"""Bounded public metadata requests and atomic response caching."""
import hashlib
from http.client import IncompleteRead
import json
from pathlib import Path
import ssl
import time
import urllib.error
import urllib.request


def fetch(url,cache,limit=64*1024**2,delay=0.5):
    cache = Path(cache)
    cache.mkdir(parents=True,exist_ok=True)
    path = cache/hashlib.sha256(url.encode()).hexdigest()
    if path.exists() and time.time()-path.stat().st_mtime < 24*3600:
        if path.stat().st_size>limit:raise RuntimeError('Metadata response exceeds byte limit')
        return path.read_bytes()
    for attempt in range(3):
        try:
            request = urllib.request.Request(url,headers={'User-Agent':'PaperAtlas/0.1 (public scholarly metadata research)'})
            with urllib.request.urlopen(request,timeout=90) as response:
                if response.status != 200:
                    raise RuntimeError('Expected complete HTTP 200 response')
                expected=int(response.headers.get('Content-Length','0'))
                if expected>limit:
                    raise RuntimeError('Metadata response exceeds byte limit')
                data = response.read(limit+1)
                if len(data)>limit:
                    raise RuntimeError('Metadata response exceeds byte limit')
                if expected and len(data)!=expected:
                    raise IncompleteRead(data,max(0,expected-len(data)))
            part = path.with_suffix('.part')
            part.write_bytes(data)
            part.replace(path)
            path.with_suffix('.json').write_text(json.dumps({'url':url,'sha256':hashlib.sha256(data).hexdigest(),'bytes':len(data),'retrieved_at':time.strftime('%Y-%m-%dT%H:%M:%SZ',time.gmtime())}))
            time.sleep(delay)
            return data
        except urllib.error.HTTPError as exc:
            if exc.code not in (429,500,502,503,504) or attempt==2:
                exc.close()
                raise
            retry_after=exc.headers.get('Retry-After','1');exc.close()
            try:retry_after=int(retry_after)
            except ValueError:retry_after=5
            time.sleep(min(30,max(2**attempt,retry_after)))
        except (urllib.error.URLError,TimeoutError,ConnectionError,IncompleteRead,ssl.SSLError) as exc:
            reason=getattr(exc,'reason',exc)
            if isinstance(reason,ssl.SSLCertVerificationError) or attempt==2:raise
            time.sleep(min(30,2**(attempt+1)))