#!/usr/bin/env python3
"""Parallel HTTP range downloader with resume. Reads manifest.json entries
{url, start, length, out}; each entry resumes from existing partial file size.
"""
import json
import os
import sys
import time
import urllib.request
from concurrent.futures import ThreadPoolExecutor

CHUNK = 8 << 20


def download(e):
    out, url, start, length = e["out"], e["url"], e["start"], e["length"]
    os.makedirs(os.path.dirname(out), exist_ok=True)
    have = os.path.getsize(out) if os.path.exists(out) else 0
    if have >= length:
        return 0
    for attempt in range(8):
        try:
            req = urllib.request.Request(url)
            req.add_header("Range", f"bytes={start+have}-{start+length-1}")
            with urllib.request.urlopen(req, timeout=120) as r:
                with open(out, "ab") as f:
                    while True:
                        buf = r.read(CHUNK)
                        if not buf:
                            break
                        f.write(buf)
                        have += len(buf)
            if have >= length:
                return length
        except Exception as ex:
            time.sleep(min(2 ** attempt, 60))
            have = os.path.getsize(out) if os.path.exists(out) else 0
    raise RuntimeError(f"failed: {out} ({have}/{length})")


def main():
    manifest = json.load(open(sys.argv[1]))
    workers = int(sys.argv[2]) if len(sys.argv) > 2 else 24
    total = sum(e["length"] for e in manifest)
    done_bytes = 0
    t0 = time.time()
    failed = []

    def wrapped(e):
        try:
            download(e)
            return e["length"]
        except Exception as ex:
            failed.append((e["out"], str(ex)))
            return 0

    with ThreadPoolExecutor(workers) as ex:
        for i, n in enumerate(ex.map(wrapped, manifest)):
            done_bytes += n
            if i % 200 == 0 or i == len(manifest) - 1:
                el = time.time() - t0
                print(f"[{i+1}/{len(manifest)}] {done_bytes/1e9:.1f} GB "
                      f"({done_bytes/max(el,1)/1e6:.0f} MB/s avg)", flush=True)
    if failed:
        print("FAILED entries:")
        for o, m in failed[:20]:
            print(" ", o, m)
        sys.exit(1)
    # verify
    bad = [e for e in manifest
           if not os.path.exists(e["out"]) or os.path.getsize(e["out"]) < e["length"]]
    print(f"verify: {len(manifest)-len(bad)}/{len(manifest)} complete")
    sys.exit(1 if bad else 0)


if __name__ == "__main__":
    main()