module Diff::LCS::Internals

Public Class Methods

analyze_patchset(patchset, depth = 0) click to toggle source

This method will analyze the provided patchset to provide a single-pass normalization (conversion of the array form of Diff::LCS::Change objects to the object form of same) and detection of whether the patchset represents changes to be made.

# File lib/diff/lcs/internals.rb, line 102
def analyze_patchset(patchset, depth = 0)
  fail "Patchset too complex" if depth > 1

  has_changes = false
  new_patchset = []

  # Format:
  # [ # patchset
  #   # hunk (change)
  #   [ # hunk
  #     # change
  #   ]
  # ]

  patchset.each do |hunk|
    case hunk
    when Diff::LCS::Change
      has_changes ||= !hunk.unchanged?
      new_patchset << hunk
    when Array
      # Detect if the 'hunk' is actually an array-format change object.
      if Diff::LCS::Change.valid_action? hunk[0]
        hunk = Diff::LCS::Change.from_a(hunk)
        has_changes ||= !hunk.unchanged?
        new_patchset << hunk
      else
        with_changes, hunk = analyze_patchset(hunk, depth + 1)
        has_changes ||= with_changes
        new_patchset.concat(hunk)
      end
    else
      fail ArgumentError, "Cannot normalise a hunk of class #{hunk.class}."
    end
  end

  [has_changes, new_patchset]
end

intuit_diff_direction(src, patchset, limit = nil) click to toggle source

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.

# File lib/diff/lcs/internals.rb, line 147
  def intuit_diff_direction(src, patchset, limit = nil)
    string = src.is_a?(String)
    count = left_match = left_miss = right_match = right_miss = 0

    patchset.each do |change|
      count += 1

      case change
      when Diff::LCS::ContextChange
        le = string ? src[change.old_position, 1] : src[change.old_position]
        re = string ? src[change.new_position, 1] : src[change.new_position]

        case change.action
        when "-" # Remove details from the old string
          if le == change.old_element
            left_match += 1
          else
            left_miss += 1
          end
        when "+"
          if re == change.new_element
            right_match += 1
          else
            right_miss += 1
          end
        when "="
          left_miss += 1 if le != change.old_element
          right_miss += 1 if re != change.new_element
        when "!"
          if le == change.old_element
            left_match += 1
          elsif re == change.new_element
            right_match += 1
          else
            left_miss += 1
            right_miss += 1
          end
        end
      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_match += 1
          else
            left_miss += 1
          end
        when "+"
          if element == change.element
            right_match += 1
          else
            right_miss += 1
          end
        when "="
          if element != change.element
            left_miss += 1
            right_miss += 1
          end
        end
      end

      break if !limit.nil? && (count > limit)
    end

    no_left = left_match.zero? && left_miss.positive?
    no_right = right_match.zero? && right_miss.positive?

    case [no_left, no_right]
    when [false, true]
      :patch
    when [true, false]
      :unpatch
    else
      case left_match <=> right_match
      when 1
        if left_miss.zero?
          :patch
        else
          :unpatch
        end
      when -1
        if right_miss.zero?
          :unpatch
        else
          :patch
        end
      else
        fail "The provided patchset does not appear to apply to the provided \
enumerable as either source or destination value."
      end
    end
  end

lcs(a, b) click to toggle source

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::Internals.lcs(a, b)
result.each_with_index do |e, i|
  assert_equal(a[i], b[e]) unless e.nil?
end

# File lib/diff/lcs/internals.rb, line 41
def lcs(a, b)
  a_start = b_start = 0
  a_finish = a.size - 1
  b_finish = b.size - 1
  vector = []

  # Collect any common elements at the beginning...
  while (a_start <= a_finish) && (b_start <= b_finish) && (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) && (b_start <= b_finish) && (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.
  # An explanation for how this works: https://codeforces.com/topic/92191
  b_matches = position_hash(b, b_start..b_finish)

  thresh = []
  links = []
  string = a.is_a?(String)

  (a_start..a_finish).each do |i|
    ai = string ? a[i, 1] : a[i]
    bm = b_matches[ai]
    k = nil
    bm.reverse_each do |j|
      # Although the threshold check is not mandatory for this to work,
      # it may have an optimization purpose
      # An attempt to remove it: https://github.com/halostatue/diff-lcs/pull/72
      # Why it is reintroduced: https://github.com/halostatue/diff-lcs/issues/78
      if k && (thresh[k] > j) && (thresh[k - 1] < j)
        thresh[k] = j
      else
        k = replace_next_larger(thresh, j, k)
      end
      links[k] = [k.positive? ? links[k - 1] : nil, i, j] unless k.nil?
    end
  end

  unless thresh.empty?
    link = links[thresh.size - 1]
    until link.nil?
      vector[link[1]] = link[2]
      link = link[0]
    end
  end

  vector
end