Subclass semver for nginx

* Because nginx versions are not exactly semver.
* Also add convenience properties to the semver version class to avoid
  leaking internals too much.

Signed-off-by: Philippe Ombredanne <pombredanne@nexb.com>

Author: pombredanne

src/univers/version_range.py

@@ -857,7 +857,7 @@ class NginxVersionRange(VersionRange):
     """
 
     scheme = "nginx"
-    version_class = versions.SemverVersion
+    version_class = versions.NginxVersion
 
     vers_by_native_comparators = {
         "==": "=",
@@ -919,12 +919,10 @@ def from_native(cls, string):
                 # suffixed version
                 vs = clauses.rstrip("+")
                 version = cls.version_class(vs)
-                is_stable = is_even(version.value.minor)
-
-                if is_stable:
+                if version.is_stable:
                     # we have a start and end in stable ranges
                     start_version = cls.version_class(vs)
-                    end_version = cls.version_class(str(start_version.value.next_minor()))
+                    end_version = cls.version_class(str(start_version.next_minor()))
                     vstart = VersionConstraint(comparator=">=", version=start_version)
                     vend = VersionConstraint(comparator="<", version=end_version)
                     constraints.extend([vstart, vend])
@@ -1036,21 +1034,6 @@ def build_range_from_github_advisory_constraint(scheme: str, string: str):
     return vrc(constraints=constraints)
 
 
-def is_even(s):
-    """
-    Return True if the string "s" is an even number and False if this is an odd
-    number. For example:
-
-    >>> is_even(4)
-    True
-    >>> is_even(123)
-    False
-    >>> is_even(0)
-    True
-    """
-    return (int(s) % 2) == 0
-
-
 RANGE_CLASS_BY_SCHEMES = {
     "npm": NpmVersionRange,
     "deb": DebianVersionRange,

src/univers/versions.py

@@ -160,6 +160,7 @@ def __ge__(self, other):
 
 @attr.s(frozen=True, order=False, hash=True)
 class GenericVersion(Version):
+
     @classmethod
     def is_valid(cls, string):
         # generic implementation ...
@@ -219,6 +220,64 @@ def is_valid(cls, string):
         except ValueError:
             return False
 
+    @property
+    def major(self):
+        return self.value and self.value.major
+
+    @property
+    def minor(self):
+        return self.value and self.value.minor
+
+    @property
+    def patch(self):
+        return self.value and self.value.patch
+
+    @property
+    def prerelease(self):
+        return self.value and self.value.prerelease
+
+    @property
+    def build(self):
+        return self.value and self.value.build
+
+    def next_major(self):
+        return self.value and self.value.next_major()
+
+    def next_minor(self):
+        return self.value and self.value.next_minor()
+
+    def next_patch(self):
+        return self.value and self.value.next_patch()
+
+
+def is_even(s):
+    """
+    Return True if the string "s" is an even number and False if this is an odd
+    number. For example:
+
+    >>> is_even(4)
+    True
+    >>> is_even(123)
+    False
+    >>> is_even(0)
+    True
+    """
+    return (int(s) % 2) == 0
+
+
+@attr.s(frozen=True, order=False, eq=False, hash=True)
+class NginxVersion(SemverVersion):
+    """
+    Semver with 3 segments and extra attribute for stable vs. unstable branches
+    """
+
+    @property
+    def is_stable(self):
+        """
+        True if this is a "stable "version
+        """
+        return is_even(self.minor)
+
 
 @attr.s(frozen=True, order=False, eq=False, hash=True)
 class RubygemsVersion(Version):