find_moved_element

find_moved_element.rb

# Algorithm to detect delete/insertion change (ie. drag-n-drop) in array of unique elements
# From https://webshittery.com/blog/cumbersome-apis/
# Note that in certain situations where there is more than one correct answer

# For example:
#
# find_moved_element([1, 2, 3, 4, 5], [1, 3, 2, 4, 5]),
#
# Correct answers include:
#
# { moved_element: 3, original_index: 2, new_index: 1 }
# { moved_element: 2, original_index: 1, new_index: 2 }
#
# Another example:
# find_moved_element([1, 2, 3, 4, 5], [2, 3, 4, 5, 1]),
#
# Only one correct answer:
# { moved_element: 1, original_index: 0, new_index: 4 }

# N.B. "moved" is defined as deleting element, then inserting element back (think of drag-n-drop).
def find_moved_element(array_orig, array_moved)
  if array_orig == array_moved
    raise 'You didnt move anything, doofus!'
  end

  orig_index_hash = {}
  array_orig.each do |el|
    orig_index_hash[el] = array_orig.index(el)
  end

  moved_index_hash = {}
  array_moved.each do |el|
    moved_index_hash[el] = array_moved.index(el)
  end

  changed_value = nil

  # Let's find the changed value!
  array_orig.each do |el|
    # Same index, means its unmoved
    next if moved_index_hash[el] == orig_index_hash[el]

    # Most of the time an absolute value difference of exactly 1
    # means this also wasn't the changed value.  There are edge-cases so lets
    # handle that below.
    next if (moved_index_hash[el] - orig_index_hash[el]).abs == 1

    changed_value = el
    break
  end

  # The edgecase occurred where the deleted element
  # was then inserted exactly one away from its original position.
  # In which case, any element that has a different index than
  # before was the one that was moved.
  unless changed_value
    array_orig.each do |el|
      # Same index, means its unmoved
      next if moved_index_hash[el] == orig_index_hash[el]

      changed_value = el
      break
    end
  end

  original_index = array_orig.index(changed_value)
  new_index = array_moved.index(changed_value)

  {
    moved_element: changed_value,
    original_index: original_index,
    new_index: new_index,
  }
end


# TEST CASES
a = [1, 2, 3, 4, 5, 6, 7]
b = [1, 3, 2, 4, 5, 6, 7]

answer = find_moved_element(a, b)

unless answer == { moved_element: 2, original_index: 1, new_index: 2 }
  raise "You dun goofed, doofus for #{b}"
end


a = [1, 2, 3, 4, 5, 6, 7]
b = [2, 3, 4, 5, 1, 6, 7]

answer = find_moved_element(a, b)

unless answer == { moved_element: 1, original_index: 0, new_index: 4 }
  raise "You dun goofed, doofus for #{b}"
end

a = [1, 2, 3, 4, 5, 6, 7]
b = [7, 1, 2, 3, 4, 5, 6]

answer = find_moved_element(a, b)
unless answer == { moved_element: 7, original_index: 6, new_index: 0 }
  raise "You dun goofed, doofus for #{b}"
end