Method: Diff::LCS.traverse_sequences

Defined in:
lib/diff/lcs.rb

.traverse_sequences(seq1, seq2, callbacks = Diff::LCS::SequenceCallbacks) ⇒ Object

#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)

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[i] and B[j] 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[i] and arrow b is pointing to B[j]. 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 arrow a and call the appropriate callback, then it will advance arrow b and call the appropriate callback.

The methods for callbacks#match, callbacks#discard_a, and callbacks#discard_b are invoked with an event comprising the action (“=”, “+”, or “-”, respectively), the indexes i and j, and the elements A[i] and B[j]. 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 will 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[j]). 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[j] 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[i]) 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[i] 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.



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# File 'lib/diff/lcs.rb', line 286

def traverse_sequences(seq1, seq2, callbacks = Diff::LCS::SequenceCallbacks) # :yields: change events
  callbacks ||= Diff::LCS::SequenceCallbacks
  matches = Diff::LCS::Internals.lcs(seq1, seq2)

  run_finished_a = run_finished_b = false
  string = seq1.is_a?(String)

  a_size = seq1.size
  b_size = seq2.size
  ai = bj = 0

  matches.each do |b_line|
    if b_line.nil?
      unless seq1[ai].nil?
        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)
      end
    else
      ax = string ? seq1[ai, 1] : seq1[ai]

      loop do
        break unless bj < b_line

        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
      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)
      bj += 1
    end
    ai += 1
  end

  # 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) || (bj < b_size)
    # last A?
    if ai == a_size && bj < b_size
      if callbacks.respond_to?(:finished_a) && !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 && ai < a_size
      if callbacks.respond_to?(:finished_b) && !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