Build alpm download URL

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

Author: TG1999

src/packageurl/contrib/purl2url.py

@@ -575,6 +575,21 @@ def _conda_base_for_channel(channel: str) -> str:
     return download_url
 
 
+@download_router.route("pkg:alpm/.*")
+def build_alpm_download_url(purl_str):
+    purl = PackageURL.from_string(purl_str)
+    name = purl.name
+    version = purl.version
+    arch = purl.qualifiers.get("arch", "any")
+
+    if not name or not version:
+        return None
+
+    first_letter = name[0]
+    url = f"https://archive.archlinux.org/packages/{first_letter}/{name}/{name}-{version}-{arch}.pkg.tar.zst"
+    return url
+
+
 def get_repo_download_url(purl):
     """
     Return ``download_url`` if present in ``purl`` qualifiers or