[Core] Add Caps Word feature to core (#16588)

Co-authored-by: precondition <57645186+precondition@users.noreply.github.com>
Co-authored-by: Drashna Jaelre <drashna@live.com>

3 files changed, 165 insertions, 0 deletions

diff --git a/docs/_summary.md b/docs/_summary.md
index 786685eba4..fed75196b4 100644
--- a/docs/_summary.md
+++ b/docs/_summary.md
@@ -75,6 +75,7 @@
 
   * Software Features
     * [Auto Shift](feature_auto_shift.md)
+    * [Caps Word](feature_caps_word.md)
     * [Combos](feature_combo.md)
     * [Debounce API](feature_debounce_type.md)
     * [Key Lock](feature_key_lock.md)
diff --git a/docs/feature_caps_word.md b/docs/feature_caps_word.md
new file mode 100644
index 0000000000..0b71119917
--- /dev/null
+++ b/docs/feature_caps_word.md
@@ -0,0 +1,156 @@
+# Caps Word
+
+It is often useful to type a single word in all capitals, for instance
+abbreviations like "QMK", or in code, identifiers like `KC_SPC`. "Caps Word" is
+a modern alternative to Caps Lock:
+
+* Letters are capitalized while active, and Caps Word automatically disables
+  itself at the end of the word. That is, it stops by default once a space or
+  any key other than `a`--`z`, `0`--`9`, `-`, `_`, delete, or backspace is
+  pressed. Caps Word also disables itself if the keyboard is idle for 5 seconds.
+  This is configurable, see below.
+
+* To avoid requiring a dedicated key for Caps Word, there is an option
+  (`BOTH_SHIFTS_TURNS_ON_CAPS_WORD`) to activate Caps Word by simultaneously
+  pressing both shift keys. See below for other options.
+
+* The implementation does not use the Caps Lock (`KC_CAPS`) keycode. Caps Word
+  works even if you're remapping Caps Lock at the OS level to Ctrl or something
+  else, as Emacs and Vim users often do.
+
+
+## How do I enable Caps Word :id=how-do-i-enable-caps-word
+
+In your `rules.mk`, add:
+
+```make
+CAPS_WORD_ENABLE = yes
+```
+
+Next, use one the following methods to activate Caps Word:
+
+* **Activate by pressing a key**: Use the `CAPS_WORD` keycode (short
+  alias `CAPSWRD`) in your keymap.
+
+* **Activate by pressing Left Shift + Right Shift**: Add `#define
+  BOTH_SHIFTS_TURNS_ON_CAPS_WORD` to config.h. You may also need to disable or
+  reconfigure Command, details below. Then, simultaneously pressing both left
+  and right shifts turns on Caps Word. This method works with the plain
+  `KC_LSFT` and `KC_RSFT` keycodes as well as one-shot shifts and Space Cadet
+  shifts. If your shift keys are mod-taps, hold both shift mod-tap keys until
+  the tapping term, then release them.
+
+* **Activate by double tapping Left Shift**: Add `#define
+  DOUBLE_TAP_SHIFT_TURNS_ON_CAPS_WORD` config.h. Then, double tapping Left Shift
+  turns on Caps Word. This method works with `KC_LSFT` or one-shot Left Shift
+  `OSM(MOD_LSFT)`. To count as a double tap, the maximum time in milliseconds
+  between taps is `TAPPING_TERM`, or if using `TAPPING_TERM_PER_KEY`, the time
+  returned by `get_tapping_term()` for the shift keycode being tapped.
+
+* **Custom activation**: You can activate Caps Word from code by calling
+  `caps_word_on()`. This may be used to activate Caps Word through [a
+  combo](feature_combo.md) or [tap dance](feature_tap_dance.md) or any means
+  you like.
+
+### Troubleshooting: Command :id=troubleshooting-command
+
+When using `BOTH_SHIFTS_TURNS_ON_CAPS_WORD`, you might see a compile message
+**"BOTH_SHIFTS_TURNS_ON_CAPS_WORD and Command should not be enabled at the same
+time, since both use the Left Shift + Right Shift key combination."**
+
+Many keyboards enable the [Command feature](feature_command.md), which by
+default is also activated using the Left Shift + Right Shift key combination. To
+fix this conflict, please disable Command by adding in rules.mk: 
+
+```make
+COMMAND_ENABLE = no
+```
+
+Or configure Command to use another key combination like Left Ctrl + Right Ctrl
+by defining `IS_COMMAND()` in config.h:
+
+```c
+// Activate Command with Left Ctrl + Right Ctrl.
+#define IS_COMMAND() (get_mods() == MOD_MASK_CTRL)
+```
+
+
+## Customizing Caps Word :id=customizing-caps-word
+
+### Idle timeout :id=idle-timeout
+
+Caps Word turns off automatically if no keys are pressed for
+`CAPS_WORD_IDLE_TIMEOUT` milliseconds. The default is 5000 (5 seconds).
+Configure the timeout duration in config.h, for instance 
+
+```c
+#define CAPS_WORD_IDLE_TIMEOUT 3000  // 3 seconds.
+```
+
+Setting `CAPS_WORD_IDLE_TIMEOUT` to 0 configures Caps Word to never time out.
+Caps Word then remains active indefinitely until a word breaking key is pressed.
+
+
+### Functions :id=functions
+
+Functions to manipulate Caps Word:
+
+| Function                | Description                                    |
+|-------------------------|------------------------------------------------|
+| `caps_word_on()`        | Turns Caps Word on.                            |
+| `caps_word_off()`       | Turns Caps Word off.                           |
+| `caps_word_toggle()`    | Toggles Caps Word.                             |
+| `is_caps_word_on()`     | Returns true if Caps Word is currently on.     |
+
+
+### Configure which keys are "word breaking" :id=configure-which-keys-are-word-breaking
+
+You can define the `caps_word_press_user(uint16_t keycode)` callback to
+configure which keys should be shifted and which keys are considered "word
+breaking" and stop Caps Word.
+
+The callback is called on every key press while Caps Word is active. When the
+key should be shifted (that is, a letter key), the callback should call
+`add_weak_mods(MOD_BIT(KC_LSFT))` to shift the key. Returning true continues the
+current "word," while returning false is "word breaking" and deactivates Caps
+Word. The default callback is
+
+```c
+bool caps_word_press_user(uint16_t keycode) {
+    switch (keycode) {
+        // Keycodes that continue Caps Word, with shift applied.
+        case KC_A ... KC_Z:
+        case KC_MINS:
+            add_weak_mods(MOD_BIT(KC_LSFT));  // Apply shift to next key.
+            return true;
+
+        // Keycodes that continue Caps Word, without shifting.
+        case KC_1 ... KC_0:
+        case KC_BSPC:
+        case KC_DEL:
+        case KC_UNDS:
+            return true;
+
+        default:
+            return false;  // Deactivate Caps Word.
+    }
+}
+```
+
+
+### Representing Caps Word state :id=representing-caps-word-state
+
+Define `caps_word_set_user(bool active)` to get callbacks when Caps Word turns
+on or off. This is useful to represent the current Caps Word state, e.g. by
+setting an LED or playing a sound. In your keymap, define
+
+```c
+void caps_word_set_user(bool active) {
+    if (active) {
+        // Do something when Caps Word activates.
+    } else {
+        // Do something when Caps Word deactivates.
+    }
+}
+```
+
diff --git a/docs/keycodes.md b/docs/keycodes.md
index 10652bdb2f..bd5af32dd3 100644
--- a/docs/keycodes.md
+++ b/docs/keycodes.md
@@ -269,6 +269,14 @@ See also: [Bluetooth](feature_bluetooth.md)
 |`OUT_USB` |USB only                                      |
 |`OUT_BT`  |Bluetooth only                                |
 
+## Caps Word :id=caps-word
+
+See also: [Caps Word](feature_caps_word.md)
+
+|Key        |Aliases  |Description                   |
+|-----------|---------|------------------------------|
+|`CAPS_WORD`|`CAPSWRD`|Toggles Caps Word             |
+
 ## Dynamic Macros :id=dynamic-macros
 
 See also: [Dynamic Macros](feature_dynamic_macros.md)