chore: establish YCode replication baseline and TODO

src/main/gateway/sse.ts

@@ -0,0 +1,69 @@
+/**
+ * 极简 SSE（text/event-stream）解析器。
+ *
+ * 只依赖 Web 标准的 ReadableStream，可直接消费 fetch 响应体。
+ * 上游既可能是标准 SSE，也可能在末尾缺少空行，因此以「块边界」为准做收尾。
+ */
+
+export interface SseEvent {
+  event?: string
+  data: string
+}
+
+export async function* parseSse(
+  stream: ReadableStream<Uint8Array>
+): AsyncGenerator<SseEvent, void, void> {
+  const reader = stream.getReader()
+  const decoder = new TextDecoder()
+  let buffer = ''
+
+  try {
+    for (;;) {
+      const { done, value } = await reader.read()
+      if (done) break
+      buffer += decoder.decode(value, { stream: true })
+
+      let index: number
+      // 事件之间以空行分隔，兼容 \r\n 与 \n
+      while ((index = findBoundary(buffer)) !== -1) {
+        const raw = buffer.slice(0, index)
+        buffer = buffer.slice(index).replace(/^(\r?\n){2}/, '')
+        const parsed = parseBlock(raw)
+        if (parsed) yield parsed
+      }
+    }
+
+    // 收尾：流结束时可能残留最后一个事件
+    buffer += decoder.decode()
+    const tail = parseBlock(buffer)
+    if (tail) yield tail
+  } finally {
+    reader.releaseLock()
+  }
+}
+
+function findBoundary(buffer: string): number {
+  const a = buffer.indexOf('\n\n')
+  const b = buffer.indexOf('\r\n\r\n')
+  if (a === -1) return b
+  if (b === -1) return a
+  return Math.min(a, b)
+}
+
+function parseBlock(block: string): SseEvent | null {
+  const lines = block.split(/\r?\n/)
+  const dataLines: string[] = []
+  let event: string | undefined
+
+  for (const line of lines) {
+    if (!line || line.startsWith(':')) continue
+    if (line.startsWith('event:')) {
+      event = line.slice(6).trim()
+    } else if (line.startsWith('data:')) {
+      dataLines.push(line.slice(5).replace(/^ /, ''))
+    }
+  }
+
+  if (dataLines.length === 0) return null
+  return { event, data: dataLines.join('\n') }
+}