Initial commitHEADmaster

1 files changed, 960 insertions, 0 deletions

diff --git a/patchtheory.py b/patchtheory.py
new file mode 100644
index 0000000..21b3e29
--- /dev/null
+++ b/patchtheory.py
@@ -0,0 +1,960 @@
+import re, operator, io, sys
+
+rem_range = re.compile(b'@@ -([0-9]+)(?:,([0-9]+))? \\+([0-9]+)(?:,([0-9]+))? @@(?: .*)?').fullmatch
+rem_diffgit = re.compile('diff --git a/([^ ]*) b/([^ ]*)').fullmatch
+
+INFINITY = 2**64
+
+class PatchError(Exception):
+    __slots__ = ('a', 'b')
+    def __init__(self, a, b, /, fn=None):
+        super().__init__(fn)
+        self.a = a
+        self.b = b
+
+    def __str__(self, /):
+        fn = self.args[0]
+        r = [f' in {fn!r}:' if fn is not None else ':']
+
+        def tabmangle(p):
+            o = io.BytesIO()
+            p.write(o.write)
+            p = o.getvalue().decode('utf-8', 'replace').splitlines()
+            return ['    ' + l for l in p]
+
+        r.append('')
+        r += tabmangle(self.a)
+
+        r.append('')
+        r += tabmangle(self.b)
+
+        return '\n'.join(r)
+
+class ConflictError(PatchError):
+    __slots__ = ()
+    
+    def __str__(self, /):
+        return 'Conflict' + PatchError.__str__(self)
+
+class MismatchError(PatchError):
+    __slots__ = ()
+    
+    def __str__(self, /):
+        return 'Mismatch' + PatchError.__str__(self)
+
+class Hunk:
+    __slots__ = ('lhs_line', 'rhs_line', 'pre_context', 'lhs', 'rhs', 'post_context')
+
+    def __init__(self, /, *, lhs_line, rhs_line, pre_context, lhs, rhs, post_context):
+        assert lhs_line > 0
+        assert rhs_line > 0
+
+        self.lhs_line = lhs_line
+        self.rhs_line = rhs_line
+        self.pre_context = pre_context
+        self.lhs = lhs
+        self.rhs = rhs
+        self.post_context = post_context
+
+    @property
+    def lhs_size(self, /):
+        return len(self.pre_context) + len(self.lhs) + len(self.post_context)
+
+    @property
+    def rhs_size(self, /):
+        return len(self.pre_context) + len(self.rhs) + len(self.post_context)
+
+    def write(self, w, /, munge=False, *, lhs_line=None, rhs_line=None, lhs_nl=True, rhs_nl=True):
+        if lhs_line is None:
+            lhs_line = self.lhs_line
+        if rhs_line is None:
+            rhs_line = self.rhs_line
+        pre_context = self.pre_context
+        lhs = self.lhs
+        rhs = self.rhs
+        post_context = self.post_context
+        clen = len(pre_context) + len(post_context)
+        llen = len(lhs) + clen
+        rlen = len(rhs) + clen
+
+        if munge:
+            w(f'@@ -xxx,{llen} +yyy,{rlen} @@'.encode('utf-8'))
+        else:
+            w(f'@@ -{lhs_line},{llen} +{rhs_line},{rlen} @@'.encode('utf-8'))
+
+        for l in pre_context:
+            w(b'\n ')
+            w(l)
+
+        for l in lhs:
+            w(b'\n-')
+            w(l)
+
+        for l in rhs:
+            w(b'\n+')
+            w(l)
+
+        for l in post_context:
+            w(b'\n ')
+            w(l)
+
+        w(b'\n')
+
+    @classmethod
+    def parse_l(tp, l, line, /):
+        m = rem_range(l)
+        if m is None:
+            raise ValueError(f'Invalid patch: bad range info {l!r}')
+        lhs_line,n2,rhs_line,n4 = m.groups()
+        lhs_line = 1 if lhs_line == b'0' else int(lhs_line)
+        rhs_line = 1 if rhs_line == b'0' else int(rhs_line)
+        n2 = int(n2) if n2 else 1
+        n4 = int(n4) if n4 else 1
+
+        hunk = []
+        prev = -1
+        lhs_nl = True
+        rhs_nl = True
+        post_nl = True
+        while True:
+            l = line()
+            if l.startswith(b'\\ '):
+                if prev == 0:
+                    lhs_nl = False
+                elif prev == 1:
+                    rhs_nl = False
+                elif prev == -1:
+                    post_nl = False
+                else:
+                    raise ValueError('Invalid patch: bad "No newline at end of file" flag')
+                prev = -2
+                continue
+            if not n2 and not n4:
+                break
+            k = l[0]
+            if k == b' '[0]:
+                if not post_nl or not lhs_nl or not rhs_nl:
+                    raise ValueError('Invalid patch: bad "No newline at end of file" flag')
+                if not n2 or not n4:
+                    raise ValueError('Invalid patch: bad line count')
+                n2 -= 1
+                n4 -= 1
+                prev = -1
+            elif k == b'-'[0]:
+                if not post_nl or not lhs_nl:
+                    raise ValueError('Invalid patch: bad "No newline at end of file" flag')
+                if not n2:
+                    raise ValueError('Invalid patch: bad line count')
+                n2 -= 1
+                prev = 0
+            elif k == b'+'[0]:
+                if not post_nl or not rhs_nl:
+                    raise ValueError('Invalid patch: bad "No newline at end of file" flag')
+                if not n4:
+                    raise ValueError('Invalid patch: bad line count')
+                n4 -= 1
+                prev = 1
+            else:
+                raise ValueError(f'Invalid patch: bad prefix character {chr(k)!r}')
+                assert False
+            hunk.append(l)
+        lret = l
+
+        strip1 = operator.itemgetter(slice(1,None))
+
+        for i,l in enumerate(hunk):
+            if l[0] != b' '[0]:
+                break
+        else:
+            raise ValueError('Invalid patch: no changed lines in hunk')
+        pre_context = (*map(strip1, hunk[:i]),)
+        del hunk[:i]
+
+        j = -1
+        while True:
+            l = hunk[j]
+            if l[0] != b' '[0]:
+                break
+            j -= 1
+        j += len(hunk) + 1
+        if not post_nl:
+            hunk[j] += b'\n\\ No newline at end of file'
+        post_context = (*map(strip1, hunk[j:]),)
+        del hunk[j:]
+
+        lhs = []
+        rhs = []
+        for l in hunk:
+            l0 = l[0]
+            if l0 != b'+'[0]:
+                lhs.append(l)
+            if l0 != b'-'[0]:
+                rhs.append(l)
+        if not lhs_nl:
+            lhs[-1] += b'\n\\ No newline at end of file'
+        if not rhs_nl:
+            rhs[-1] += b'\n\\ No newline at end of file'
+
+        return lret, tp(
+            lhs_line = lhs_line,
+            rhs_line = rhs_line,
+            pre_context = pre_context,
+            lhs = (*map(strip1, lhs),),
+            rhs = (*map(strip1, rhs),),
+            post_context = post_context,
+        )
+
+    def reduce_context(self, n=3):
+        pre = self.pre_context
+        lhs = self.lhs
+        rhs = self.rhs
+        post = self.post_context
+
+        i = 0
+        j = -1
+        m = min(len(lhs), len(rhs))
+        while i - j <= m:
+            if lhs[i] == rhs[i]:
+                i += 1
+                if i - j > m:
+                    break
+                if lhs[j] == rhs[j]:
+                    j -= 1
+            else:
+                if lhs[j] != rhs[j]:
+                    break
+                j -= 1
+        j += 1
+        jl = len(lhs) + j
+        jr = len(rhs) + j
+
+        assert lhs[:i] == rhs[:i]
+        assert lhs[jl:] == rhs[jr:]
+        pre = pre + lhs[:i]
+        post = lhs[jl:] + post
+        lhs = lhs[i:jl]
+        rhs = rhs[i:jr]
+
+        if not lhs and not rhs:
+            return None
+
+        assert lhs != rhs
+        assert not lhs or not rhs or lhs[0] != rhs[0] and lhs[-1] != rhs[-1]
+
+        lhs_line = self.lhs_line
+        rhs_line = self.rhs_line
+
+        if n is None:
+            pass
+        elif not n:
+            d = len(pre)
+            lhs_line += d
+            rhs_line += d
+            pre = ()
+            post = ()
+        else:
+            if len(pre) > n:
+                d = len(pre) - n
+                lhs_line += d
+                rhs_line += d
+                pre = pre[d:]
+
+            if len(post) > n:
+                post = post[:n]
+
+        return Hunk(
+            lhs_line = lhs_line,
+            rhs_line = rhs_line,
+            pre_context = pre,
+            lhs = lhs,
+            rhs = rhs,
+            post_context = post,
+        )
+
+def join_hunks(hs_a, hs_b):
+    hs_a = iter(hs_a)
+    hs_b = iter(hs_b)
+
+    need_a = True
+    need_b = True
+    have_cur = False
+
+    def make_cur(n):
+        if not have_cur:
+            return None
+        return Hunk(
+            lhs_line = cur_lhs_line,
+            rhs_line = cur_rhs_line,
+            pre_context = (),
+            lhs = tuple(cur_lhs),
+            rhs = tuple(cur_rhs),
+            post_context = (),
+        ).reduce_context(n)
+
+    delta = 0
+    r = []
+
+    while True:
+        if need_a:
+            need_a = False
+            a = next(hs_a, None)
+
+        if need_b:
+            need_b = False
+            b = next(hs_b, None)
+
+        if not have_cur:
+            if a is not None and (b is None or a.rhs_line <= b.lhs_line):
+                cur_lhs_line = a.lhs_line
+                cur_rhs_line = a.lhs_line + delta
+                cur_lhs = [*a.pre_context, *a.lhs, *a.post_context]
+                cur_rhs = [*a.pre_context, *a.rhs, *a.post_context]
+                need_a = True
+            elif b is not None:
+                cur_lhs_line = b.rhs_line - delta
+                cur_rhs_line = b.rhs_line
+                cur_lhs = [*b.pre_context, *b.lhs, *b.post_context]
+                cur_rhs = [*b.pre_context, *b.rhs, *b.post_context]
+                need_b = True
+            else:
+                return (*r,)
+            have_cur = True
+            continue
+
+        cur_lhs_end = cur_lhs_line + len(cur_lhs)
+        cur_rhs_end = cur_rhs_line + len(cur_rhs)
+
+        if a is not None and a.lhs_line <= cur_lhs_end:
+            lhs = [*a.pre_context, *a.lhs, *a.post_context]
+            rhs = [*a.pre_context, *a.rhs, *a.post_context]
+
+            i = a.lhs_line - cur_lhs_line
+            k = i + len(rhs)
+
+            if len(cur_lhs) >= k:
+                if cur_lhs[i:k] != rhs:
+                    raise MismatchError(make_cur(None), a)
+                cur_lhs[i:k] = lhs
+            else:
+                j = len(cur_lhs) - i
+                if cur_lhs[i:] != rhs[:j]:
+                    raise MismatchError(make_cur(None), a)
+                cur_lhs[i:] = lhs
+                cur_rhs.extend(rhs[j:])
+            need_a = True
+            continue
+
+        if b is not None and b.rhs_line <= cur_rhs_end:
+            lhs = [*b.pre_context, *b.lhs, *b.post_context]
+            rhs = [*b.pre_context, *b.rhs, *b.post_context]
+            i = b.rhs_line - cur_rhs_line
+            k = i + len(lhs)
+
+            if len(cur_rhs) >= k:
+                if cur_rhs[i:k] != lhs:
+                    raise MismatchError(make_cur(None), b)
+                cur_rhs[i:k] = rhs
+            else:
+                j = len(cur_rhs) - i
+                if cur_rhs[i:] != lhs[:j]:
+                    raise MismatchError(make_cur(None), b)
+                cur_rhs[i:] = rhs
+                cur_lhs.extend(lhs[j:])
+            need_b = True
+            continue
+
+        cur = make_cur(3)
+        have_cur = False
+        if cur is None:
+            continue
+        r.append(cur)
+        delta += len(cur.rhs) - len(cur.lhs)
+
+def commute_hunks(hs_a, hs_b):
+    hs_a = iter(hs_a)
+    hs_b = iter(hs_b)
+
+    need_a = True
+    need_b = True
+
+    delta_a = 0
+    delta_b = 0
+    ra = []
+    rb = []
+    wa = ra.append
+    wb = rb.append
+
+    while True:
+        if need_a:
+            need_a = False
+            try:
+                a = next(hs_a)
+            except StopIteration:
+                if b is None and not need_b:
+                    break
+                a = None
+                a_begin = INFINITY
+                a_pre = ()
+                a_post = ()
+                a_end = INFINITY
+            else:
+                a_pre = a.pre_context
+                a_post = a.post_context
+                a_begin = a.rhs_line + len(a_pre)
+                a_end = a_begin + len(a.rhs)
+
+        if need_b:
+            need_b = False
+            try:
+                b = next(hs_b)
+            except StopIteration:
+                if a is None:
+                    break
+                b = None
+                b_begin = INFINITY
+                b_pre = ()
+                b_post = ()
+                b_end = INFINITY
+            else:
+                b_pre = b.pre_context
+                b_post = b.post_context
+                b_begin = b.lhs_line + len(b_pre)
+                b_end = b_begin + len(b.lhs)
+
+        if a_end <= b_begin:
+            gap = b_begin - a_end
+
+            if (n := len(a_post) + len(b_pre) - gap) > 0:
+                assert (a_pre + a.rhs + a_post)[-n:] == (b_pre + b.lhs + b_post)[:n]
+                if (n := len(a_post) - gap) > 0:
+                    a_post = a_post[:gap] + (b.rhs + b_post)[:n]
+                if (n := len(b_pre) - gap) > 0:
+                    b_pre = (a_pre + a.lhs)[-n:] + b_pre[n:]
+
+            wa(Hunk(
+                lhs_line = a.lhs_line + delta_b,
+                rhs_line = a.rhs_line + delta_b,
+                pre_context = a_pre,
+                lhs = a.lhs,
+                rhs = a.rhs,
+                post_context = a_post,
+            ))
+            delta_a += len(a.rhs) - len(a.lhs)
+            need_a = True
+            continue
+
+        if b_end <= a_begin:
+            gap = a_begin - b_end
+
+            if (n := len(b_post) + len(a_pre) - gap) > 0:
+                assert (b_pre + b.lhs + b_post)[-n:] == (a_pre + a.rhs + a_post)[:n]
+                if (n := len(b_post) - gap) > 0:
+                    b_post = b_post[:gap] + (a.lhs + a_post)[:n]
+                if (n := len(a_pre) - gap) > 0:
+                    a_pre = (b_pre + b.rhs)[-n:] + a_pre[n:]
+
+            wb(Hunk(
+                lhs_line = b.lhs_line - delta_a,
+                rhs_line = b.rhs_line - delta_a,
+                pre_context = b_pre,
+                lhs = b.lhs,
+                rhs = b.rhs,
+                post_context = b_post,
+            ))
+            delta_b += len(b.rhs) - len(b.lhs)
+            need_b = True
+            continue
+
+        raise ConflictError(a, b)
+
+    return (*rb,), (*ra,)
+
+class FileDiff:
+    __slots__ = ('src_name', 'dst_name', 'src_mode', 'dst_mode', 'hunks',)
+
+    def __init__(self, /, *, src_name, dst_name, src_mode, dst_mode, hunks):
+        assert (src_mode is ...) == (dst_mode is ...)
+        assert (src_name is None) == (src_mode is None)
+        assert (dst_name is None) == (dst_mode is None)
+
+        self.src_name = src_name
+        self.dst_name = dst_name
+        if src_mode == dst_mode:
+            self.src_mode = ...
+            self.dst_mode = ...
+        else:
+            self.src_mode = src_mode
+            self.dst_mode = dst_mode
+        self.hunks = hunks
+
+    @property
+    def op_kind_name(self):
+        if self.src_name is None:
+            return 'create'
+        if self.dst_name is None:
+            return 'delete'
+        is_chmod = self.dst_mode != self.dst_mode
+        if self.dst_name != self.dst_name:
+            return 'chmod+rename' if is_chmod else 'rename'
+        if self.hunks:
+            return 'chmod+modify' if is_chmod else 'modify'
+        return 'chmod' if is_chmod else 'noop'
+
+    def write(self, w):
+        src_name = self.src_name
+        dst_name = self.dst_name
+
+        if src_name is None:
+            assert dst_name is not None
+            w(f'diff --git a/{dst_name} b/{dst_name}\n'.encode('utf-8'))
+        elif dst_name is None:
+            w(f'diff --git a/{src_name} b/{src_name}\n'.encode('utf-8'))
+        else:
+            w(f'diff --git a/{src_name} b/{dst_name}\n'.encode('utf-8'))
+
+        if src_name is not None and dst_name is not None and src_name != dst_name:
+            w(f'rename from {src_name}\nrename to {dst_name}\n'.encode('utf-8'))
+
+        m = self.src_mode
+        if dst_name is None:
+            assert m is not None
+            w(f'deleted file mode {m}\n'.encode('utf-8'))
+        elif m not in (None, ...):
+            w(f'old mode {m}\n'.encode('utf-8'))
+
+        m = self.dst_mode
+        if src_name is None:
+            assert m is not None
+            w(f'new file mode {m}\n'.encode('utf-8'))
+        elif m not in (None, ...):
+            w(f'new mode {m}\n'.encode('utf-8'))
+
+        h = self.hunks
+        if h:
+            if src_name is None:
+                src_name = '/dev/null'
+                lhs_line = 0
+            else:
+                src_name = f'a/{src_name}'
+                lhs_line = None
+            if dst_name is None:
+                dst_name = '/dev/null'
+                rhs_line = 0
+            else:
+                dst_name = f'b/{dst_name}'
+                rhs_line = None
+
+            w(f'--- {src_name}\n+++ {dst_name}\n'.encode('utf-8'))
+
+            for h in h:
+                h.write(w, lhs_line=lhs_line, rhs_line=rhs_line)
+
+    def __neg__(self, /):
+        tp = type(self)
+
+class Diff:
+    __slots__ = ('files',)
+
+    def __init__(self):
+        self.files = ()
+
+    def __bool__(self):
+        return not not self.files
+
+    @classmethod
+    def _sort_files(tp, files):
+        r = object.__new__(tp)
+        r.files = (*sorted(files, key=lambda f: (f.src_name or '', f.dst_name or '')),)
+        return r
+
+    @classmethod
+    def parse_l(tp, l, line):
+        files = []
+
+        while True:
+            l = l.decode('utf-8').rstrip()
+            if l == '--' or not l:
+                break
+            m = rem_diffgit(l)
+            if m is None:
+                raise ValueError(f'Invalid patch: expected \'diff --git ...\' header, got {l!r}')
+
+            header = l
+
+            src_name,dst_name = m.groups()
+            src_exists = ...
+            dst_exists = ...
+
+            if not src_exists and not dst_exists:
+                raise RuntimeError('wtf')
+            if (not src_exists or not dst_exists) and src_name != dst_name:
+                raise RuntimeError('wtf')
+
+            has_diff = False
+            has_chmod = False
+            has_rename = False
+            dst_mode = ...
+            src_mode = ...
+
+            while True:
+                l = line()
+                if l.startswith(b'--- a/'):
+                    if src_exists is False: raise RuntimeError('wtf')
+                    src_exists = True
+                    if l[6:].decode('utf-8') != src_name:
+                        raise RuntimeError('wtf')
+                    has_diff = True
+                    continue
+                if l == b'--- /dev/null':
+                    if src_exists is True: raise RuntimeError('wtf')
+                    src_exists = False
+                    has_diff = True
+                    continue
+                if l.startswith(b'+++ b/'):
+                    if dst_exists is False: raise RuntimeError('wtf')
+                    dst_exists = True
+                    if l[6:].decode('utf-8') != dst_name:
+                        raise RuntimeError('wtf')
+                    has_diff = True
+                    continue
+                if l == b'+++ /dev/null':
+                    if dst_exists is True: raise RuntimeError('wtf')
+                    dst_exists = False
+                    has_diff = True
+                    continue
+                if l.startswith(b'index '):
+                    continue
+                if l.startswith(b'new file mode '):
+                    if src_exists is True: raise RuntimeError('wtf')
+                    if dst_exists is False: raise RuntimeError('wtf')
+                    src_exists = False
+                    dst_exists=True
+                    dst_mode = l[14:].decode('ascii')
+                    src_mode = None
+                    continue
+                if l.startswith(b'new mode '):
+                    dst_mode = l[9:].decode('ascii')
+                    has_chmod = True
+                    continue
+                if l.startswith(b'deleted file mode '):
+                    if src_exists is False: raise RuntimeError('wtf')
+                    if dst_exists is True: raise RuntimeError('wtf')
+                    src_exists = True
+                    dst_exists = False
+                    src_mode = l[18:].decode('ascii')
+                    dst_mode = None
+                    continue
+                if l.startswith(b'old mode '):
+                    if dst_exists is False:
+                        raise RuntimeError('File deleted')
+                    src_mode = l[9:].decode('ascii')
+                    has_chmod = True
+                    dst_exists = True
+                    continue
+                if l.startswith(b'similarity index '):
+                    has_rename = True
+                    continue
+                if l.startswith(b'rename from '):
+                    has_rename = True
+                    rename_from = l[12:].decode('utf-8')
+                    continue
+                if l.startswith(b'rename to '):
+                    rename_to = l[10:].decode('utf-8')
+                    has_rename = True
+                    continue
+                break
+
+            if has_chmod or has_rename:
+                if src_exists is False: raise RuntimeError('wtf')
+                if dst_exists is False: raise RuntimeError('wtf')
+                src_exists = True
+                dst_exists = True
+
+            if has_chmod:
+                assert src_mode is not ...
+                assert dst_mode is not ...
+
+            assert src_exists is not ...
+            if not src_exists:
+                assert dst_mode is not ...
+
+            assert dst_exists is not ...
+            if not dst_exists:
+                assert src_mode is not ...
+
+            if has_rename:
+                if src_name == dst_name or not src_exists or not dst_exists:
+                    raise RuntimeError(f'Bad rename: {src_name!r} -> {dst_name!r}')
+                assert rename_from == src_name
+                assert rename_to == dst_name
+
+            if not src_exists: src_name = None
+            if not dst_exists: dst_name = None
+
+            assert (src_mode is ...) == (dst_mode is ...)
+            assert has_diff or has_chmod or has_rename or not dst_exists
+
+            #want_header = fmt_diff_git(src_name, dst_name)
+            #if header != want_header:
+            #    raise ValueError(f'Invalid patch: expected {want_header!r} header, got {header!r}')
+
+            if has_diff:
+                hunks = []
+
+                while True:
+                    l,hunk = Hunk.parse_l(l, line)
+                    hunk = hunk.reduce_context()
+                    if hunk is not None:
+                        hunks.append(hunk)
+                    if not l.startswith(b'@@ '):
+                        break
+
+                if src_name is None:
+                    assert len(hunks) == 1
+                    assert len(hunks[0].pre_context) == 0
+                    assert len(hunks[0].post_context) == 0
+                    assert len(hunks[0].lhs) == 0
+                    assert hunks[0].lhs_line == 1
+                    hunks[0].lhs_line = 1
+
+                if dst_name is None:
+                    assert len(hunks) == 1
+                    assert len(hunks[0].pre_context) == 0
+                    assert len(hunks[0].post_context) == 0
+                    assert len(hunks[0].rhs) == 0
+                    assert hunks[0].rhs_line == 1
+
+                hunks = (*hunks,)
+
+            else:
+                hunks = ()
+
+            files.append(FileDiff(
+                src_name = src_name,
+                dst_name = dst_name,
+                src_mode = src_mode,
+                dst_mode = dst_mode,
+                hunks = hunks,
+            ))
+
+        try:
+            l = line().rstrip()
+        except StopIteration:
+            pass
+        else:
+            if l[0] not in b'0123456789':
+                l = l.decode('utf-8')
+                raise ValueError('Invalid patch: expected git version, got {l!r}')
+
+
+            while True:
+                try:
+                    l = line()
+                except StopIteration:
+                    break
+                if l.strip():
+                    l = l.decode('utf-8')
+                    raise ValueError('Invalid patch: got trailing garbage {l!r}')
+
+        return tp._sort_files(files)
+
+    @classmethod
+    def parse(tp, body):
+        line = iter(body.split(b'\n')).__next__
+        return tp.parse_l(line(), line)
+
+    def write(self, w, /):
+        for f in self.files:
+            f.write(w)
+
+    def write_munged(self, w, /):
+        for f in self.files:
+            w((f.src_name or '').encode('utf-8'))
+            w(b'\n')
+            w((f.dst_name or '').encode('utf-8'))
+            w(b'\n')
+            for h in f.hunks:
+                h.write(w, True)
+            w(b'\n')
+
+    def __neg__(self):
+        return Diff._sort_files((*(FileDiff(
+            src_name = f.dst_name,
+            dst_name = f.src_name,
+            src_mode = f.dst_mode,
+            dst_mode = f.src_mode,
+            hunks = (*(Hunk(
+                lhs_line = h.rhs_line,
+                rhs_line = h.lhs_line,
+                pre_context = h.pre_context,
+                lhs = h.rhs,
+                rhs = h.lhs,
+                post_context = h.post_context,
+            ) for h in f.hunks),),
+        ) for f in self.files),))
+
+    def __add__(a, b):
+        tp = type(a)
+        if type(b) is not tp:
+            raise TypeError
+
+        r = []
+        w = r.append
+
+        fdiff = FileDiff
+        def join(fa, fb):
+            src_name = fa.src_name
+            dst_name = fb.dst_name
+            src_mode = fa.src_mode
+            dst_mode = fb.dst_mode
+            if src_mode is ...:
+                if dst_mode is not ...:
+                    src_mode = fb.src_mode
+            elif dst_mode is ...:
+                dst_mode = fa.dst_mode
+            elif fa.dst_mode != fb.src_mode:
+                raise ValueError('mismatch')
+
+            hunks = (*join_hunks(fa.hunks, fb.hunks),)
+
+            if src_name is None and dst_name is None:
+                if hunks:
+                    raise ValueError('mismatch')
+                return
+
+            if src_name == dst_name and src_mode == dst_mode and not hunks:
+                return
+
+            w(fdiff(
+                src_name = src_name,
+                dst_name = dst_name,
+                src_mode = src_mode,
+                dst_mode = dst_mode,
+                hunks = hunks,
+            ))
+
+        _pair_diffs(a, b, on_lhs=w, on_rhs=w, on_pair=join, on_re=join)
+        return tp._sort_files(r)
+
+    @classmethod
+    def commute(tp, a, b):
+        if type(a) is not tp or type(b) is not tp:
+            raise TypeError
+
+        ra = []
+        rb = []
+        wa = ra.append
+        wb = rb.append
+        fdiff = FileDiff
+
+        def join(fa, fb):
+            name1 = fa.src_name
+            name2 = fa.dst_name
+            name3 = fb.dst_name
+            if name1 == name2:
+                name2 = name3
+            elif name2 == name3:
+                name2 = name1
+            else:
+                raise ValueError('cannot commute {name2!r}: {fa.op_kind_name} / {fb.op_kind_name}')
+
+            mode1 = fa.src_mode
+            mode3 = fb.dst_mode
+            if mode1 is ...:
+                if mode3 is ...:
+                    mode2 = ...
+                else:
+                    mode1 = fb.src_mode
+                    mode2 = mode3
+
+            elif mode3 is ...:
+                mode3 = fa.dst_mode
+                mode2 = mode1
+
+            else:
+                mode2 = fb.dst_mode
+                if fa.src_mode != mode2:
+                    raise ValueError('mismatch')
+                if mode1 == mode2:
+                    mode2 = mode3
+                elif mode2 == mode3:
+                    mode2 = mode1
+                else:
+                    raise ValueError('cannot commute {name2!r}: chmod/chmod conflict')
+
+            try:
+                hb,ha = commute_hunks(fa.hunks, fb.hunks)
+            except PatchError as err:
+                err.args = (name1, name2, name3)
+                raise
+
+            wa(fdiff(
+                src_name = name1,
+                dst_name = name2,
+                src_mode = mode1,
+                dst_mode = mode2,
+                hunks = ha,
+            ))
+            wb(fdiff(
+                src_name = name2,
+                dst_name = name3,
+                src_mode = mode2,
+                dst_mode = mode3,
+                hunks = hb,
+            ))
+
+        _pair_diffs(a, b, on_lhs=wa, on_rhs=wb, on_re=join, on_pair=join)
+        return tp._sort_files(rb), tp._sort_files(ra)
+
+def _pair_diffs(a, b, *, on_lhs, on_rhs, on_pair, on_re):
+    a_del = {}
+    b_new = {}
+
+    a_dst = {}
+    b_dst = set()
+    for f in a.files:
+        dst = f.dst_name
+        if dst is None:
+            a_del[f.src_name] = f
+            continue
+        a_dst[dst] = f
+
+    a = a_dst.pop
+    for f in b.files:
+        dst = f.dst_name
+        if dst is not None:
+            b_dst.add(dst)
+
+        src = f.src_name
+        if src is None:
+            b_new[dst] = f
+            continue
+
+        try:
+            f2 = a(src)
+        except KeyError:
+            pass
+        else:
+            on_pair(f2, f)
+            continue
+        on_rhs(f)
+
+    for dst in a_dst.keys() & b_dst:
+        raise ValueError(f'mismatch for {dst!r}')
+
+    b = b_new.pop
+    for src,f in a_del.items():
+        try:
+            f2 = b(src)
+        except KeyError:
+            pass
+        else:
+            on_re(f, f2)
+            continue
+        on_lhs(f)
+
+    for f in b_new.values():
+        on_rhs(f)
+
+    for f in a_dst.values():
+        on_lhs(f)