perf(lua): optimize vim.deep_equal #15236

By remembering the keys already compared in repeating a comparison is
avoided. Thanks: https://stackoverflow.com/a/32660766

runtime/doc/lua.txt

@@ -1268,7 +1268,17 @@ schedule_wrap({cb})                                      *vim.schedule_wrap()*
 
 
 deep_equal({a}, {b})                                        *vim.deep_equal()*
-                TODO: Documentation
+                Deep compare values for equality
+
+                Tables are compared recursively unless they both provide the `eq` methamethod.
+                All other types are compared using the equality `==` operator.
+
+                Parameters: ~
+                    {a}  first value
+                    {b}  second value
+
+                Return: ~
+                    `true` if values are equals, else `false` .
 
 deepcopy({orig})                                              *vim.deepcopy()*
                 Returns a deep copy of the given object. Non-table objects are

runtime/lua/vim/shared.lua

@@ -267,18 +267,23 @@ function vim.tbl_deep_extend(behavior, ...)
 end
 
 --- Deep compare values for equality
+---
+--- Tables are compared recursively unless they both provide the `eq` methamethod.
+--- All other types are compared using the equality `==` operator.
+---@param a first value
+---@param b second value
+---@returns `true` if values are equals, else `false`.
 function vim.deep_equal(a, b)
   if a == b then return true end
   if type(a) ~= type(b) then return false end
   if type(a) == 'table' then
-    -- TODO improve this algorithm's performance.
     for k, v in pairs(a) do
       if not vim.deep_equal(v, b[k]) then
         return false
       end
     end
-    for k, v in pairs(b) do
-      if not vim.deep_equal(v, a[k]) then
+    for k, _ in pairs(b) do
+      if a[k] == nil then
         return false
       end
     end