Feature: add export compression policy module

src/documents/export/compression.py

@@ -0,0 +1,101 @@
+from __future__ import annotations
+
+import importlib
+import zipfile
+
+# ZIP_ZSTANDARD exists only on Python 3.14+ (PEP 784). None elsewhere.
+ZSTD: int | None = getattr(zipfile, "ZIP_ZSTANDARD", None)
+
+# CLI choices are fixed across runtimes so argparse never hides zstd; runtime
+# availability is enforced separately in compression_available().
+COMPRESSION_CHOICES: tuple[str, ...] = (
+    "stored",
+    "deflated",
+    "bzip2",
+    "lzma",
+    "zstd",
+)
+
+# Method name -> zipfile compression constant (zstd only when supported).
+COMPRESSION_METHODS: dict[str, int] = {
+    "stored": zipfile.ZIP_STORED,
+    "deflated": zipfile.ZIP_DEFLATED,
+    "bzip2": zipfile.ZIP_BZIP2,
+    "lzma": zipfile.ZIP_LZMA,
+}
+if ZSTD is not None:
+    COMPRESSION_METHODS["zstd"] = ZSTD
+
+# Inclusive (min, max) level bounds per method; None => level not applicable.
+# Verified on CPython 3.14.3.
+LEVEL_BOUNDS: dict[str, tuple[int, int] | None] = {
+    "stored": None,
+    "deflated": (0, 9),
+    "bzip2": (1, 9),
+    "lzma": None,
+    "zstd": (-131072, 22),
+}
+
+# zipfile compress_type id -> method name. 93 = current zstd id, 20 = legacy
+# zstd id that zipfile can still read.
+_COMPRESS_TYPE_TO_METHOD: dict[int, str] = {
+    zipfile.ZIP_STORED: "stored",
+    zipfile.ZIP_DEFLATED: "deflated",
+    zipfile.ZIP_BZIP2: "bzip2",
+    zipfile.ZIP_LZMA: "lzma",
+    93: "zstd",
+    20: "zstd",
+}
+
+
+def compression_available(method: str) -> bool:
+    """Whether the running interpreter can actually use the given method."""
+    if method in ("stored", "deflated"):
+        # zlib is a hard CPython dependency; stored needs nothing.
+        return True
+    if method == "bzip2":
+        return _module_importable("bz2")
+    if method == "lzma":
+        return _module_importable("lzma")
+    if method == "zstd":
+        return ZSTD is not None and _module_importable("compression.zstd")
+    return False
+
+
+def _module_importable(name: str) -> bool:
+    try:
+        importlib.import_module(name)
+    except ImportError:
+        return False
+    return True
+
+
+def level_error(method: str, level: int | None) -> str | None:
+    """Return a human message if (method, level) is invalid, else None."""
+    if level is None:
+        return None
+    bounds = LEVEL_BOUNDS[method]
+    if bounds is None:
+        return f"--zip-compression-level has no effect for '{method}'"
+    low, high = bounds
+    if not (low <= level <= high):
+        return (
+            f"--zip-compression-level for '{method}' must be between {low} and {high}"
+        )
+    return None
+
+
+def compress_type_readable(compress_type: int) -> bool:
+    """Whether this interpreter can decompress an entry of the given type."""
+    method = _COMPRESS_TYPE_TO_METHOD.get(compress_type)
+    if method is None:
+        return False
+    return compression_available(method)
+
+
+def unreadable_method_names(compress_types: set[int]) -> set[str]:
+    """Map a set of compress_type ids to human method names for error messages."""
+    names: set[str] = set()
+    for ct in compress_types:
+        names.add(_COMPRESS_TYPE_TO_METHOD.get(ct, f"method {ct}"))
+    return names

src/documents/tests/export/test_compression.py

@@ -0,0 +1,98 @@
+import sys
+import zipfile
+
+import pytest
+
+from documents.export import compression
+
+
+class TestCompressionMethods:
+    def test_choices_always_include_zstd(self) -> None:
+        # zstd is offered regardless of runtime; availability is checked separately
+        assert compression.COMPRESSION_CHOICES == (
+            "stored",
+            "deflated",
+            "bzip2",
+            "lzma",
+            "zstd",
+        )
+
+    @pytest.mark.parametrize(
+        ("name", "constant"),
+        [
+            ("stored", zipfile.ZIP_STORED),
+            ("deflated", zipfile.ZIP_DEFLATED),
+            ("bzip2", zipfile.ZIP_BZIP2),
+            ("lzma", zipfile.ZIP_LZMA),
+        ],
+    )
+    def test_method_maps_to_zipfile_constant(self, name: str, constant: int) -> None:
+        assert compression.COMPRESSION_METHODS[name] == constant
+
+    def test_stored_and_deflated_always_available(self) -> None:
+        assert compression.compression_available("stored")
+        assert compression.compression_available("deflated")
+
+    def test_zstd_availability_tracks_runtime(self) -> None:
+        expected: bool = sys.version_info >= (3, 14)
+        assert compression.compression_available("zstd") == expected
+
+
+class TestLevelError:
+    @pytest.mark.parametrize(
+        ("method", "level"),
+        [
+            ("deflated", 0),
+            ("deflated", 9),
+            ("bzip2", 1),
+            ("bzip2", 9),
+            ("deflated", None),
+            ("stored", None),
+        ],
+    )
+    def test_valid_levels_return_none(self, method: str, level: int | None) -> None:
+        assert compression.level_error(method, level) is None
+
+    @pytest.mark.parametrize(
+        ("method", "level"),
+        [
+            ("deflated", 10),
+            ("deflated", -1),
+            ("bzip2", 0),
+            ("bzip2", 10),
+        ],
+    )
+    def test_out_of_range_levels_return_message(
+        self,
+        method: str,
+        level: int,
+    ) -> None:
+        msg: str | None = compression.level_error(method, level)
+        assert msg is not None
+        assert "between" in msg
+
+    @pytest.mark.parametrize("method", ["stored", "lzma"])
+    def test_level_on_levelless_method_is_rejected(self, method: str) -> None:
+        msg: str | None = compression.level_error(method, 5)
+        assert msg is not None
+        assert "no effect" in msg
+
+
+class TestCompressTypeReadable:
+    @pytest.mark.parametrize("ct", [zipfile.ZIP_STORED, zipfile.ZIP_DEFLATED])
+    def test_stored_and_deflated_always_readable(self, ct: int) -> None:
+        assert compression.compress_type_readable(ct)
+
+    def test_zstd_compress_type_readability_tracks_runtime(self) -> None:
+        # 93 = ZIP_ZSTANDARD; 20 = legacy zstd method id (read-only)
+        expected: bool = sys.version_info >= (3, 14)
+        assert compression.compress_type_readable(93) == expected
+        assert compression.compress_type_readable(20) == expected
+
+    def test_unknown_compress_type_is_unreadable(self) -> None:
+        assert not compression.compress_type_readable(9999)
+
+    def test_unreadable_method_names_lists_methods(self) -> None:
+        # An unknown method id maps to no name and is reported generically.
+        names: set[str] = compression.unreadable_method_names({9999})
+        assert names == {"method 9999"}