diff --git a/README.rst b/README.rst
index 5236df9..6983f7e 100644
--- a/README.rst
+++ b/README.rst
@@ -52,8 +52,8 @@ which is compatible with python packaging software
 
 
 The PyCalVer package provides the ``pycalver`` command and
-module to generate version strings which follow the format:
-``v{calendar_version}.{build_number}[-{release_tag}]``
+module to generate version strings which follow the following
+format: ``v{calendar_version}.{build_number}[-{release_tag}]``
 
 Some examples:
 
@@ -69,9 +69,9 @@ Some examples:
     v202207.18134
 
 
-The ``pycalver bump`` command will parse specified/configured
-files for such strings and rewrite them with an updated version
-string.
+The ``pycalver bump`` command will parse the files you configure
+in ``setup.cfg`` for such strings and rewrite them with an
+updated version string.
 
 The format accepted by PyCalVer can be parsed with this regular
 expression:
diff --git a/src/pycalver/__init__.py b/src/pycalver/__init__.py
index ba36471..378e88d 100644
--- a/src/pycalver/__init__.py
+++ b/src/pycalver/__init__.py
@@ -8,4 +8,4 @@ import os
 
 __version__ = "v201809.0002-beta"
 
-DEBUG = os.environ.get("PYCALVER_DEBUG", "0") == "1"
\ No newline at end of file
+DEBUG = os.environ.get("PYCALVER_DEBUG", "0") == "1"
diff --git a/src/pycalver/__main__.py b/src/pycalver/__main__.py
index a73be68..74d0a98 100644
--- a/src/pycalver/__main__.py
+++ b/src/pycalver/__main__.py
@@ -10,8 +10,6 @@ import os
 import sys
 import click
 import logging
-import difflib
-import typing as typ
 
 from . import DEBUG
 from . import vcs
@@ -56,13 +54,13 @@ def cli():
 def show() -> None:
     _init_loggers(verbose=False)
 
-    cfg = config.parse()
+    cfg: config.MaybeConfig = config.parse()
     if cfg is None:
         log.error("Could not parse configuration from setup.cfg")
         sys.exit(1)
 
-    print(f"Current Version: {cfg['current_version']}")
-    print(f"PEP440 Version: {cfg['pep440_version']}")
+    print(f"Current Version: {cfg.current_version}")
+    print(f"PEP440 Version: {cfg.pep440_version}")
 
 
 @cli.command()
@@ -99,7 +97,7 @@ def init(dry: bool) -> None:
     """Initialize [pycalver] configuration in setup.cfg"""
     _init_loggers(verbose=False)
 
-    cfg = config.parse()
+    cfg: config.MaybeConfig = config.parse()
     if cfg:
         log.error("Configuration already initialized in setup.cfg")
         sys.exit(1)
@@ -179,16 +177,14 @@ def bump(
         log.error(f"Valid arguments are: {', '.join(parse.VALID_RELESE_VALUES)}")
         sys.exit(1)
 
-    cfg = config.parse()
+    cfg: config.MaybeConfig = config.parse()
 
     if cfg is None:
         log.error("Could not parse configuration from setup.cfg")
         sys.exit(1)
 
-    old_version = cfg["current_version"]
+    old_version = cfg.current_version
     new_version = version.bump(old_version, release=release)
-    new_version_nfo = parse.parse_version_info(new_version)
-    new_version_fmt_kwargs = new_version_nfo._asdict()
 
     log.info(f"Old Version: {old_version}")
     log.info(f"New Version: {new_version}")
@@ -196,53 +192,12 @@ def bump(
     if dry:
         log.info("Running with '--dry', showing diffs instead of updating files.")
 
+    file_patterns = cfg.file_patterns
+    filepaths = set(file_patterns.keys())
+
     _vcs = vcs.get_vcs()
-    dirty_files = _vcs.dirty_files()
-
-    if dirty_files:
-        log.warn(f"{_vcs.__name__} working directory is not clean:")
-        for file in dirty_files:
-            log.warn("    " + file)
-
-    if not allow_dirty and dirty_files:
-        sys.exit(1)
-
-    pattern_files = cfg["file_patterns"].keys()
-    dirty_pattern_files = set(dirty_files) & set(pattern_files)
-    if dirty_pattern_files:
-        log.error("Not commiting when pattern files are dirty:")
-        for file in dirty_pattern_files:
-            log.warn("    " + file)
-        sys.exit(1)
-
-    matches: typ.List[parse.PatternMatch]
-    for filepath, patterns in cfg["file_patterns"].items():
-        with io.open(filepath, mode="rt", encoding="utf-8") as fh:
-            content = fh.read()
-
-        old_lines = content.splitlines()
-        new_lines = old_lines.copy()
-
-        matches = parse.parse_patterns(old_lines, patterns)
-        for m in matches:
-            replacement = m.pattern.format(**new_version_fmt_kwargs)
-            span_l, span_r = m.span
-            new_line = m.line[:span_l] + replacement + m.line[span_r:]
-            new_lines[m.lineno] = new_line
-
-        if dry or verbose:
-            print("\n".join(difflib.unified_diff(
-                old_lines,
-                new_lines,
-                lineterm="",
-                fromfile="a/" + filepath,
-                tofile="b/" + filepath,
-            )))
-
-        if not dry:
-            new_content = "\n".join(new_lines)
-            with io.open(filepath, mode="wt", encoding="utf-8") as fh:
-                fh.write(new_content)
+    _vcs.assert_not_dirty(filepaths, allow_dirty)
+    rewrite.rewrite(new_version, file_patterns, dry, verbose)
 
     if dry:
         return
diff --git a/src/pycalver/config.py b/src/pycalver/config.py
index 83d1aa6..061cbe2 100644
--- a/src/pycalver/config.py
+++ b/src/pycalver/config.py
@@ -18,38 +18,48 @@ from .parse import PYCALVER_RE
 log = logging.getLogger("pycalver.config")
 
 
-def parse(config_file="setup.cfg") -> typ.Optional[typ.Dict[str, typ.Any]]:
-    if not os.path.exists(config_file):
-        log.error("File not found: setup.cfg")
-        return None
+class Config(typ.NamedTuple):
 
+    current_version : str
+    pep440_version  : str
+
+    tag             : bool
+    commit          : bool
+
+    file_patterns   : typ.Dict[str, typ.List[str]]
+
+
+MaybeConfig = typ.Optional[Config]
+
+
+def _parse_buffer(cfg_buffer: io.StringIO) -> MaybeConfig:
     cfg_parser = configparser.RawConfigParser("")
-    with io.open(config_file, mode="rt", encoding="utf-8") as fh:
-        cfg_parser.readfp(fh)
+    cfg_parser.readfp(cfg_buffer)
 
     if "pycalver" not in cfg_parser:
         log.error("setup.cfg does not contain a [pycalver] section.")
         return None
 
-    cfg = dict(cfg_parser.items("pycalver"))
+    base_cfg = dict(cfg_parser.items("pycalver"))
 
-    if "current_version" not in cfg:
+    if "current_version" not in base_cfg:
         log.error("setup.cfg does not have 'pycalver.current_version'")
         return None
 
-    current_version = cfg["current_version"]
+    current_version = base_cfg["current_version"]
     if PYCALVER_RE.match(current_version) is None:
         log.error(f"setup.cfg 'pycalver.current_version is invalid")
         log.error(f"current_version = {current_version}")
         return None
 
-    cfg["pep440_version"] = str(pkg_resources.parse_version(current_version))
+    pep440_version = str(pkg_resources.parse_version(current_version))
 
-    cfg["tag"] = cfg.get("tag", "").lower() in ("yes", "true", "1", "on")
-    cfg["commit"] = cfg.get("commit", "").lower() in ("yes", "true", "1", "on")
+    tag = base_cfg.get("tag", "").lower() in ("yes", "true", "1", "on")
+    commit = base_cfg.get("commit", "").lower() in ("yes", "true", "1", "on")
 
-    cfg["file_patterns"] = {}
+    file_patterns: typ.Dict[str, typ.List[str]] = {}
 
+    section_name: str
     for section_name in cfg_parser.sections():
         if not section_name.startswith("pycalver:file:"):
             continue
@@ -59,25 +69,39 @@ def parse(config_file="setup.cfg") -> typ.Optional[typ.Dict[str, typ.Any]]:
             log.error(f"No such file: {filepath} from {section_name} in setup.cfg")
             return None
 
-        section = dict(cfg_parser.items(section_name))
+        section: typ.Dict[str, str] = dict(cfg_parser.items(section_name))
+        patterns = section.get("patterns")
 
-        if "patterns" in section:
-            cfg["file_patterns"][filepath] = [
+        if patterns is None:
+            file_patterns[filepath] = ["{version}", "{pep440_version}"]
+        else:
+            file_patterns[filepath] = [
                 line.strip()
-                for line in section["patterns"].splitlines()
+                for line in patterns.splitlines()
                 if line.strip()
             ]
-        else:
-            cfg["file_patterns"][filepath] = ["{version}", "{pep440_version}"]
 
-    if not cfg["file_patterns"]:
-        cfg["file_patterns"]["setup.cfg"] = ["{version}", "{pep440_version}"]
+    if not file_patterns:
+        file_patterns["setup.cfg"] = ["{version}", "{pep440_version}"]
 
+    cfg = Config(current_version, pep440_version, tag, commit, file_patterns)
     log.debug(f"Config Parsed: {cfg}")
-
     return cfg
 
 
+def parse(config_file="setup.cfg") -> MaybeConfig:
+    if not os.path.exists(config_file):
+        log.error("File not found: setup.cfg")
+        return None
+
+    cfg_buffer = io.StringIO()
+    with io.open(config_file, mode="rt", encoding="utf-8") as fh:
+        cfg_buffer.write(fh.read())
+
+    cfg_buffer.seek(0)
+    return _parse_buffer(cfg_buffer)
+
+
 def default_config_lines() -> typ.List[str]:
     initial_version = dt.datetime.now().strftime("v%Y%m.0001-dev")
 
diff --git a/src/pycalver/lex_id.py b/src/pycalver/lex_id.py
index 00b8453..3fcc3c6 100644
--- a/src/pycalver/lex_id.py
+++ b/src/pycalver/lex_id.py
@@ -85,6 +85,9 @@ MINIMUM_ID = "0"
 
 def next_id(prev_id: str) -> str:
     num_digits = len(prev_id)
+    if prev_id.count("9") == num_digits:
+        raise OverflowError("max lexical version reached: " + prev_id)
+
     _prev_id = int(prev_id, 10)
     _next_id = int(_prev_id) + 1
     next_id = f"{_next_id:0{num_digits}}"
diff --git a/src/pycalver/rewrite.py b/src/pycalver/rewrite.py
index f9649b8..a783a66 100644
--- a/src/pycalver/rewrite.py
+++ b/src/pycalver/rewrite.py
@@ -5,17 +5,52 @@
 # (C) 2018 Manuel Barkhau (@mbarkhau)
 # SPDX-License-Identifier: MIT
 
-import logging
+import io
 import difflib
+import logging
+import typing as typ
+
+from . import parse
 
 log = logging.getLogger("pycalver.rewrite")
 
 
-def rewrite_file(file: str, pattern: str, dry=False) -> None:
-    difflib.unified_diff(
-        file_content_before.splitlines(),
-        file_content_after.splitlines(),
-        lineterm="",
-        fromfile="a/" + file,
-        tofile="b/" + file,
-    )
+def rewrite(
+    new_version: str,
+    file_patterns: typ.Dict[str, str],
+    dry=False,
+    verbose=False,
+) -> None:
+    new_version_nfo = parse.parse_version_info(new_version)
+    new_version_fmt_kwargs = new_version_nfo._asdict()
+
+    matches: typ.List[parse.PatternMatch]
+    for filepath, patterns in file_patterns.items():
+        with io.open(filepath, mode="rt", encoding="utf-8") as fh:
+            content = fh.read()
+
+        old_lines = content.splitlines()
+        new_lines = old_lines.copy()
+
+        matches = parse.parse_patterns(old_lines, patterns)
+        for m in matches:
+            replacement = m.pattern.format(**new_version_fmt_kwargs)
+            span_l, span_r = m.span
+            new_line = m.line[:span_l] + replacement + m.line[span_r:]
+            new_lines[m.lineno] = new_line
+
+        if dry or verbose:
+            print("\n".join(difflib.unified_diff(
+                old_lines,
+                new_lines,
+                lineterm="",
+                fromfile="a/" + filepath,
+                tofile="b/" + filepath,
+            )))
+
+        if dry:
+            continue
+
+        new_content = "\n".join(new_lines)
+        with io.open(filepath, mode="wt", encoding="utf-8") as fh:
+            fh.write(new_content)
diff --git a/src/pycalver/vcs.py b/src/pycalver/vcs.py
index 2be6797..78aec0c 100644
--- a/src/pycalver/vcs.py
+++ b/src/pycalver/vcs.py
@@ -9,8 +9,10 @@
 # MIT License - (C) 2013-2014 Filip Noetzel
 
 import os
+import sys
 import logging
 import tempfile
+import typing as typ
 import subprocess as sp
 
 
@@ -52,6 +54,25 @@ class BaseVCS:
             if not line.strip().startswith(b"??")
         ]
 
+    @classmethod
+    def assert_not_dirty(cls, filepaths: typ.Set[str], allow_dirty=False) -> None:
+        dirty_files = cls.dirty_files()
+
+        if dirty_files:
+            log.warn(f"{cls.__name__} working directory is not clean:")
+            for file in dirty_files:
+                log.warn("    " + file)
+
+        if not allow_dirty and dirty_files:
+            sys.exit(1)
+
+        dirty_pattern_files = set(dirty_files) & filepaths
+        if dirty_pattern_files:
+            log.error("Not commiting when pattern files are dirty:")
+            for file in dirty_pattern_files:
+                log.warn("    " + file)
+            sys.exit(1)
+
 
 class Git(BaseVCS):
 
diff --git a/src/pycalver/version.py b/src/pycalver/version.py
index 80a517c..b2bd2dc 100644
--- a/src/pycalver/version.py
+++ b/src/pycalver/version.py
@@ -14,15 +14,25 @@ log = logging.getLogger("pycalver.version")
 
 
 def current_calver() -> str:
-    return dt.datetime.utcnow().strftime("v%Y%m")
+    return dt.date.today().strftime("v%Y%m")
 
 
-def bump(old_version: str, release: str=None) -> str:
+def bump(old_version: str, *, release: str=None) -> str:
     # old_version is assumed to be a valid calver string,
     # validated in pycalver.config.parse.
     old_ver = parse.parse_version_info(old_version)
 
     new_calver = current_calver()
+
+    if old_ver.calver > new_calver:
+        log.warning(
+            f"'version.bump' called with '{old_version}', " +
+            f"which is from the future, " +
+            f"maybe your system clock is out of sync."
+        )
+        # leave calver as is (don't go back in time)
+        new_calver = old_ver.calver
+
     new_build = lex_id.next_id(old_ver.build[1:])
     if release is None:
         if old_ver.release:
@@ -30,6 +40,8 @@ def bump(old_version: str, release: str=None) -> str:
             new_release = old_ver.release[1:]
         else:
             new_release = None
+    elif release == "final":
+        new_release = None
     else:
         new_release = release
 
@@ -37,38 +49,3 @@ def bump(old_version: str, release: str=None) -> str:
     if new_release:
         new_version += "-" + new_release
     return new_version
-
-
-def incr_version(old_version: str, *, tag: str="__sentinel__") -> str:
-    maybe_match: MaybeMatch = VERSION_RE.search(old_version)
-
-    if maybe_match is None:
-        raise ValueError(f"Invalid version string: {old_version}")
-
-    prev_version_info: PyCalVerInfo = maybe_match.groupdict()
-
-    prev_calver: str = prev_version_info["calver"]
-    next_calver: str = current_calver()
-
-    prev_build: str = prev_version_info["build"]
-    if prev_calver > next_calver:
-        log.warning(
-            f"'incr_version' called with '{old_version}', " +
-            f"which is from the future, " +
-            f"maybe your system clock is out of sync."
-        )
-        next_calver = prev_calver   # leave calver as is
-
-    next_build = lex_id.next_id(prev_build)
-    new_version = f"{next_calver}.{next_build}"
-    if tag != "__sentinel__":
-        if tag is None:
-            pass    # tag explicitly ignored/removed
-        else:
-            new_version += "-" + tag
-    elif "tag" in prev_version_info:
-        # preserve previous tag
-        new_version += "-" + prev_version_info["tag"]
-
-    assert old_version < new_version, f"{old_version}  {new_version}"
-    return new_version
diff --git a/test/test_config.py b/test/test_config.py
index e69de29..f65452f 100644
--- a/test/test_config.py
+++ b/test/test_config.py
@@ -0,0 +1,5 @@
+from pycalver import config
+
+
+def test_parse():
+    pass
diff --git a/test/test_lex_id.py b/test/test_lex_id.py
index 37a2a4a..d454fb8 100644
--- a/test/test_lex_id.py
+++ b/test/test_lex_id.py
@@ -7,11 +7,23 @@ def test_next_id_basic():
     assert lex_id.next_id("09") == "110"
 
 
+def test_next_id_overflow():
+    try:
+        prev_id = "9999"
+        next_id = lex_id.next_id(prev_id)
+        assert False, (prev_id, "->", next_id)
+    except OverflowError:
+        pass
+
+
 def test_next_id_random():
     for i in range(1000):
         prev_id = str(random.randint(1, 100000))
-        next_id = lex_id.next_id(prev_id)
-        assert prev_id < next_id
+        try:
+            next_id = lex_id.next_id(prev_id)
+            assert prev_id < next_id
+        except OverflowError:
+            assert len(prev_id) == prev_id.count("9")
 
 
 def test_ord_val():
diff --git a/test/test_version.py b/test/test_version.py
index 089a3a5..e2408c8 100644
--- a/test/test_version.py
+++ b/test/test_version.py
@@ -1,4 +1,6 @@
+import random
 import datetime as dt
+
 from pycalver import version
 
 
@@ -9,29 +11,47 @@ def test_current_calver():
     assert v[1:].isdigit()
 
 
-# def test_calver():
-#     import random
+def test_bump_beta():
+    calver = version.current_calver()
+    cur_version = calver + ".0001-beta"
+    assert cur_version < version.bump(cur_version)
+    assert version.bump(cur_version).endswith("-beta")
+    assert version.bump(cur_version, release="alpha").endswith("-alpha")
+    assert version.bump(cur_version, release="final").endswith("0002")
 
-#     first_version_str = "v201808.0001-dev"
-#     padding = len(first_version_str) + 3
-#     version_str = first_version_str
 
-#     def _current_calver() -> str:
-#         _current_calver.delta += dt.timedelta(days=int(random.random() * 5))
+def test_bump_final():
+    calver = version.current_calver()
+    cur_version = calver + ".0001"
+    assert cur_version < version.bump(cur_version)
+    assert version.bump(cur_version, release="alpha").endswith("-alpha")
+    assert version.bump(cur_version, release="final").endswith("0002")
+    assert version.bump(cur_version).endswith("0002")
 
-#         return (dt.datetime.utcnow() + _current_calver.delta).strftime("v%Y%m")
 
-#     _current_calver.delta = dt.timedelta(days=1)
+def test_bump_future():
+    future_date = dt.datetime.today() + dt.timedelta(days=300)
+    future_calver = future_date.strftime("v%Y%m")
+    cur_version = future_calver + ".0001"
+    assert cur_version < version.bump(cur_version)
 
-#     global current_calver
-#     current_calver = _current_calver
 
-#     for i in range(1050):
-#         version_str = incr_version(version_str, tag=random.choice([
-#             None, "alpha", "beta", "rc"
-#         ]))
-#         print(f"{version_str:<{padding}}", end=" ")
-#         if (i + 1) % 8 == 0:
-#             print()
+def test_bump_random():
+    cur_date = dt.date.today()
+    cur_version = cur_date.strftime("v%Y%m") + ".0001-dev"
 
-#     print()
+    def _mock_current_calver():
+        return cur_date.strftime("v%Y%m")
+
+    _orig_current_calver = version.current_calver
+    version.current_calver = _mock_current_calver
+    try:
+        for i in range(1000):
+            cur_date += dt.timedelta(days=int((1 + random.random()) ** 10))
+            new_version = version.bump(cur_version, release=random.choice([
+                None, "alpha", "beta", "rc", "final"
+            ]))
+            assert cur_version < new_version
+            cur_version = new_version
+    finally:
+        version.current_calver = _orig_current_calver