24H only is handled by disabling changing the setting

watch-faces/clock/world_clock_face.h

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+/*
+ * MIT License
+ *
+ * Copyright (c) 2022 Joey Castillo
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+#ifndef WORLD_CLOCK_FACE_H_
+#define WORLD_CLOCK_FACE_H_
+
+/*
+ * WORLD CLOCK FACE
+ *
+ * The World Clock watch face looks similar to the Simple Clock watch face,
+ * but you’ll notice that at first launch the day of week indicators are blank.
+ * That’s because this watch face does not display the day of the week.
+ * Instead, you may customize these letters to display the name of a time zone
+ * of your choosing.
+ *
+ * To customize this watch face, press and hold the ALARM button. The first
+ * letter in the top row will begin flashing. Press the ALARM button repeatedly
+ * to advance through the available letters in the first slot, then press the
+ * LIGHT button to move to the second letter. Finally, press LIGHT again to move
+ * to the time zone setting, and press ALARM to cycle through the available time
+ * zones. Press LIGHT one last time to return to the world clock display.
+ *
+ * Note that the second slot cannot display all letters or numbers. Also note
+ * that at this time, time zones do not automatically update for daylight saving
+ * time; you will need to manually adjust this field each spring and fall.
+ */
+
+#include "movement.h"
+
+typedef union {
+    struct {
+        uint8_t char_0;
+        uint8_t char_1;
+        uint8_t timezone_index;
+    } bit;
+    uint32_t reg;
+} world_clock_settings_t;
+
+typedef struct {
+    world_clock_settings_t settings;
+    uint8_t backup_register;
+    uint8_t current_screen;
+    uint32_t previous_date_time;
+} world_clock_state_t;
+
+void world_clock_face_setup(movement_settings_t *settings, uint8_t watch_face_index, void ** context_ptr);
+void world_clock_face_activate(movement_settings_t *settings, void *context);
+bool world_clock_face_loop(movement_event_t event, movement_settings_t *settings, void *context);
+void world_clock_face_resign(movement_settings_t *settings, void *context);
+
+uint8_t world_clock_face_get_weekday(uint16_t day, uint16_t month, uint16_t year);
+
+#define world_clock_face ((const watch_face_t){ \
+    world_clock_face_setup, \
+    world_clock_face_activate, \
+    world_clock_face_loop, \
+    world_clock_face_resign, \
+    NULL, \
+})
+
+#endif // WORLD_CLOCK_FACE_H_