"""A small HTML entry shell using the public ASGI middleware interface."""
from pathlib import Path
import re


class EntryShell:
    """Only decorate the document; never buffer API, SSE, media or asset responses."""
    def __init__(self, app, shell_path):
        self.app = app
        self.shell = Path(shell_path).read_bytes()

    async def __call__(self, scope, receive, send):
        if scope['type'] != 'http' or scope.get('method') != 'GET' or scope.get('path') != '/':
            await self.app(scope, receive, send)
            return
        start = None
        chunks = []
        decorate = False

        async def intercept(message):
            nonlocal start, decorate
            if message['type'] == 'http.response.start':
                headers = dict(message.get('headers', []))
                decorate = (message['status'] == 200 and
                            headers.get(b'content-type', b'').startswith(b'text/html') and
                            b'content-encoding' not in headers)
                if decorate:
                    start = message
                else:
                    await send(message)
            elif message['type'] == 'http.response.body' and decorate:
                chunks.append(message.get('body', b''))
                if not message.get('more_body', False):
                    body = re.sub(rb'(<body\b[^>]*>)', lambda match: match[0] + self.shell,
                                  b''.join(chunks), count=1, flags=re.I)
                    headers = [(k, v) for k, v in start['headers']
                               if k.lower() not in {b'content-length', b'etag', b'cache-control'}]
                    headers += [(b'content-length', str(len(body)).encode()),
                                (b'cache-control', b'no-store')]
                    await send({**start, 'headers': headers})
                    await send({'type': 'http.response.body', 'body': body})
            else:
                await send(message)

        await self.app(scope, receive, intercept)