defmodule Diff do
  alias Diff.Matrix
  alias Diff.Diffable
  @moduledoc """
  Functions for performing diffs.
  """

  defmodule Insert do
    defstruct [:element, :index, :length]
  end

  defmodule Delete do
    defstruct [:element, :index, :length]
  end

  defmodule Modified do
    defstruct [:element, :old_element, :index, :length]
  end

  defmodule Unchanged do
    defstruct [:element, :index, :length]
  end

  defmodule Ignored do
    defstruct [:element, :index, :length]
  end

  @doc"""
  Applies with patches with supplied annotation (top and tail)
  This is used to generate visual diffs, etc
  Shares the same code as patch
  """
  def annotated_patch(original, patches, annotations, from_list_fn \\ fn(list) -> list end) do
    apply_patches(original, patches, annotations, from_list_fn)
  end

  @doc """
  Applies the patches from a previous diff to the given string.
  Will return the patched version as a list unless a from_list_fn/1 is supplied.
  This function will takes the patched list as input and outputs the result.
  """
  def patch(original, patches, from_list_fn \\ fn(list) -> list end) do
    apply_patches(original, patches, [], from_list_fn)
  end

  defp apply_patches(original, patches, annotations, from_list_fn) do
    original = Diffable.to_list(original)

    patchfn = fn(patch, {increment, changed}) ->
      do_patch({increment, changed}, patch, annotations)
    end

    increment = 0
    {_, returnlist} = Enum.reduce(patches, {increment, original}, patchfn)
    from_list_fn.(returnlist)
  end

  defp do_patch({incr, original}, %Diff.Insert{element: element, index: index},
    annotations) do
    { left, right } = Enum.split(original, index + incr)
    {newelement, newincr} = annotate(element, :insert, annotations, incr)
    return = left ++ newelement ++ right
    {newincr, return}
  end

  defp do_patch({incr, original}, %Diff.Delete{ element: element, index: index,
                                        length: length }, annotations) do
    { left, deleted } = Enum.split(original, index + incr)
    { actuallydeleted, right } = Enum.split(deleted, length)
    case element do
      ^actuallydeleted ->
        {newelement, newincr} = annotate(element, :deleted, annotations, incr)
        return = left ++ newelement ++ right
        {newincr, return}
      _other ->
        exit("failed delete")
    end
  end

  defp do_patch({incr, original},
    %Diff.Modified{ element: element, old_element: _,
                    index: index, length: length},
    annotations) do
    { left, deleted } = Enum.split(original, index + incr)
    { _, right } = Enum.split(deleted, length)
    {newelement, newincr} = annotate(element, :modified, annotations, incr)
    return = left ++ newelement ++ right
    {newincr, return}
  end

  defp do_patch({incr, original}, %Diff.Unchanged{}, _annotations) do
    {incr, original}
  end

  defp do_patch({incr, original}, %Diff.Ignored{element: element, index: index},
    annotations) do
    { left, right } = Enum.split(original, index + incr)
    {newelement, newincr} = annotate(element, :ignored, annotations, incr)
    return = left ++ newelement ++ right
    {newincr, return}
  end

  @doc"""
  Creates a list of changes from the orginal binary to the changed one.
  Takes the following options:

  * `:keep_unchanged` - Keeps unchanged binary parts in the returned patches
  * `ignore` - Takes a regex and ignores matches
  """
  def diff(original, changed, options \\ []) do
    original = Diffable.to_list(original)
    changed = Diffable.to_list(changed)

    original_length = length(original)
    changed_length = length(changed)

    longest_common_subsequence(original, changed, original_length, changed_length)
    |> build_diff(original, changed, original_length, changed_length, [], options)
    |> build_changes(options)
  end

  defp longest_common_subsequence(x, y, x_length, y_length) do

    matrix = Matrix.new(x_length + 1, y_length + 1)

    # a reduction over a 2D array requires a closure inside an anonymous function
    # sorry but there is nothing to be done about that
    rowreductionFn = fn(i, matrix) ->

      # setup the second closure
      columnreductionFn = fn(j, matrix) ->
      if Enum.fetch!(x, i-1) == Enum.fetch!(y, j-1) do
        value = Matrix.get(matrix, i-1, j-1)
        Matrix.put(matrix, i, j, value + 1)
      else
        original_value = Matrix.get(matrix, i, j-1)
        changed_value = Matrix.get(matrix, i - 1, j)

        Matrix.put(matrix, i, j, max(original_value, changed_value))
      end
      end

      Enum.reduce(1..y_length, matrix, columnreductionFn)

    end

    _matrix = Enum.reduce(1..x_length, matrix, rowreductionFn)

  end

  defp build_diff(matrix, x, y, i, j, edits, options) do
    cond do
      i > 0 and j > 0 and Enum.fetch!(x, i-1) == Enum.fetch!(y, j-1) ->
        newedits = if Dict.get(options, :keep_unchanged, false) do
            edits ++ [{:unchanged, Enum.fetch!(x, i-1), i-1}]
          else
            edits
          end
        build_diff(matrix, x, y, i-1, j-1, newedits, options)
      j > 0 and (i == 0 or Matrix.get(matrix, i, j-1) >= Matrix.get(matrix,i-1, j)) ->
        newedit = {:insert, Enum.fetch!(y, j-1), j-1}
        build_diff(matrix, x, y, i, j-1, edits ++ [newedit], options)
      i > 0 and (j == 0 or Matrix.get(matrix, i, j-1) < Matrix.get(matrix, i-1, j)) ->
        newdelete = {:delete, Enum.fetch!(x, i-1), j}
        build_diff(matrix, x, y, i-1, j, edits ++ [newdelete], options)
      true ->
        edits |> Enum.reverse
    end
  end

  defp build_changes(edits, options) do

    # we now have a set of individual letter changes
    # but if there is a series of inserts or deletes then
    # we need to reduce them into single multichar changes
    mergeindividualchangesFn = fn({type, char, index}, changes) ->
      if changes == [] do
        changes ++ [make_change(type, char, index)]
      else
        change = List.last(changes)
        regex = Dict.get(options, :ignore)
        cond do
          regex && Regex.match?(regex, char) ->
            changes ++ [make_change(:ignored, char, index)]
            # one branch for deletes
            is_type(change, type) && type == :delete && index == change.index ->
            change = if regex && Regex.match?(regex, Enum.join(change.element)) do
                %Ignored{ element: change.element, index: change.index,
                          length: change.length }
              else
                %{change | element: change.element ++ [char], length:
                  change.length + 1 }
              end
            List.replace_at(changes, length(changes)-1, change)
            # a different branch for everyone else
            is_type(change, type) && type != :delete && index == (change.index + change.length) ->
            change = if regex && Regex.match?(regex, Enum.join(change.element)) do
                %Ignored{ element: change.element, index: change.index,
                          length: change.length }
              else
                %{change | element: change.element ++ [char], length:
                  change.length + 1 }
              end
            List.replace_at(changes, length(changes)-1, change)
          true ->
            changes ++ [make_change(type, char, index)]
        end
      end
    end

    # if we change a single letter it will be a consecutive delete/insert
    # this reduction merges them into a single modified statement
    makemodifiedFn = fn(x, changes) ->
      if changes == [] do
        [x]
      else
        last_change = List.last(changes)
        if is_type(last_change, :delete)
        and is_type(x, :insert)
        and last_change.index == x.index
        and last_change.length == x.length do
          last_change = %Modified{ element: x.element, old_element: last_change.element,
                                   index: x.index, length: x.length }
          List.replace_at(changes, length(changes) - 1, last_change)
        else
          changes ++ [x]
        end
      end
    end

    # Now do both these sets of reduction on the edits
    Enum.reduce(edits, [], mergeindividualchangesFn)
    |> Enum.reduce([], makemodifiedFn)
  end

  defp make_change(:insert, char, index) do
    %Insert{ element: [char], index: index, length: 1 }
  end

  defp make_change(:delete, char, index) do
    %Delete{ element: [char], index: index, length: 1 }
  end

  defp make_change(:unchanged, char, index) do
    %Unchanged{ element: [char], index: index, length: 1 }
  end

  defp make_change(:ignored, char, index) do
    %Ignored{ element: [char], index: index, length: 1 }
  end

  defp is_type(%Insert{}, :insert) do
    true
  end

  defp is_type(%Delete{}, :delete) do
    true
  end

  defp is_type(%Unchanged{}, :unchanged) do
    true
  end

  defp is_type(%Ignored{}, :ignored) do
    true
  end

  defp is_type(_, _) do
    false
  end

  defp annotate(list, type, annotations, increment) do
    annotation = for a <- annotations,
      Map.get(a, type) != nil, do: Map.get(a, type)
    case {type, annotation} do
      {:deleted, []}           -> {[],   increment}
      {_,        []}           -> {list, increment}
      {:deleted, [annotation]} -> apply_deletion(list,   annotation, increment)
      {_,        [annotation]} -> apply_annotation(list, annotation, increment)
    end
  end

  defp apply_deletion(list, annotation, increment) do
    {[annotation.before] ++ list ++ [annotation.after], increment + 2}
  end

  defp apply_annotation(list, annotation, increment) do
    {[annotation.before] ++ list ++ [annotation.after], increment + 2}
  end

end