version: Add version parsing and formating

src/borg/version.py

@@ -0,0 +1,49 @@
+import re
+
+
+def parse_version(version):
+    """
+    simplistic parser for setuptools_scm versions
+
+    supports final versions and alpha ('a'), beta ('b') and rc versions. It just discards commits since last tag
+    and git revision hash.
+
+    Output is a version tuple containing integers. It ends with one or two elements that ensure that relational
+    operators yield correct relations for alpha, beta and rc versions too. For final versions the last element
+    is a -1, for prerelease versions the last two elements are a smaller negative number and the number of e.g.
+    the beta.
+
+    Note, this sorts version 1.0 before 1.0.0.
+
+    This version format is part of the remote protocol, don‘t change in breaking ways.
+    """
+
+    parts = version.split('+')[0].split('.')
+    if parts[-1].startswith('dev'):
+        del parts[-1]
+    version = [int(segment) for segment in parts[:-1]]
+
+    prerelease = re.fullmatch('([0-9]+)(a|b|rc)([0-9]+)', parts[-1])
+    if prerelease:
+        version_type = {'a': -4, 'b': -3, 'rc': -2}[prerelease.group(2)]
+        version += [int(prerelease.group(1)), version_type, int(prerelease.group(3))]
+    else:
+        version += [int(parts[-1]), -1]
+
+    return tuple(version)
+
+
+def format_version(version):
+    """a reverse for parse_version (obviously without the dropped information)"""
+    f = []
+    it = iter(version)
+    while True:
+        part = next(it)
+        if part >= 0:
+            f += str(part)
+        elif part == -1:
+            break
+        else:
+            f[-1] = f[-1] + {-2: 'rc', -3: 'b', -4: 'a'}[part] + str(next(it))
+            break
+    return '.'.join(f)