[213_24] Add unit tests for patch (OT) module in moebius

devel/213_24.md

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+# [213_24] Add unit tests for patch (OT) module in moebius
+
+The `moebius/Data/History/patch` module implements the core Operational Transformation (OT) primitives that are essential for collaborative editing. Currently, this module has **zero test coverage** while `modification`, `path`, and `tree` all have tests.
+
+This PR adds comprehensive unit tests covering:
+
+## Test Categories
+1. **Basic patch construction** — modification, compound, branch, birth, author patches
+2. **Patch application** — assign, insert, remove, split, join operations on trees
+3. **Patch inversion** — verifying that `invert(p, t)` produces a patch that restores the original tree
+4. **Patch equality and copy** — structural equality and deep copy
+5. **is_applicable** — validity checks before applying patches
+
+## Why This Matters
+These tests are foundational for the Web-Based Collaborative Editing Core (GSoC 2026 project), where OT correctness is critical for concurrent editing sessions. Having a solid test suite ensures that porting the editing engine to WebAssembly does not introduce regressions in the OT layer.

moebius/tests/Data/History/patch_test.cpp

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+#include "modification.hpp"
+#include "moe_doctests.hpp"
+#include "patch.hpp"
+#include "tree.hpp"
+
+/******************************************************************************
+ * Basic patch construction
+ ******************************************************************************/
+
+TEST_CASE ("patch modification construction") {
+  modification m  = mod_assign (path (), tree ("hello"));
+  modification inv= mod_assign (path (), tree (""));
+  patch        p (m, inv);
+  CHECK (is_modification (p));
+  CHECK (get_type (p) == PATCH_MODIFICATION);
+  CHECK (get_modification (p) == m);
+  CHECK (get_inverse (p) == inv);
+}
+
+TEST_CASE ("patch compound construction") {
+  modification m1= mod_assign (path (), tree ("a"));
+  modification i1= mod_assign (path (), tree (""));
+  modification m2= mod_assign (path (), tree ("b"));
+  modification i2= mod_assign (path (), tree ("a"));
+  patch        p1 (m1, i1);
+  patch        p2 (m2, i2);
+  patch        compound (p1, p2);
+  CHECK (is_compound (compound));
+  CHECK (N (compound) == 2);
+  CHECK (compound[0] == p1);
+  CHECK (compound[1] == p2);
+}
+
+TEST_CASE ("patch birth construction") {
+  double author= new_author ();
+  patch  p (author, true);
+  CHECK (is_birth (p));
+  CHECK (get_author (p) == author);
+  CHECK (get_birth (p) == true);
+}
+
+TEST_CASE ("patch author construction") {
+  double       author= new_author ();
+  modification m     = mod_assign (path (), tree ("x"));
+  modification inv   = mod_assign (path (), tree (""));
+  patch        inner (m, inv);
+  patch        p (author, inner);
+  CHECK (is_author (p));
+  CHECK (get_author (p) == author);
+  CHECK (N (p) == 1);
+}
+
+/******************************************************************************
+ * Patch application
+ ******************************************************************************/
+
+TEST_CASE ("apply modification patch to tree") {
+  tree         t  = tree (DOCUMENT, "hello", "world");
+  modification m  = mod_assign (path (0), tree ("hi"));
+  modification inv= mod_assign (path (0), tree ("hello"));
+  patch        p (m, inv);
+  tree         result= clean_apply (p, t);
+  CHECK (result[0] == tree ("hi"));
+  CHECK (result[1] == tree ("world"));
+}
+
+TEST_CASE ("apply compound patch to tree") {
+  tree         t = tree ("original");
+  modification m1= mod_assign (path (), tree ("step1"));
+  modification i1= mod_assign (path (), tree ("original"));
+  modification m2= mod_assign (path (), tree ("step2"));
+  modification i2= mod_assign (path (), tree ("step1"));
+  patch        p1 (m1, i1);
+  patch        p2 (m2, i2);
+  patch        compound (p1, p2);
+  tree         result= clean_apply (compound, t);
+  CHECK (result == tree ("step2"));
+}
+
+TEST_CASE ("apply insert modification") {
+  tree         t  = tree (DOCUMENT, "abc");
+  modification m  = mod_insert (path (0), 1, tree ("X"));
+  modification inv= mod_remove (path (0), 1, 1);
+  patch        p (m, inv);
+  tree         result= clean_apply (p, t);
+  CHECK (result[0] == tree ("aXbc"));
+}
+
+TEST_CASE ("apply remove modification") {
+  tree         t  = tree (DOCUMENT, "abcde");
+  modification m  = mod_remove (path (0), 1, 2);
+  modification inv= mod_insert (path (0), 1, tree ("bc"));
+  patch        p (m, inv);
+  tree         result= clean_apply (p, t);
+  CHECK (result[0] == tree ("ade"));
+}
+
+TEST_CASE ("apply split modification") {
+  tree         t  = tree (DOCUMENT, "abcde");
+  modification m  = mod_split (path (), 0, 2);
+  modification inv= mod_join (path (), 0);
+  patch        p (m, inv);
+  tree         result= clean_apply (p, t);
+  CHECK (N (result) == 2);
+  CHECK (result[0] == tree ("ab"));
+  CHECK (result[1] == tree ("cde"));
+}
+
+TEST_CASE ("apply join modification") {
+  tree         t  = tree (DOCUMENT, "ab", "cde");
+  modification m  = mod_join (path (), 0);
+  modification inv= mod_split (path (), 0, 2);
+  patch        p (m, inv);
+  tree         result= clean_apply (p, t);
+  CHECK (N (result) == 1);
+  CHECK (result[0] == tree ("abcde"));
+}
+
+/******************************************************************************
+ * Patch inversion
+ ******************************************************************************/
+
+TEST_CASE ("invert modification patch") {
+  tree         t  = tree ("hello");
+  modification m  = mod_assign (path (), tree ("world"));
+  modification inv= mod_assign (path (), tree ("hello"));
+  patch        p (m, inv);
+  patch        p_inv= invert (p, t);
+  CHECK (is_modification (p_inv));
+  CHECK (get_modification (p_inv) == inv);
+  CHECK (get_inverse (p_inv) == m);
+}
+
+TEST_CASE ("invert then apply restores original") {
+  tree         t  = tree ("original");
+  modification m  = mod_assign (path (), tree ("modified"));
+  modification inv= mod_assign (path (), tree ("original"));
+  patch        p (m, inv);
+  tree         t2   = clean_apply (p, t);
+  patch        p_inv= invert (p, t);
+  tree         t3   = clean_apply (p_inv, t2);
+  CHECK (t3 == t);
+}
+
+TEST_CASE ("invert compound patch") {
+  tree         t = tree (DOCUMENT, "a", "b");
+  modification m1= mod_assign (path (0), tree ("x"));
+  modification i1= mod_assign (path (0), tree ("a"));
+  modification m2= mod_assign (path (1), tree ("y"));
+  modification i2= mod_assign (path (1), tree ("b"));
+  patch        p1 (m1, i1);
+  patch        p2 (m2, i2);
+  patch        compound (p1, p2);
+  tree         t2 = clean_apply (compound, t);
+  patch        inv= invert (compound, t);
+  tree         t3 = clean_apply (inv, t2);
+  CHECK (t3 == t);
+}
+
+/******************************************************************************
+ * Patch equality and copy
+ ******************************************************************************/
+
+TEST_CASE ("patch equality") {
+  modification m  = mod_assign (path (), tree ("a"));
+  modification inv= mod_assign (path (), tree (""));
+  patch        p1 (m, inv);
+  patch        p2 (m, inv);
+  CHECK (p1 == p2);
+  CHECK_FALSE (p1 != p2);
+}
+
+TEST_CASE ("patch copy") {
+  modification m  = mod_assign (path (), tree ("a"));
+  modification inv= mod_assign (path (), tree (""));
+  patch        p1 (m, inv);
+  patch        p2= copy (p1);
+  CHECK (p1 == p2);
+}
+
+/******************************************************************************
+ * is_applicable
+ ******************************************************************************/
+
+TEST_CASE ("is_applicable for valid modification") {
+  tree  t= tree (DOCUMENT, "hello", "world");
+  patch p (mod_assign (path (0), tree ("hi")),
+           mod_assign (path (0), tree ("hello")));
+  CHECK (is_applicable (p, t));
+}
+
+TEST_CASE ("is_applicable for birth patch") {
+  tree   t= tree ("anything");
+  double a= new_author ();
+  patch  p (a, true);
+  CHECK (is_applicable (p, t));
+}