new file mode 100644
--- /dev/null
+++ b/maps.py
@@ -0,0 +1,89 @@
+''' Module for self-contained maps. '''
+
+import os
+from mercurial import util as hgutil
+
+class AuthorMap(dict):
+    '''A mapping from Subversion-style authors to Mercurial-style
+    authors, and back. The data is stored persistently on disk.
+    
+    If the 'hgsubversion.defaultauthors' configuration option is set to false,
+    attempting to obtain an unknown author will fail with an Abort.
+    '''
+
+    def __init__(self, ui, path, defaulthost=None):
+        '''Initialise a new AuthorMap.
+        
+        The ui argument is used to print diagnostic messages.
+        
+        The path argument is the location of the backing store,
+        typically .hg/authormap.
+        '''
+        self.ui = ui
+        self.path = path
+        if defaulthost:
+            self.defaulthost = '@%s' % defaulthost.lstrip('@')
+        else:
+            self.defaulthost = ''
+        self.super = super(AuthorMap, self)
+        self.super.__init__()
+        self.load(path)
+
+    def load(self, path):
+        ''' Load mappings from a file at the specified path. '''
+        if os.path.exists(path):
+            self.ui.note('Reading authormap from %s\n' % path)
+            f = open(path, 'r')
+            for number, line in enumerate(f):
+                if not line.strip():
+                    continue
+                try:
+                    srcauth, dstauth = line.split('=', 1)
+                    srcauth = srcauth.strip()
+                    dstauth = dstauth.strip()
+                    if srcauth in self and dstauth != self[srcauth]:
+                        self.ui.warn(('Overriding author mapping for "%s" from '
+                                      + '"%s" to "%s"\n')
+                                     % (srcauth, self[srcauth], dstauth))
+                    else:
+                        self[srcauth] = dstauth
+                except IndexError:
+                    self.ui.warn('Ignoring line %i in author map %s: %s\n'
+                                 % (number, path, line.rstrip()))
+            f.close()
+
+    def __setitem__(self, key, value):
+        ''' Similar to dict.__setitem__, but also updates the new mapping in the
+        backing store. '''
+        self.super.__setitem__(key, value)
+
+        self.ui.debug(('Writing author map to %s\n') % self.path)
+        f = open(self.path, 'w+')
+        for k, v in self.iteritems():
+            f.write("%s=%s\n" % (k, v))
+        f.close()
+
+    def __getitem__(self, author):
+        ''' Similar to dict.__getitem__, except in case of an unknown author.
+        In such cases, a new value is generated and added to the dictionary
+        as well as the backing store. '''
+        if author in self:
+            result = self.super.__getitem__(author)
+        elif self.ui.configbool('hgsubversion', 'defaultauthors', True):
+            # TODO: should we treat missing authors specially?
+            self[author] = result = '%s%s' % (author, self.defaulthost)
+            self.ui.warn('Substituting author "%s" for default "%s"\n'
+                         % (author, result))
+        else:
+            raise hgutil.Abort('Author %s has no entry in the author map!'
+                               % author)
+        self.ui.debug('Mapping author "%s" to "%s"\n' % (author, result))
+        return result
+
+    def reverselookup(self, author):
+        for svnauthor, hgauthor in self.iteritems():
+            if author == hgauthor:
+                return svnauthor
+        else:
+            # Mercurial incorrectly splits at e.g. '.', so we roll our own.
+            return author.rsplit('@', 1)[0]