Adds nvim_get_hl_by_name/by_id

...in order to retrieve highlights.

Added test/functional/api/highlight_spec.lua
HL_NORMAL is not really a good name, since it's more like an empty attribute than the normal's one.
If one pays attention, syn_cterm_attr2entry is never called with attr=0 because it's always special cased before.
I suggest in subsequent PRs we remove the ATTR_OFF and just insert an EMPTY ATTR/RESET_ATTR/UNINITIALIZED for id 0.

src/nvim/api/ui.c

@@ -242,39 +242,7 @@ static void push_call(UI *ui, char *name, Array args)
 static void remote_ui_highlight_set(UI *ui, HlAttrs attrs)
 {
   Array args = ARRAY_DICT_INIT;
-  Dictionary hl = ARRAY_DICT_INIT;
-
-  if (attrs.bold) {
-    PUT(hl, "bold", BOOLEAN_OBJ(true));
-  }
-
-  if (attrs.underline) {
-    PUT(hl, "underline", BOOLEAN_OBJ(true));
-  }
-
-  if (attrs.undercurl) {
-    PUT(hl, "undercurl", BOOLEAN_OBJ(true));
-  }
-
-  if (attrs.italic) {
-    PUT(hl, "italic", BOOLEAN_OBJ(true));
-  }
-
-  if (attrs.reverse) {
-    PUT(hl, "reverse", BOOLEAN_OBJ(true));
-  }
-
-  if (attrs.foreground != -1) {
-    PUT(hl, "foreground", INTEGER_OBJ(attrs.foreground));
-  }
-
-  if (attrs.background != -1) {
-    PUT(hl, "background", INTEGER_OBJ(attrs.background));
-  }
-
-  if (attrs.special != -1) {
-    PUT(hl, "special", INTEGER_OBJ(attrs.special));
-  }
+  Dictionary hl = hlattrs2dict(attrs);
 
   ADD(args, DICTIONARY_OBJ(hl));
   push_call(ui, "highlight_set", args);

src/nvim/api/vim.c

@@ -33,6 +33,7 @@
 #include "nvim/syntax.h"
 #include "nvim/getchar.h"
 #include "nvim/os/input.h"
+#include "nvim/ui.h"
 
 #define LINE_BUFFER_SIZE 4096
 
@@ -55,6 +56,42 @@ void nvim_command(String command, Error *err)
   try_end(err);
 }
 
+/// Retrieves highlight description from its name
+///
+/// @param name Highlight group name
+/// @return a highlight description e.g. {'bold': true, 'bg': 123, 'fg': 42}
+/// @see nvim_get_hl_by_id
+Dictionary nvim_get_hl_by_name(String name, Error *err)
+  FUNC_API_SINCE(3)
+{
+  Dictionary result = ARRAY_DICT_INIT;
+  int id = syn_name2id((const char_u *)name.data);
+
+  if (id == 0) {
+    api_set_error(err, kErrorTypeException, "Invalid highlight name %s",
+                  name.data);
+    return result;
+  }
+  result = nvim_get_hl_by_id(id, err);
+  return result;
+}
+
+/// Retrieves highlight description from its id
+///
+/// @param hl_id highlight id as returned by hlID()
+/// @see nvim_get_hl_by_name
+Dictionary nvim_get_hl_by_id(Integer hl_id, Error *err)
+  FUNC_API_SINCE(3)
+{
+  Dictionary dic = ARRAY_DICT_INIT;
+  if (syn_get_final_id((int)hl_id) == 0) {
+    api_set_error(err, kErrorTypeException, "Invalid highlight id %d", hl_id);
+    return dic;
+  }
+  int attrcode = syn_id2attr((int)hl_id);
+  return get_attr_by_id(attrcode, err);
+}
+
 /// Passes input keys to Nvim.
 /// On VimL error: Does not fail, but updates v:errmsg.
 ///

src/nvim/syntax.c

@@ -42,6 +42,7 @@
 #include "nvim/ui.h"
 #include "nvim/os/os.h"
 #include "nvim/os/time.h"
+#include "nvim/api/private/helpers.h"
 
 static bool did_syntax_onoff = false;
 
@@ -6622,7 +6623,6 @@ do_highlight(char_u *line, int forceit, int init) {
     syn_unadd_group();
   } else {
     if (is_normal_group) {
-      HL_TABLE()[idx].sg_attr = 0;
       // Need to update all groups, because they might be using "bg" and/or
       // "fg", which have been changed now.
       highlight_attr_set_all();
@@ -6826,7 +6826,7 @@ int hl_combine_attr(int char_attr, int prim_attr)
     // Copy all attributes from char_aep to the new entry
     new_en = *char_aep;
   } else {
-    memset(&new_en, 0, sizeof(new_en));
+    new_en = (attrentry_T)ATTRENTRY_INIT;
   }
 
   spell_aep = syn_cterm_attr2entry(prim_attr);
@@ -6859,6 +6859,7 @@ int hl_combine_attr(int char_attr, int prim_attr)
 
 /// \note this function does not apply exclusively to cterm attr contrary
 /// to what its name implies
+/// \warn don't call it with attr 0 (i.e., the null attribute)
 attrentry_T *syn_cterm_attr2entry(int attr)
 {
   attr -= ATTR_OFF;
@@ -7103,22 +7104,15 @@ syn_list_header(int did_header, int outlen, int id)
   return newline;
 }
 
-/*
- * Set the attribute numbers for a highlight group.
- * Called after one of the attributes has changed.
- */
-static void 
-set_hl_attr (
-    int idx                    /* index in array */
-)
+/// Set the attribute numbers for a highlight group.
+/// Called after one of the attributes has changed.
+/// @param idx corrected highlight index
+static void
+set_hl_attr(int idx)
 {
   attrentry_T at_en = ATTRENTRY_INIT;
   struct hl_group     *sgp = HL_TABLE() + idx;
 
-  // The "Normal" group doesn't need an attribute number
-  if (sgp->sg_name_u != NULL && STRCMP(sgp->sg_name_u, "NORMAL") == 0) {
-    return;
-  }
 
   at_en.cterm_ae_attr = sgp->sg_cterm;
   at_en.cterm_fg_color = sgp->sg_cterm_fg;

src/nvim/tui/tui.c

@@ -288,7 +288,7 @@ static void terminfo_stop(UI *ui)
 static void tui_terminal_start(UI *ui)
 {
   TUIData *data = ui->data;
-  data->print_attrs = EMPTY_ATTRS;
+  data->print_attrs = HLATTRS_INIT;
   ugrid_init(&data->grid);
   terminfo_start(ui);
   update_size(ui);
@@ -628,7 +628,7 @@ static void clear_region(UI *ui, int top, int bot, int left, int right)
   if (grid->bg == -1 && right == ui->width -1) {
     // Background is set to the default color and the right edge matches the
     // screen end, try to use terminal codes for clearing the requested area.
-    HlAttrs clear_attrs = EMPTY_ATTRS;
+    HlAttrs clear_attrs = HLATTRS_INIT;
     clear_attrs.foreground = grid->fg;
     clear_attrs.background = grid->bg;
     update_attrs(ui, clear_attrs);
@@ -926,7 +926,7 @@ static void tui_scroll(UI *ui, Integer count)
     cursor_goto(ui, grid->top, grid->left);
     // also set default color attributes or some terminals can become funny
     if (scroll_clears_to_current_colour) {
-      HlAttrs clear_attrs = EMPTY_ATTRS;
+      HlAttrs clear_attrs = HLATTRS_INIT;
       clear_attrs.foreground = grid->fg;
       clear_attrs.background = grid->bg;
       update_attrs(ui, clear_attrs);

src/nvim/ugrid.c

@@ -16,7 +16,7 @@
 
 void ugrid_init(UGrid *grid)
 {
-  grid->attrs = EMPTY_ATTRS;
+  grid->attrs = HLATTRS_INIT;
   grid->fg = grid->bg = -1;
   grid->cells = NULL;
 }
@@ -118,7 +118,7 @@ UCell *ugrid_put(UGrid *grid, uint8_t *text, size_t size)
 
 static void clear_region(UGrid *grid, int top, int bot, int left, int right)
 {
-  HlAttrs clear_attrs = EMPTY_ATTRS;
+  HlAttrs clear_attrs = HLATTRS_INIT;
   clear_attrs.foreground = grid->fg;
   clear_attrs.background = grid->bg;
   UGRID_FOREACH_CELL(grid, top, bot, left, right, {

src/nvim/ugrid.h

@@ -21,8 +21,6 @@ struct ugrid {
   UCell **cells;
 };
 
-#define EMPTY_ATTRS ((HlAttrs){ false, false, false, false, false, -1, -1, -1 })
-
 #define UGRID_FOREACH_CELL(grid, top, bot, left, right, code) \
   do { \
     for (int row = top; row <= bot; row++) { \

src/nvim/ui.c

@@ -166,6 +166,115 @@ void ui_event(char *name, Array args)
   }
 }
 
+/// Retrieves attribute description from its id
+///
+/// @param attr_id attribute id
+Dictionary get_attr_by_id(Integer attr_id, Error *err)
+{
+  HlAttrs attrs = HLATTRS_INIT;
+  Dictionary dic = ARRAY_DICT_INIT;
+
+  if (attr_id == 0) {
+    goto end;
+  }
+
+  attrentry_T *aep = syn_cterm_attr2entry((int)attr_id);
+  if (!aep) {
+    api_set_error(err, kErrorTypeException,
+                  "Invalid attribute id %d", attr_id);
+    return dic;
+  }
+
+  attrs = attrentry2hlattrs(aep, p_tgc);
+
+end:
+  return hlattrs2dict(attrs);
+}
+
+
+/// Converts an attrentry_T into an HlAttrs
+///
+/// @param[in] aep data to convert
+/// @param use_rgb use 'gui*' settings if true, else resorts to 'cterm*'
+HlAttrs attrentry2hlattrs(const attrentry_T *aep, bool use_rgb)
+{
+  assert(aep);
+
+  HlAttrs attrs = HLATTRS_INIT;
+  int mask = 0;
+
+  mask = use_rgb ? aep->rgb_ae_attr : aep->cterm_ae_attr;
+
+  attrs.bold = mask & HL_BOLD;
+  attrs.underline = mask & HL_UNDERLINE;
+  attrs.undercurl = mask & HL_UNDERCURL;
+  attrs.italic = mask & HL_ITALIC;
+  attrs.reverse = mask & (HL_INVERSE | HL_STANDOUT);
+
+  if (use_rgb) {
+    if (aep->rgb_fg_color != -1) {
+      attrs.foreground = aep->rgb_fg_color;
+    }
+
+    if (aep->rgb_bg_color != -1) {
+      attrs.background = aep->rgb_bg_color;
+    }
+
+    if (aep->rgb_sp_color != -1) {
+      attrs.special = aep->rgb_sp_color;
+    }
+  } else {
+    if (cterm_normal_fg_color != aep->cterm_fg_color) {
+      attrs.foreground = aep->cterm_fg_color - 1;
+    }
+
+    if (cterm_normal_bg_color != aep->cterm_bg_color) {
+        attrs.background = aep->cterm_bg_color - 1;
+    }
+  }
+
+  return attrs;
+}
+
+Dictionary hlattrs2dict(HlAttrs attrs)
+{
+  Dictionary hl = ARRAY_DICT_INIT;
+
+  if (attrs.bold) {
+    PUT(hl, "bold", BOOLEAN_OBJ(true));
+  }
+
+  if (attrs.underline) {
+    PUT(hl, "underline", BOOLEAN_OBJ(true));
+  }
+
+  if (attrs.undercurl) {
+    PUT(hl, "undercurl", BOOLEAN_OBJ(true));
+  }
+
+  if (attrs.italic) {
+    PUT(hl, "italic", BOOLEAN_OBJ(true));
+  }
+
+  if (attrs.reverse) {
+    PUT(hl, "reverse", BOOLEAN_OBJ(true));
+  }
+
+  if (attrs.foreground != -1) {
+    PUT(hl, "foreground", INTEGER_OBJ(attrs.foreground));
+  }
+
+  if (attrs.background != -1) {
+    PUT(hl, "background", INTEGER_OBJ(attrs.background));
+  }
+
+  if (attrs.special != -1) {
+    PUT(hl, "special", INTEGER_OBJ(attrs.special));
+  }
+
+  return hl;
+}
+
 void ui_refresh(void)
 {
   if (!ui_active()) {
@@ -405,54 +514,20 @@ void ui_flush(void)
 
 static void set_highlight_args(int attr_code)
 {
-  HlAttrs rgb_attrs = { false, false, false, false, false, -1, -1, -1 };
+  HlAttrs rgb_attrs = HLATTRS_INIT;
   HlAttrs cterm_attrs = rgb_attrs;
 
   if (attr_code == HL_NORMAL) {
     goto end;
   }
-
-  int rgb_mask = 0;
-  int cterm_mask = 0;
   attrentry_T *aep = syn_cterm_attr2entry(attr_code);
 
   if (!aep) {
     goto end;
   }
 
-  rgb_mask = aep->rgb_ae_attr;
-  cterm_mask = aep->cterm_ae_attr;
-
-  rgb_attrs.bold = rgb_mask & HL_BOLD;
-  rgb_attrs.underline = rgb_mask & HL_UNDERLINE;
-  rgb_attrs.undercurl = rgb_mask & HL_UNDERCURL;
-  rgb_attrs.italic = rgb_mask & HL_ITALIC;
-  rgb_attrs.reverse = rgb_mask & (HL_INVERSE | HL_STANDOUT);
-  cterm_attrs.bold = cterm_mask & HL_BOLD;
-  cterm_attrs.underline = cterm_mask & HL_UNDERLINE;
-  cterm_attrs.undercurl = cterm_mask & HL_UNDERCURL;
-  cterm_attrs.italic = cterm_mask & HL_ITALIC;
-  cterm_attrs.reverse = cterm_mask & (HL_INVERSE | HL_STANDOUT);
-
-  if (aep->rgb_fg_color != normal_fg) {
-    rgb_attrs.foreground = aep->rgb_fg_color;
-  }
-
-  if (aep->rgb_bg_color != normal_bg) {
-    rgb_attrs.background = aep->rgb_bg_color;
-  }
-
-  if (aep->rgb_sp_color != normal_sp) {
-    rgb_attrs.special = aep->rgb_sp_color;
-  }
-
-  if (cterm_normal_fg_color != aep->cterm_fg_color) {
-    cterm_attrs.foreground = aep->cterm_fg_color - 1;
-  }
-
-  if (cterm_normal_bg_color != aep->cterm_bg_color) {
-    cterm_attrs.background = aep->cterm_bg_color - 1;
-  }
+  rgb_attrs = attrentry2hlattrs(aep, true);
+  cterm_attrs = attrentry2hlattrs(aep, false);
 
 end:
   UI_CALL(highlight_set, (ui->rgb ? rgb_attrs : cterm_attrs));

src/nvim/ui.h

@@ -21,6 +21,9 @@ typedef struct {
   int foreground, background, special;
 } HlAttrs;
 
+#define HLATTRS_INIT \
+  ((HlAttrs){ false, false, false, false, false, -1, -1, -1 })
+
 typedef struct ui_t UI;
 
 struct ui_t {

test/functional/api/highlight_spec.lua

@@ -0,0 +1,54 @@
+
+local helpers = require('test.functional.helpers')(after_each)
+local clear, nvim = helpers.clear, helpers.nvim
+local Screen = require('test.functional.ui.screen')
+local eq, eval = helpers.eq, helpers.eval
+local command = helpers.command
+local ok = helpers.ok
+local meths = helpers.meths
+
+
+describe('highlight api', function()
+
+  before_each(function()
+    clear('--cmd', 'set termguicolors')
+  end)
+
+  it("nvim_get_hl_by_id", function()
+    local expected_hl = { background = Screen.colors.Yellow,
+                          foreground = Screen.colors.Red,
+                          special = Screen.colors.Blue
+                        }
+
+    command('hi NewHighlight guifg=red guibg=yellow guisp=blue')
+
+    local hl_id = eval("hlID('NewHighlight')")
+    eq(expected_hl, nvim("get_hl_by_id", hl_id))
+
+    -- assume there is no hl with 30000
+    local err, emsg = pcall(meths.get_hl_by_id, 30000)
+    eq(false, err)
+    ok(string.find(emsg, 'Invalid highlight id') ~= nil)
+  end)
+
+  it("nvim_get_hl_by_name", function()
+    local expected_hl = { background = Screen.colors.Yellow,
+                          foreground = Screen.colors.Red }
+
+    -- test "Normal" hl defaults
+    eq({}, nvim("get_hl_by_name", 'Normal'))
+
+    command('hi NewHighlight guifg=red guibg=yellow')
+    eq(expected_hl, nvim("get_hl_by_name", 'NewHighlight'))
+
+    command('hi Normal guifg=red guibg=yellow')
+    eq(expected_hl, nvim("get_hl_by_name", 'Normal'))
+    local err, emsg = pcall(meths.get_hl_by_name , 'unknown_highlight')
+    eq(false, err)
+    ok(string.find(emsg, 'Invalid highlight name') ~= nil)
+  end)
+
+
+
+end)
+