feat(ui): allow to get the highlight namespace

runtime/doc/api.txt

@@ -979,6 +979,19 @@ nvim_get_hl_id_by_name({name})                      *nvim_get_hl_id_by_name()*
 
     similar to |hlID()|, but allocates a new ID if not present.
 
+nvim_get_hl_ns({*opts})                                     *nvim_get_hl_ns()*
+    Gets the active highlight namespace.
+
+    Parameters: ~
+      • {opts}  Optional parameters
+                • winid: (number) |window-ID| for retrieving a window's
+                  highlight namespace. A value of -1 is returned when
+                  |nvim_win_set_hl_ns()| has not been called for the window
+                  (or was called with a namespace of -1).
+
+    Return: ~
+        Namespace id, or -1
+
 nvim_get_keymap({mode})                                    *nvim_get_keymap()*
     Gets a list of global (non-buffer-local) |mapping| definitions.
 

runtime/lua/vim/_meta/api.lua

@@ -1258,6 +1258,16 @@ function vim.api.nvim_get_hl_by_name(name, rgb) end
 --- @return integer
 function vim.api.nvim_get_hl_id_by_name(name) end
 
+--- Gets the active highlight namespace.
+---
+--- @param opts vim.api.keyset.get_ns Optional parameters
+---             • winid: (number) `window-ID` for retrieving a window's
+---               highlight namespace. A value of -1 is returned when
+---               `nvim_win_set_hl_ns()` has not been called for the window
+---               (or was called with a namespace of -1).
+--- @return integer
+function vim.api.nvim_get_hl_ns(opts) end
+
 --- Gets a list of global (non-buffer-local) `mapping` definitions.
 ---
 --- @param mode string Mode short-name ("n", "i", "v", ...)

runtime/lua/vim/_meta/api_keysets.lua

@@ -136,6 +136,9 @@ error('Cannot require a meta file')
 --- @field link? boolean
 --- @field create? boolean
 
+--- @class vim.api.keyset.get_ns
+--- @field winid? integer
+
 --- @class vim.api.keyset.highlight
 --- @field bold? boolean
 --- @field standout? boolean

src/nvim/api/keysets.h

@@ -195,6 +195,11 @@ typedef struct {
   Boolean create;
 } Dict(get_highlight);
 
+typedef struct {
+  OptionalKeys is_set__get_ns_;
+  Window winid;
+} Dict(get_ns);
+
 typedef struct {
   OptionalKeys is_set__win_text_height_;
   Integer start_row;

src/nvim/api/vim.c

@@ -170,6 +170,29 @@ void nvim_set_hl(Integer ns_id, String name, Dict(highlight) *val, Error *err)
   }
 }
 
+/// Gets the active highlight namespace.
+///
+/// @param opts Optional parameters
+///           - winid: (number) |window-ID| for retrieving a window's highlight
+///             namespace. A value of -1 is returned when |nvim_win_set_hl_ns()|
+///             has not been called for the window (or was called with a namespace
+///             of -1).
+/// @param[out] err Error details, if any
+/// @return Namespace id, or -1
+Integer nvim_get_hl_ns(Dict(get_ns) *opts, Error *err)
+  FUNC_API_SINCE(12)
+{
+  if (HAS_KEY(opts, get_ns, winid)) {
+    win_T *win = find_window_by_handle(opts->winid, err);
+    if (!win) {
+      return 0;
+    }
+    return win->w_ns_hl;
+  } else {
+    return ns_hl_global;
+  }
+}
+
 /// Set active namespace for highlights defined with |nvim_set_hl()|. This can be set for
 /// a single window, see |nvim_win_set_hl_ns()|.
 ///

test/functional/api/highlight_spec.lua

@@ -637,3 +637,19 @@ describe('API: get highlight', function()
     eq({link ='Foo', default = true}, meths.get_hl(0, {name = 'Bar'}))
   end)
 end)
+
+describe('API: set/get highlight namespace', function()
+  it('set/get highlight namespace', function()
+    eq(0, meths.get_hl_ns({}))
+    local ns = meths.create_namespace('')
+    meths.set_hl_ns(ns)
+    eq(ns, meths.get_hl_ns({}))
+  end)
+
+  it('set/get window highlight namespace', function()
+    eq(-1, meths.get_hl_ns({winid = 0}))
+    local ns = meths.create_namespace('')
+    meths.win_set_hl_ns(0, ns)
+    eq(ns, meths.get_hl_ns({winid = 0}))
+  end)
+end)