When diffing lists where each element has a distinct key, we can refine the diffing patch by introducing two composite edit moves: swaps and moves.

Swaps exchange the position of two elements. Swap cost is set to 2 * change - epsilon. Moves change the position of one element. Move cost is set to delete + addition - epsilon.

When the cost delete + addition is greater than change and with those specific weights, the optimal patch with Swaps and Moves can be computed directly and cheaply from the original optimal patch.

type 'a with_pos = {
  1. pos : int;
  2. data : 'a;
}
val with_pos : 'a list -> 'a with_pos list
type ('l, 'r, 'diff) mismatch =
  1. | Name of {
    1. pos : int;
    2. got : string;
    3. expected : string;
    4. types_match : bool;
    }
  2. | Type of {
    1. pos : int;
    2. got : 'l;
    3. expected : 'r;
    4. reason : 'diff;
    }
type ('l, 'r, 'diff) change =
  1. | Change of ('l, 'r, 'diff) mismatch
  2. | Swap of {
    1. pos : int * int;
    2. first : string;
    3. last : string;
    }
  3. | Move of {
    1. name : string;
    2. got : int;
    3. expected : int;
    }
  4. | Insert of {
    1. pos : int;
    2. insert : 'r;
    }
  5. | Delete of {
    1. pos : int;
    2. delete : 'l;
    }

This specialized version of changes introduces two composite changes: Move and Swap

val prefix : Format.formatter -> ('l, 'r, 'diff) change -> unit
module Define (D : Diffing.Defs with type eq := unit) : sig ... end