✨ Add split sink routing helpers

bitlogger/BitLogger_wbtest.mbt

@@ -182,6 +182,58 @@ test "callback sink receives record" {
   inspect(captured_message.val, content="hello")
 }
 
+test "split sink routes records by predicate" {
+  let left_messages : Ref[Array[String]] = Ref::new([])
+  let right_messages : Ref[Array[String]] = Ref::new([])
+  let logger = Logger::new(
+    split_sink(
+      callback_sink(fn(rec) {
+        left_messages.val.push(rec.message)
+      }),
+      callback_sink(fn(rec) {
+        right_messages.val.push(rec.message)
+      }),
+      fn(rec) {
+        rec.target == "audit"
+      },
+    ),
+    min_level=Level::Info,
+    target="main",
+  )
+  logger.info("drop to right")
+  logger.log(Level::Info, "keep on left", target="audit")
+  inspect(left_messages.val.length(), content="1")
+  inspect(left_messages.val[0], content="keep on left")
+  inspect(right_messages.val.length(), content="1")
+  inspect(right_messages.val[0], content="drop to right")
+}
+
+test "split_by_level routes warn and error separately" {
+  let high_messages : Ref[Array[String]] = Ref::new([])
+  let low_messages : Ref[Array[String]] = Ref::new([])
+  let logger = Logger::new(
+    split_by_level(
+      callback_sink(fn(rec) {
+        high_messages.val.push(rec.level.label() + ":" + rec.message)
+      }),
+      callback_sink(fn(rec) {
+        low_messages.val.push(rec.level.label() + ":" + rec.message)
+      }),
+      min_level=Level::Warn,
+    ),
+    min_level=Level::Trace,
+    target="split",
+  )
+  logger.info("info")
+  logger.warn("warn")
+  logger.error("error")
+  inspect(high_messages.val.length(), content="2")
+  inspect(high_messages.val[0], content="WARN:warn")
+  inspect(high_messages.val[1], content="ERROR:error")
+  inspect(low_messages.val.length(), content="1")
+  inspect(low_messages.val[0], content="INFO:info")
+}
+
 test "callback sink sees child target and context logger shape" {
   let captured_target : Ref[String] = Ref::new("")
   let captured_message : Ref[String] = Ref::new("")

bitlogger/README.mbt.md

@@ -24,6 +24,8 @@ BitLogger 是一个基于 MoonBit 的结构化日志库。
 - `json_console_sink()` 提供 JSON 控制台输出
 - sink composition via `fanout_sink(...)`
 - `fanout_sink(...)` 支持多 sink 组合
+- sink routing via `split_sink(...)` and `split_by_level(...)`
+- `split_sink(...)`、`split_by_level(...)` 支持按谓词或 level 将日志路由到不同 sink
 - custom callback sink via `callback_sink(...)`
 - `callback_sink(...)` 支持自定义外部集成
 - buffered sink via `buffered_sink(...)`
@@ -142,6 +144,24 @@ test {
 }
 ```
 
+```mbt check
+test {
+  let logger = Logger::new(
+    split_by_level(
+      callback_sink(fn(rec) {
+        println("high priority: \{rec.level.label()} \{rec.message}")
+      }),
+      console_sink(),
+      min_level=Level::Warn,
+    ),
+    min_level=Level::Trace,
+    target="split",
+  )
+  logger.info("normal output")
+  logger.warn("warning output")
+}
+```
+
 ```mbt check
 test {
   let formatter = text_formatter(

bitlogger/sinks.mbt

@@ -247,6 +247,34 @@ pub impl[A : Sink, B : Sink] Sink for FanoutSink[A, B] with write(self, rec) {
   self.right.write({ ..rec })
 }
 
+pub struct SplitSink[A, B] {
+  left : A
+  right : B
+  predicate : (Record) -> Bool
+}
+
+pub fn[A, B] split_sink(left : A, right : B, predicate : (Record) -> Bool) -> SplitSink[A, B] {
+  { left, right, predicate }
+}
+
+pub fn[A, B] split_by_level(
+  left : A,
+  right : B,
+  min_level~ : Level = Level::Warn,
+) -> SplitSink[A, B] {
+  split_sink(left, right, fn(rec) {
+    rec.level.enabled(min_level)
+  })
+}
+
+pub impl[A : Sink, B : Sink] Sink for SplitSink[A, B] with write(self, rec) {
+  if (self.predicate)(rec) {
+    self.left.write(rec)
+  } else {
+    self.right.write(rec)
+  }
+}
+
 pub struct CallbackSink {
   callback : (Record) -> Unit
 }