compatibility: Implement a version comparator.

zerver/views/compatibility.py

@@ -1,10 +1,53 @@
 
 from django.http import HttpResponse, HttpRequest
-from typing import Any, List, Dict, Optional
+import re
+from typing import Any, List, Dict, Optional, Tuple, Union
 
 from zerver.lib.response import json_error, json_success
 from zerver.lib.user_agent import parse_user_agent
 
+def pop_int(ver: str) -> Tuple[Optional[int], str]:
+    match = re.search(r'^(\d+)(.*)', ver)
+    if match is None:
+        return None, ver
+    return int(match.group(1)), match.group(2)
+
+def version_lt(ver1: str, ver2: str) -> Optional[bool]:
+    '''
+    Compare two Zulip-style version strings.
+
+    Versions are dot-separated sequences of decimal integers,
+    followed by arbitrary trailing decoration.  Comparison is
+    lexicographic on the integer sequences, and refuses to
+    guess how any trailing decoration compares to any other,
+    to further numerals, or to nothing.
+
+    Returns:
+      True if ver1 < ver2
+      False if ver1 >= ver2
+      None if can't tell.
+    '''
+    while True:
+        # Pull off the leading numeral from each version.
+        maj1, rest1 = pop_int(ver1)
+        maj2, rest2 = pop_int(ver2)
+        if maj1 is None or maj2 is None:
+            # One or both has no leading numeral; some unknown format.
+            return None
+        if maj1 < maj2:
+            return True
+        if maj1 > maj2:
+            return False
+        # Leading numerals are equal.
+
+        if rest1 == rest2:
+            return False
+        if not(rest1.startswith('.') and rest2.startswith('.')):
+            return None
+        ver1 = rest1[1:]
+        ver2 = rest2[1:]
+
+
 def check_global_compatibility(request: HttpRequest) -> HttpResponse:
     user_agent = parse_user_agent(request.META["HTTP_USER_AGENT"])
     if user_agent['name'] == "ZulipInvalid":