Adding Tim Peters' ndiff utility.

This is handy for comparing plain-text documentation files,
since it displays intra-line differences.

1 files changed, 667 insertions, 0 deletions

diff --git a/Tools/scripts/ndiff.py b/Tools/scripts/ndiff.py
new file mode 100755
index 0000000..2ba5e53
--- /dev/null
+++ b/Tools/scripts/ndiff.py
@@ -0,0 +1,667 @@
+#! /usr/bin/env python
+
+# Released to the public domain $JustDate:  3/16/98 $,
+# by Tim Peters (email tim_one@email.msn.com).
+
+# ndiff file1 file2 -- a human-friendly file differencer.
+
+# $Revision$
+# $NoKeywords: $
+
+# SequenceMatcher tries to compute a "human-friendly diff" between
+# two sequences (chiefly picturing a file as a sequence of lines,
+# and a line as a sequence of characters, here).  Unlike UNIX(tm) diff,
+# e.g., the fundamental notion is the longest *contiguous* & junk-free
+# matching subsequence.  That's what catches peoples' eyes.  The
+# Windows(tm) windiff has another interesting notion, pairing up elements
+# that appear uniquely in each sequence.  That, and the method here,
+# appear to yield more intuitive difference reports than does diff.  This
+# method appears to be the least vulnerable to synching up on blocks
+# of "junk lines", though (like blank lines in ordinary text files,
+# or maybe "<P>" lines in HTML files).  That may be because this is
+# the only method of the 3 that has a *concept* of "junk" <wink>.
+#
+# Note that ndiff makes no claim to produce a *minimal* diff.  To the
+# contrary, minimal diffs are often counter-intuitive, because they
+# synch up anywhere possible, sometimes accidental matches 100 pages
+# apart.  Restricting synch points to contiguous matches preserves some
+# notion of locality, at the occasional cost of producing a longer diff.
+#
+# With respect to junk, an earlier verion of ndiff simply refused to
+# *start* a match with a junk element.  The result was cases like this:
+#     before: private Thread currentThread;
+#     after:  private volatile Thread currentThread;
+# If you consider whitespace to be junk, the longest continguous match
+# not starting with junk is "e Thread currentThread".  So ndiff reported
+# that "e volatil" was inserted between the 't' and the 'e' in "private".
+# While an accurate view, to people that's absurd.  The current version
+# looks for matching blocks that are entirely junk-free, then extends the
+# longest one of those as far as possible but only with matching junk.
+# So now "currentThread" is matched, then extended to suck up the
+# preceding blank; then "private" is matched, and extended to suck up the
+# following blank; then "Thread" is matched; and finally ndiff reports
+# that "volatile " was inserted before "Thread".  The only quibble
+# remaining is that perhaps it was really the case that " volative"
+# was inserted after "private".  I can live with that <wink>.
+#
+# NOTE on the output:  From an ndiff report,
+# 1) The first file can be recovered by retaining only lines that begin
+#    with "  " or "- ", and deleting those 2-character prefixes.
+# 2) The second file can be recovered similarly, but by retaining only
+#    "  " and "+ " lines.
+# 3) Lines beginning with "? " attempt to guide the eye to intraline
+#    differences, and were not present in either input file.
+#
+# NOTE on junk:  the module-level names
+#    IS_LINE_JUNK
+#    IS_CHARACTER_JUNK
+# can be set to any functions you like.  The first one should accept
+# a single string argument, and return true iff the string is junk.
+# The default is whether the regexp r"\s*#?\s*$" matches (i.e., a
+# line without visible characters, except for at most one splat).
+# The second should accept a string of length 1 etc.  The default is
+# whether the character is a blank or tab (note: bad idea to include
+# newline in this!).
+#
+# After setting those, you can call fcompare(f1name, f2name) with the
+# names of the files you want to compare.  The difference report
+# is sent to stdout.  Or you can call main(), which expects to find
+# (exactly) the two file names in sys.argv.
+
+import string
+TRACE = 0
+
+# define what "junk" means
+import re
+
+def IS_LINE_JUNK(line, pat=re.compile(r"\s*#?\s*$").match):
+    return pat(line) is not None
+
+def IS_CHARACTER_JUNK(ch, ws=" \t"):
+    return ch in ws
+
+del re
+
+class SequenceMatcher:
+    def __init__(self, isjunk=None, a='', b=''):
+        # Members:
+        # a
+        #      first sequence
+        # b
+        #      second sequence; differences are computed as "what do
+        #      we need to do to 'a' to change it into 'b'?"
+        # b2j
+        #      for x in b, b2j[x] is a list of the indices (into b)
+        #      at which x appears; junk elements do not appear
+        # b2jhas
+        #      b2j.has_key
+        # fullbcount
+        #      for x in b, fullbcount[x] == the number of times x
+        #      appears in b; only materialized if really needed (used
+        #      only for computing quick_ratio())
+        # matching_blocks
+        #      a list of (i, j, k) triples, where a[i:i+k] == b[j:j+k];
+        #      ascending & non-overlapping in i and in j; terminated by
+        #      a dummy (len(a), len(b), 0) sentinel
+        # opcodes
+        #      a list of (tag, i1, i2, j1, j2) tuples, where tag is
+        #      one of
+        #          'replace'   a[i1:i2] should be replaced by b[j1:j2]
+        #          'delete'    a[i1:i2] should be deleted
+        #          'insert'    b[j1:j2] should be inserted
+        #          'equal'     a[i1:i2] == b[j1:j2]
+        # isjunk
+        #      a user-supplied function taking a sequence element and
+        #      returning true iff the element is "junk" -- this has
+        #      subtle but helpful effects on the algorithm, which I'll
+        #      get around to writing up someday <0.9 wink>.
+        #      DON'T USE!  Only __chain_b uses this.  Use isbjunk.
+        # isbjunk
+        #      for x in b, isbjunk(x) == isjunk(x) but much faster;
+        #      it's really the has_key method of a hidden dict.
+        #      DOES NOT WORK for x in a!
+
+        self.isjunk = isjunk
+        self.a = self.b = None
+        self.set_seqs(a, b)
+
+    def set_seqs(self, a, b):
+        self.set_seq1(a)
+        self.set_seq2(b)
+
+    def set_seq1(self, a):
+        if a is self.a:
+            return
+        self.a = a
+        self.matching_blocks = self.opcodes = None
+
+    def set_seq2(self, b):
+        if b is self.b:
+            return
+        self.b = b
+        self.matching_blocks = self.opcodes = None
+        self.fullbcount = None
+        self.__chain_b()
+
+    # for each element x in b, set b2j[x] to a list of the indices in
+    # b where x appears; the indices are in increasing order; note that
+    # the number of times x appears in b is len(b2j[x]) ...
+    # when self.isjunk is defined, junk elements don't show up in this
+    # map at all, which stops the central find_longest_match method
+    # from starting any matching block at a junk element ...
+    # also creates the fast isbjunk function ...
+    # note that this is only called when b changes; so for cross-product
+    # kinds of matches, it's best to call set_seq2 once, then set_seq1
+    # repeatedly
+
+    def __chain_b(self):
+        # Because isjunk is a user-defined (not C) function, and we test
+        # for junk a LOT, it's important to minimize the number of calls.
+        # Before the tricks described here, __chain_b was by far the most
+        # time-consuming routine in the whole module!  If anyone sees
+        # Jim Roskind, thank him again for profile.py -- I never would
+        # have guessed that.
+        # The first trick is to build b2j ignoring the possibility
+        # of junk.  I.e., we don't call isjunk at all yet.  Throwing
+        # out the junk later is much cheaper than building b2j "right"
+        # from the start.
+        b = self.b
+        self.b2j = b2j = {}
+        self.b2jhas = b2jhas = b2j.has_key
+        for i in xrange(0, len(b)):
+            elt = b[i]
+            if b2jhas(elt):
+                b2j[elt].append(i)
+            else:
+                b2j[elt] = [i]
+
+        # Now b2j.keys() contains elements uniquely, and especially when
+        # the sequence is a string, that's usually a good deal smaller
+        # than len(string).  The difference is the number of isjunk calls
+        # saved.
+        isjunk, junkdict = self.isjunk, {}
+        if isjunk:
+            for elt in b2j.keys():
+                if isjunk(elt):
+                    junkdict[elt] = 1   # value irrelevant; it's a set
+                    del b2j[elt]
+
+        # Now for x in b, isjunk(x) == junkdict.has_key(x), but the
+        # latter is much faster.  Note too that while there may be a
+        # lot of junk in the sequence, the number of *unique* junk
+        # elements is probably small.  So the memory burden of keeping
+        # this dict alive is likely trivial compared to the size of b2j.
+        self.isbjunk = junkdict.has_key
+
+    def find_longest_match(self, alo, ahi, blo, bhi):
+        """Find longest matching block in a[alo:ahi] and b[blo:bhi].
+
+        If isjunk is not defined:
+
+        Return (i,j,k) such that a[i:i+k] is equal to b[j:j+k], where
+            alo <= i <= i+k <= ahi
+            blo <= j <= j+k <= bhi
+        and for all (i',j',k') meeting those conditions,
+            k >= k'
+            i <= i'
+            and if i == i', j <= j'
+        In other words, of all maximal matching blocks, returns one
+        that starts earliest in a, and of all those maximal matching
+        blocks that start earliest in a, returns the one that starts
+        earliest in b.
+
+        If isjunk is defined, first the longest matching block is
+        determined as above, but with the additional restriction that
+        no junk element appears in the block.  Then that block is
+        extended as far as possible by matching (only) junk elements on
+        both sides.  So the resulting block never matches on junk except
+        as identical junk happens to be adjacent to an "interesting"
+        match.
+
+        If no blocks match, returns (alo, blo, 0).
+        """
+
+        # CAUTION:  stripping common prefix or suffix would be incorrect.
+        # E.g.,
+        #    ab
+        #    acab
+        # Longest matching block is "ab", but if common prefix is
+        # stripped, it's "a" (tied with "b").  UNIX(tm) diff does so
+        # strip, so ends up claiming that ab is changed to acab by
+        # inserting "ca" in the middle.  That's minimal but unintuitive:
+        # "it's obvious" that someone inserted "ac" at the front.
+        # Windiff ends up at the same place as diff, but by pairing up
+        # the unique 'b's and then matching the first two 'a's.
+
+        # find longest junk-free match
+        a, b, b2j, isbjunk = self.a, self.b, self.b2j, self.isbjunk
+        besti, bestj, bestsize = alo, blo, 0
+        for i in xrange(alo, ahi):
+            # check for longest match starting at a[i]
+            if i + bestsize >= ahi:
+                # we're too far right to get a new best
+                break
+            # look at all instances of a[i] in b; note that because
+            # b2j has no junk keys, the loop is skipped if a[i] is junk
+            for j in b2j.get(a[i], []):
+                # a[i] matches b[j]
+                if j < blo:
+                    continue
+                if j + bestsize >= bhi:
+                    # we're too far right to get a new best, here or
+                    # anywhere to the right
+                    break
+                if a[i + bestsize] != b[j + bestsize]:
+                    # can't be longer match; this test is not necessary
+                    # for correctness, but is a huge win for efficiency
+                    continue
+                # set k to length of match
+                k = 1   # a[i] == b[j] already known
+                while i + k < ahi and j + k < bhi and \
+                      a[i+k] == b[j+k] and not isbjunk(b[j+k]):
+                    k = k + 1
+                if k > bestsize:
+                    besti, bestj, bestsize = i, j, k
+                    if i + bestsize >= ahi:
+                        # only time in my life I really wanted a
+                        # labelled break <wink> -- we're done with
+                        # both loops now
+                        break
+
+        # Now that we have a wholly interesting match (albeit possibly
+        # empty!), we may as well suck up the matching junk on each
+        # side of it too.  Can't think of a good reason not to, and it
+        # saves post-processing the (possibly considerable) expense of
+        # figuring out what to do with it.  In the case of an empty
+        # interesting match, this is clearly the right thing to do,
+        # because no other kind of match is possible in the regions.
+        while besti > alo and bestj > blo and \
+              isbjunk(b[bestj-1]) and \
+              a[besti-1] == b[bestj-1]:
+            besti, bestj, bestsize = besti-1, bestj-1, bestsize+1
+        while besti+bestsize < ahi and bestj+bestsize < bhi and \
+              isbjunk(b[bestj+bestsize]) and \
+              a[besti+bestsize] == b[bestj+bestsize]:
+            bestsize = bestsize + 1
+
+        if TRACE:
+            print "get_matching_blocks", alo, ahi, blo, bhi
+            print "    returns", besti, bestj, bestsize
+        return besti, bestj, bestsize
+
+#   A different implementation, using a binary doubling technique that
+#   does far fewer element compares (trades 'em for integer compares),
+#   and has n*lg n worst-case behavior.  Alas, the code is much harder
+#   to follow (the details are tricky!), and in most cases I've seen,
+#   it takes at least 50% longer than the "clever dumb" method above;
+#   probably due to creating layers of small dicts.
+#   NOTE:  this no longer matches the version above wrt junk; remains
+#   too unpromising to update it; someday, though ...
+
+#    def find_longest_match(self, alo, ahi, blo, bhi):
+#        """Find longest matching block in a[alo:ahi] and b[blo:bhi].
+#
+#        Return (i,j,k) such that a[i:i+k] is equal to b[j:j+k], where
+#            alo <= i <= i+k <= ahi
+#            blo <= j <= j+k <= bhi
+#        and for all (i',j',k') meeting those conditions,
+#            k >= k'
+#            i <= i'
+#            and if i == i', j <= j'
+#        In other words, of all maximal matching blocks, returns one
+#        that starts earliest in a, and of all those maximal matching
+#        blocks that start earliest in a, returns the one that starts
+#        earliest in b.
+#
+#        If no blocks match, returns (alo, blo, 0).
+#        """
+#
+#        a, b2j = self.a, self.b2j
+#        # alljs[size][i] is a set of all j's s.t. a[i:i+len] matches
+#        # b[j:j+len]
+#        alljs = {}
+#        alljs[1] = js = {}
+#        ahits = {}
+#        for i in xrange(alo, ahi):
+#            elt = a[i]
+#            if ahits.has_key(elt):
+#                js[i] = ahits[elt]
+#                continue
+#            if b2j.has_key(elt):
+#                in_range = {}
+#                for j in b2j[elt]:
+#                    if j >= blo:
+#                        if j >= bhi:
+#                            break
+#                        in_range[j] = 1
+#                if in_range:
+#                    ahits[elt] = js[i] = in_range
+#        del ahits
+#        size = 1
+#        while js:
+#            oldsize = size
+#            size = size + size
+#            oldjs = js
+#            alljs[size] = js = {}
+#            for i in oldjs.keys():
+#                # i has matches of size oldsize
+#                if not oldjs.has_key(i + oldsize):
+#                    # can't double it
+#                    continue
+#                second_js = oldjs[i + oldsize]
+#                answer = {}
+#                for j in oldjs[i].keys():
+#                    if second_js.has_key(j + oldsize):
+#                        answer[j] = 1
+#                if answer:
+#                    js[i] = answer
+#        del alljs[size]
+#        size = size >> 1    # max power of 2 with a match
+#        if not size:
+#            return alo, blo, 0
+#        besti, bestj, bestsize = alo, blo, 0
+#        fatis = alljs[size].keys()
+#        fatis.sort()
+#        for i in fatis:
+#            # figure out longest match starting at a[i]
+#            totalsize = halfsize = size
+#            # i has matches of len totalsize at the indices in js
+#            js = alljs[size][i].keys()
+#            while halfsize > 1:
+#                halfsize = halfsize >> 1
+#                # is there a match of len halfsize starting at
+#                # i + totalsize?
+#                newjs = []
+#                if alljs[halfsize].has_key(i + totalsize):
+#                    second_js = alljs[halfsize][i + totalsize]
+#                    for j in js:
+#                        if second_js.has_key(j + totalsize):
+#                            newjs.append(j)
+#                if newjs:
+#                    totalsize = totalsize + halfsize
+#                    js = newjs
+#            if totalsize > bestsize:
+#                besti, bestj, bestsize = i, min(js), totalsize
+#        return besti, bestj, bestsize
+
+    def get_matching_blocks(self):
+        if self.matching_blocks is not None:
+            return self.matching_blocks
+        self.matching_blocks = []
+        la, lb = len(self.a), len(self.b)
+        self.__helper(0, la, 0, lb, self.matching_blocks)
+        self.matching_blocks.append( (la, lb, 0) )
+        if TRACE:
+            print '*** matching blocks', self.matching_blocks
+        return self.matching_blocks
+
+    # builds list of matching blocks covering a[alo:ahi] and
+    # b[blo:bhi], appending them in increasing order to answer
+
+    def __helper(self, alo, ahi, blo, bhi, answer):
+        i, j, k = x = self.find_longest_match(alo, ahi, blo, bhi)
+        # a[alo:i] vs b[blo:j] unknown
+        # a[i:i+k] same as b[j:j+k]
+        # a[i+k:ahi] vs b[j+k:bhi] unknown
+        if k:
+            if alo < i and blo < j:
+                self.__helper(alo, i, blo, j, answer)
+            answer.append( x )
+            if i+k < ahi and j+k < bhi:
+                self.__helper(i+k, ahi, j+k, bhi, answer)
+
+    def ratio(self):
+        """Return a measure of the sequences' similarity (float in [0,1]).
+
+        Where T is the total number of elements in both sequences, and
+        M is the number of matches, this is 2*M / T.
+        Note that this is 1 if the sequences are identical, and 0 if
+        they have nothing in common.
+        """
+
+        matches = reduce(lambda sum, triple: sum + triple[-1],
+                         self.get_matching_blocks(), 0)
+        return 2.0 * matches / (len(self.a) + len(self.b))
+
+    def quick_ratio(self):
+        """Return an upper bound on ratio() relatively quickly."""
+        # viewing a and b as multisets, set matches to the cardinality
+        # of their intersection; this counts the number of matches
+        # without regard to order, so is clearly an upper bound
+        if self.fullbcount is None:
+            self.fullbcount = fullbcount = {}
+            for elt in self.b:
+                fullbcount[elt] = fullbcount.get(elt, 0) + 1
+        fullbcount = self.fullbcount
+        # avail[x] is the number of times x appears in 'b' less the
+        # number of times we've seen it in 'a' so far ... kinda
+        avail = {}
+        availhas, matches = avail.has_key, 0
+        for elt in self.a:
+            if availhas(elt):
+                numb = avail[elt]
+            else:
+                numb = fullbcount.get(elt, 0)
+            avail[elt] = numb - 1
+            if numb > 0:
+                matches = matches + 1
+        return 2.0 * matches / (len(self.a) + len(self.b))
+
+    def real_quick_ratio(self):
+        """Return an upper bound on ratio() very quickly"""
+        la, lb = len(self.a), len(self.b)
+        # can't have more matches than the number of elements in the
+        # shorter sequence
+        return 2.0 * min(la, lb) / (la + lb)
+
+    def get_opcodes(self):
+        if self.opcodes is not None:
+            return self.opcodes
+        i = j = 0
+        self.opcodes = answer = []
+        for ai, bj, size in self.get_matching_blocks():
+            # invariant:  we've pumped out correct diffs to change
+            # a[:i] into b[:j], and the next matching block is
+            # a[ai:ai+size] == b[bj:bj+size].  So we need to pump
+            # out a diff to change a[i:ai] into b[j:bj], pump out
+            # the matching block, and move (i,j) beyond the match
+            tag = ''
+            if i < ai and j < bj:
+                tag = 'replace'
+            elif i < ai:
+                tag = 'delete'
+            elif j < bj:
+                tag = 'insert'
+            if tag:
+                answer.append( (tag, i, ai, j, bj) )
+            i, j = ai+size, bj+size
+            # the list of matching blocks is terminated by a
+            # sentinel with size 0
+            if size:
+                answer.append( ('equal', ai, i, bj, j) )
+        return answer
+
+# meant for dumping lines
+def dump(tag, x, lo, hi):
+    for i in xrange(lo, hi):
+        print tag, x[i],
+
+# figure out which mark to stick under characters in lines that
+# have changed (blank = same, - = deleted, + = inserted, ^ = replaced)
+_combine = { '  ': ' ',
+             '. ': '-',
+             ' .': '+',
+             '..': '^' }
+
+def plain_replace(a, alo, ahi, b, blo, bhi):
+    assert alo < ahi and blo < bhi
+    # dump the shorter block first -- reduces the burden on short-term
+    # memory if the blocks are of very different sizes
+    if bhi - blo < ahi - alo:
+        dump('+', b, blo, bhi)
+        dump('-', a, alo, ahi)
+    else:
+        dump('-', a, alo, ahi)
+        dump('+', b, blo, bhi)
+
+# When replacing one block of lines with another, this guy searches
+# the blocks for *similar* lines; the best-matching pair (if any) is
+# used as a synch point, and intraline difference marking is done on
+# the similar pair.  Lots of work, but often worth it.
+
+def fancy_replace(a, alo, ahi, b, blo, bhi):
+    if TRACE:
+        print '*** fancy_replace', alo, ahi, blo, bhi
+        dump('>', a, alo, ahi)
+        dump('<', b, blo, bhi)
+
+    # don't synch up unless the lines have a similarity score of at
+    # least cutoff; best_ratio tracks the best score seen so far
+    best_ratio, cutoff = 0.74, 0.75
+    cruncher = SequenceMatcher(IS_CHARACTER_JUNK)
+    eqi, eqj = None, None   # 1st indices of equal lines (if any)
+
+    # search for the pair that matches best without being identical
+    # (identical lines must be junk lines, & we don't want to synch up
+    # on junk -- unless we have to)
+    for j in xrange(blo, bhi):
+        bj = b[j]
+        cruncher.set_seq2(bj)
+        for i in xrange(alo, ahi):
+            ai = a[i]
+            if ai == bj:
+                if eqi is None:
+                    eqi, eqj = i, j
+                continue
+            cruncher.set_seq1(ai)
+            # computing similarity is expensive, so use the quick
+            # upper bounds first -- have seen this speed up messy
+            # compares by a factor of 3.
+            # note that ratio() is only expensive to compute the first
+            # time it's called on a sequence pair; the expensive part
+            # of the computation is cached by cruncher
+            if cruncher.real_quick_ratio() > best_ratio and \
+                  cruncher.quick_ratio() > best_ratio and \
+                  cruncher.ratio() > best_ratio:
+                best_ratio, best_i, best_j = cruncher.ratio(), i, j
+    if best_ratio < cutoff:
+        # no non-identical "pretty close" pair
+        if eqi is None:
+            # no identical pair either -- treat it as a straight replace
+            plain_replace(a, alo, ahi, b, blo, bhi)
+            return
+        # no close pair, but an identical pair -- synch up on that
+        best_i, best_j, best_ratio = eqi, eqj, 1.0
+    else:
+        # there's a close pair, so forget the identical pair (if any)
+        eqi = None
+
+    # a[best_i] very similar to b[best_j]; eqi is None iff they're not
+    # identical
+    if TRACE:
+        print '*** best_ratio', best_ratio, best_i, best_j
+        dump('>', a, best_i, best_i+1)
+        dump('<', b, best_j, best_j+1)
+
+    # pump out diffs from before the synch point
+    fancy_helper(a, alo, best_i, b, blo, best_j)
+
+    # do intraline marking on the synch pair
+    aelt, belt = a[best_i], b[best_j]
+    if eqi is None:
+        # pump out a '-', '+', '?' triple for the synched lines;
+        atags = btags = ""
+        cruncher.set_seqs(aelt, belt)
+        for tag, ai1, ai2, bj1, bj2 in cruncher.get_opcodes():
+            la, lb = ai2 - ai1, bj2 - bj1
+            if tag == 'replace':
+                atags = atags + '.' * la
+                btags = btags + '.' * lb
+            elif tag == 'delete':
+                atags = atags + '.' * la
+            elif tag == 'insert':
+                btags = btags + '.' * lb
+            elif tag == 'equal':
+                atags = atags + ' ' * la
+                btags = btags + ' ' * lb
+            else:
+                raise ValueError, 'unknown tag ' + `tag`
+        la, lb = len(atags), len(btags)
+        if la < lb:
+            atags = atags + ' ' * (lb - la)
+        elif lb < la:
+            btags = btags + ' ' * (la - lb)
+        combined = map(lambda x,y: _combine[x+y], atags, btags)
+        print '-', aelt, '+', belt, '?', \
+              string.rstrip(string.join(combined, ''))
+    else:
+        # the synch pair is identical
+        print ' ', aelt,
+
+    # pump out diffs from after the synch point
+    fancy_helper(a, best_i+1, ahi, b, best_j+1, bhi)
+
+def fancy_helper(a, alo, ahi, b, blo, bhi):
+    if alo < ahi:
+        if blo < bhi:
+            fancy_replace(a, alo, ahi, b, blo, bhi)
+        else:
+            dump('-', a, alo, ahi)
+    elif blo < bhi:
+        dump('+', b, blo, bhi)
+
+# open a file & return the file object; gripe and return 0 if it
+# couldn't be opened
+def fopen(fname):
+    try:
+        return open(fname, 'r')
+    except IOError, detail:
+        print "couldn't open " + fname + ": " + `detail`
+        return 0
+
+# open two files & spray the diff to stdout; return false iff a problem
+def fcompare(f1name, f2name):
+    f1 = fopen(f1name)
+    f2 = fopen(f2name)
+    if not f1 or not f2:
+        return 0
+
+    a = f1.readlines(); f1.close()
+    b = f2.readlines(); f2.close()
+
+    cruncher = SequenceMatcher(IS_LINE_JUNK, a, b)
+    for tag, alo, ahi, blo, bhi in cruncher.get_opcodes():
+        if tag == 'replace':
+            fancy_replace(a, alo, ahi, b, blo, bhi)
+        elif tag == 'delete':
+            dump('-', a, alo, ahi)
+        elif tag == 'insert':
+            dump('+', b, blo, bhi)
+        elif tag == 'equal':
+            dump(' ', a, alo, ahi)
+        else:
+            raise ValueError, 'unknown tag ' + `tag`
+
+    return 1
+
+# get file names from argv & compare; return false iff a problem
+def main():
+    from sys import argv
+    if len(argv) != 3:
+        print 'need 2 args'
+        return 0
+    [f1name, f2name] = argv[1:3]
+    print '-:', f1name
+    print '+:', f2name
+    return fcompare(f1name, f2name)
+
+if __name__ == '__main__':
+    if 1:
+        main()
+    else:
+        import profile, pstats
+        statf = "ndiff.pro"
+        profile.run("main()", statf)
+        stats = pstats.Stats(statf)
+        stats.strip_dirs().sort_stats('time').print_stats()
+