[Core] get_keycode_string(): function to format keycodes as strings, for more readable debug logging. (#24787)

* keycode_string(): Format keycodes as strings.

This adds the `keycode_string()` function described in
https://getreuer.info/posts/keyboards/keycode-string/index.html
as a core feature.

* Fix formatting.

* keycode_string review revisions.

* Rename keycode_string() -> get_keycode_string() for consistency with
  existing string utils like get_u8_str().

* Revise custom keycode names with separate _user and _kb tables.

* Correct indent in builddefs/generic_features.mk.

Co-authored-by: Ryan <fauxpark@gmail.com>

* Add KC_NUHS, KC_NUBS, and KC_CAPS.

* Fix linking error with custom names.

* Attempt at simplifying interface.

* Formatting fix.

* Several fixes and revisions.

* Don't use PSTR in KEYCODE_STRING_NAME, since this fails to build on
  AVR. Store custom names in RAM.
* Revise the internal table of common keycode names to use its own
  storage representation, still in PROGMEM, and now more efficiently
  stored flat in 8 bytes per entry.
* Support Swap Hands keycodes and a few other keycodes.

* Revert "Formatting fix."

This reverts commit 2a2771068c7ee545ffac4103aa07e847a9ec3816.

* Revert "Attempt at simplifying interface."

This reverts commit 8eaf67de76e75bc92d106a8b0decc893fbc65fa5.

* Simplify custom names API by sigprof's suggestion.

* Support more keycodes.

* Add QK_LOCK keycode.
* Add Secure keycodes.
* Add Joystick keycodes.
* Add Programmable Button keycodes.
* Add macro MC_ keycodes.
* For remaining keys in known code ranges, stringify them as
  "QK_<feature>+<number>". For instance, "QK_MIDI+7".

* Bug fix and a few improvements.

* Fix missing right-hand bit when displaying 5-bit mods numerically.
* Support KC_HYPR, KC_MEH, HYPR_T(kc), MEH_T(kc).
* Exclude one-shot keycodes when NO_ACTION_ONESHOT is defined.

---------

Co-authored-by: Ryan <fauxpark@gmail.com>

1 files changed, 564 insertions, 0 deletions

diff --git a/quantum/keycode_string.c b/quantum/keycode_string.c
new file mode 100644
index 0000000000..18044f2ef6
--- /dev/null
+++ b/quantum/keycode_string.c
@@ -0,0 +1,564 @@
+// Copyright 2024-2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "keycode_string.h"
+
+#include <string.h>
+#include "bitwise.h"
+#include "keycode.h"
+#include "progmem.h"
+#include "quantum_keycodes.h"
+#include "util.h"
+
+typedef int_fast8_t index_t;
+
+// clang-format off
+/** Packs a 7-char keycode name, ignoring the third char, as 3 words. */
+#define KEYCODE_NAME7(c0, c1, unused_c2, c3, c4, c5, c6) \
+    ((uint16_t)c0) | (((uint16_t)c1) << 8),              \
+    ((uint16_t)c3) | (((uint16_t)c4) << 8),              \
+    ((uint16_t)c5) | (((uint16_t)c6) << 8)
+
+/**
+ * @brief Names of some common keycodes.
+ *
+ * Each (keycode, name) entry is stored flat in 8 bytes in PROGMEM. Names in
+ * this table must be at most 7 chars long and have an underscore '_' for the
+ * third char. This underscore is assumed and not actually stored.
+ *
+ * To save memory, feature-specific key entries are ifdef'd to include them only
+ * when their feature is enabled.
+ */
+static const uint16_t common_names[] PROGMEM = {
+    KC_TRNS, KEYCODE_NAME7('K', 'C', '_', 'T', 'R', 'N', 'S'),
+    KC_ENT , KEYCODE_NAME7('K', 'C', '_', 'E', 'N', 'T',  0 ),
+    KC_ESC , KEYCODE_NAME7('K', 'C', '_', 'E', 'S', 'C',  0 ),
+    KC_BSPC, KEYCODE_NAME7('K', 'C', '_', 'B', 'S', 'P', 'C'),
+    KC_TAB , KEYCODE_NAME7('K', 'C', '_', 'T', 'A', 'B',  0 ),
+    KC_SPC , KEYCODE_NAME7('K', 'C', '_', 'S', 'P', 'C',  0 ),
+    KC_MINS, KEYCODE_NAME7('K', 'C', '_', 'M', 'I', 'N', 'S'),
+    KC_EQL , KEYCODE_NAME7('K', 'C', '_', 'E', 'Q', 'L',  0 ),
+    KC_LBRC, KEYCODE_NAME7('K', 'C', '_', 'L', 'B', 'R', 'C'),
+    KC_RBRC, KEYCODE_NAME7('K', 'C', '_', 'R', 'B', 'R', 'C'),
+    KC_BSLS, KEYCODE_NAME7('K', 'C', '_', 'B', 'S', 'L', 'S'),
+    KC_NUHS, KEYCODE_NAME7('K', 'C', '_', 'N', 'U', 'H', 'S'),
+    KC_SCLN, KEYCODE_NAME7('K', 'C', '_', 'S', 'C', 'L', 'N'),
+    KC_QUOT, KEYCODE_NAME7('K', 'C', '_', 'Q', 'U', 'O', 'T'),
+    KC_GRV , KEYCODE_NAME7('K', 'C', '_', 'G', 'R', 'V',  0 ),
+    KC_COMM, KEYCODE_NAME7('K', 'C', '_', 'C', 'O', 'M', 'M'),
+    KC_DOT , KEYCODE_NAME7('K', 'C', '_', 'D', 'O', 'T',  0 ),
+    KC_SLSH, KEYCODE_NAME7('K', 'C', '_', 'S', 'L', 'S', 'H'),
+    KC_CAPS, KEYCODE_NAME7('K', 'C', '_', 'C', 'A', 'P', 'S'),
+    KC_PSCR, KEYCODE_NAME7('K', 'C', '_', 'P', 'S', 'C', 'R'),
+    KC_PAUS, KEYCODE_NAME7('K', 'C', '_', 'P', 'A', 'U', 'S'),
+    KC_INS , KEYCODE_NAME7('K', 'C', '_', 'I', 'N', 'S',  0 ),
+    KC_HOME, KEYCODE_NAME7('K', 'C', '_', 'H', 'O', 'M', 'E'),
+    KC_PGUP, KEYCODE_NAME7('K', 'C', '_', 'P', 'G', 'U', 'P'),
+    KC_DEL , KEYCODE_NAME7('K', 'C', '_', 'D', 'E', 'L',  0 ),
+    KC_END , KEYCODE_NAME7('K', 'C', '_', 'E', 'N', 'D',  0 ),
+    KC_PGDN, KEYCODE_NAME7('K', 'C', '_', 'P', 'G', 'D', 'N'),
+    KC_RGHT, KEYCODE_NAME7('K', 'C', '_', 'R', 'G', 'H', 'T'),
+    KC_LEFT, KEYCODE_NAME7('K', 'C', '_', 'L', 'E', 'F', 'T'),
+    KC_DOWN, KEYCODE_NAME7('K', 'C', '_', 'D', 'O', 'W', 'N'),
+    KC_UP  , KEYCODE_NAME7('K', 'C', '_', 'U', 'P',  0 ,  0 ),
+    KC_NUBS, KEYCODE_NAME7('K', 'C', '_', 'N', 'U', 'B', 'S'),
+    KC_HYPR, KEYCODE_NAME7('K', 'C', '_', 'H', 'Y', 'P', 'R'),
+    KC_MEH , KEYCODE_NAME7('K', 'C', '_', 'M', 'E', 'H',  0 ),
+#ifdef EXTRAKEY_ENABLE
+    KC_WHOM, KEYCODE_NAME7('K', 'C', '_', 'W', 'H', 'O', 'M'),
+    KC_WBAK, KEYCODE_NAME7('K', 'C', '_', 'W', 'B', 'A', 'K'),
+    KC_WFWD, KEYCODE_NAME7('K', 'C', '_', 'W', 'F', 'W', 'D'),
+    KC_WSTP, KEYCODE_NAME7('K', 'C', '_', 'W', 'S', 'T', 'P'),
+    KC_WREF, KEYCODE_NAME7('K', 'C', '_', 'W', 'R', 'E', 'F'),
+    KC_MNXT, KEYCODE_NAME7('K', 'C', '_', 'M', 'N', 'X', 'T'),
+    KC_MPRV, KEYCODE_NAME7('K', 'C', '_', 'M', 'P', 'R', 'V'),
+    KC_MPLY, KEYCODE_NAME7('K', 'C', '_', 'M', 'P', 'L', 'Y'),
+    KC_MUTE, KEYCODE_NAME7('K', 'C', '_', 'M', 'U', 'T', 'E'),
+    KC_VOLU, KEYCODE_NAME7('K', 'C', '_', 'V', 'O', 'L', 'U'),
+    KC_VOLD, KEYCODE_NAME7('K', 'C', '_', 'V', 'O', 'L', 'D'),
+#endif // EXTRAKEY_ENABLE
+#ifdef MOUSEKEY_ENABLE
+    MS_LEFT, KEYCODE_NAME7('M', 'S', '_', 'L', 'E', 'F', 'T'),
+    MS_RGHT, KEYCODE_NAME7('M', 'S', '_', 'R', 'G', 'H', 'T'),
+    MS_UP  , KEYCODE_NAME7('M', 'S', '_', 'U', 'P',  0 ,  0 ),
+    MS_DOWN, KEYCODE_NAME7('M', 'S', '_', 'D', 'O', 'W', 'N'),
+    MS_WHLL, KEYCODE_NAME7('M', 'S', '_', 'W', 'H', 'L', 'L'),
+    MS_WHLR, KEYCODE_NAME7('M', 'S', '_', 'W', 'H', 'L', 'R'),
+    MS_WHLU, KEYCODE_NAME7('M', 'S', '_', 'W', 'H', 'L', 'U'),
+    MS_WHLD, KEYCODE_NAME7('M', 'S', '_', 'W', 'H', 'L', 'D'),
+#endif // MOUSEKEY_ENABLE
+#ifdef SWAP_HANDS_ENABLE
+    SH_ON  , KEYCODE_NAME7('S', 'H', '_', 'O', 'N',  0 ,  0 ),
+    SH_OFF , KEYCODE_NAME7('S', 'H', '_', 'O', 'F', 'F',  0 ),
+    SH_MON , KEYCODE_NAME7('S', 'H', '_', 'M', 'O', 'N',  0 ),
+    SH_MOFF, KEYCODE_NAME7('S', 'H', '_', 'M', 'O', 'F', 'F'),
+    SH_TOGG, KEYCODE_NAME7('S', 'H', '_', 'T', 'O', 'G', 'G'),
+    SH_TT  , KEYCODE_NAME7('S', 'H', '_', 'T', 'T',  0 ,  0 ),
+#    if !defined(NO_ACTION_ONESHOT)
+    SH_OS  , KEYCODE_NAME7('S', 'H', '_', 'O', 'S',  0 ,  0 ),
+#    endif // !defined(NO_ACTION_ONESHOT)
+#endif // SWAP_HANDS_ENABLE
+#ifdef LEADER_ENABLE
+    QK_LEAD, KEYCODE_NAME7('Q', 'K', '_', 'L', 'E', 'A', 'D'),
+#endif // LEADER_ENABLE
+#ifdef KEY_LOCK_ENABLE
+    QK_LOCK, KEYCODE_NAME7('Q', 'K', '_', 'L', 'O', 'C', 'K'),
+#endif // KEY_LOCK_ENABLE
+#ifdef TRI_LAYER_ENABLE
+    TL_LOWR, KEYCODE_NAME7('T', 'L', '_', 'L', 'O', 'W', 'R'),
+    TL_UPPR, KEYCODE_NAME7('T', 'L', '_', 'U', 'P', 'P', 'R'),
+#endif // TRI_LAYER_ENABLE
+#ifdef GRAVE_ESC_ENABLE
+    QK_GESC, KEYCODE_NAME7('Q', 'K', '_', 'G', 'E', 'S', 'C'),
+#endif // GRAVE_ESC_ENABLE
+#ifdef CAPS_WORD_ENABLE
+    CW_TOGG, KEYCODE_NAME7('C', 'W', '_', 'T', 'O', 'G', 'G'),
+#endif // CAPS_WORD_ENABLE
+#ifdef SECURE_ENABLE
+    SE_LOCK, KEYCODE_NAME7('S', 'E', '_', 'L', 'O', 'C', 'K'),
+    SE_UNLK, KEYCODE_NAME7('S', 'E', '_', 'U', 'N', 'L', 'K'),
+    SE_TOGG, KEYCODE_NAME7('S', 'E', '_', 'T', 'O', 'G', 'G'),
+    SE_REQ , KEYCODE_NAME7('S', 'E', '_', 'R', 'E', 'Q',  0 ),
+#endif // SECURE_ENABLE
+#ifdef LAYER_LOCK_ENABLE
+    QK_LLCK, KEYCODE_NAME7('Q', 'K', '_', 'L', 'L', 'C', 'K'),
+#endif // LAYER_LOCK_ENABLE
+    EE_CLR , KEYCODE_NAME7('E', 'E', '_', 'C', 'L', 'R',  0 ),
+    QK_BOOT, KEYCODE_NAME7('Q', 'K', '_', 'B', 'O', 'O', 'T'),
+    DB_TOGG, KEYCODE_NAME7('D', 'B', '_', 'T', 'O', 'G', 'G'),
+};
+// clang-format on
+
+/** Users can override this to define names of additional keycodes. */
+__attribute__((weak)) const keycode_string_name_t* keycode_string_names_data_user = NULL;
+__attribute__((weak)) uint16_t                     keycode_string_names_size_user = 0;
+/** Keyboard vendors can override this to define names of additional keycodes. */
+__attribute__((weak)) const keycode_string_name_t* keycode_string_names_data_kb = NULL;
+__attribute__((weak)) uint16_t                     keycode_string_names_size_kb = 0;
+/** Names of the 4 mods on each hand. */
+static const char mod_names[] PROGMEM = "CTL\0SFT\0ALT\0GUI";
+/** Internal buffer for holding a stringified keycode. */
+static char buffer[32];
+#define BUFFER_MAX_LEN (sizeof(buffer) - 1)
+static index_t buffer_len;
+
+/** Finds the name of a keycode in `common_names` or returns NULL. */
+static const char* search_common_names(uint16_t keycode) {
+    static uint8_t buffer[8];
+
+    for (int_fast16_t offset = 0; offset < ARRAY_SIZE(common_names); offset += 4) {
+        if (keycode == pgm_read_word(common_names + offset)) {
+            const uint16_t w0 = pgm_read_word(common_names + offset + 1);
+            const uint16_t w1 = pgm_read_word(common_names + offset + 2);
+            const uint16_t w2 = pgm_read_word(common_names + offset + 3);
+            buffer[0]         = (uint8_t)w0;
+            buffer[1]         = (uint8_t)(w0 >> 8);
+            buffer[2]         = '_';
+            buffer[3]         = (uint8_t)w1;
+            buffer[4]         = (uint8_t)(w1 >> 8);
+            buffer[5]         = (uint8_t)w2;
+            buffer[6]         = (uint8_t)(w2 >> 8);
+            buffer[7]         = 0;
+            return (const char*)buffer;
+        }
+    }
+
+    return NULL;
+}
+
+/**
+ * @brief Finds the name of a keycode in table or returns NULL.
+ *
+ * @param data   Pointer to table to be searched.
+ * @param size   Numer of entries in the table.
+ * @return Name string for the keycode, or NULL if not found.
+ */
+static const char* search_table(const keycode_string_name_t* data, uint16_t size, uint16_t keycode) {
+    if (data != NULL) {
+        for (uint16_t i = 0; i < size; ++i) {
+            if (data[i].keycode == keycode) {
+                return data[i].name;
+            }
+        }
+    }
+    return NULL;
+}
+
+/** Formats `number` in `base`, either 10 or 16. */
+static char* number_string(uint16_t number, int8_t base) {
+    static char result[7];
+    result[sizeof(result) - 1] = '\0';
+    index_t i                  = sizeof(result) - 1;
+    do {
+        const uint8_t digit = number % base;
+        number /= base;
+        result[--i] = (digit < 10) ? (char)(digit + UINT8_C('0')) : (char)(digit + (UINT8_C('A') - 10));
+    } while (number > 0 && i > 0);
+
+    if (base == 16 && i >= 2) {
+        result[--i] = 'x';
+        result[--i] = '0';
+    }
+    return result + i;
+}
+
+/** Appends `str` to `buffer`, truncating if the result would overflow. */
+static void append(const char* str) {
+    char*   dest = buffer + buffer_len;
+    index_t i;
+    for (i = 0; buffer_len + i < BUFFER_MAX_LEN && str[i]; ++i) {
+        dest[i] = str[i];
+    }
+    buffer_len += i;
+    buffer[buffer_len] = '\0';
+}
+
+/** Same as append(), but where `str` is a PROGMEM string. */
+static void append_P(const char* str) {
+    char*   dest = buffer + buffer_len;
+    index_t i;
+    for (i = 0; buffer_len + i < BUFFER_MAX_LEN; ++i) {
+        const char c = pgm_read_byte(&str[i]);
+        if (c == '\0') {
+            break;
+        }
+        dest[i] = c;
+    }
+    buffer_len += i;
+    buffer[buffer_len] = '\0';
+}
+
+/** Appends a single char to `buffer` if there is space. */
+static void append_char(char c) {
+    if (buffer_len < BUFFER_MAX_LEN) {
+        buffer[buffer_len]   = c;
+        buffer[++buffer_len] = '\0';
+    }
+}
+
+/** Formats `number` in `base`, either 10 or 16, and appends it to `buffer`. */
+static void append_number(uint16_t number, int8_t base) {
+    append(number_string(number, base));
+}
+
+/** Stringifies 5-bit mods and appends it to `buffer`. */
+static void append_5_bit_mods(uint8_t mods) {
+    const bool    is_rhs = mods > 15;
+    const uint8_t csag   = mods & 15;
+    if (csag != 0 && (csag & (csag - 1)) == 0) { // One mod is set.
+        append_P(PSTR("MOD_"));
+        append_char(is_rhs ? 'R' : 'L');
+        append_P(&mod_names[4 * biton(csag)]);
+    } else { // Fallback: write the mod as a hex value.
+        append_number(mods, 16);
+    }
+}
+
+/**
+ * @brief Writes a keycode of the format `name` + "(" + `param` + ")".
+ * @note `name` is a PROGMEM string, `param` is not.
+ */
+static void append_unary_keycode(const char* name, const char* param) {
+    append_P(name);
+    append_char('(');
+    append(param);
+    append_char(')');
+}
+
+/**
+ * @brief Writes a keycode of the format `name` + `number`.
+ * @note `name` is a PROGMEM string.
+ */
+static void append_numbered_keycode(const char* name, uint16_t number) {
+    append_P(name);
+    append_number(number, 10);
+}
+
+/** Stringifies `keycode` and appends it to `buffer`. */
+static void append_keycode(uint16_t keycode) {
+    // In case there is overlap among tables, search `keycode_string_names_user`
+    // first so that it takes precedence.
+    const char* keycode_name = search_table(keycode_string_names_data_user, keycode_string_names_size_user, keycode);
+    if (keycode_name) {
+        append(keycode_name);
+        return;
+    }
+    keycode_name = search_table(keycode_string_names_data_kb, keycode_string_names_size_kb, keycode);
+    if (keycode_name) {
+        append(keycode_name);
+        return;
+    }
+    keycode_name = search_common_names(keycode);
+    if (keycode_name) {
+        append(keycode_name);
+        return;
+    }
+
+    if (keycode <= 255) { // Basic keycodes.
+        switch (keycode) {
+            // Modifiers KC_LSFT, KC_RCTL, etc.
+            case MODIFIER_KEYCODE_RANGE: {
+                const uint8_t i      = keycode - KC_LCTL;
+                const bool    is_rhs = i > 3;
+                append_P(PSTR("KC_"));
+                append_char(is_rhs ? 'R' : 'L');
+                append_P(&mod_names[4 * (i & 3)]);
+            }
+                return;
+
+            // Letters A-Z.
+            case KC_A ... KC_Z:
+                append_P(PSTR("KC_"));
+                append_char((char)(keycode + (UINT8_C('A') - KC_A)));
+                return;
+
+            // Digits 0-9 (NOTE: Unlike the ASCII order, KC_0 comes *after* KC_9.)
+            case KC_1 ... KC_0:
+                append_numbered_keycode(PSTR("KC_"), (keycode - (KC_1 - 1)) % 10);
+                return;
+
+            // Keypad digits.
+            case KC_KP_1 ... KC_KP_0:
+                append_numbered_keycode(PSTR("KC_KP_"), (keycode - (KC_KP_1 - 1)) % 10);
+                return;
+
+            // Function keys. F1-F12 and F13-F24 are coded in separate ranges.
+            case KC_F1 ... KC_F12:
+                append_numbered_keycode(PSTR("KC_F"), keycode - (KC_F1 - 1));
+                return;
+
+            case KC_F13 ... KC_F24:
+                append_numbered_keycode(PSTR("KC_F"), keycode - (KC_F13 - 13));
+                return;
+        }
+    }
+
+    // clang-format off
+    switch (keycode) {
+        // A modified keycode, like S(KC_1) for Shift + 1 = !. This implementation
+        // only covers modified keycodes where one modifier is applied, e.g. a
+        // Ctrl + Shift + kc or Hyper + kc keycode is not formatted.
+        case QK_MODS ... QK_MODS_MAX: {
+            uint8_t mods = QK_MODS_GET_MODS(keycode);
+            const bool is_rhs = mods > 15;
+            mods &= 15;
+            if (mods != 0 && (mods & (mods - 1)) == 0) { // One mod is set.
+                const char* name = &mod_names[4 * biton(mods)];
+                if (is_rhs) {
+                    append_char('R');
+                    append_P(name);
+                } else {
+                    append_char(pgm_read_byte(&name[0]));
+                }
+                append_char('(');
+                append_keycode(QK_MODS_GET_BASIC_KEYCODE(keycode));
+                append_char(')');
+                return;
+            }
+        } break;
+
+#if !defined(NO_ACTION_ONESHOT)
+        case QK_ONE_SHOT_MOD ... QK_ONE_SHOT_MOD_MAX: // One-shot mod OSM(mod) key.
+            append_P(PSTR("OSM("));
+            append_5_bit_mods(QK_ONE_SHOT_MOD_GET_MODS(keycode));
+            append_char(')');
+            return;
+#endif // !defined(NO_ACTION_ONESHOT)
+
+        // Various layer switch keys.
+        case QK_LAYER_TAP ... QK_LAYER_TAP_MAX: // Layer-tap LT(layer,kc) key.
+            append_P(PSTR("LT("));
+            append_number(QK_LAYER_TAP_GET_LAYER(keycode), 10);
+            append_char(',');
+            append_keycode(QK_LAYER_TAP_GET_TAP_KEYCODE(keycode));
+            append_char(')');
+            return;
+
+        case QK_LAYER_MOD ... QK_LAYER_MOD_MAX: // LM(layer,mod) key.
+            append_P(PSTR("LM("));
+            append_number(QK_LAYER_MOD_GET_LAYER(keycode), 10);
+            append_char(',');
+            append_5_bit_mods(QK_LAYER_MOD_GET_MODS(keycode));
+            append_char(')');
+            return;
+
+        case QK_TO ... QK_TO_MAX: // TO(layer) key.
+            append_unary_keycode(PSTR("TO"), number_string(QK_TO_GET_LAYER(keycode), 10));
+            return;
+
+        case QK_MOMENTARY ... QK_MOMENTARY_MAX: // MO(layer) key.
+            append_unary_keycode(PSTR("MO"), number_string(QK_MOMENTARY_GET_LAYER(keycode), 10));
+            return;
+
+        case QK_DEF_LAYER ... QK_DEF_LAYER_MAX: // DF(layer) key.
+            append_unary_keycode(PSTR("DF"), number_string(QK_DEF_LAYER_GET_LAYER(keycode), 10));
+            return;
+
+        case QK_TOGGLE_LAYER ... QK_TOGGLE_LAYER_MAX: // TG(layer) key.
+            append_unary_keycode(PSTR("TG"), number_string(QK_TOGGLE_LAYER_GET_LAYER(keycode), 10));
+            return;
+
+#if !defined(NO_ACTION_ONESHOT)
+        case QK_ONE_SHOT_LAYER ... QK_ONE_SHOT_LAYER_MAX: // OSL(layer) key.
+            append_unary_keycode(PSTR("OSL"), number_string(QK_ONE_SHOT_LAYER_GET_LAYER(keycode), 10));
+            return;
+#endif // !defined(NO_ACTION_ONESHOT)
+
+        case QK_LAYER_TAP_TOGGLE ... QK_LAYER_TAP_TOGGLE_MAX: // TT(layer) key.
+            append_unary_keycode(PSTR("TT"), number_string(QK_LAYER_TAP_TOGGLE_GET_LAYER(keycode), 10));
+            return;
+
+        case QK_PERSISTENT_DEF_LAYER ... QK_PERSISTENT_DEF_LAYER_MAX: // PDF(layer) key.
+            append_unary_keycode(PSTR("PDF"), number_string(QK_PERSISTENT_DEF_LAYER_GET_LAYER(keycode), 10));
+            return;
+
+        // Mod-tap MT(mod,kc) key. This implementation formats the MT keys where
+        // one modifier is applied. For MT keys with multiple modifiers, the mod
+        // arg is written numerically as a hex code.
+        case QK_MOD_TAP ... QK_MOD_TAP_MAX: {
+            uint8_t mods = QK_MOD_TAP_GET_MODS(keycode);
+            const bool is_rhs = mods > 15;
+            const uint8_t csag = mods & 15;
+            if (csag != 0 && (csag & (csag - 1)) == 0) { // One mod is set.
+                append_char(is_rhs ? 'R' : 'L');
+                append_P(&mod_names[4 * biton(csag)]);
+                append_P(PSTR("_T("));
+            } else if (mods == MOD_HYPR) {
+                append_P(PSTR("HYPR_T("));
+            } else if (mods == MOD_MEH) {
+                append_P(PSTR("MEH_T("));
+            } else {
+                append_P(PSTR("MT("));
+                append_number(mods, 16);
+                append_char(',');
+            }
+            append_keycode(QK_MOD_TAP_GET_TAP_KEYCODE(keycode));
+            append_char(')');
+        }   return;
+
+        case QK_TAP_DANCE ... QK_TAP_DANCE_MAX: // Tap dance TD(i) key.
+            append_unary_keycode(PSTR("TD"), number_string(QK_TAP_DANCE_GET_INDEX(keycode), 10));
+            return;
+
+#ifdef UNICODE_ENABLE
+        case QK_UNICODE ... QK_UNICODE_MAX: // Unicode UC(codepoint) key.
+            append_unary_keycode(PSTR("UC"), number_string(QK_UNICODE_GET_CODE_POINT(keycode), 16));
+            return;
+#elif defined(UNICODEMAP_ENABLE)
+        case QK_UNICODEMAP ... QK_UNICODEMAP_MAX: // Unicode Map UM(i) key.
+            append_unary_keycode(PSTR("UM"), number_string(QK_UNICODEMAP_GET_INDEX(keycode), 10));
+            return;
+
+        case QK_UNICODEMAP_PAIR ... QK_UNICODEMAP_PAIR_MAX: { // UP(i,j) key.
+            const uint8_t i = QK_UNICODEMAP_PAIR_GET_UNSHIFTED_INDEX(keycode);
+            const uint8_t j = QK_UNICODEMAP_PAIR_GET_SHIFTED_INDEX(keycode);
+            append_P(PSTR("UP("));
+            append_number(i, 10);
+            append_char(',');
+            append_number(j, 10);
+            append_char(')');
+        }   return;
+#endif
+#ifdef MOUSEKEY_ENABLE
+        case MS_BTN1 ... MS_BTN8: // Mouse button keycode.
+            append_numbered_keycode(PSTR("MS_BTN"), keycode - (MS_BTN1 - 1));
+            return;
+#endif // MOUSEKEY_ENABLE
+#ifdef SWAP_HANDS_ENABLE
+        case QK_SWAP_HANDS ... QK_SWAP_HANDS_MAX: // Swap Hands SH_T(kc) key.
+            if (!IS_SWAP_HANDS_KEYCODE(keycode)) {
+                append_P(PSTR("SH_T("));
+                append_keycode(QK_SWAP_HANDS_GET_TAP_KEYCODE(keycode));
+                append_char(')');
+                return;
+            }
+            break;
+#endif // SWAP_HANDS_ENABLE
+#ifdef JOYSTICK_ENABLE
+        case JOYSTICK_KEYCODE_RANGE: // Joystick JS_ key.
+            append_numbered_keycode(PSTR("JS_"), keycode - JS_0);
+            return;
+#endif // JOYSTICK_ENABLE
+#ifdef PROGRAMMABLE_BUTTON_ENABLE
+        case PROGRAMMABLE_BUTTON_KEYCODE_RANGE: // Programmable button PB_ key.
+            append_numbered_keycode(PSTR("PB_"), keycode - (PB_1 - 1));
+            return;
+#endif // PROGRAMMABLE_BUTTON_ENABLE
+
+        case MACRO_KEYCODE_RANGE: // Macro range MC_ keycode.
+            append_numbered_keycode(PSTR("MC_"), keycode - MC_0);
+            return;
+
+        case KB_KEYCODE_RANGE: // Keyboard range keycode.
+            append_numbered_keycode(PSTR("QK_KB_"), keycode - QK_KB_0);
+            return;
+
+        case USER_KEYCODE_RANGE: // User range keycode.
+            append_numbered_keycode(PSTR("QK_USER_"), keycode - QK_USER_0);
+            return;
+
+        // It would take a nontrivial amount of string data to cover some
+        // feature-specific keycodes, such as those for MIDI and lighting. As a
+        // fallback while still providing some information, we stringify
+        // remaining keys in known code ranges as "QK_<feature>+<number>".
+#ifdef MAGIC_ENABLE
+        case MAGIC_KEYCODE_RANGE:
+            append_numbered_keycode(PSTR("QK_MAGIC+"), keycode - QK_MAGIC);
+            return;
+#endif // MAGIC_ENABLE
+#ifdef MIDI_ENABLE
+        case MIDI_KEYCODE_RANGE:
+            append_numbered_keycode(PSTR("QK_MIDI+"), keycode - QK_MIDI);
+            return;
+#endif // MIDI_ENABLE
+#ifdef SEQUENCER_ENABLE
+        case SEQUENCER_KEYCODE_RANGE:
+            append_numbered_keycode(PSTR("QK_SEQUENCER+"), keycode - QK_SEQUENCER);
+            return;
+#endif // SEQUENCER_ENABLE
+#ifdef AUDIO_ENABLE
+        case AUDIO_KEYCODE_RANGE:
+            append_numbered_keycode(PSTR("QK_AUDIO+"), keycode - QK_AUDIO);
+            return;
+#endif // AUDIO_ENABLE
+#if defined(BACKLIGHT_ENABLE) || defined(LED_MATRIX_ENABLE) || defined(RGBLIGHT_ENABLED) || defined(RGB_MATRIX_ENABLE) // Lighting-related features.
+        case QK_LIGHTING ... QK_LIGHTING_MAX:
+            append_numbered_keycode(PSTR("QK_LIGHTING+"), keycode - QK_LIGHTING);
+            return;
+#endif // defined(BACKLIGHT_ENABLE) || defined(LED_MATRIX_ENABLE) || defined(RGBLIGHT_ENABLED) || defined(RGB_MATRIX_ENABLE)
+#ifdef STENO_ENABLE
+        case STENO_KEYCODE_RANGE:
+            append_numbered_keycode(PSTR("QK_STENO+"), keycode - QK_STENO);
+            return;
+#endif // AUDIO_ENABLE
+#ifdef BLUETOOTH_ENABLE
+        case CONNECTION_KEYCODE_RANGE:
+            append_numbered_keycode(PSTR("QK_CONNECTION+"), keycode - QK_CONNECTION);
+            return;
+#endif // BLUETOOTH_ENABLE
+        case QUANTUM_KEYCODE_RANGE:
+            append_numbered_keycode(PSTR("QK_QUANTUM+"), keycode - QK_QUANTUM);
+            return;
+    }
+    // clang-format on
+
+    append_number(keycode, 16); // Fallback: write keycode as hex value.
+}
+
+const char* get_keycode_string(uint16_t keycode) {
+    buffer_len = 0;
+    buffer[0]  = '\0';
+    append_keycode(keycode);
+    return buffer;
+}