Add support for modmap None (#291)

Unlike current xkbcommon, X11’s xkbcomp allows to remove entries in
the modifiers’ map using “modifier_map None { … }”.

“None” is translated to the special value “XkbNoModifier” defined in
“X11/extensions/XKB.h”. Then it relies on the fact that in "CopyModMapDef",
the following code:

    1U << entry->modifier

ends up being zero when “entry->modifier” is “XkbNoModifier” (i.e. 0xFF).
Indeed, it relies on the overflow behaviour of the left shift, which in
practice resolves to use only the 5 low bits of the shift amount, i.e.
0x1F here. Then the result of “1U << 0xFF” is cast to “char”, i.e. 0.

This is a good trick but too magical, so in libxkbcommon we will use
an explicit test against our new constant XKB_MOD_NONE.

4 files changed, 30 insertions, 9 deletions

diff --git a/src/keymap.h b/src/keymap.h
index 7c5341d..f7ea5bd 100644
--- a/src/keymap.h
+++ b/src/keymap.h
@@ -109,6 +109,9 @@
 /* Don't allow more leds than we can hold in xkb_led_mask_t. */
 #define XKB_MAX_LEDS ((xkb_led_index_t) (sizeof(xkb_led_mask_t) * 8))
 
+/* Special value to handle modMap None {…} */
+#define XKB_MOD_NONE 0xffffffffU
+
 /* These should all go away. */
 enum mod_type {
     MOD_REAL = (1 << 0),
diff --git a/src/text.c b/src/text.c
index 60edb03..290a5ae 100644
--- a/src/text.c
+++ b/src/text.c
@@ -216,6 +216,9 @@ ModIndexText(struct xkb_context *ctx, const struct xkb_mod_set *mods,
     if (ndx == XKB_MOD_INVALID)
         return "none";
 
+    if (ndx == XKB_MOD_NONE)
+        return "None";
+
     if (ndx >= mods->num_mods)
         return NULL;
 
diff --git a/src/xkbcomp/ast.h b/src/xkbcomp/ast.h
index 6c51ce4..8b0901f 100644
--- a/src/xkbcomp/ast.h
+++ b/src/xkbcomp/ast.h
@@ -303,6 +303,7 @@ typedef struct {
 typedef struct {
     ParseCommon common;
     enum merge_mode merge;
+    // NOTE: Can also be “None”, rather than a modifier name.
     xkb_atom_t modifier;
     ExprDef *keys;
 } ModMapDef;
diff --git a/src/xkbcomp/symbols.c b/src/xkbcomp/symbols.c
index eb78412..f990529 100644
--- a/src/xkbcomp/symbols.c
+++ b/src/xkbcomp/symbols.c
@@ -162,6 +162,8 @@ ClearKeyInfo(KeyInfo *keyi)
 typedef struct {
     enum merge_mode merge;
     bool haveSymbol;
+    // NOTE: Can also be XKB_MOD_NONE, meaning
+    //       “don’t add a modifier to the modmap”.
     xkb_mod_index_t modifier;
     union {
         xkb_atom_t keyName;
@@ -1152,14 +1154,21 @@ HandleModMapDef(SymbolsInfo *info, ModMapDef *def)
     xkb_mod_index_t ndx;
     bool ok;
     struct xkb_context *ctx = info->ctx;
-
-    ndx = XkbModNameToIndex(&info->mods, def->modifier, MOD_REAL);
-    if (ndx == XKB_MOD_INVALID) {
-        log_err(info->ctx,
-                "Illegal modifier map definition; "
-                "Ignoring map for non-modifier \"%s\"\n",
-                xkb_atom_text(ctx, def->modifier));
-        return false;
+    const char *modifier_name = xkb_atom_text(ctx, def->modifier);
+
+    if (istreq(modifier_name, "none")) {
+        // Handle special "None" entry
+        ndx = XKB_MOD_NONE;
+    } else {
+        // Handle normal entry
+        ndx = XkbModNameToIndex(&info->mods, def->modifier, MOD_REAL);
+        if (ndx == XKB_MOD_INVALID) {
+            log_err(info->ctx,
+                    "Illegal modifier map definition; "
+                    "Ignoring map for non-modifier \"%s\"\n",
+                    xkb_atom_text(ctx, def->modifier));
+            return false;
+        }
     }
 
     ok = true;
@@ -1523,7 +1532,12 @@ CopyModMapDefToKeymap(struct xkb_keymap *keymap, SymbolsInfo *info,
         }
     }
 
-    key->modmap |= (1u << entry->modifier);
+    // Handle modMap None
+    if (entry->modifier != XKB_MOD_NONE) {
+        // Convert modifier index to modifier mask
+        key->modmap |= (1u << entry->modifier);
+    }
+
     return true;
 }