#!/usr/bin/env python3

import json
import struct
import sys


# Read a message from stdin and decode it.
def getMessage():
    rawLength = sys.stdin.buffer.read(4)
    if len(rawLength) == 0:
        sys.exit(0)
    messageLength = struct.unpack("@I", rawLength)[0]
    message = sys.stdin.buffer.read(messageLength).decode("utf-8")
    return json.loads(message)


# Encode a message for transmission, given its content.
def encodeMessage(messageContent):
    encodedContent = json.dumps(messageContent, separators=(",", ":")).encode("utf-8")
    encodedLength = struct.pack("@I", len(encodedContent))
    return {"length": encodedLength, "content": encodedContent}


# Send an encoded message to stdout
def sendMessage(encodedMessage):
    sys.stdout.buffer.write(encodedMessage["length"])
    sys.stdout.buffer.write(encodedMessage["content"])
    sys.stdout.buffer.flush()


while True:
    receivedMessage = getMessage()
    if receivedMessage == "ping":
        sendMessage(encodeMessage("native app sends pong"))