From 4ca27aec303f54dc9ee670c67c2fefa80dd004a1 Mon Sep 17 00:00:00 2001
From: SangTran-127 <tranquangsang12.7@gmail.com>
Date: Sun, 9 Aug 2026 18:32:44 +0700
Subject: feat: add KeyByOperator and wordCounter for stateful processing

---
 core/operator/stateful_map.go | 75 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 75 insertions(+)
 create mode 100644 core/operator/stateful_map.go

(limited to 'core/operator/stateful_map.go')

diff --git a/core/operator/stateful_map.go b/core/operator/stateful_map.go
new file mode 100644
index 0000000..7263170
--- /dev/null
+++ b/core/operator/stateful_map.go
@@ -0,0 +1,75 @@
+package operator
+
+import (
+	"context"
+	"fmt"
+
+	"goflink/core"
+)
+
+// RichMapFunc is a stateful map on the user side: Open grabs the state handles
+// off ctx, Map runs per record with the backend already pointed at that
+// record's key.
+type RichMapFunc[IN, OUT any] interface {
+	Open(ctx context.Context) error
+	Map(ctx context.Context, in IN) (OUT, error)
+	Close(ctx context.Context) error
+}
+
+type StatefulMapOperator[IN, OUT any] struct {
+	userFunc RichMapFunc[IN, OUT]
+	backend  core.StateBackend
+}
+
+func NewStatefulMapOperator[IN, OUT any](fn RichMapFunc[IN, OUT]) *StatefulMapOperator[IN, OUT] {
+	return &StatefulMapOperator[IN, OUT]{userFunc: fn}
+}
+
+func (s *StatefulMapOperator[IN, OUT]) Open(ctx context.Context) error {
+	// Get backend state from context via using context KV
+	be, ok := core.ExtractStateBackend(ctx)
+	if !ok || be == nil {
+		return fmt.Errorf("stateful map: no state backend in context")
+	}
+
+	s.backend = be
+	// The user func opens its ValueState off the same ctx.
+	return s.userFunc.Open(ctx)
+}
+
+func (s *StatefulMapOperator[IN, OUT]) Process(ctx context.Context, record core.StreamRecord[IN], out Emitter[OUT]) error {
+	if record.Type != core.RecordData {
+		return out.Emit(ctx, core.StreamRecord[OUT]{
+			Type:      record.Type,
+			Timestamp: record.Timestamp,
+			Key:       record.Key,
+			BarrierID: record.BarrierID,
+		})
+	}
+
+	// Without a key every record would silently share the "" state slot.
+	if record.Key == "" {
+		return fmt.Errorf("stateful map: record %v has no key, keyBy it first", record.Value)
+	}
+
+	if err := s.backend.SetCurrentKey(record.Key); err != nil {
+		return fmt.Errorf("set current key %q: %w", record.Key, err)
+	}
+
+	// run the user transform
+	res, err := s.userFunc.Map(ctx, record.Value)
+	if err != nil {
+		return fmt.Errorf("cannot process value %v: %w", record.Value, err)
+	}
+
+	return out.Emit(ctx, core.StreamRecord[OUT]{
+		Type:      core.RecordData,
+		Timestamp: record.Timestamp,
+		Key:       record.Key,
+		Value:     res,
+	})
+}
+
+func (s *StatefulMapOperator[IN, OUT]) Close(ctx context.Context) error {
+	return s.userFunc.Close(ctx)
+}
-- 
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