(t *testing.T) {
+			t.Parallel()
+
+			m := memory.New(objectFormat)
+			mainID := objectFormat.Sum([]byte("main"))
+			devID := objectFormat.Sum([]byte("dev"))
+			nextMainID := objectFormat.Sum([]byte("next-main"))
+			nextDevID := objectFormat.Sum([]byte("next-dev"))
+			aliasID := objectFormat.Sum([]byte("alias"))
+
+			seed(t, m, func(tx store.Transaction) {
+				err := tx.Create("refs/heads/main", mainID)
+				if err != nil {
+					t.Fatalf("Create(main): %v", err)
+				}
+
+				err = tx.Create("refs/heads/dev", devID)
+				if err != nil {
+					t.Fatalf("Create(dev): %v", err)
+				}
+
+				err = tx.CreateSymbolic("refs/heads/alias", "refs/heads/main")
+				if err != nil {
+					t.Fatalf("CreateSymbolic(alias): %v", err)
+				}
+			})
+
+			batch, err := m.BeginBatch()
+			if err != nil {
+				t.Fatalf("BeginBatch: %v", err)
+			}
+
+			err = batch.Update("refs/heads/main", nextMainID, mainID)
+			if err != nil {
+				t.Fatalf("Update(main): %v", err)
+			}
+
+			// Updates the symbolic alias in deref mode,
+			// which resolves to refs/heads/main
+			// and therefore duplicates the first operation.
+			err = batch.Update("refs/heads/alias", aliasID, mainID)
+			if err != nil {
+				t.Fatalf("Update(alias): %v", err)
+			}
+
+			err = batch.Update("refs/heads/dev", nextDevID, devID)
+			if err != nil {
+				t.Fatalf("Update(dev): %v", err)
+			}
+
+			results, err := batch.Apply()
+			if err != nil {
+				t.Fatalf("Apply: %v", err)
+			}
+
+			if len(results) != 3 {
+				t.Fatalf("len(results) = %d, want 3", len(results))
+			}
+
+			if results[0].Status != store.BatchStatusApplied {
+				t.Fatalf("results[0].Status = %v, want applied", results[0].Status)
+			}
+
+			if results[1].Status != store.BatchStatusRejected {
+				t.Fatalf("results[1].Status = %v, want rejected", results[1].Status)
+			}
+
+			if !errors.Is(results[1].Error, store.ErrDuplicateUpdate) {
+				t.Fatalf("results[1].Error = %v, want ErrDuplicateUpdate", results[1].Error)
+			}
+
+			if results[2].Status != store.BatchStatusApplied {
+				t.Fatalf("results[2].Status = %v, want applied", results[2].Status)
+			}
+
+			if got := resolveDirect(t, m, "refs/heads/main").ID; got != nextMainID {
+				t.Fatalf("main after batch = %v, want %v", got, nextMainID)
+			}
+
+			if got := resolveDirect(t, m, "refs/heads/dev").ID; got != nextDevID {
+				t.Fatalf("dev after batch = %v, want %v", got, nextDevID)
+			}
+
+			resolved, err := m.Resolve("refs/heads/alias")
+			if err != nil {
+				t.Fatalf("Resolve(alias): %v", err)
+			}
+
+			symbolic, ok := resolved.(ref.Symbolic)
+			if !ok {
+				t.Fatalf("Resolve(alias) = %T, want ref.Symbolic", resolved)
+			}
+
+			if symbolic.Target != "refs/heads/main" {
+				t.Fatalf("alias target = %q, want refs/heads/main", symbolic.Target)
+			}
+		})
+	}
+}
diff --git a/ref/store/memory/doc.go b/ref/store/memory/doc.go
new file mode 100644
index 00000000..37a829b0
--- /dev/null
+++ b/ref/store/memory/doc.go
@@ -0,0 +1,2 @@
+// Package memory provides one in-memory reference store.
+package memory
diff --git a/ref/store/memory/helpers_test.go b/ref/store/memory/helpers_test.go
new file mode 100644
index 00000000..a7973f13
--- /dev/null
+++ b/ref/store/memory/helpers_test.go
@@ -0,0 +1,46 @@
+package memory_test
+
+import (
+	"testing"
+
+	"lindenii.org/go/furgit/ref"
+	"lindenii.org/go/furgit/ref/store"
+	"lindenii.org/go/furgit/ref/store/memory"
+)
+
+// resolveDirect resolves name and asserts that it is a direct reference.
+//
+// Unlike Memory.ResolveToDirect, it does not follow symbolic references.
+func resolveDirect(t *testing.T, memory *memory.Memory, name string) ref.Direct {
+	t.Helper()
+
+	resolved, err := memory.Resolve(name)
+	if err != nil {
+		t.Fatalf("Resolve(%q): %v", name, err)
+	}
+
+	direct, ok := resolved.(ref.Direct)
+	if !ok {
+		t.Fatalf("Resolve(%q) = %T, want ref.Direct", name, resolved)
+	}
+
+	return direct
+}
+
+// seed runs fn against a fresh transaction and commits it,
+// failing the test on any error.
+func seed(t *testing.T, memory *memory.Memory, fn func(tx store.Transaction)) {
+	t.Helper()
+
+	tx, err := memory.BeginTransaction()
+	if err != nil {
+		t.Fatalf("BeginTransaction: %v", err)
+	}
+
+	fn(tx)
+
+	err = tx.Commit()
+	if err != nil {
+		t.Fatalf("Commit: %v", err)
+	}
+}
diff --git a/ref/store/memory/memory.go b/ref/store/memory/memory.go
new file mode 100644
index 00000000..3c8f4968
--- /dev/null
+++ b/ref/store/memory/memory.go
@@ -0,0 +1,43 @@
+package memory
+
+import (
+	"sync"
+
+	"lindenii.org/go/furgit/object/id"
+	"lindenii.org/go/furgit/ref/store"
+)
+
+// Memory reads and writes one in-memory Git reference namespace.
+//
+// Labels: Close-Caller.
+type Memory struct {
+	mu           sync.RWMutex //exhaustruct:optional
+	objectFormat id.ObjectFormat
+	refs         map[string]storedRef
+}
+
+var (
+	_ store.Reader        = (*Memory)(nil)
+	_ store.Transactioner = (*Memory)(nil)
+	_ store.Batcher       = (*Memory)(nil)
+)
+
+// New builds one empty in-memory reference store for one object format.
+func New(objectFormat id.ObjectFormat) *Memory {
+	return &Memory{
+		objectFormat: objectFormat,
+		refs:         make(map[string]storedRef),
+	}
+}
+
+// ObjectFormat returns the object format used by the store.
+func (memory *Memory) ObjectFormat() id.ObjectFormat {
+	return memory.objectFormat
+}
+
+// Close closes the in-memory reference store.
+//
+// Labels: MT-Unsafe.
+func (memory *Memory) Close() error {
+	return nil
+}
diff --git a/ref/store/memory/read.go b/ref/store/memory/read.go
new file mode 100644
index 00000000..540b7576
--- /dev/null
+++ b/ref/store/memory/read.go
@@ -0,0 +1,78 @@
+package memory
+
+import (
+	"fmt"
+
+	"lindenii.org/go/furgit/ref"
+	"lindenii.org/go/furgit/ref/store"
+)
+
+// Resolve resolves one reference name from the in-memory namespace.
+func (memory *Memory) Resolve(name string) (ref.Ref, error) {
+	memory.mu.RLock()
+	defer memory.mu.RUnlock()
+
+	return publicRef(name, memory.refs[name])
+}
+
+// ResolveToDirect resolves symbolic references
+// until one direct reference is reached.
+func (memory *Memory) ResolveToDirect(name string) (ref.Direct, error) {
+	memory.mu.RLock()
+	defer memory.mu.RUnlock()
+
+	return memory.resolveToDirectLocked(name)
+}
+
+func (memory *Memory) resolveToDirectLocked(name string) (ref.Direct, error) {
+	cur := name
+	seen := make(map[string]struct{})
+
+	for {
+		if _, ok := seen[cur]; ok {
+			return ref.Direct{}, fmt.Errorf("%w: at %q", store.ErrSymbolicCycle, cur)
+		}
+
+		seen[cur] = struct{}{}
+
+		resolved, err := publicRef(cur, memory.refs[cur])
+		if err != nil {
+			return ref.Direct{}, err
+		}
+
+		switch resolved := resolved.(type) {
+		case ref.Direct:
+			return resolved, nil
+		case ref.Symbolic:
+			if resolved.Target == "" {
+				return ref.Direct{}, fmt.Errorf(
+					"%w: symbolic reference %q has empty target",
+					store.ErrInvalidValue, resolved.Name(),
+				)
+			}
+
+			cur = resolved.Target
+		default:
+			panic(fmt.Sprintf("ref/store/memory: unsupported reference type %T", resolved))
+		}
+	}
+}
+
+func publicRef(name string, stored storedRef) (ref.Ref, error) {
+	switch stored.kind {
+	case storedDirect:
+		direct := ref.Direct{RefName: name, ID: stored.id, Peeled: nil}
+		if stored.peeled != nil {
+			peeled := *stored.peeled
+			direct.Peeled = &peeled
+		}
+
+		return direct, nil
+	case storedSymbolic:
+		return ref.Symbolic{RefName: name, Target: stored.target}, nil
+	case storedMissing:
+		return nil, store.ErrReferenceNotFound
+	default:
+		panic(fmt.Sprintf("ref/store/memory: unsupported stored reference kind %d", stored.kind))
+	}
+}
diff --git a/ref/store/memory/read_test.go b/ref/store/memory/read_test.go
new file mode 100644
index 00000000..5c082794
--- /dev/null
+++ b/ref/store/memory/read_test.go
@@ -0,0 +1,80 @@
+package memory_test
+
+import (
+	"errors"
+	"testing"
+
+	"lindenii.org/go/furgit/object/id"
+	"lindenii.org/go/furgit/ref"
+	"lindenii.org/go/furgit/ref/store"
+	"lindenii.org/go/furgit/ref/store/memory"
+)
+
+func TestResolveSymbolic(t *testing.T) {
+	t.Parallel()
+
+	for _, objectFormat := range id.SupportedObjectFormats() {
+		t.Run(objectFormat.String(), func(t *testing.T) {
+			t.Parallel()
+
+			m := memory.New(objectFormat)
+			mainID := objectFormat.Sum([]byte("main"))
+
+			seed(t, m, func(tx store.Transaction) {
+				err := tx.Create("refs/heads/main", mainID)
+				if err != nil {
+					t.Fatalf("Create(main): %v", err)
+				}
+
+				err = tx.CreateSymbolic("HEAD", "refs/heads/main")
+				if err != nil {
+					t.Fatalf("CreateSymbolic(HEAD): %v", err)
+				}
+			})
+
+			head, err := m.Resolve("HEAD")
+			if err != nil {
+				t.Fatalf("Resolve(HEAD): %v", err)
+			}
+
+			symbolic, ok := head.(ref.Symbolic)
+			if !ok {
+				t.Fatalf("Resolve(HEAD) = %T, want ref.Symbolic", head)
+			}
+
+			if symbolic.Target != "refs/heads/main" {
+				t.Fatalf("HEAD target = %q, want refs/heads/main", symbolic.Target)
+			}
+
+			direct, err := m.ResolveToDirect("HEAD")
+			if err != nil {
+				t.Fatalf("ResolveToDirect(HEAD): %v", err)
+			}
+
+			if direct.ID != mainID {
+				t.Fatalf("ResolveToDirect(HEAD) ID = %v, want %v", direct.ID, mainID)
+			}
+		})
+	}
+}
+
+func TestResolveMissing(t *testing.T) {
+	t.Parallel()
+
+	for _, objectFormat := range id.SupportedObjectFormats() {
+		t.Run(objectFormat.String(), func(t *testing.T) {
+			t.Parallel()
+
+			m := memory.New(objectFormat)
+
+			_, err := m.Resolve("refs/heads/absent")
+			if err == nil {
+				t.Fatalf("Resolve(absent) succeeded, want ErrReferenceNotFound")
+			}
+
+			if !errors.Is(err, store.ErrReferenceNotFound) {
+				t.Fatalf("Resolve(absent) err = %v, want ErrReferenceNotFound", err)
+			}
+		})
+	}
+}
diff --git a/ref/store/memory/ref.go b/ref/store/memory/ref.go
new file mode 100644
index 00000000..1286c358
--- /dev/null
+++ b/ref/store/memory/ref.go
@@ -0,0 +1,43 @@
+package memory
+
+import "lindenii.org/go/furgit/object/id"
+
+// storedRef is the internal representation of one reference.
+//
+// Unlike the public ref values,
+// it carries no name of its own;
+// the name is the map key.
+type storedRef struct {
+	kind   storedKind
+	id     id.ObjectID  //exhaustruct:optional
+	target string       //exhaustruct:optional
+	peeled *id.ObjectID //exhaustruct:optional
+}
+
+type storedKind uint8
+
+const (
+	storedMissing storedKind = iota
+	storedDirect
+	storedSymbolic
+)
+
+func cloneStoredRef(stored storedRef) storedRef {
+	if stored.peeled == nil {
+		return stored
+	}
+
+	peeled := *stored.peeled
+	stored.peeled = &peeled
+
+	return stored
+}
+
+func cloneRefs(refs map[string]storedRef) map[string]storedRef {
+	cloned := make(map[string]storedRef, len(refs))
+	for name, stored := range refs {
+		cloned[name] = cloneStoredRef(stored)
+	}
+
+	return cloned
+}
diff --git a/ref/store/memory/transaction.go b/ref/store/memory/transaction.go
new file mode 100644
index 00000000..e68f8cab
--- /dev/null
+++ b/ref/store/memory/transaction.go
@@ -0,0 +1,95 @@
+package memory
+
+import (
+	"lindenii.org/go/furgit/object/id"
+	"lindenii.org/go/furgit/ref/store"
+)
+
+// Transaction stages in-memory updates for one atomic commit.
+type Transaction struct {
+	store *Memory
+	ops   []queuedUpdate
+}
+
+var _ store.Transaction = (*Transaction)(nil)
+
+// BeginTransaction creates one new in-memory transaction.
+func (memory *Memory) BeginTransaction() (store.Transaction, error) {
+	return &Transaction{
+		store: memory,
+		ops:   make([]queuedUpdate, 0, 8),
+	}, nil
+}
+
+// Create queues a direct reference creation.
+func (tx *Transaction) Create(name string, newID id.ObjectID) error {
+	return tx.queue(queuedUpdate{name: name, kind: updateCreate, newID: newID})
+}
+
+// Update queues a direct reference update.
+func (tx *Transaction) Update(name string, newID, oldID id.ObjectID) error {
+	return tx.queue(queuedUpdate{name: name, kind: updateReplace, newID: newID, oldID: oldID})
+}
+
+// Delete queues a direct reference deletion.
+func (tx *Transaction) Delete(name string, oldID id.ObjectID) error {
+	return tx.queue(queuedUpdate{name: name, kind: updateDelete, oldID: oldID})
+}
+
+// Verify queues a direct reference verification.
+func (tx *Transaction) Verify(name string, oldID id.ObjectID) error {
+	return tx.queue(queuedUpdate{name: name, kind: updateVerify, oldID: oldID})
+}
+
+// CreateSymbolic queues a symbolic reference creation.
+func (tx *Transaction) CreateSymbolic(name, newTarget string) error {
+	return tx.queue(queuedUpdate{name: name, kind: updateCreateSymbolic, newTarget: newTarget})
+}
+
+// UpdateSymbolic queues a symbolic reference update.
+func (tx *Transaction) UpdateSymbolic(name, newTarget, oldTarget string) error {
+	return tx.queue(queuedUpdate{name: name, kind: updateReplaceSymbolic, newTarget: newTarget, oldTarget: oldTarget})
+}
+
+// DeleteSymbolic queues a symbolic reference deletion.
+func (tx *Transaction) DeleteSymbolic(name, oldTarget string) error {
+	return tx.queue(queuedUpdate{name: name, kind: updateDeleteSymbolic, oldTarget: oldTarget})
+}
+
+// VerifySymbolic queues a symbolic reference verification.
+func (tx *Transaction) VerifySymbolic(name, oldTarget string) error {
+	return tx.queue(queuedUpdate{name: name, kind: updateVerifySymbolic, oldTarget: oldTarget})
+}
+
+// Commit validates and applies the queued updates atomically.
+func (tx *Transaction) Commit() error {
+	tx.store.mu.Lock()
+	defer tx.store.mu.Unlock()
+
+	prepared, _, err := prepareUpdates(tx.store.refs, tx.ops)
+	if err != nil {
+		return err
+	}
+
+	next := cloneRefs(tx.store.refs)
+	applyPreparedUpdates(next, prepared)
+	tx.store.refs = next
+
+	return nil
+}
+
+// Abort abandons the transaction.
+func (tx *Transaction) Abort() error {
+	return nil
+}
+
+func (tx *Transaction) queue(op queuedUpdate) error {
+	err := validateQueuedUpdate(tx.store.objectFormat, op)
+	if err != nil {
+		return err
+	}
+
+	tx.ops = append(tx.ops, op)
+
+	return nil
+}
diff --git a/ref/store/memory/transaction_test.go b/ref/store/memory/transaction_test.go
new file mode 100644
index 00000000..75ac3f88
--- /dev/null
+++ b/ref/store/memory/transaction_test.go
@@ -0,0 +1,96 @@
+package memory_test
+
+import (
+	"errors"
+	"testing"
+
+	"lindenii.org/go/furgit/object/id"
+	"lindenii.org/go/furgit/ref/store"
+	"lindenii.org/go/furgit/ref/store/memory"
+)
+
+func TestTransactionRejectLeavesStoreUnchanged(t *testing.T) {
+	t.Parallel()
+
+	for _, objectFormat := range id.SupportedObjectFormats() {
+		t.Run(objectFormat.String(), func(t *testing.T) {
+			t.Parallel()
+
+			m := memory.New(objectFormat)
+			mainID := objectFormat.Sum([]byte("main"))
+			devID := objectFormat.Sum([]byte("dev"))
+			nextID := objectFormat.Sum([]byte("next"))
+			wrongOld := objectFormat.Sum([]byte("wrong"))
+
+			seed(t, m, func(tx store.Transaction) {
+				err := tx.Create("refs/heads/main", mainID)
+				if err != nil {
+					t.Fatalf("Create(main): %v", err)
+				}
+
+				err = tx.Create("refs/heads/dev", devID)
+				if err != nil {
+					t.Fatalf("Create(dev): %v", err)
+				}
+			})
+
+			tx, err := m.BeginTransaction()
+			if err != nil {
+				t.Fatalf("BeginTransaction: %v", err)
+			}
+
+			err = tx.Update("refs/heads/main", nextID, mainID)
+			if err != nil {
+				t.Fatalf("Update(main): %v", err)
+			}
+
+			err = tx.Update("refs/heads/dev", nextID, wrongOld)
+			if err != nil {
+				t.Fatalf("Update(dev): %v", err)
+			}
+
+			err = tx.Commit()
+			if err == nil {
+				t.Fatalf("Commit succeeded, want WrongOldIDError")
+			}
+
+			if _, ok := errors.AsType[*store.WrongOldIDError](err); !ok {
+				t.Fatalf("Commit error = %T %v, want *store.WrongOldIDError", err, err)
+			}
+
+			if got := resolveDirect(t, m, "refs/heads/main").ID; got != mainID {
+				t.Fatalf("main after rejected transaction = %v, want %v", got, mainID)
+			}
+
+			if got := resolveDirect(t, m, "refs/heads/dev").ID; got != devID {
+				t.Fatalf("dev after rejected transaction = %v, want %v", got, devID)
+			}
+		})
+	}
+}
+
+func TestTransactionRejectsForeignObjectFormat(t *testing.T) {
+	t.Parallel()
+
+	for _, objectFormat := range id.SupportedObjectFormats() {
+		t.Run(objectFormat.String(), func(t *testing.T) {
+			t.Parallel()
+
+			m := memory.New(objectFormat)
+
+			tx, err := m.BeginTransaction()
+			if err != nil {
+				t.Fatalf("BeginTransaction: %v", err)
+			}
+
+			err = tx.Create("refs/heads/main", id.ObjectID{})
+			if err == nil {
+				t.Fatalf("Create with unset ID succeeded, want ErrInvalidValue")
+			}
+
+			if !errors.Is(err, store.ErrInvalidValue) {
+				t.Fatalf("Create error = %v, want ErrInvalidValue", err)
+			}
+		})
+	}
+}
diff --git a/ref/store/memory/update.go b/ref/store/memory/update.go
new file mode 100644
index 00000000..8e8c6e30
--- /dev/null
+++ b/ref/store/memory/update.go
@@ -0,0 +1,372 @@
+package memory
+
+import (
+	"errors"
+	"fmt"
+
+	"lindenii.org/go/furgit/object/id"
+	refname "lindenii.org/go/furgit/ref/name"
+	"lindenii.org/go/furgit/ref/store"
+)
+
+type updateKind uint8
+
+const (
+	updateCreate updateKind = iota
+	updateReplace
+	updateDelete
+	updateVerify
+	updateCreateSymbolic
+	updateReplaceSymbolic
+	updateDeleteSymbolic
+	updateVerifySymbolic
+)
+
+type queuedUpdate struct {
+	name      string
+	kind      updateKind
+	newID     id.ObjectID //exhaustruct:optional
+	oldID     id.ObjectID //exhaustruct:optional
+	newTarget string      //exhaustruct:optional
+	oldTarget string      //exhaustruct:optional
+}
+
+type resolvedUpdateTarget struct {
+	name string
+	ref  storedRef
+}
+
+type preparedUpdate struct {
+	op     queuedUpdate
+	target resolvedUpdateTarget
+}
+
+// validateQueuedUpdate checks one operation at queue time,
+// rejecting malformed names and values
+// before they can enter a transaction or batch.
+func validateQueuedUpdate(objectFormat id.ObjectFormat, op queuedUpdate) error {
+	switch op.kind {
+	case updateCreate, updateReplace:
+		err := refname.ValidateUpdateName(op.name, true)
+		if err != nil {
+			return fmt.Errorf("ref/store/memory: %w", err)
+		}
+
+		if op.newID.ObjectFormat() != objectFormat {
+			return fmt.Errorf("%w: object id format mismatch", store.ErrInvalidValue)
+		}
+	case updateDelete, updateVerify:
+		err := refname.ValidateUpdateName(op.name, false)
+		if err != nil {
+			return fmt.Errorf("ref/store/memory: %w", err)
+		}
+
+		if op.oldID.ObjectFormat() != objectFormat {
+			return fmt.Errorf("%w: object id format mismatch", store.ErrInvalidValue)
+		}
+	case updateCreateSymbolic, updateReplaceSymbolic:
+		err := refname.ValidateUpdateName(op.name, true)
+		if err != nil {
+			return fmt.Errorf("ref/store/memory: %w", err)
+		}
+
+		if op.newTarget == "" {
+			return fmt.Errorf("%w: empty symbolic target", store.ErrInvalidValue)
+		}
+
+		err = refname.ValidateSymbolicTarget(op.name, op.newTarget)
+		if err != nil {
+			return fmt.Errorf("ref/store/memory: %w", err)
+		}
+	case updateDeleteSymbolic, updateVerifySymbolic:
+		err := refname.ValidateUpdateName(op.name, false)
+		if err != nil {
+			return fmt.Errorf("ref/store/memory: %w", err)
+		}
+	default:
+		panic(fmt.Sprintf("ref/store/memory: unsupported update operation %d", op.kind))
+	}
+
+	if op.kind == updateReplaceSymbolic || op.kind == updateDeleteSymbolic || op.kind == updateVerifySymbolic {
+		if op.oldTarget == "" {
+			return fmt.Errorf("%w: empty symbolic old target", store.ErrInvalidValue)
+		}
+	}
+
+	return nil
+}
+
+// prepareUpdates resolves, conflict-checks, and verifies a queued operation
+// set against refs without mutating it.
+// On failure it returns the name of the offending operation alongside the error.
+func prepareUpdates(refs map[string]storedRef, ops []queuedUpdate) ([]preparedUpdate, string, error) {
+	prepared, name, err := resolvePreparedUpdates(refs, ops)
+	if err != nil {
+		return prepared, name, err
+	}
+
+	deleted, written := collectPreparedWrites(prepared)
+	existing := collectVisibleNames(refs)
+
+	for _, name := range written {
+		err = verifyRefnameAvailable(name, existing, written, deleted)
+		if err != nil {
+			return prepared, name, err
+		}
+	}
+
+	name, err = verifyPreparedUpdates(refs, prepared)
+	if err != nil {
+		return prepared, name, err
+	}
+
+	return prepared, "", nil
+}
+
+func resolvePreparedUpdates(refs map[string]storedRef, ops []queuedUpdate) ([]preparedUpdate, string, error) {
+	prepared := make([]preparedUpdate, 0, len(ops))
+	targets := make(map[string]struct{}, len(ops))
+
+	for _, op := range ops {
+		target, err := resolveQueuedUpdateTarget(refs, op)
+		if err != nil {
+			return prepared, op.name, err
+		}
+
+		if _, exists := targets[target.name]; exists {
+			return prepared, op.name, store.ErrDuplicateUpdate
+		}
+
+		targets[target.name] = struct{}{}
+		prepared = append(prepared, preparedUpdate{op: op, target: target})
+	}
+
+	return prepared, "", nil
+}
+
+func resolveQueuedUpdateTarget(refs map[string]storedRef, op queuedUpdate) (resolvedUpdateTarget, error) {
+	switch op.kind {
+	case updateCreate:
+		return resolveOrdinaryTarget(refs, op.name, true)
+	case updateReplace, updateDelete, updateVerify:
+		return resolveOrdinaryTarget(refs, op.name, false)
+	case updateCreateSymbolic, updateReplaceSymbolic, updateDeleteSymbolic, updateVerifySymbolic:
+		return resolvedUpdateTarget{name: op.name, ref: directRead(refs, op.name)}, nil
+	default:
+		panic(fmt.Sprintf("ref/store/memory: unsupported update operation %d", op.kind))
+	}
+}
+
+func resolveOrdinaryTarget(refs map[string]storedRef, name string, allowMissing bool) (resolvedUpdateTarget, error) {
+	cur := name
+	seen := make(map[string]struct{})
+
+	for {
+		if _, ok := seen[cur]; ok {
+			return resolvedUpdateTarget{}, fmt.Errorf("%w: at %q", store.ErrSymbolicCycle, cur)
+		}
+
+		seen[cur] = struct{}{}
+
+		refState := directRead(refs, cur)
+		switch refState.kind {
+		case storedMissing:
+			if !allowMissing {
+				return resolvedUpdateTarget{}, store.ErrReferenceNotFound
+			}
+
+			return resolvedUpdateTarget{name: cur, ref: refState}, nil
+		case storedDirect:
+			return resolvedUpdateTarget{name: cur, ref: refState}, nil
+		case storedSymbolic:
+			if refState.target == "" {
+				return resolvedUpdateTarget{}, fmt.Errorf(
+					"%w: symbolic reference has empty target", store.ErrInvalidValue,
+				)
+			}
+
+			cur = refState.target
+		default:
+			panic(fmt.Sprintf("ref/store/memory: unsupported stored reference kind %d", refState.kind))
+		}
+	}
+}
+
+func directRead(refs map[string]storedRef, name string) storedRef {
+	stored, ok := refs[name]
+	if !ok {
+		return storedRef{kind: storedMissing}
+	}
+
+	return cloneStoredRef(stored)
+}
+
+func collectPreparedWrites(prepared []preparedUpdate) (deleted map[string]struct{}, written []string) {
+	deleted = make(map[string]struct{})
+	written = make([]string, 0, len(prepared))
+
+	for _, item := range prepared {
+		switch item.op.kind {
+		case updateDelete, updateDeleteSymbolic:
+			deleted[item.target.name] = struct{}{}
+		case updateCreate, updateReplace, updateCreateSymbolic, updateReplaceSymbolic:
+			written = append(written, item.target.name)
+		case updateVerify, updateVerifySymbolic:
+		default:
+			panic(fmt.Sprintf("ref/store/memory: unsupported update operation %d", item.op.kind))
+		}
+	}
+
+	return deleted, written
+}
+
+func collectVisibleNames(refs map[string]storedRef) map[string]struct{} {
+	names := make(map[string]struct{}, len(refs))
+	for name := range refs {
+		names[name] = struct{}{}
+	}
+
+	return names
+}
+
+func verifyRefnameAvailable(name string, existing map[string]struct{}, writes []string, deleted map[string]struct{}) error {
+	for existingName := range existing {
+		if existingName == name {
+			continue
+		}
+
+		if _, skip := deleted[existingName]; skip {
+			continue
+		}
+
+		if refnamesConflict(name, existingName) {
+			return &store.NameConflictError{Other: existingName}
+		}
+	}
+
+	for _, other := range writes {
+		if other == name {
+			continue
+		}
+
+		if refnamesConflict(name, other) {
+			return &store.NameConflictError{Other: other}
+		}
+	}
+
+	return nil
+}
+
+func refnamesConflict(left, right string) bool {
+	return left == right ||
+		hasPathPrefix(left, right) ||
+		hasPathPrefix(right, left)
+}
+
+func hasPathPrefix(name, prefix string) bool {
+	return len(name) > len(prefix) &&
+		name[len(prefix)] == '/' &&
+		name[:len(prefix)] == prefix
+}
+
+func verifyPreparedUpdates(refs map[string]storedRef, prepared []preparedUpdate) (string, error) {
+	for i := range prepared {
+		item := &prepared[i]
+		item.target.ref = directRead(refs, item.target.name)
+
+		err := verifyPreparedUpdateCurrent(*item)
+		if err != nil {
+			return item.op.name, err
+		}
+	}
+
+	return "", nil
+}
+
+func verifyPreparedUpdateCurrent(item preparedUpdate) error {
+	switch item.op.kind {
+	case updateCreate, updateCreateSymbolic:
+		if item.target.ref.kind != storedMissing {
+			return store.ErrCreateExists
+		}
+
+		return nil
+	case updateReplace, updateDelete, updateVerify:
+		if item.target.ref.kind == storedMissing {
+			return store.ErrReferenceNotFound
+		}
+
+		if item.target.ref.kind != storedDirect {
+			return store.ErrExpectedDirect
+		}
+
+		if item.target.ref.id != item.op.oldID {
+			return &store.WrongOldIDError{Actual: item.target.ref.id, Expected: item.op.oldID}
+		}
+
+		return nil
+	case updateReplaceSymbolic, updateDeleteSymbolic, updateVerifySymbolic:
+		if item.target.ref.kind == storedMissing {
+			return store.ErrReferenceNotFound
+		}
+
+		if item.target.ref.kind != storedSymbolic {
+			return store.ErrExpectedSymbolic
+		}
+
+		if item.target.ref.target != item.op.oldTarget {
+			return &store.WrongOldTargetError{Actual: item.target.ref.target, Expected: item.op.oldTarget}
+		}
+
+		return nil
+	default:
+		panic(fmt.Sprintf("ref/store/memory: unsupported update operation %d", item.op.kind))
+	}
+}
+
+func applyPreparedUpdates(refs map[string]storedRef, prepared []preparedUpdate) {
+	for _, item := range prepared {
+		switch item.op.kind {
+		case updateCreate, updateReplace:
+			refs[item.target.name] = storedRef{kind: storedDirect, id: item.op.newID}
+		case updateCreateSymbolic, updateReplaceSymbolic:
+			refs[item.target.name] = storedRef{kind: storedSymbolic, target: item.op.newTarget}
+		case updateDelete, updateDeleteSymbolic:
+			delete(refs, item.target.name)
+		case updateVerify, updateVerifySymbolic:
+		default:
+			panic(fmt.Sprintf("ref/store/memory: unsupported update operation %d", item.op.kind))
+		}
+	}
+}
+
+// isBatchRejected reports whether err is a per-operation rejection
+// that should drop only the offending operation,
+// rather than a fatal failure that aborts the whole batch.
+func isBatchRejected(err error) bool {
+	switch {
+	case errors.Is(err, store.ErrReferenceNotFound),
+		errors.Is(err, store.ErrCreateExists),
+		errors.Is(err, store.ErrDuplicateUpdate),
+		errors.Is(err, store.ErrExpectedDirect),
+		errors.Is(err, store.ErrExpectedSymbolic),
+		errors.Is(err, store.ErrInvalidValue),
+		errors.Is(err, store.ErrSymbolicCycle),
+		errors.Is(err, refname.ErrInvalidName):
+		return true
+	}
+
+	if _, ok := errors.AsType[*store.NameConflictError](err); ok {
+		return true
+	}
+
+	if _, ok := errors.AsType[*store.WrongOldIDError](err); ok {
+		return true
+	}
+
+	if _, ok := errors.AsType[*store.WrongOldTargetError](err); ok {
+		return true
+	}
+
+	return false
+}