# Module: Diff::LCS

Included in:
Array, String
Defined in:
lib/diff/lcs.rb,
lib/diff/lcs.rb,
lib/diff/lcs.rb,
lib/diff/lcs/version.rb,
lib/diff/lcs/callbacks.rb

## Overview

Computes "intelligent" differences between two sequenced Enumerables. This is an implementation of the McIlroy-Hunt "diff" algorithm for Enumerable objects that include Diffable.

Based on Mario I. Wolczko's <mario@wolczko.com> Smalltalk version (1.2, 1993) and Ned Konz's <perl@bike-nomad.com> Perl version (Algorithm::Diff).

## Synopsis

``````require 'diff/lcs'

seq1 = %w(a b c e h j l m n p)
seq2 = %w(b c d e f j k l m r s t)

lcs = Diff::LCS.LCS(seq1, seq2)
diffs = Diff::LCS.diff(seq1, seq2)
sdiff = Diff::LCS.sdiff(seq1, seq2)
seq = Diff::LCS.traverse_sequences(seq1, seq2, callback_obj)
bal = Diff::LCS.traverse_balanced(seq1, seq2, callback_obj)
seq2 == Diff::LCS.patch(seq1, diffs)
seq2 == Diff::LCS.patch!(seq1, diffs)
seq1 == Diff::LCS.unpatch(seq2, diffs)
seq1 == Diff::LCS.unpatch!(seq2, diffs)
seq2 == Diff::LCS.patch(seq1, sdiff)
seq2 == Diff::LCS.patch!(seq1, sdiff)
seq1 == Diff::LCS.unpatch(seq2, sdiff)
seq1 == Diff::LCS.unpatch!(seq2, sdiff)``````

Alternatively, objects can be extended with Diff::LCS:

``````seq1.extend(Diff::LCS)
lcs = seq1.lcs(seq2)
diffs = seq1.diff(seq2)
sdiff = seq1.sdiff(seq2)
seq = seq1.traverse_sequences(seq2, callback_obj)
bal = seq1.traverse_balanced(seq2, callback_obj)
seq2 == seq1.patch(diffs)
seq2 == seq1.patch!(diffs)
seq1 == seq2.unpatch(diffs)
seq1 == seq2.unpatch!(diffs)
seq2 == seq1.patch(sdiff)
seq2 == seq1.patch!(sdiff)
seq1 == seq2.unpatch(sdiff)
seq1 == seq2.unpatch!(sdiff)``````

Default extensions are provided for Array and String objects through the use of 'diff/lcs/array' and 'diff/lcs/string'.

## Introduction (by Mark-Jason Dominus)

The following text is from the Perl documentation. The only changes have been to make the text appear better in Rdoc.

I once read an article written by the authors of diff; they said that they hard worked very hard on the algorithm until they found the right one.

I think what they ended up using (and I hope someone will correct me, because I am not very confident about this) was the `longest common subsequence' method. In the LCS problem, you have two sequences of items:

``````a b c d f g h j q z
a b c d e f g i j k r x y z``````

and you want to find the longest sequence of items that is present in both original sequences in the same order. That is, you want to find a new sequence S which can be obtained from the first sequence by deleting some items, and from the second sequence by deleting other items. You also want S to be as long as possible. In this case S is:

``a b c d f g j z``

From there it's only a small step to get diff-like output:

``````e   h i   k   q r x y
+   - +   +   - + + +``````

This module solves the LCS problem. It also includes a canned function to generate diff-like output.

It might seem from the example above that the LCS of two sequences is always pretty obvious, but that's not always the case, especially when the two sequences have many repeated elements. For example, consider

``````a x b y c z p d q
a b c a x b y c z``````

A naive approach might start by matching up the a and b that appear at the beginning of each sequence, like this:

``````a x b y c         z p d q
a   b   c a b y c z``````

This finds the common subsequence a b c z. But actually, the LCS is +a x b y c z+:

``````      a x b y c z p d q
a b c a x b y c z``````

## Author

This version is by Austin Ziegler <diff-lcs@halostatue.ca>.

It is based on the Perl Algorithm::Diff by Ned Konz <perl@bike-nomad.com>, copyright &copy; 2000 - 2002 and the Smalltalk diff version by Mario I. Wolczko <mario@wolczko.com>, copyright &copy;

1. Documentation includes work by Mark-Jason Dominus.

## Licence

Copyright &copy; 2004 Austin Ziegler This program is free software; you can redistribute it and/or modify it under the same terms as Ruby, or alternatively under the Perl Artistic licence.

## Credits

Much of the documentation is taken directly from the Perl Algorithm::Diff implementation and was written originally by Mark-Jason Dominus <mjd-perl-diff@plover.com> and later by Ned Konz. The basic Ruby implementation was re-ported from the Smalltalk implementation, available at st.cs.uiuc.edu/pub/Smalltalk/MANCHESTER/manchester/4.0/diff.st

#sdiff and #traverse_balanced were written for the Perl version by Mike Schilli <m@perlmeister.com>.

"The algorithm is described in A Fast Algorithm for Computing Longest Common Subsequences, CACM, vol.20, no.5, pp.350-353, May 1977, with a few minor improvements to improve the speed."

## Constant Summary

PATCH_MAP =

:nodoc:

```{ #:nodoc:
:patch => { '+' => '+', '-' => '-', '!' => '!', '=' => '=' },
:unpatch => { '+' => '-', '-' => '+', '!' => '!', '=' => '=' }
}```
VERSION =
`'1.1.2'`
SequenceCallbacks =

An alias for DefaultCallbacks that is used in Diff::LCS#traverse_sequences.

``Diff::LCS.LCS(seq1, seq2, Diff::LCS::SequenceCallbacks)``
`DefaultCallbacks`
BalancedCallbacks =

An alias for DefaultCallbacks that is used in Diff::LCS#traverse_balanced.

``Diff::LCS.LCS(seq1, seq2, Diff::LCS::BalancedCallbacks)``
`DefaultCallbacks`

## Class Method Summary (collapse)

• Examine the patchset and the source to see in which direction the patch should be applied.

• If vector maps the matching elements of another collection onto this Enumerable, compute the inverse vector that maps this Enumerable onto the collection.

• Compute the longest common subsequence between the sequenced Enumerables a and b.

• Normalize the patchset.

• Returns a hash mapping each element of an Enumerable to the set of positions it occupies in the Enumerable, optionally restricted to the elements specified in the range of indexes specified by interval.

• Find the place at which value would normally be inserted into the Enumerable.

• Diff::LCS.diff computes the smallest set of additions and deletions necessary to turn the first sequence into the second, and returns a description of these changes.

• Given two sequenced Enumerables, LCS returns an Array containing their longest common subsequences.

• Given a patchset, convert the current version to the new version.

• Given a set of patchset, convert the current version to the next version.

• Diff::LCS.sdiff computes all necessary components to show two sequences and their minimized differences side by side, just like the Unix utility sdiff does:.

• #traverse_balanced is an alternative to #traverse_sequences.

• Diff::LCS.traverse_sequences is the most general facility provided by this module; diff and LCS are implemented as calls to it.

• Given a set of patchset, convert the current version to the prior version.

## Instance Method Summary (collapse)

• Returns the difference set between self and other.

• Returns an Array containing the longest common subsequence(s) between self and other.

• Attempts to patch a copy of self with the provided patchset.

• Attempts to patch self with the provided patchset.

• Returns the balanced ("side-by-side") difference set between self and other.

• Traverses the discovered longest common subsequences between self and other using the alternate, balanced algorithm.

• Traverses the discovered longest common subsequences between self and other.

• Attempts to unpatch a copy of self with the provided patchset.

• Attempts to unpatch self with the provided patchset.

## Class Method Details

### + (Object) __diff_direction(src, patchset, limit = nil)

Examine the patchset and the source to see in which direction the patch should be applied.

WARNING: By default, this examines the whole patch, so this could take some time. This also works better with Diff::LCS::ContextChange or Diff::LCS::Change as its source, as an array will cause the creation of one of the above.

 ``` 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016``` ```# File 'lib/diff/lcs.rb', line 931 def __diff_direction(src, patchset, limit = nil) count = left = left_miss = right = right_miss = 0 string = src.kind_of?(String) patchset.each do |change| count += 1 case change when Diff::LCS::Change # With a simplistic change, we can't tell the difference between # the left and right on '!' actions, so we ignore those. On '=' # actions, if there's a miss, we miss both left and right. element = string ? src[change.position, 1] : src[change.position] case change.action when '-' if element == change.element left += 1 else left_miss += 1 end when '+' if element == change.element right += 1 else right_miss += 1 end when '=' if element != change.element left_miss += 1 right_miss += 1 end end when Diff::LCS::ContextChange case change.action when '-' # Remove details from the old string element = string ? src[change.old_position, 1] : src[change.old_position] if element == change.old_element left += 1 else left_miss += 1 end when '+' element = string ? src[change.new_position, 1] : src[change.new_position] if element == change.new_element right += 1 else right_miss += 1 end when '=' le = string ? src[change.old_position, 1] : src[change.old_position] re = string ? src[change.new_position, 1] : src[change.new_position] left_miss += 1 if le != change.old_element right_miss += 1 if re != change.new_element when '!' element = string ? src[change.old_position, 1] : src[change.old_position] if element == change.old_element left += 1 else element = string ? src[change.new_position, 1] : src[change.new_position] if element == change.new_element right += 1 else left_miss += 1 right_miss += 1 end end end end break if not limit.nil? and count > limit end no_left = (left == 0) and (left_miss >= 0) no_right = (right == 0) and (right_miss >= 0) case [no_left, no_right] when [false, true] return :patch when [true, false] return :unpatch else raise "The provided patchset does not appear to apply to the provided value as either source or destination value." end end```

### + (Object) __inverse_vector(a, vector)

If vector maps the matching elements of another collection onto this Enumerable, compute the inverse vector that maps this Enumerable onto the collection. (Currently unused.)

 ``` 904 905 906 907 908 909 910``` ```# File 'lib/diff/lcs.rb', line 904 def __inverse_vector(a, vector) inverse = a.dup (0 ... vector.size).each do |ii| inverse[vector[ii]] = ii unless vector[ii].nil? end inverse end```

### + (Object) __lcs(a, b)

Compute the longest common subsequence between the sequenced Enumerables a and b. The result is an array whose contents is such that

``````result = Diff::LCS.__lcs(a, b)
result.each_with_index do |e, ii|
assert_equal(a[ii], b[e]) unless e.nil?
end``````
 ``` 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861``` ```# File 'lib/diff/lcs.rb', line 808 def __lcs(a, b) a_start = b_start = 0 a_finish = a.size - 1 b_finish = b.size - 1 vector = [] # Prune off any common elements at the beginning... while (a_start <= a_finish) and (b_start <= b_finish) and (a[a_start] == b[b_start]) vector[a_start] = b_start a_start += 1 b_start += 1 end # Now the end... while (a_start <= a_finish) and (b_start <= b_finish) and (a[a_finish] == b[b_finish]) vector[a_finish] = b_finish a_finish -= 1 b_finish -= 1 end # Now, compute the equivalence classes of positions of elements. b_matches = Diff::LCS.__position_hash(b, b_start .. b_finish) thresh = [] links = [] (a_start .. a_finish).each do |ii| ai = a.kind_of?(String) ? a[ii, 1] : a[ii] bm = b_matches[ai] kk = nil bm.reverse_each do |jj| if kk and (thresh[kk] > jj) and (thresh[kk - 1] < jj) thresh[kk] = jj else kk = Diff::LCS.__replace_next_larger(thresh, jj, kk) end links[kk] = [ (kk > 0) ? links[kk - 1] : nil, ii, jj ] unless kk.nil? end end unless thresh.empty? link = links[thresh.size - 1] while not link.nil? vector[link[1]] = link[2] link = link[0] end end vector end```

### + (Object) __normalize_patchset(patchset)

Normalize the patchset. A patchset is always a sequence of changes, but how those changes are represented may vary, depending on how they were generated. In all cases we support, we also support the array representation of the changes. The formats are:

``````[ # patchset <- Diff::LCS.diff(a, b)
[ # one or more hunks
Diff::LCS::Change # one or more changes
] ]

[ # patchset, equivalent to the above
[ # one or more hunks
[ action, line, value ] # one or more changes
] ]

[ # patchset <- Diff::LCS.diff(a, b, Diff::LCS::ContextDiffCallbacks)
#       OR <- Diff::LCS.sdiff(a, b, Diff::LCS::ContextDiffCallbacks)
[ # one or more hunks
Diff::LCS::ContextChange # one or more changes
] ]

[ # patchset, equivalent to the above
[ # one or more hunks
[ action, [ old line, old value ], [ new line, new value ] ]
# one or more changes
] ]

[ # patchset <- Diff::LCS.sdiff(a, b)
#       OR <- Diff::LCS.diff(a, b, Diff::LCS::SDiffCallbacks)
Diff::LCS::ContextChange # one or more changes
]

[ # patchset, equivalent to the above
[ action, [ old line, old value ], [ new line, new value ] ]
# one or more changes
]``````

The result of this will be either of the following.

``````[ # patchset
Diff::LCS::ContextChange # one or more changes
]

[ # patchset
Diff::LCS::Change # one or more changes
]``````

If either of the above is provided, it will be returned as such.

 ``` 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095``` ```# File 'lib/diff/lcs.rb', line 1067 def __normalize_patchset(patchset) patchset.map do |hunk| case hunk when Diff::LCS::ContextChange, Diff::LCS::Change hunk when Array if (not hunk[0].kind_of?(Array)) and hunk[1].kind_of?(Array) and hunk[2].kind_of?(Array) Diff::LCS::ContextChange.from_a(hunk) else hunk.map do |change| case change when Diff::LCS::ContextChange, Diff::LCS::Change change when Array # change[1] will ONLY be an array in a ContextChange#to_a call. # In Change#to_a, it represents the line (singular). if change[1].kind_of?(Array) Diff::LCS::ContextChange.from_a(change) else Diff::LCS::Change.from_a(change) end end end end else raise ArgumentError, "Cannot normalise a hunk of class #{hunk.class}." end end.flatten end```

### + (Object) __position_hash(enum, interval = 0 .. -1)

Returns a hash mapping each element of an Enumerable to the set of positions it occupies in the Enumerable, optionally restricted to the elements specified in the range of indexes specified by interval.

 ``` 915 916 917 918 919 920 921 922``` ```# File 'lib/diff/lcs.rb', line 915 def __position_hash(enum, interval = 0 .. -1) hash = Hash.new { |hh, kk| hh[kk] = [] } interval.each do |ii| kk = enum.kind_of?(String) ? enum[ii, 1] : enum[ii] hash[kk] << ii end hash end```

### + (Object) __replace_next_larger(enum, value, last_index = nil)

Find the place at which value would normally be inserted into the Enumerable. If that place is already occupied by value, do nothing and return nil. If the place does not exist (i.e., it is off the end of the Enumerable), add it to the end. Otherwise, replace the element at that point with value. It is assumed that the Enumerable's values are numeric.

This operation preserves the sort order.

 ``` 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899``` ```# File 'lib/diff/lcs.rb', line 871 def __replace_next_larger(enum, value, last_index = nil) # Off the end? if enum.empty? or (value > enum[-1]) enum << value return enum.size - 1 end # Binary search for the insertion point last_index ||= enum.size first_index = 0 while (first_index <= last_index) ii = (first_index + last_index) >> 1 found = enum[ii] if value == found return nil elsif value > found first_index = ii + 1 else last_index = ii - 1 end end # The insertion point is in first_index; overwrite the next larger # value. enum[first_index] = value return first_index end```

### + (Object) diff(seq1, seq2, callbacks = nil, &block)

Diff::LCS.diff computes the smallest set of additions and deletions necessary to turn the first sequence into the second, and returns a description of these changes.

See Diff::LCS::DiffCallbacks for the default behaviour. An alternate behaviour may be implemented with Diff::LCS::ContextDiffCallbacks. If a Class argument is provided for callbacks, #diff will attempt to initialise it. If the callbacks object (possibly initialised) responds to #finish, it will be called.

 ``` 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267``` ```# File 'lib/diff/lcs.rb', line 246 def diff(seq1, seq2, callbacks = nil, &block) # :yields diff changes: callbacks ||= Diff::LCS::DiffCallbacks if callbacks.kind_of?(Class) cb = callbacks.new rescue callbacks callbacks = cb end traverse_sequences(seq1, seq2, callbacks) callbacks.finish if callbacks.respond_to?(:finish) if block_given? res = callbacks.diffs.map do |hunk| if hunk.kind_of?(Array) hunk = hunk.map { |block| yield block } else yield hunk end end res else callbacks.diffs end end```

### + (Object) LCS(seq1, seq2, &block)

Given two sequenced Enumerables, LCS returns an Array containing their longest common subsequences.

``lcs = Diff::LCS.LCS(seq1, seq2)``

This array whose contents is such that:

``````lcs.each_with_index do |ee, ii|
assert(ee.nil? || (seq1[ii] == seq2[ee]))
end``````

If a block is provided, the matching subsequences will be yielded from seq1 in turn and may be modified before they are placed into the returned Array of subsequences.

 ``` 222 223 224 225 226 227 228 229 230 231 232 233 234 235``` ```# File 'lib/diff/lcs.rb', line 222 def LCS(seq1, seq2, &block) #:yields seq1[ii] for each matched: matches = Diff::LCS.__lcs(seq1, seq2) ret = [] matches.each_with_index do |ee, ii| unless matches[ii].nil? if block_given? ret << (yield seq1[ii]) else ret << seq1[ii] end end end ret end```

### + (Object) patch(src, patchset, direction = nil)

Given a patchset, convert the current version to the new version. If direction is not specified (must be :patch or :unpatch), then discovery of the direction of the patch will be attempted.

 ``` 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786``` ```# File 'lib/diff/lcs.rb', line 689 def patch(src, patchset, direction = nil) string = src.kind_of?(String) # Start with a new empty type of the source's class res = src.class.new # Normalize the patchset. patchset = __normalize_patchset(patchset) direction ||= Diff::LCS.__diff_direction(src, patchset) direction ||= :patch ai = bj = 0 patchset.each do |change| # Both Change and ContextChange support #action action = PATCH_MAP[direction][change.action] case change when Diff::LCS::ContextChange case direction when :patch el = change.new_element op = change.old_position np = change.new_position when :unpatch el = change.old_element op = change.new_position np = change.old_position end case action when '-' # Remove details from the old string while ai < op res << (string ? src[ai, 1] : src[ai]) ai += 1 bj += 1 end ai += 1 when '+' while bj < np res << (string ? src[ai, 1] : src[ai]) ai += 1 bj += 1 end res << el bj += 1 when '=' # This only appears in sdiff output with the SDiff callback. # Therefore, we only need to worry about dealing with a single # element. res << el ai += 1 bj += 1 when '!' while ai < op res << (string ? src[ai, 1] : src[ai]) ai += 1 bj += 1 end bj += 1 ai += 1 res << el end when Diff::LCS::Change case action when '-' while ai < change.position res << (string ? src[ai, 1] : src[ai]) ai += 1 bj += 1 end ai += 1 when '+' while bj < change.position res << (string ? src[ai, 1] : src[ai]) ai += 1 bj += 1 end bj += 1 res << change.element end end end while ai < src.size res << (string ? src[ai, 1] : src[ai]) ai += 1 bj += 1 end res end```

### + (Object) patch!(src, patchset)

Given a set of patchset, convert the current version to the next version. Does no auto-discovery.

 ``` 796 797 798``` ```# File 'lib/diff/lcs.rb', line 796 def patch!(src, patchset) Diff::LCS.patch(src, patchset, :patch) end```

### + (Object) sdiff(seq1, seq2, callbacks = nil, &block)

Diff::LCS.sdiff computes all necessary components to show two sequences and their minimized differences side by side, just like the Unix utility sdiff does:

``````old        <     -
same             same
before     |     after
-          >     new``````

See Diff::LCS::SDiffCallbacks for the default behaviour. An alternate behaviour may be implemented with Diff::LCS::ContextDiffCallbacks. If a Class argument is provided for callbacks, #diff will attempt to initialise it. If the callbacks object (possibly initialised) responds to #finish, it will be called.

 ``` 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304``` ```# File 'lib/diff/lcs.rb', line 283 def sdiff(seq1, seq2, callbacks = nil, &block) #:yields diff changes: callbacks ||= Diff::LCS::SDiffCallbacks if callbacks.kind_of?(Class) cb = callbacks.new rescue callbacks callbacks = cb end traverse_balanced(seq1, seq2, callbacks) callbacks.finish if callbacks.respond_to?(:finish) if block_given? res = callbacks.diffs.map do |hunk| if hunk.kind_of?(Array) hunk = hunk.map { |block| yield block } else yield hunk end end res else callbacks.diffs end end```

### + (Object) traverse_balanced(seq1, seq2, callbacks = Diff::LCS::BalancedCallbacks)

#traverse_balanced is an alternative to #traverse_sequences. It uses a different algorithm to iterate through the entries in the computed longest common subsequence. Instead of viewing the changes as insertions or deletions from one of the sequences, #traverse_balanced will report changes between the sequences. To represent a

The arguments to #traverse_balanced are the two sequences to traverse and a callback object, like this:

``traverse_balanced(seq1, seq2, Diff::LCS::ContextDiffCallbacks.new)``

#sdiff is implemented with #traverse_balanced.

## Callback Methods

Optional callback methods are emphasized.

 callbacks#match Called when a and b are pointing to common elements in A and B. callbacks#discard_a Called when a is pointing to an element not in B. callbacks#discard_b Called when b is pointing to an element not in A. callbacks#change Called when a and b are pointing to the same relative position, but A[a] and B[b] are not the same; a change has occurred.

#traverse_balanced might be a bit slower than #traverse_sequences, noticable only while processing huge amounts of data.

The sdiff function of this module is implemented as call to #traverse_balanced.

## Algorithm

``````a---+
v
A = a b c e h j l m n p
B = b c d e f j k l m r s t
^
b---+``````

### Matches

If there are two arrows (a and b) pointing to elements of sequences A and B, the arrows will initially point to the first elements of their respective sequences. #traverse_sequences will advance the arrows through the sequences one element at a time, calling a method on the user-specified callback object before each advance. It will advance the arrows in such a way that if there are elements A[ii] and B[jj] which are both equal and part of the longest common subsequence, there will be some moment during the execution of #traverse_sequences when arrow a is pointing to A[ii] and arrow b is pointing to B[jj]. When this happens, #traverse_sequences will call callbacks#match and then it will advance both arrows.

Otherwise, one of the arrows is pointing to an element of its sequence that is not part of the longest common subsequence. #traverse_sequences will advance that arrow and will call callbacks#discard_a or callbacks#discard_b, depending on which arrow it advanced.

### Changes

If both a and b point to elements that are not part of the longest common subsequence, then #traverse_sequences will try to call callbacks#change and advance both arrows. If callbacks#change is not implemented, then callbacks#discard_a and callbacks#discard_b will be called in turn.

The methods for callbacks#match, callbacks#discard_a, callbacks#discard_b, and callbacks#change are invoked with an event comprising the action ("=", "+", "-", or "!", respectively), the indicies ii and jj, and the elements A[ii] and B[jj]. Return values are discarded by #traverse_balanced.

### Context

Note that ii and jj may not be the same index position, even if a and b are considered to be pointing to matching or changed elements.

 ``` 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678``` ```# File 'lib/diff/lcs.rb', line 577 def traverse_balanced(seq1, seq2, callbacks = Diff::LCS::BalancedCallbacks) matches = Diff::LCS.__lcs(seq1, seq2) a_size = seq1.size b_size = seq2.size ai = bj = mb = 0 ma = -1 string = seq1.kind_of?(String) # Process all the lines in the match vector. loop do # Find next match indices +ma+ and +mb+ loop do ma += 1 break unless ma < matches.size and matches[ma].nil? end break if ma >= matches.size # end of matches? mb = matches[ma] # Change(seq2) while (ai < ma) or (bj < mb) ax = string ? seq1[ai, 1] : seq1[ai] bx = string ? seq2[bj, 1] : seq2[bj] case [(ai < ma), (bj < mb)] when [true, true] if callbacks.respond_to?(:change) event = Diff::LCS::ContextChange.new('!', ai, ax, bj, bx) event = yield event if block_given? callbacks.change(event) ai += 1 bj += 1 else event = Diff::LCS::ContextChange.new('-', ai, ax, bj, bx) event = yield event if block_given? callbacks.discard_a(event) ai += 1 ax = string ? seq1[ai, 1] : seq1[ai] event = Diff::LCS::ContextChange.new('+', ai, ax, bj, bx) event = yield event if block_given? callbacks.discard_b(event) bj += 1 end when [true, false] event = Diff::LCS::ContextChange.new('-', ai, ax, bj, bx) event = yield event if block_given? callbacks.discard_a(event) ai += 1 when [false, true] event = Diff::LCS::ContextChange.new('+', ai, ax, bj, bx) event = yield event if block_given? callbacks.discard_b(event) bj += 1 end end # Match ax = string ? seq1[ai, 1] : seq1[ai] bx = string ? seq2[bj, 1] : seq2[bj] event = Diff::LCS::ContextChange.new('=', ai, ax, bj, bx) event = yield event if block_given? callbacks.match(event) ai += 1 bj += 1 end while (ai < a_size) or (bj < b_size) ax = string ? seq1[ai, 1] : seq1[ai] bx = string ? seq2[bj, 1] : seq2[bj] case [(ai < a_size), (bj < b_size)] when [true, true] if callbacks.respond_to?(:change) event = Diff::LCS::ContextChange.new('!', ai, ax, bj, bx) event = yield event if block_given? callbacks.change(event) ai += 1 bj += 1 else event = Diff::LCS::ContextChange.new('-', ai, ax, bj, bx) event = yield event if block_given? callbacks.discard_a(event) ai += 1 ax = string ? seq1[ai, 1] : seq1[ai] event = Diff::LCS::ContextChange.new('+', ai, ax, bj, bx) event = yield event if block_given? callbacks.discard_b(event) bj += 1 end when [true, false] event = Diff::LCS::ContextChange.new('-', ai, ax, bj, bx) event = yield event if block_given? callbacks.discard_a(event) ai += 1 when [false, true] event = Diff::LCS::ContextChange.new('+', ai, ax, bj, bx) event = yield event if block_given? callbacks.discard_b(event) bj += 1 end end end```

### + (Object) traverse_sequences(seq1, seq2, callbacks = Diff::LCS::SequenceCallbacks, &block)

Diff::LCS.traverse_sequences is the most general facility provided by this module; diff and LCS are implemented as calls to it.

The arguments to #traverse_sequences are the two sequences to traverse, and a callback object, like this:

``traverse_sequences(seq1, seq2, Diff::LCS::ContextDiffCallbacks.new)``

#diff is implemented with #traverse_sequences.

## Callback Methods

Optional callback methods are emphasized.

 callbacks#match Called when a and b are pointing to common elements in A and B. callbacks#discard_a Called when a is pointing to an element not in B. callbacks#discard_b Called when b is pointing to an element not in A. callbacks#finished_a Called when a has reached the end of sequence A. callbacks#finished_b Called when b has reached the end of sequence B.

## Algorithm

``````a---+
v
A = a b c e h j l m n p
B = b c d e f j k l m r s t
^
b---+``````

If there are two arrows (a and b) pointing to elements of sequences A and B, the arrows will initially point to the first elements of their respective sequences. #traverse_sequences will advance the arrows through the sequences one element at a time, calling a method on the user-specified callback object before each advance. It will advance the arrows in such a way that if there are elements A[ii] and B[jj] which are both equal and part of the longest common subsequence, there will be some moment during the execution of #traverse_sequences when arrow a is pointing to A[ii] and arrow b is pointing to B[jj]. When this happens, #traverse_sequences will call callbacks#match and then it will advance both arrows.

Otherwise, one of the arrows is pointing to an element of its sequence that is not part of the longest common subsequence. #traverse_sequences will advance that arrow and will call callbacks#discard_a or callbacks#discard_b, depending on which arrow it advanced. If both arrows point to elements that are not part of the longest common subsequence, then #traverse_sequences will advance one of them and call the appropriate callback, but it is not specified which it will call.

The methods for callbacks#match, callbacks#discard_a, and callbacks#discard_b are invoked with an event comprising the action ("=", "+", or "-", respectively), the indicies ii and jj, and the elements A[ii] and B[jj]. Return values are discarded by #traverse_sequences.

### End of Sequences

If arrow a reaches the end of its sequence before arrow b does, #traverse_sequence try to call callbacks#finished_a with the last index and element of A (A[-1]) and the current index and element of B (B[jj]). If callbacks#finished_a does not exist, then callbacks#discard_b will be called on each element of B until the end of the sequence is reached (the call will be done with A[-1] and B[jj] for each element).

If b reaches the end of B before a reaches the end of A, callbacks#finished_b will be called with the current index and element of A (A[ii]) and the last index and element of B (A[-1]). Again, if callbacks#finished_b does not exist on the callback object, then callbacks#discard_a will be called on each element of A until the end of the sequence is reached (A[ii] and B[-1]).

There is a chance that one additional callbacks#discard_a or callbacks#discard_b will be called after the end of the sequence is reached, if a has not yet reached the end of A or b has not yet reached the end of B.

 ``` 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493``` ```# File 'lib/diff/lcs.rb', line 387 def traverse_sequences(seq1, seq2, callbacks = Diff::LCS::SequenceCallbacks, &block) #:yields change events: matches = Diff::LCS.__lcs(seq1, seq2) run_finished_a = run_finished_b = false string = seq1.kind_of?(String) a_size = seq1.size b_size = seq2.size ai = bj = 0 (0 .. matches.size).each do |ii| b_line = matches[ii] ax = string ? seq1[ii, 1] : seq1[ii] bx = string ? seq2[bj, 1] : seq2[bj] if b_line.nil? unless ax.nil? event = Diff::LCS::ContextChange.new('-', ii, ax, bj, bx) event = yield event if block_given? callbacks.discard_a(event) end else loop do break unless bj < b_line bx = string ? seq2[bj, 1] : seq2[bj] event = Diff::LCS::ContextChange.new('+', ii, ax, bj, bx) event = yield event if block_given? callbacks.discard_b(event) bj += 1 end bx = string ? seq2[bj, 1] : seq2[bj] event = Diff::LCS::ContextChange.new('=', ii, ax, bj, bx) event = yield event if block_given? callbacks.match(event) bj += 1 end ai = ii end ai += 1 # The last entry (if any) processed was a match. +ai+ and +bj+ point # just past the last matching lines in their sequences. while (ai < a_size) or (bj < b_size) # last A? if ai == a_size and bj < b_size if callbacks.respond_to?(:finished_a) and not run_finished_a ax = string ? seq1[-1, 1] : seq1[-1] bx = string ? seq2[bj, 1] : seq2[bj] event = Diff::LCS::ContextChange.new('>', (a_size - 1), ax, bj, bx) event = yield event if block_given? callbacks.finished_a(event) run_finished_a = true else ax = string ? seq1[ai, 1] : seq1[ai] loop do bx = string ? seq2[bj, 1] : seq2[bj] event = Diff::LCS::ContextChange.new('+', ai, ax, bj, bx) event = yield event if block_given? callbacks.discard_b(event) bj += 1 break unless bj < b_size end end end # last B? if bj == b_size and ai < a_size if callbacks.respond_to?(:finished_b) and not run_finished_b ax = string ? seq1[ai, 1] : seq1[ai] bx = string ? seq2[-1, 1] : seq2[-1] event = Diff::LCS::ContextChange.new('<', ai, ax, (b_size - 1), bx) event = yield event if block_given? callbacks.finished_b(event) run_finished_b = true else bx = string ? seq2[bj, 1] : seq2[bj] loop do ax = string ? seq1[ai, 1] : seq1[ai] event = Diff::LCS::ContextChange.new('-', ai, ax, bj, bx) event = yield event if block_given? callbacks.discard_a(event) ai += 1 break unless bj < b_size end end end if ai < a_size ax = string ? seq1[ai, 1] : seq1[ai] bx = string ? seq2[bj, 1] : seq2[bj] event = Diff::LCS::ContextChange.new('-', ai, ax, bj, bx) event = yield event if block_given? callbacks.discard_a(event) ai += 1 end if bj < b_size ax = string ? seq1[ai, 1] : seq1[ai] bx = string ? seq2[bj, 1] : seq2[bj] event = Diff::LCS::ContextChange.new('+', ai, ax, bj, bx) event = yield event if block_given? callbacks.discard_b(event) bj += 1 end end end```

### + (Object) unpatch!(src, patchset)

Given a set of patchset, convert the current version to the prior version. Does no auto-discovery.

 ``` 790 791 792``` ```# File 'lib/diff/lcs.rb', line 790 def unpatch!(src, patchset) Diff::LCS.patch(src, patchset, :unpatch) end```

## Instance Method Details

### - (Object) diff(other, callbacks = nil, &block)

Returns the difference set between self and other. See Diff::LCS#diff.

 ``` 156 157 158``` ```# File 'lib/diff/lcs.rb', line 156 def diff(other, callbacks = nil, &block) Diff::LCS::diff(self, other, callbacks, &block) end```

### - (Object) lcs(other, &block)

Returns an Array containing the longest common subsequence(s) between self and other. See Diff::LCS#LCS.

``lcs = seq1.lcs(seq2)``
 ``` 150 151 152``` ```# File 'lib/diff/lcs.rb', line 150 def lcs(other, &block) #:yields self[ii] if there are matched subsequences: Diff::LCS.LCS(self, other, &block) end```

### - (Object) patch(patchset)

Attempts to patch a copy of self with the provided patchset. See Diff::LCS#patch.

 ``` 183 184 185``` ```# File 'lib/diff/lcs.rb', line 183 def patch(patchset) Diff::LCS::patch(self.dup, patchset) end```

### - (Object) patch!(patchset)

Attempts to patch self with the provided patchset. See Diff::LCS#patch!. Does no autodiscovery.

 ``` 195 196 197``` ```# File 'lib/diff/lcs.rb', line 195 def patch!(patchset) Diff::LCS::patch!(self, patchset) end```

### - (Object) sdiff(other, callbacks = nil, &block)

Returns the balanced ("side-by-side") difference set between self and other. See Diff::LCS#sdiff.

 ``` 162 163 164``` ```# File 'lib/diff/lcs.rb', line 162 def sdiff(other, callbacks = nil, &block) Diff::LCS::sdiff(self, other, callbacks, &block) end```

### - (Object) traverse_balanced(other, callbacks = nil, &block)

Traverses the discovered longest common subsequences between self and other using the alternate, balanced algorithm. See Diff::LCS#traverse_balanced.

 ``` 176 177 178 179``` ```# File 'lib/diff/lcs.rb', line 176 def traverse_balanced(other, callbacks = nil, &block) traverse_balanced(self, other, callbacks || Diff::LCS::YieldingCallbacks, &block) end```

### - (Object) traverse_sequences(other, callbacks = nil, &block)

Traverses the discovered longest common subsequences between self and other. See Diff::LCS#traverse_sequences.

 ``` 168 169 170 171``` ```# File 'lib/diff/lcs.rb', line 168 def traverse_sequences(other, callbacks = nil, &block) traverse_sequences(self, other, callbacks || Diff::LCS::YieldingCallbacks, &block) end```

### - (Object) unpatch(patchset)

Attempts to unpatch a copy of self with the provided patchset. See Diff::LCS#patch.

 ``` 189 190 191``` ```# File 'lib/diff/lcs.rb', line 189 def unpatch(patchset) Diff::LCS::unpatch(self.dup, patchset) end```

### - (Object) unpatch!(patchset)

Attempts to unpatch self with the provided patchset. See Diff::LCS#unpatch. Does no autodiscovery.

 ``` 201 202 203``` ```# File 'lib/diff/lcs.rb', line 201 def unpatch!(patchset) Diff::LCS::unpatch!(self, patchset) end```