package sexp_diff

This module can be used for computing the diff of two sexps. It can identify insertion, deletion, and modification anywhere in the sexp. If you want to display the diff in a human-readable format, take a look at Sexp_diff_display.

This library intended for comparing serialized representations of the same OCaml type. It doesn't handle the case where a sub-sexp moves to a different depth in the updated sexp. Note that this case never occurs when comparing sexp representations of the same OCaml type.

A common approach for diffing sexps is to serialize the sexps as strings and then diff the strings. While this is a reasonable approach, it means the diff algorithm is unable to use the tree structure to produce a better diff. Nodes which are very far apart in the tree can be close together in the serialized representation of the tree.

This library computes the diff on the tree directly, without serializing the tree.

The complexity is O(nm), where n is the number of nodes in the original sexp and m is the number of nodes in the updated sexp.

Actually it's a bit better than that: if we denote by n_i the number of nodes in the first sexp at depth i, and m_i the number of nodes in the second sexp at depth i, the complexity is O(sum_i (n_i * m_i)).

If the original and updated sexps are 10KB or less, this library will run in less than a second on them. Otherwise, this library may or may not be fast enough, and you should probably test its performance before using it.

It's very similar the algorithm that's described in

      Stanley M. Selkow (1977)
      The Tree-to-tree Editing Problem
      Information Processing Letters, 6(6)
      https://pdfs.semanticscholar.org/2e2e/47f3748368797c9d51b08e938dfb930b97c3.pdf

The differences are that we have a 'replace' operation which is distinguished from 'add' and 'delete', and in the paper every node can have a label, while with sexps only leaves (aka. atoms) can have labels.

To explain exactly what this module does, we can define the functions to_original: Diff.t -> Sexp.t and to_updated: Diff.t -> Sexp.t. They are defined as follows:

let extract ~project diff =
  let rec f = function
    | Delete x       -> project (Some x, None)
    | Add x          -> project (None,   Some x)
    | Replace (x, y) -> project (Some x, Some y)
    | Same x         -> Some x
    | Enclose diffs  ->
      let sexps = List.filter_map diffs ~f in
      Some (Sexp.List sexps)
  in
  Option.value_exn (f diff)
;;

let to_original = extract ~project:fst
let to_updated  = extract ~project:snd

Then we guarantee that: to_original (diff ~original ~updated) = original and to_updated (diff ~original ~updated) = updated

We also guarantee the following: Diff.apply_exn (diff ~original ~updated) original = updated and Diff.apply_exn (Diff.invert (diff ~original ~updated)) updated = original