feat(common): add parseSSEStream function

This function parses the raw buffer recieved from sse requests and emits completed chunks at O(n)

Author: apaparazzi0329

lib/common.ts

@@ -1,4 +1,5 @@
 import os = require('os');
+import { EventEmitter } from 'events';
 
 // tslint:disable-next-line:no-var-requires
 const pkg = require('../package.json');
@@ -30,3 +31,101 @@ export function getSdkHeaders(serviceName: string, serviceVersion: string, opera
 
   return headers;
 }
+
+export function parseSSEStream(stream: NodeJS.ReadableStream) {
+  let eventMemoryKey = ''
+  let dataMemoryKey = ''
+  let stringBuffer = ''
+  let eventMessage = ''
+  const eventEmitter = new EventEmitter()
+
+  stream.on('data', (chunk: Buffer) => {
+    const chunkString = chunk.toString()
+    
+    for (const char of chunkString) {
+      // Looks for "event: "
+      if (char === 'e' && eventMemoryKey === '') {
+        eventMemoryKey += char
+      } else if (char === 'v' && eventMemoryKey === 'e') {
+        eventMemoryKey += char
+      } else if (char === 'e' && eventMemoryKey === 'ev') {
+        eventMemoryKey += char
+      } else if (char === 'n' && eventMemoryKey === 'eve') {
+        eventMemoryKey += char
+      } else if (char === 't' && eventMemoryKey === 'even') {
+        eventMemoryKey += char
+      } else if (char === ':' && eventMemoryKey === 'event') {
+        eventMemoryKey += char
+      } else if (char === ' ' && eventMemoryKey === 'event:') {
+        eventMemoryKey += char
+      } else if (eventMemoryKey === 'event: ' && dataMemoryKey !== 'data: ') {
+        stringBuffer += char // Start saving characters in string buffer
+      } else if (eventMemoryKey === 'event: ' && dataMemoryKey === 'data: ') {
+        // Must remove extra 'event: ' added to stringBuffer
+        const trimmedStringBuffer = stringBuffer.substring(0, stringBuffer.length - 'event: '.length)
+
+        try {
+          const parsedBuffer = JSON.parse(trimmedStringBuffer)
+          eventEmitter.emit('data', {
+            event: eventMessage,
+            data: parsedBuffer
+          })
+        } catch(e) {
+          if (!(e instanceof SyntaxError)) {
+            throw Error('Unexpected error when trying to emit data event')
+          }
+          // stringBuffer was not parseable, we must continue and wait for more chunks
+        }
+    
+        stringBuffer = char
+        dataMemoryKey = ''
+      } else {
+        eventMemoryKey = ''
+      }
+      // Looks for "data: "
+      if (char === 'd' && eventMemoryKey === 'event: ') {
+        dataMemoryKey += char
+      } else if (char === 'a' && dataMemoryKey === 'd') {
+        dataMemoryKey += char
+      } else if (char === 't' && dataMemoryKey === 'da') {
+        dataMemoryKey += char
+      } else if (char === 'a' && dataMemoryKey === 'dat') {
+        dataMemoryKey += char
+      } else if (char === ':' && dataMemoryKey === 'data') {
+        dataMemoryKey += char
+      } else if (char === ' ' && dataMemoryKey === 'data:') {
+        dataMemoryKey += char
+    
+        // Must remove extra 'data: ' and '/n' added to stringBuffer
+        stringBuffer = stringBuffer.substring(0, stringBuffer.length - 'data: '.length - 1)
+    
+        // Save and clear stringBuffer
+        eventMessage = stringBuffer
+        stringBuffer = ''
+        eventMemoryKey = ''
+      } else if (dataMemoryKey === 'data: ') {
+        stringBuffer += char // Start saving characters in string buffer
+      }
+      else {
+        dataMemoryKey = ''
+      }
+    }
+    
+    try {
+      const parsedBuffer = JSON.parse(stringBuffer)
+      eventEmitter.emit('data', {
+        event: eventMessage,
+        data: parsedBuffer
+      })
+
+      stringBuffer = ''
+    } catch(e) {
+      if (!(e instanceof SyntaxError)) {
+        throw Error('Unexpected error when trying to emit data event')
+      }
+       // stringBuffer was not parseable, we must continue and wait for more chunks
+    }
+  });
+
+  return eventEmitter
+}