"""Tests for Kompress compressor.

Covers:
- Lazy imports: module importable without torch installed
- is_kompress_available(): correct detection of [ml] extra
- KompressConfig / KompressResult: dataclass defaults
- KompressCompressor: passthrough for short content, fallback on error
- Transform interface: apply() method
"""

import logging
from types import SimpleNamespace
from unittest.mock import MagicMock, patch

# ── Import safety (the whole point of the fix) ─────────────────────────


class TestLazyImports:
    """The module must be importable without torch/transformers."""

    def test_is_kompress_available_importable(self) -> None:
        """is_kompress_available can be imported even without torch."""
        from headroom.transforms.kompress_compressor import is_kompress_available

        # Should return bool (True or False depending on environment)
        result = is_kompress_available()
        assert isinstance(result, bool)

    def test_module_import_without_torch(self) -> None:
        """Importing the module with torch blocked should not raise."""
        import sys

        # Block torch AND onnxruntime imports
        with patch.dict(
            sys.modules,
            {"torch": None, "torch.nn": None, "onnxruntime": None},
        ):
            from headroom.transforms.kompress_compressor import (
                _is_pytorch_available,
            )

            # Without both torch and onnxruntime, should return False
            assert _is_pytorch_available() is False
            # Note: is_kompress_available() may still return True if onnxruntime
            # was already imported before patching. Test the individual checkers.

    def test_dataclasses_importable_without_torch(self) -> None:
        """KompressConfig, KompressResult, KompressCompressor are importable without torch."""
        from headroom.transforms.kompress_compressor import (
            KompressCompressor,  # noqa: F401
            KompressConfig,
            KompressResult,
        )

        # These don't need torch to instantiate
        config = KompressConfig()
        assert config.device == "auto"
        assert config.enable_ccr is True

        result = KompressResult(
            compressed="hello",
            original="hello world",
            original_tokens=2,
            compressed_tokens=1,
            compression_ratio=0.5,
        )
        assert result.tokens_saved == 1
        assert result.savings_percentage == 50.0


class TestKompressBackendSelection:
    def test_selected_backend_aliases(self, monkeypatch) -> None:
        import headroom.transforms.kompress_compressor as kmod

        monkeypatch.setenv("HEADROOM_KOMPRESS_BACKEND", "mps")
        assert kmod._selected_backend() == "pytorch_mps"

        monkeypatch.setenv("HEADROOM_KOMPRESS_BACKEND", "coreml")
        assert kmod._selected_backend() == "onnx_coreml"

        monkeypatch.setenv("HEADROOM_KOMPRESS_BACKEND", "cpu")
        assert kmod._selected_backend() == "onnx_cpu"

        monkeypatch.setenv("HEADROOM_KOMPRESS_BACKEND", "unknown")
        assert kmod._selected_backend() == "auto"

    def test_unrecognized_backend_warns_and_falls_back_to_auto(self, monkeypatch, caplog) -> None:
        import headroom.transforms.kompress_compressor as kmod

        monkeypatch.setenv("HEADROOM_KOMPRESS_BACKEND", "tpu")
        with caplog.at_level(logging.WARNING, logger=kmod.logger.name):
            assert kmod._selected_backend() == "auto"

        assert any(
            "unrecognized" in record.getMessage() and "tpu" in record.getMessage()
            for record in caplog.records
        )

    def test_valid_backend_values_do_not_warn(self, monkeypatch, caplog) -> None:
        import headroom.transforms.kompress_compressor as kmod

        with caplog.at_level(logging.WARNING, logger=kmod.logger.name):
            for value in ("auto", "onnx", "cpu", "coreml", "mps", "torch", "ONNX-CPU"):
                monkeypatch.setenv("HEADROOM_KOMPRESS_BACKEND", value)
                kmod._selected_backend()
            monkeypatch.delenv("HEADROOM_KOMPRESS_BACKEND", raising=False)
            kmod._selected_backend()

        assert not caplog.records

    def test_forced_pytorch_mps_backend_uses_mps_device(self, monkeypatch) -> None:
        import headroom.transforms.kompress_compressor as kmod

        calls: list[tuple[str, str]] = []
        monkeypatch.setenv("HEADROOM_KOMPRESS_BACKEND", "pytorch_mps")
        monkeypatch.setattr(kmod, "_kompress_cache", {})
        monkeypatch.setattr(
            kmod,
            "_load_kompress_pytorch",
            lambda model_id, device, *, allow_download=True: (
                calls.append((model_id, device)) or ("model", "tokenizer", "pytorch")
            ),
        )

        assert kmod._load_kompress("model-a", device="auto") == ("model", "tokenizer", "pytorch")
        assert calls == [("model-a", "mps")]

    def test_forced_coreml_backend_uses_onnx_coreml(self, monkeypatch) -> None:
        import headroom.transforms.kompress_compressor as kmod

        calls: list[tuple[str, bool]] = []
        monkeypatch.setenv("HEADROOM_KOMPRESS_BACKEND", "onnx_coreml")
        monkeypatch.setattr(kmod, "_kompress_cache", {})
        monkeypatch.setattr(
            kmod,
            "_load_kompress_onnx",
            lambda model_id, *, use_coreml=False, allow_download=True: (
                calls.append((model_id, use_coreml)) or ("model", "tokenizer", "onnx_coreml")
            ),
        )

        assert kmod._load_kompress("model-b") == ("model", "tokenizer", "onnx_coreml")
        assert calls == [("model-b", True)]

    def test_auto_backend_preserves_onnx_first(self, monkeypatch) -> None:
        import headroom.transforms.kompress_compressor as kmod

        calls: list[str] = []
        monkeypatch.delenv("HEADROOM_KOMPRESS_BACKEND", raising=False)
        monkeypatch.setattr(kmod, "_kompress_cache", {})
        monkeypatch.setattr(kmod, "_is_onnx_available", lambda: True)
        monkeypatch.setattr(kmod, "_is_pytorch_available", lambda: True)
        monkeypatch.setattr(
            kmod,
            "_load_kompress_onnx",
            lambda model_id, *, use_coreml=False, allow_download=True: (
                calls.append("onnx") or ("model", "tokenizer", "onnx")
            ),
        )
        monkeypatch.setattr(
            kmod,
            "_load_kompress_pytorch",
            lambda model_id, device, *, allow_download=True: (
                calls.append("pytorch") or ("model", "tokenizer", "pytorch")
            ),
        )

        assert kmod._load_kompress("model-c") == ("model", "tokenizer", "onnx")
        assert calls == ["onnx"]

    def test_onnx_session_options_read_thread_caps(self, monkeypatch) -> None:
        import headroom.transforms.kompress_compressor as kmod

        created: list[SimpleNamespace] = []

        class FakeSessionOptions:
            def __init__(self) -> None:
                self.intra_op_num_threads = None
                self.inter_op_num_threads = None
                self.enable_cpu_mem_arena = True
                self.enable_mem_pattern = True

        fake_ort = SimpleNamespace(
            SessionOptions=lambda: created.append(FakeSessionOptions()) or created[-1]
        )
        monkeypatch.setenv("HEADROOM_KOMPRESS_ONNX_INTRA_THREADS", "2")
        monkeypatch.setenv("HEADROOM_KOMPRESS_ONNX_INTER_THREADS", "1")

        options = kmod._onnx_session_options(fake_ort)

        assert options.intra_op_num_threads == 2
        assert options.inter_op_num_threads == 1
        assert options.enable_cpu_mem_arena is False
        assert options.enable_mem_pattern is False


# ── KompressResult ──────────────────────────────────────────────────────


class TestKompressResult:
    def test_tokens_saved(self) -> None:
        from headroom.transforms.kompress_compressor import KompressResult

        r = KompressResult(
            compressed="a b",
            original="a b c d",
            original_tokens=4,
            compressed_tokens=2,
            compression_ratio=0.5,
        )
        assert r.tokens_saved == 2

    def test_tokens_saved_no_negative(self) -> None:
        from headroom.transforms.kompress_compressor import KompressResult

        r = KompressResult(
            compressed="a b c d e",
            original="a b c",
            original_tokens=3,
            compressed_tokens=5,
            compression_ratio=1.67,
        )
        assert r.tokens_saved == 0

    def test_savings_percentage_zero_tokens(self) -> None:
        from headroom.transforms.kompress_compressor import KompressResult

        r = KompressResult(
            compressed="",
            original="",
            original_tokens=0,
            compressed_tokens=0,
            compression_ratio=1.0,
        )
        assert r.savings_percentage == 0.0

    def test_default_model(self) -> None:
        from headroom.transforms.kompress_compressor import HF_MODEL_ID, KompressResult

        r = KompressResult(
            compressed="x",
            original="x y",
            original_tokens=2,
            compressed_tokens=1,
            compression_ratio=0.5,
        )
        assert r.model_used == HF_MODEL_ID


# ── KompressCompressor (without model) ──────────────────────────────────


class TestKompressCompressorPassthrough:
    """Test compressor behavior that doesn't require the actual model."""

    def test_short_content_passthrough(self) -> None:
        """Content under 10 words should pass through unchanged."""
        from headroom.transforms.kompress_compressor import KompressCompressor

        compressor = KompressCompressor()
        result = compressor.compress("hello world")
        assert result.compressed == "hello world"
        assert result.compression_ratio == 1.0
        assert result.original_tokens == 2
        assert result.compressed_tokens == 2

    def test_empty_content_passthrough(self) -> None:
        from headroom.transforms.kompress_compressor import KompressCompressor

        compressor = KompressCompressor()
        result = compressor.compress("")
        assert result.compressed == ""
        assert result.compression_ratio == 1.0

    def test_fallback_on_model_error(self) -> None:
        """If _load_kompress fails, compress should return passthrough."""
        from headroom.transforms.kompress_compressor import KompressCompressor

        compressor = KompressCompressor()
        long_text = " ".join(f"word{i}" for i in range(20))

        with patch(
            "headroom.transforms.kompress_compressor._load_kompress",
            side_effect=RuntimeError("no model"),
        ):
            result = compressor.compress(long_text)
            assert result.compressed == long_text
            assert result.compression_ratio == 1.0


# ── Transform interface ─────────────────────────────────────────────────


class TestKompressTransformInterface:
    def test_apply_short_messages_unchanged(self) -> None:
        """Messages with <10 words should pass through apply() unchanged."""
        from headroom.transforms.kompress_compressor import KompressCompressor

        compressor = KompressCompressor()
        messages = [
            {"role": "user", "content": "hello"},
            {"role": "tool", "content": "short"},
        ]
        tokenizer = MagicMock()
        tokenizer.count_text = MagicMock(return_value=5)

        result = compressor.apply(messages, tokenizer)
        assert len(result.messages) == 2
        assert result.messages[0]["content"] == "hello"
        assert result.messages[1]["content"] == "short"

    def test_apply_preserves_user_messages(self) -> None:
        """User messages should never be compressed."""
        from headroom.transforms.kompress_compressor import KompressCompressor

        compressor = KompressCompressor()
        long_text = " ".join(f"word{i}" for i in range(50))
        messages = [{"role": "user", "content": long_text}]
        tokenizer = MagicMock()
        tokenizer.count_text = MagicMock(return_value=50)

        with patch(
            "headroom.transforms.kompress_compressor._load_kompress",
            side_effect=RuntimeError("should not be called"),
        ):
            result = compressor.apply(messages, tokenizer)
            assert result.messages[0]["content"] == long_text


# ── compress_batch ──────────────────────────────────────────────────────


class TestKompressCompressorBatch:
    """Tests for the batched compression API (compress_batch).

    These exercise the non-model paths — passthrough handling, argument
    validation, order preservation, and fallback behavior on model-load
    failure. The actual batched inference path is covered by integration
    tests that require the model to be downloaded.
    """

    def test_empty_batch_returns_empty_list(self) -> None:
        from headroom.transforms.kompress_compressor import KompressCompressor

        compressor = KompressCompressor()
        result = compressor.compress_batch([])
        assert result == []

    def test_all_short_texts_passthrough_without_model(self) -> None:
        """Texts under 10 words must passthrough; model never loaded."""
        from headroom.transforms.kompress_compressor import KompressCompressor

        compressor = KompressCompressor()
        contents = ["hello", "world", "short text here"]

        with patch(
            "headroom.transforms.kompress_compressor._load_kompress",
            side_effect=AssertionError("model should not be loaded for short texts"),
        ):
            results = compressor.compress_batch(contents)

        assert len(results) == 3
        for i, r in enumerate(results):
            assert r.compressed == contents[i]
            assert r.compression_ratio == 1.0

    def test_order_preserved(self) -> None:
        """Output order must match input order even when model load fails."""
        from headroom.transforms.kompress_compressor import KompressCompressor

        compressor = KompressCompressor()
        long_texts = [
            " ".join(f"alpha{i}" for i in range(20)),
            " ".join(f"beta{i}" for i in range(20)),
            " ".join(f"gamma{i}" for i in range(20)),
        ]

        with patch(
            "headroom.transforms.kompress_compressor._load_kompress",
            side_effect=RuntimeError("no model"),
        ):
            results = compressor.compress_batch(long_texts)

        assert len(results) == 3
        assert results[0].original.startswith("alpha0")
        assert results[1].original.startswith("beta0")
        assert results[2].original.startswith("gamma0")

    def test_mixed_short_and_long_passthrough_on_model_failure(self) -> None:
        """Short texts passthrough; long texts fall back to passthrough on model failure."""
        from headroom.transforms.kompress_compressor import KompressCompressor

        compressor = KompressCompressor()
        contents = [
            "short",
            " ".join(f"word{i}" for i in range(20)),  # triggers model path
            "also short",
        ]

        with patch(
            "headroom.transforms.kompress_compressor._load_kompress",
            side_effect=RuntimeError("no model"),
        ):
            results = compressor.compress_batch(contents)

        assert len(results) == 3
        assert results[0].compressed == "short"
        assert results[0].compression_ratio == 1.0
        assert results[1].compression_ratio == 1.0  # passthrough fallback
        assert results[2].compressed == "also short"

    def test_ratio_list_length_mismatch_raises(self) -> None:
        """If target_ratio is a list it must match contents length."""
        from headroom.transforms.kompress_compressor import KompressCompressor

        compressor = KompressCompressor()
        contents = ["a b c", "d e f"]

        # Too short
        try:
            compressor.compress_batch(contents, target_ratio=[0.5])
            raise AssertionError("expected ValueError for length mismatch")
        except ValueError as e:
            assert "length" in str(e).lower()

        # Too long
        try:
            compressor.compress_batch(contents, target_ratio=[0.5, 0.5, 0.5])
            raise AssertionError("expected ValueError for length mismatch")
        except ValueError as e:
            assert "length" in str(e).lower()

    def test_batch_of_one_equivalent_to_single_compress_on_short_text(self) -> None:
        """Batch-of-one with short text should produce identical passthrough."""
        from headroom.transforms.kompress_compressor import KompressCompressor

        compressor = KompressCompressor()
        text = "hello world"

        single = compressor.compress(text)
        batch = compressor.compress_batch([text])

        assert len(batch) == 1
        assert batch[0].compressed == single.compressed
        assert batch[0].compression_ratio == single.compression_ratio
        assert batch[0].original_tokens == single.original_tokens

    def test_uniform_ratio_scalar(self) -> None:
        """A scalar target_ratio must apply to every text in the batch."""
        from headroom.transforms.kompress_compressor import KompressCompressor

        compressor = KompressCompressor()
        # Short texts — passthrough regardless of ratio
        contents = ["short a", "short b", "short c"]

        results = compressor.compress_batch(contents, target_ratio=0.3)

        assert len(results) == 3
        for r, original in zip(results, contents, strict=True):
            assert r.compressed == original  # short passthrough

    def test_per_item_ratio_list_with_nones(self) -> None:
        """A list of ratios with some None entries must be accepted."""
        from headroom.transforms.kompress_compressor import KompressCompressor

        compressor = KompressCompressor()
        contents = ["short a", "short b", "short c"]
        ratios: list[float | None] = [0.5, None, 0.25]

        # Short texts always passthrough; validating the list shape alone.
        results = compressor.compress_batch(contents, target_ratio=ratios)
        assert len(results) == 3


# ── unload_kompress_model ───────────────────────────────────────────────


class TestUnloadKompressModel:
    def test_unload_when_no_model(self) -> None:
        import headroom.transforms.kompress_compressor as kmod
        from headroom.transforms.kompress_compressor import unload_kompress_model

        # Ensure no model is loaded (previous tests may have set the cache)
        kmod._kompress_cache.clear()

        # Should return False when no model is loaded
        assert unload_kompress_model() is False


# ── onnx_coreml backend gating (issue #2442) ────────────────────────────


class TestOnnxBackendPrefixGating:
    """Non-CPU ONNX backends (onnx_coreml, onnx_cpu) must take the ONNX path.

    The bug: sites gated on the exact string ``backend == "onnx"`` misclassified
    ``onnx_coreml`` as PyTorch and called ``next(model.parameters())`` on the
    ``_OnnxModel`` wrapper, which has no ``.parameters()`` — crashing every call
    and silently disabling Kompress. The fix uses ``backend.startswith("onnx")``.
    """

    class _FakeOnnxModel:
        """Mimics the ONNX wrapper: has get_keep_mask but no .parameters()."""

        def get_keep_mask(self, input_ids, attention_mask):  # noqa: ANN001, ANN201
            return [[True]]

    @staticmethod
    def _fake_tokenizer(words, **kwargs):  # noqa: ANN001, ANN205
        # ONNX path must request numpy tensors, never torch.
        assert kwargs.get("return_tensors") == "np"
        return {"input_ids": [[1, 2]], "attention_mask": [[1, 1]]}

    def test_timed_canary_onnx_coreml_skips_pytorch_device_dispatch(self) -> None:
        from headroom.transforms.kompress_compressor import KompressCompressor

        compressor = KompressCompressor()
        model = self._FakeOnnxModel()  # no .parameters()

        # Must not raise AttributeError: '_OnnxModel' object has no attribute
        # 'parameters'; returns a float wall-clock duration.
        elapsed = compressor._timed_canary(model, self._fake_tokenizer, "onnx_coreml")
        assert isinstance(elapsed, float)

    def test_timed_canary_pytorch_still_dispatches_to_device(self) -> None:
        # Negative control: the PyTorch branch DOES touch .parameters(), so the
        # paramless fake model raises there — proving the test above is only
        # green because onnx_coreml correctly skips that branch.
        import pytest

        from headroom.transforms.kompress_compressor import KompressCompressor

        compressor = KompressCompressor()
        model = self._FakeOnnxModel()

        def pt_tokenizer(words, **kwargs):  # noqa: ANN001, ANN202
            assert kwargs.get("return_tensors") == "pt"
            return {"input_ids": [[1, 2]], "attention_mask": [[1, 1]]}

        with pytest.raises(AttributeError):
            compressor._timed_canary(model, pt_tokenizer, "pytorch")


class TestPytorchWeightLoading:
    """_load_pytorch_weights must load the merged v2 checkpoint format correctly,
    fall back to the plain format only when the repo genuinely has no merged.pt,
    and refuse to run on a state-dict mismatch instead of silently ignoring it.
    """

    @staticmethod
    def _make_model(torch):
        import torch.nn as nn

        model = nn.Module()
        model.encoder = nn.Linear(4, 4)
        model.token_head = nn.Linear(4, 2)
        model.span_conv = nn.Sequential(nn.Conv1d(4, 4, 1), nn.GELU())
        return model

    def test_merged_checkpoint_loads_into_matching_submodules(self, tmp_path, monkeypatch) -> None:
        import pytest

        torch = pytest.importorskip("torch")
        import headroom.transforms.kompress_compressor as kmod

        model = self._make_model(torch)
        ckpt_path = tmp_path / "merged.pt"
        torch.save(
            {
                "encoder_state_dict": model.encoder.state_dict(),
                "token_head_state_dict": model.token_head.state_dict(),
                "span_conv_state_dict": model.span_conv.state_dict(),
            },
            ckpt_path,
        )

        fresh_model = self._make_model(torch)
        monkeypatch.setattr(kmod, "hf_hub_download_local_first", lambda *a, **k: str(ckpt_path))

        kmod._load_pytorch_weights(fresh_model, "some/repo", allow_download=True)

        for name, param in model.encoder.state_dict().items():
            assert torch.equal(param, fresh_model.encoder.state_dict()[name])

    def test_merged_checkpoint_missing_section_raises(self, tmp_path, monkeypatch) -> None:
        import pytest

        torch = pytest.importorskip("torch")
        import headroom.transforms.kompress_compressor as kmod

        ckpt_path = tmp_path / "merged.pt"
        torch.save({"encoder_state_dict": {}}, ckpt_path)

        fresh_model = self._make_model(torch)
        monkeypatch.setattr(kmod, "hf_hub_download_local_first", lambda *a, **k: str(ckpt_path))

        with pytest.raises(RuntimeError, match="missing"):
            kmod._load_pytorch_weights(fresh_model, "some/repo", allow_download=True)

    def test_merged_checkpoint_key_mismatch_raises_instead_of_silently_dropping(
        self, tmp_path, monkeypatch
    ) -> None:
        """Regression test for the bug this loader used to have: loading a
        state-dict that does not match the module tree (e.g. an unmerged PEFT
        checkpoint) must fail loudly, not silently skip the mismatched keys.
        """
        import pytest

        torch = pytest.importorskip("torch")
        import headroom.transforms.kompress_compressor as kmod

        ckpt_path = tmp_path / "merged.pt"
        torch.save(
            {
                # Wrong prefix, mimics the unmerged PEFT structure documented
                # in scripts/export_kompress_v2_onnx.py.
                "encoder_state_dict": {"base_model.model.weight": torch.zeros(4, 4)},
                "token_head_state_dict": {},
                "span_conv_state_dict": {},
            },
            ckpt_path,
        )

        fresh_model = self._make_model(torch)
        monkeypatch.setattr(kmod, "hf_hub_download_local_first", lambda *a, **k: str(ckpt_path))

        with pytest.raises(RuntimeError, match="state_dict mismatch"):
            kmod._load_pytorch_weights(fresh_model, "some/repo", allow_download=True)

    def test_missing_merged_pt_falls_back_to_plain_safetensors(self, tmp_path, monkeypatch) -> None:
        import pytest

        torch = pytest.importorskip("torch")
        safetensors_torch = pytest.importorskip("safetensors.torch")
        import headroom.transforms.kompress_compressor as kmod

        model = self._make_model(torch)
        weights_path = tmp_path / "model.safetensors"
        safetensors_torch.save_file(dict(model.state_dict()), str(weights_path))

        fresh_model = self._make_model(torch)

        def fake_download(model_id, filename, *, allow_network=True, **kwargs):  # noqa: ANN001
            if filename == "merged.pt":
                raise kmod.EntryNotFoundError("no merged.pt in this repo")
            assert filename == "model.safetensors"
            return str(weights_path)

        monkeypatch.setattr(kmod, "hf_hub_download_local_first", fake_download)

        kmod._load_pytorch_weights(fresh_model, "some/v1/repo", allow_download=True)

        for name, param in model.state_dict().items():
            assert torch.equal(param, fresh_model.state_dict()[name])

    def test_plain_safetensors_key_mismatch_raises(self, tmp_path, monkeypatch) -> None:
        import pytest

        torch = pytest.importorskip("torch")
        safetensors_torch = pytest.importorskip("safetensors.torch")
        import headroom.transforms.kompress_compressor as kmod

        weights_path = tmp_path / "model.safetensors"
        safetensors_torch.save_file({"totally.unrelated.key": torch.zeros(2)}, str(weights_path))

        fresh_model = self._make_model(torch)

        def fake_download(model_id, filename, *, allow_network=True, **kwargs):  # noqa: ANN001
            if filename == "merged.pt":
                raise kmod.EntryNotFoundError("no merged.pt in this repo")
            return str(weights_path)

        monkeypatch.setattr(kmod, "hf_hub_download_local_first", fake_download)

        with pytest.raises(RuntimeError, match="state_dict mismatch"):
            kmod._load_pytorch_weights(fresh_model, "some/v1/repo", allow_download=True)

    def test_cache_only_miss_raises_kompress_model_not_cached(self, monkeypatch) -> None:
        import pytest

        pytest.importorskip("torch")
        import headroom.transforms.kompress_compressor as kmod

        def fake_download(model_id, filename, *, allow_network=True, **kwargs):  # noqa: ANN001
            raise kmod.LocalEntryNotFoundError("not cached")

        monkeypatch.setattr(kmod, "hf_hub_download_local_first", fake_download)
        monkeypatch.setattr(kmod, "hf_entry_known_absent", lambda *a, **k: False)

        with pytest.raises(kmod.KompressModelNotCached):
            kmod._load_pytorch_weights(SimpleNamespace(), "some/repo", allow_download=False)

    def test_cache_only_defers_instead_of_using_stale_plain_checkpoint(self, monkeypatch) -> None:
        """Regression test: if merged.pt is not cached yet and we have no
        confirmation it is genuinely absent upstream, a stale model.safetensors
        left over from a previous (pre-fix) run must NOT be used as a silent
        fallback - that would reintroduce the original bug for exactly the
        upgrade scenario that motivated this fix. It must defer instead.
        """
        import pytest

        pytest.importorskip("torch")
        import headroom.transforms.kompress_compressor as kmod

        plain_download_calls: list[str] = []

        def fake_download(model_id, filename, *, allow_network=True, **kwargs):  # noqa: ANN001
            if filename == "merged.pt":
                raise kmod.LocalEntryNotFoundError("merged.pt not cached yet")
            plain_download_calls.append(filename)
            return "/fake/cached/model.safetensors"

        monkeypatch.setattr(kmod, "hf_hub_download_local_first", fake_download)
        # Nothing has ever confirmed merged.pt is absent from this repo -
        # simulates a v2-style repo mid-upgrade, not a v1-style repo.
        monkeypatch.setattr(kmod, "hf_entry_known_absent", lambda *a, **k: False)

        with pytest.raises(kmod.KompressModelNotCached):
            kmod._load_pytorch_weights(
                SimpleNamespace(), "chopratejas/kompress-v2-base", allow_download=False
            )

        assert plain_download_calls == [], (
            "must not fall back to model.safetensors without confirming merged.pt is absent"
        )

    def test_cache_only_uses_plain_checkpoint_when_merged_pt_confirmed_absent(
        self, tmp_path, monkeypatch
    ) -> None:
        import pytest

        torch = pytest.importorskip("torch")
        safetensors_torch = pytest.importorskip("safetensors.torch")
        import headroom.transforms.kompress_compressor as kmod

        model = self._make_model(torch)
        weights_path = tmp_path / "model.safetensors"
        safetensors_torch.save_file(dict(model.state_dict()), str(weights_path))

        fresh_model = self._make_model(torch)

        def fake_download(model_id, filename, *, allow_network=True, **kwargs):  # noqa: ANN001
            if filename == "merged.pt":
                raise kmod.LocalEntryNotFoundError("merged.pt not cached")
            return str(weights_path)

        monkeypatch.setattr(kmod, "hf_hub_download_local_first", fake_download)
        # A prior real network lookup already confirmed this repo has no
        # merged.pt at all (the v1-style case), so the plain fallback is safe.
        monkeypatch.setattr(kmod, "hf_entry_known_absent", lambda *a, **k: True)

        kmod._load_pytorch_weights(fresh_model, "chopratejas/kompress-base", allow_download=False)

        for name, param in model.state_dict().items():
            assert torch.equal(param, fresh_model.state_dict()[name])

    def test_cache_only_raises_when_confirmed_absent_but_plain_also_missing(
        self, monkeypatch
    ) -> None:
        """merged.pt confirmed absent, but the plain fallback isn't cached either:
        still nothing to load from, so this must defer rather than raise a
        confusing lower-level error.
        """
        import pytest

        pytest.importorskip("torch")
        import headroom.transforms.kompress_compressor as kmod

        def fake_download(model_id, filename, *, allow_network=True, **kwargs):  # noqa: ANN001
            raise kmod.LocalEntryNotFoundError(f"{filename} not cached")

        monkeypatch.setattr(kmod, "hf_hub_download_local_first", fake_download)
        monkeypatch.setattr(kmod, "hf_entry_known_absent", lambda *a, **k: True)

        with pytest.raises(kmod.KompressModelNotCached):
            kmod._load_pytorch_weights(
                SimpleNamespace(), "chopratejas/kompress-base", allow_download=False
            )

    def test_genuine_download_failure_propagates_instead_of_falling_back(self, monkeypatch) -> None:
        """A real network/download failure (not a 404, not a cache miss under
        allow_download=False) must propagate as-is, not be swallowed into a
        silent fallback to the plain format.
        """
        import pytest

        pytest.importorskip("torch")
        import headroom.transforms.kompress_compressor as kmod

        def fake_download(model_id, filename, *, allow_network=True, **kwargs):  # noqa: ANN001
            raise OSError("connection reset")

        monkeypatch.setattr(kmod, "hf_hub_download_local_first", fake_download)

        with pytest.raises(OSError, match="connection reset"):
            kmod._load_pytorch_weights(SimpleNamespace(), "some/repo", allow_download=True)


class TestLoadKompressPytorchCaching:
    def test_returns_cached_entry_without_reloading(self, monkeypatch) -> None:
        import pytest

        pytest.importorskip("torch")
        import headroom.transforms.kompress_compressor as kmod

        sentinel = ("cached-model", "cached-tokenizer", "pytorch")
        monkeypatch.setattr(kmod, "_kompress_cache", {"some/repo": sentinel})

        def boom(*a, **k):  # noqa: ANN001, ANN202
            raise AssertionError("should not attempt to reload an already-cached model")

        monkeypatch.setattr(kmod, "_load_pytorch_weights", boom)

        assert kmod._load_kompress_pytorch("some/repo") == sentinel