A ring buffer / linked list for buffering ambiguous keys in QMK's config.h

config.h

// a linked list of pending key events (press or release) which we haven't processed yet.
typedef struct _pending_key_t {
    uint16_t keycode;
    keyrecord_t record;
    struct _pending_key_t *next;
} pending_key_t;

// worst case is 10 down strokes and 1 up stroke before we can start disambiguating.
#define RINGSIZE 11

// a ring buffer and linked list to store pending key events (presses and releases).
// (basically, this is a fixed-allocation linked list.)
typedef {
    // the actual key events.
    pending_key_t items[RINGSIZE];
    // the index of the oldest item, or -1 if no items.
    int8_t ifirst = -1;
    // the index of the most recently added item, or -1 if no items.
    int8_t ilast = -1;
    // the number of items in the ring.
    uint8_t count = 0;
    // the head of the linked list.
    pending_key_t *head = NULL;
} kring_t;

// safe accessor to the i-th item of the linked list (returns pointer or NULL).
pending_key_t* kr_get(kring_t *ring, uint8_t i) {
    if (i >= ring->count) {
        return NULL;
    }
    uint8_t j = (ring->ifirst + i) % RINGSIZE;
    return &(ring->items[j]);
}

// remove the oldest item from the ring (the head of the list).
void kr_pop(kring_t *ring) {
    if (ring->count > 0) {
        ring->ifirst += 1;
        ring->head = ring->head->next;
        ring->count -= 1;
    }
}

// add an item to the ring (append to the list).
void kr_append(kring_t *ring, uint16_t keycode, keyrecord_t *record) {
    if (ring->count >= RINGSIZE) {
        // uh oh, we overflowed the capacity of our buffer :(
        return;
    }

    // if the ring is empty, insert at index 0.
    if (ring->count == 0) {
        ring->count += 1;
        ring->first = 0;
        ring->last = 0;
        ring->head = &(ring->items[0]);
    }
    // else, append it onto the end.
    else {
        ring->count += 1;
        ring->last += 1;
        ring->last %= RINGSIZE;
    }

    // the index at which we should insert this item.
    int8_t i = ring->ilast;

    // insert the item.
    ring->items[i].keycode = keycode;
    ring->items[i].record.event = record->event;
#ifndef NO_ACTION_TAPPING
    ring->items[i].record.tap = record->tap;
#endif
    ring->items[i].next = NULL;

    // update the previous item to point to this item.
    if (ring->count > 1) {
        kr_get(ring, ring->count-2)->next = kr_get(ring, ring->count-1);
    }
}

kring_t pending;