+XKB_EXPORT size_t
+xkb_keymap_key_get_mods_for_level(struct xkb_keymap *keymap,
+ xkb_keycode_t kc,
+ xkb_layout_index_t layout,
+ xkb_level_index_t level,
+ xkb_mod_mask_t *masks_out,
+ size_t masks_size)
+{
+ const struct xkb_key *key = XkbKey(keymap, kc);
+ if (!key)
+ return 0;
+
+ layout = XkbWrapGroupIntoRange(layout, key->num_groups,
+ key->out_of_range_group_action,
+ key->out_of_range_group_number);
+ if (layout == XKB_LAYOUT_INVALID)
+ return 0;
+
+ if (level >= XkbKeyNumLevels(key, layout))
+ return 0;
+
+ const struct xkb_key_type *type = key->groups[layout].type;
+
+ size_t count = 0;
+
+ /*
+ * If the active set of modifiers doesn't match any explicit entry of
+ * the key type, the resulting level is 0 (i.e. Level 1).
+ * So, if we are asked to find the modifiers for level==0, we can offer
+ * an ~infinite supply, which is not very workable.
+ * What we do instead, is special case the empty set of modifiers for
+ * this purpose. If the empty set isn't explicit mapped to a level, we
+ * take it to map to Level 1.
+ * This is almost always what we want. If applicable, given it priority
+ * over other ways to generate the level.
+ */
+ if (level == 0) {
+ bool empty_mapped = false;
+ for (unsigned i = 0; i < type->num_entries && count < masks_size; i++)
+ if (entry_is_active(&type->entries[i]) &&
+ type->entries[i].mods.mask == 0) {
+ empty_mapped = true;
+ break;
+ }
+ if (!empty_mapped && count < masks_size) {
+ masks_out[count++] = 0;
+ }
+ }
+
+ /* Now search explicit mappings. */
+ for (unsigned i = 0; i < type->num_entries && count < masks_size; i++) {
+ if (entry_is_active(&type->entries[i]) &&
+ type->entries[i].level == level) {
+ masks_out[count++] = type->entries[i].mods.mask;
+ }
+ }
+
+ return count;
+}
+