Optimise slightly the iterator

Avoid unnecessary String.length calls as these are very expensive

Author: everzet

lib/text_delta/iterator.ex

@@ -52,22 +52,28 @@ defmodule TextDelta.Iterator do
  end
 
  def next({[head_a | _], [head_b | _]} = sets, skip_type) do
- comparison = Operation.compare(head_a, head_b)
  skip = Operation.type(head_a) == skip_type
- do_next(sets, comparison, skip)
+ len_a = Operation.length(head_a)
+ len_b = Operation.length(head_b)
+
+ cond do
+ len_a > len_b -> do_next(sets, :gt, len_b, skip)
+ len_a < len_b -> do_next(sets, :lt, len_a, skip)
+ true -> do_next(sets, :eq, 0, skip)
+ end
  end
 
- defp do_next({[head_a | tail_a], [head_b | tail_b]}, :gt, false) do
- {head_a, remainder_a} = Operation.slice(head_a, Operation.length(head_b))
+ defp do_next({[head_a | tail_a], [head_b | tail_b]}, :gt, len, false) do
+ {head_a, remainder_a} = Operation.slice(head_a, len)
  {{head_a, [remainder_a | tail_a]}, {head_b, tail_b}}
  end
 
- defp do_next({[head_a | tail_a], [head_b | tail_b]}, :lt, _) do
- {head_b, remainder_b} = Operation.slice(head_b, Operation.length(head_a))
+ defp do_next({[head_a | tail_a], [head_b | tail_b]}, :lt, len, _) do
+ {head_b, remainder_b} = Operation.slice(head_b, len)
  {{head_a, tail_a}, {head_b, [remainder_b | tail_b]}}
  end
 
- defp do_next({[head_a | tail_a], [head_b | tail_b]}, _, _) do
+ defp do_next({[head_a | tail_a], [head_b | tail_b]}, _, _, _) do
  {{head_a, tail_a}, {head_b, tail_b}}
  end
 end