1 files changed, 73 insertions, 0 deletions
diff --git a/lib/websocket/frame.js b/lib/websocket/frame.js
new file mode 100644
index 0000000..d867ad1
--- /dev/null
+++ b/lib/websocket/frame.js
@@ -0,0 +1,73 @@
+'use strict'
+
+const { maxUnsigned16Bit } = require('./constants')
+
+/** @type {import('crypto')} */
+let crypto
+try {
+  crypto = require('crypto')
+} catch {
+
+}
+
+class WebsocketFrameSend {
+  /**
+   * @param {Buffer|undefined} data
+   */
+  constructor (data) {
+    this.frameData = data
+    this.maskKey = crypto.randomBytes(4)
+  }
+
+  createFrame (opcode) {
+    const bodyLength = this.frameData?.byteLength ?? 0
+
+    /** @type {number} */
+    let payloadLength = bodyLength // 0-125
+    let offset = 6
+
+    if (bodyLength > maxUnsigned16Bit) {
+      offset += 8 // payload length is next 8 bytes
+      payloadLength = 127
+    } else if (bodyLength > 125) {
+      offset += 2 // payload length is next 2 bytes
+      payloadLength = 126
+    }
+
+    const buffer = Buffer.allocUnsafe(bodyLength + offset)
+
+    // Clear first 2 bytes, everything else is overwritten
+    buffer[0] = buffer[1] = 0
+    buffer[0] |= 0x80 // FIN
+    buffer[0] = (buffer[0] & 0xF0) + opcode // opcode
+
+    /*! ws. MIT License. Einar Otto Stangvik <einaros@gmail.com> */
+    buffer[offset - 4] = this.maskKey[0]
+    buffer[offset - 3] = this.maskKey[1]
+    buffer[offset - 2] = this.maskKey[2]
+    buffer[offset - 1] = this.maskKey[3]
+
+    buffer[1] = payloadLength
+
+    if (payloadLength === 126) {
+      buffer.writeUInt16BE(bodyLength, 2)
+    } else if (payloadLength === 127) {
+      // Clear extended payload length
+      buffer[2] = buffer[3] = 0
+      buffer.writeUIntBE(bodyLength, 4, 6)
+    }
+
+    buffer[1] |= 0x80 // MASK
+
+    // mask body
+    for (let i = 0; i < bodyLength; i++) {
+      buffer[offset + i] = this.frameData[i] ^ this.maskKey[i % 4]
+    }
+
+    return buffer
+  }
+}
+
+module.exports = {
+  WebsocketFrameSend
+}