Add batch 2 support for purl to download URL

Signed-off-by: Tushar Goel <tushar.goel.dav@gmail.com>

Author: TG1999

src/fetchcode/cran.py

@@ -0,0 +1,42 @@
+# fetchcode is a free software tool from nexB Inc. and others.
+# Visit https://github.com/aboutcode-org/fetchcode for support and download.
+#
+# Copyright (c) nexB Inc. and others. All rights reserved.
+# http://nexb.com and http://aboutcode.org
+#
+# This software is licensed under the Apache License version 2.0.
+#
+# You may not use this software except in compliance with the License.
+# You may obtain a copy of the License at:
+# http://apache.org/licenses/LICENSE-2.0
+# Unless required by applicable law or agreed to in writing, software distributed
+# under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+# CONDITIONS OF ANY KIND, either express or implied. See the License for the
+# specific language governing permissions and limitations under the License.
+
+from packageurl import PackageURL
+
+from fetchcode.utils import _http_exists
+
+
+class Cran:
+    def get_download_url(purl: str):
+        """
+        Resolve a CRAN PURL to a verified, downloadable source tarball URL.
+        Tries current contrib first, then Archive.
+        """
+        p = PackageURL.from_string(purl)
+        if not p.name or not p.version:
+            return None
+
+        current_url = f"https://cran.r-project.org/src/contrib/{p.name}_{p.version}.tar.gz"
+        if _http_exists(current_url):
+            return current_url
+
+        archive_url = (
+            f"https://cran.r-project.org/src/contrib/Archive/{p.name}/{p.name}_{p.version}.tar.gz"
+        )
+        if _http_exists(archive_url):
+            return archive_url
+
+        return None

src/fetchcode/utils.py

@@ -243,3 +243,14 @@ def get_first_three_md5_hash_characters(podname):
     create a hash (using md5) of it and take the first three characters."
     """
     return md5_hasher(podname.encode("utf-8")).hexdigest()[0:3]
+
+
+def _http_exists(url: str) -> bool:
+    """
+    Lightweight existence check using a ranged GET so CDNs/servers that ignore HEAD still work.
+    """
+    try:
+        resp = make_head_request(url, headers={"Range": "bytes=0-0"})
+        return resp is not None and resp.status_code in (200, 206)
+    except Exception:
+        return False

tests/test_cran.py

@@ -0,0 +1,86 @@
+# fetchcode is a free software tool from nexB Inc. and others.
+# Visit https://github.com/aboutcode-org/fetchcode for support and download.
+#
+# Copyright (c) nexB Inc. and others. All rights reserved.
+# http://nexb.com and http://aboutcode.org
+#
+# This software is licensed under the Apache License version 2.0.
+#
+# You may not use this software except in compliance with the License.
+# You may obtain a copy of the License at:
+# http://apache.org/licenses/LICENSE-2.0
+# Unless required by applicable law or agreed to in writing, software distributed
+# under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+# CONDITIONS OF ANY KIND, either express or implied. See the License for the
+# specific language governing permissions and limitations under the License.
+
+from unittest.mock import patch
+
+import pytest
+
+from fetchcode.cran import Cran
+
+get_download_url = Cran.get_download_url
+
+
+@pytest.fixture
+def valid_purl():
+    return "pkg:cran/dplyr@1.0.0"
+
+
+def test_current_url_exists(valid_purl):
+    current_url = "https://cran.r-project.org/src/contrib/dplyr_1.0.0.tar.gz"
+
+    with patch("fetchcode.cran._http_exists", return_value=True) as mock_check:
+        result = get_download_url(valid_purl)
+        assert result == current_url
+        mock_check.assert_called_once_with(current_url)
+
+
+def test_fallback_to_archive(valid_purl):
+    current_url = "https://cran.r-project.org/src/contrib/dplyr_1.0.0.tar.gz"
+    archive_url = "https://cran.r-project.org/src/contrib/Archive/dplyr/dplyr_1.0.0.tar.gz"
+
+    def side_effect(url):
+        return url == archive_url
+
+    with patch("fetchcode.cran._http_exists", side_effect=side_effect) as mock_check:
+        result = get_download_url(valid_purl)
+        assert result == archive_url
+        assert mock_check.call_count == 2
+        mock_check.assert_any_call(current_url)
+        mock_check.assert_any_call(archive_url)
+
+
+def test_neither_url_exists(valid_purl):
+    with patch("fetchcode.cran._http_exists", return_value=False) as mock_check:
+        result = get_download_url(valid_purl)
+        assert result is None
+        assert mock_check.call_count == 2
+
+
+def test_missing_version_returns_none():
+    result = get_download_url("pkg:cran/dplyr")
+    assert result is None
+
+
+def test_version_with_dash():
+    purl = "pkg:cran/somepkg@1.2-3"
+
+    with patch("fetchcode.cran._http_exists", return_value=True) as mock_check:
+        result = get_download_url(purl)
+        assert result == "https://cran.r-project.org/src/contrib/somepkg_1.2-3.tar.gz"
+        mock_check.assert_called_once_with(
+            "https://cran.r-project.org/src/contrib/somepkg_1.2-3.tar.gz"
+        )
+
+
+def test_name_with_dot():
+    purl = "pkg:cran/foo.bar@2.0.1"
+
+    with patch("fetchcode.cran._http_exists", return_value=True) as mock_check:
+        result = get_download_url(purl)
+        assert result == "https://cran.r-project.org/src/contrib/foo.bar_2.0.1.tar.gz"
+        mock_check.assert_called_once_with(
+            "https://cran.r-project.org/src/contrib/foo.bar_2.0.1.tar.gz"
+        )