"""Retry transient network failures with exponential backoff.

Shared by the JSON client (POST) and the QCEW CSV client (GET). Retries only
genuinely transient conditions — connection drops, timeouts, and 429/5xx — never
4xx client errors or a BLS quota rejection (those won't fix themselves).
"""
import time

import requests

# HTTP statuses worth retrying (429 = throttled, 5xx = server-side transient).
RETRY_STATUS = {429, 500, 502, 503, 504}

# Network-level exceptions worth retrying.
RETRY_EXCEPTIONS = (
    requests.exceptions.ConnectionError,
    requests.exceptions.Timeout,
    requests.exceptions.ChunkedEncodingError,
)


def _retry_after(resp) -> float | None:
    """Honor a numeric Retry-After header if the server sent one."""
    ra = getattr(resp, "headers", {}).get("Retry-After") if resp is not None else None
    if ra and str(ra).isdigit():
        return float(ra)
    return None


def with_retries(send, *, retries=2, backoff=0.5, backoff_max=8.0, sleep=time.sleep):
    """
    Call ``send()`` (which returns a requests.Response), retrying transient
    failures. Backoff is ``backoff * 2**attempt`` seconds, capped at ``backoff_max``.

    Args:
        send:        zero-arg callable issuing the request and returning a Response.
        retries:     max retries after the first attempt (so retries=2 -> 3 tries).
        backoff:     base backoff in seconds.
        backoff_max: cap on a single backoff interval.
        sleep:       sleep function (injectable for tests).
    """
    attempt = 0
    while True:
        try:
            resp = send()
        except RETRY_EXCEPTIONS:
            if attempt >= retries:
                raise
            sleep(min(backoff * (2 ** attempt), backoff_max))
            attempt += 1
            continue

        if getattr(resp, "status_code", None) in RETRY_STATUS and attempt < retries:
            sleep(_retry_after(resp) or min(backoff * (2 ** attempt), backoff_max))
            attempt += 1
            continue

        return resp