From 921415e08e4f696e285562ed8f49112db0de5332 Mon Sep 17 00:00:00 2001
From: John Estabrook <jestabro@vyos.io>
Date: Wed, 5 Aug 2026 20:59:08 -0500
Subject: T9169: factor out diff calculation module

---
 src/diff.ml | 74 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 74 insertions(+)
 create mode 100644 src/diff.ml

(limited to 'src/diff.ml')

diff --git a/src/diff.ml b/src/diff.ml
new file mode 100644
index 0000000..6a87d93
--- /dev/null
+++ b/src/diff.ml
@@ -0,0 +1,74 @@
+type change = Unchanged | Added | Subtracted | Updated of string list
+
+exception Incommensurable
+exception Empty_comparison
+
+let name_of n = Vytree.name_of_node n
+let data_of n = Vytree.data_of_node n
+let children_of n = Vytree.children_of_node n
+
+let (^~) (node : Config_tree.t) (node' : Config_tree.t) =
+  name_of node = name_of node' &&
+  (data_of node).values <> (data_of node').values
+
+let left_opt_pairs n m =
+    (children_of n) |> List.map (fun x ->
+        let maybe_node =
+            (children_of m) |> List.find_opt (fun y ->
+                name_of y = name_of x) in
+        (Some x, maybe_node))
+
+let right_opt_pairs n m =
+    (children_of m) |> List.map (fun y ->
+        let maybe_node =
+            (children_of n) |> List.find_opt (fun x ->
+                name_of x = name_of y) in
+        (maybe_node, Some y))
+
+let opt_tuple_cmp t1 t2 =
+    let opt_tuple_val t =
+        match t with
+        | None, None -> ""
+        | Some x, None -> name_of x
+        | None, Some y -> name_of y
+        | Some x, Some _ -> name_of x
+    in Util.lexical_numeric_compare (opt_tuple_val t1) (opt_tuple_val t2)
+
+let opt_zip n m =
+    left_opt_pairs n m @ right_opt_pairs n m |> List.sort_uniq opt_tuple_cmp
+
+let get_opt_name left_opt right_opt =
+    match left_opt, right_opt with
+    | Some left_node, None -> name_of left_node
+    | None, Some right_node -> name_of right_node
+    | Some left_node, Some _ -> name_of left_node
+    | None, None -> raise Empty_comparison
+
+let update_path path left_opt right_opt =
+    let name = get_opt_name left_opt right_opt in
+    if name = "" then path
+    else path @ [name]
+
+module type Place = sig
+    type t
+    val diff_func : ?recurse:bool -> string list -> t -> change -> t
+end
+
+module Diff (P: Place) = struct
+    let rec diff_calc (path : string list) (res: P.t) ((left_node_opt, right_node_opt) : Config_tree.t option * Config_tree.t option) =
+        let path = update_path path left_node_opt right_node_opt in
+        match left_node_opt, right_node_opt with
+        | None, None -> raise Empty_comparison
+        | Some _, None -> P.diff_func path res Subtracted
+        | None, Some _ -> P.diff_func path res Added
+        | Some left_node, Some right_node when left_node = right_node ->
+            P.diff_func ~recurse:true path res Unchanged
+        | Some left_node, Some right_node when left_node ^~ right_node ->
+            let values = (data_of right_node).values in
+            P.diff_func path res (Updated values)
+        | Some left_node, Some right_node ->
+            let ret = P.diff_func ~recurse:false path res Unchanged in
+            List.fold_left (diff_calc path) ret (opt_zip left_node right_node)
+
+    let diff (init: P.t) l r = diff_calc [] init (Option.some l, Option.some r)
+end
-- 
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