"""
A seek on a copied stream reports where the picture actually begins.

Copied video has to start on a keyframe, so `-ss t` with `-c:v copy` delivers
the keyframe at or before `t` — up to a whole GOP earlier. The node reported
`t` regardless, and the client sets `SourceBuffer.timestampOffset` from that
number, so everything downstream believed the picture stood a few seconds
further along than it did.

That was a wrong label while only the scrubber read it. It became a wrong
*answer* when subtitles arrived: their cues carry the source's own absolute
timestamps, so the mismatch put every line on screen a GOP before it was
spoken. Measured on a real H264 title, seeking to 600 s, 2650 s and 5000 s
landed on keyframes 0.82 s, 1.56 s and 4.64 s earlier.

**The assertion is on the decoded picture, not on the number.** A test that
only compared `stream_init["start"]` against an expected keyframe would agree
with the implementation by construction — both would be reading the same
ffprobe. The first frame delivered is decoded and matched against the source
frame at the position the node claims, which is a statement about what was
served rather than about what was computed.
"""

import shutil
import subprocess
from pathlib import Path

import pytest
from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey

from meshbay_common.crypto import generate_gek
from meshbay_common.webcrypto import chunk_key_aes, decrypt_chunk_aes
from meshbay_node.indexer.group_index import GroupIndex
from meshbay_node.transport.webrtc_server import (
    WebRTCPeerSession,
    _keyframe_at_or_before,
)

from conftest import needs_subprocess, one_root

_HAVE_FFMPEG = shutil.which("ffmpeg") and shutil.which("ffprobe")
pytestmark = [
    pytest.mark.skipif(not _HAVE_FFMPEG, reason="ffmpeg/ffprobe not installed"),
    needs_subprocess,
]

# 25 fps with a keyframe every 250 frames — one every 10 s, which is what a
# real WEB-DL looks like and what makes the gap large enough to see.
_GOP_SECONDS = 10
_SEEK_TO = 15.0                  # between the keyframes at 10 s and 20 s
_EXPECTED_KEYFRAME = 10.0


def _make_h264_clip(path: Path) -> None:
    """30 s of H264 the streaming path will copy rather than re-encode.

    The picture has to differ from one second to the next, or a frame from the
    keyframe and a frame from the requested position would compare equal and
    the test would pass against the bug it exists for.
    """
    subprocess.run(
        ["ffmpeg", "-hide_banner", "-loglevel", "error", "-y",
         "-f", "lavfi", "-i", "testsrc=size=320x240:rate=25:duration=30",
         "-f", "lavfi", "-i", "sine=duration=30",
         "-c:v", "libx264", "-preset", "ultrafast", "-pix_fmt", "yuv420p",
         "-g", str(_GOP_SECONDS * 25), "-keyint_min", str(_GOP_SECONDS * 25),
         "-sc_threshold", "0",
         "-c:a", "aac", "-shortest", str(path)],
        check=True, capture_output=True)


def _session(video_path: Path, gek: bytes):
    import blake3
    file_bytes = video_path.read_bytes()
    file_id = blake3.blake3(file_bytes).hexdigest()

    sk_node = Ed25519PrivateKey.generate()
    index = GroupIndex(group_id="g" * 32, sk_node=sk_node, gek=gek)
    from meshbay_common.protocol import IndexEntry
    index.add_entry(IndexEntry(
        id=file_id, name=video_path.name, path=video_path.parent.name,
        size=len(file_bytes), type="video", added_at=0))

    session = WebRTCPeerSession.__new__(WebRTCPeerSession)
    session._ctx = {
        "roots": one_root(video_path.parent),
        "index": index,
        "gek": gek,
        "sk_node": sk_node,
        "max_concurrent_streams": 4,
    }
    session._group_id = None
    session._user_id = "tester"
    session._stream_stopped = False
    session._stream_keepalives = 0
    session.sent = []
    session._send = session.sent.append
    session._audit = lambda *a, **k: None
    return session, file_id


def _reassemble(sent: list[dict], gek: bytes, file_id: str) -> bytes:
    file_hash = bytes.fromhex(file_id)
    out = b""
    for m in sorted((m for m in sent if m.get("type") == "stream_data"),
                    key=lambda m: m["segment_index"]):
        key = chunk_key_aes(gek, file_hash, m["segment_index"])
        out += decrypt_chunk_aes(key, m["nonce"], m["ct"])
    return out


def _frame_md5(path: Path, at: float | None = None) -> str:
    """One decoded frame as raw pixels, hashed. `at` None means the first."""
    args = ["ffmpeg", "-hide_banner", "-loglevel", "error"]
    if at is not None:
        args += ["-ss", f"{at:.6f}"]
    args += ["-i", str(path), "-frames:v", "1", "-f", "rawvideo",
             "-pix_fmt", "rgb24", "-"]
    proc = subprocess.run(args, check=True, capture_output=True)
    import hashlib
    return hashlib.md5(proc.stdout).hexdigest()


@pytest.mark.asyncio
async def test_the_lookup_finds_the_keyframe_the_seek_will_land_on(tmp_path):
    clip = tmp_path / "clip.mp4"
    _make_h264_clip(clip)

    assert await _keyframe_at_or_before(clip, _SEEK_TO) == pytest.approx(
        _EXPECTED_KEYFRAME, abs=0.05)
    # A position that *is* a keyframe answers itself, not the one before.
    assert await _keyframe_at_or_before(clip, 20.0) == pytest.approx(20.0, abs=0.05)
    # Before the first one there is nothing earlier to find.
    assert await _keyframe_at_or_before(clip, 0) == 0.0


@pytest.mark.asyncio
async def test_a_seek_reports_where_the_picture_begins_not_where_it_was_asked(tmp_path):
    clip = tmp_path / "clip.mp4"
    _make_h264_clip(clip)
    gek = generate_gek()
    session, file_id = _session(clip, gek)

    await session._stream_video_inner(
        {"file_id": file_id, "start": _SEEK_TO, "credits": 0})

    errors = [m for m in session.sent if m.get("type") == "error"]
    assert not errors, errors
    init = next(m for m in session.sent if m.get("type") == "stream_init")

    assert init["start"] == pytest.approx(_EXPECTED_KEYFRAME, abs=0.05), (
        f"the node announced {init['start']}, but a copied stream cannot "
        f"begin anywhere but the keyframe at {_EXPECTED_KEYFRAME}")
    assert init["start"] < _SEEK_TO, (
        "this fixture must have a keyframe gap to report, or the test cannot "
        "tell the fix from the fault")


@pytest.mark.asyncio
async def test_the_announced_position_is_the_picture_that_was_served(tmp_path):
    """Decoded and compared against the source — see this module's docstring."""
    clip = tmp_path / "clip.mp4"
    _make_h264_clip(clip)
    gek = generate_gek()
    session, file_id = _session(clip, gek)

    await session._stream_video_inner(
        {"file_id": file_id, "start": _SEEK_TO, "credits": 0})

    init = next(m for m in session.sent if m.get("type") == "stream_init")
    served = tmp_path / "served.mp4"
    served.write_bytes(_reassemble(session.sent, gek, file_id))

    assert _frame_md5(served) == _frame_md5(clip, at=init["start"]), (
        "the first frame delivered is not the source frame at the position "
        "the node announced, so `start` still does not name what was served")
    # And it is emphatically not the frame at the position asked for, or the
    # comparison above would hold for the old behaviour too.
    assert _frame_md5(served) != _frame_md5(clip, at=_SEEK_TO), (
        "the fixture's picture does not change across the keyframe gap, so "
        "this comparison proves nothing")