写指针 == (读指针 异或 缓冲区长度)，镜像指示法构造环形缓冲区里的语句构造，不懂

代码如下：
// This approach adds one bit to end and start pointers
// Circular buffer object
typedef struct {
    int size; // maximum number of elements
    int start; // index of oldest element
    int end; // index at which to write new element
    ElemType *elems; // vector of elements
} CircularBuffer;
 
void cbInit(CircularBuffer *cb, int size) {
    cb->size = size;
    cb->start = 0;
    cb->end = 0;
    cb->elems = (ElemType *)calloc(cb->size, sizeof(ElemType));
}
 
void cbPrint(CircularBuffer *cb) {
    printf("size = 0x%x, start = %d, end = %d\n", cb->size, cb->start, cb->end);
}
 
int cbIsFull(CircularBuffer *cb) {
// This inverts the most significant bit of start before comparison
    return cb->end == (cb->start ^ cb->size);
}
int cbIsEmpty(CircularBuffer *cb) {
    return cb->end == cb->start;
}

//增大
int cbIncr(CircularBuffer *cb, int p) {
// start and end pointers incrementation is done modulo 2*size
 
    return (p + 1) & (2 * cb->size - 1);
}
void cbWrite(CircularBuffer *cb, ElemType *elem) {
    cb->elems[cb->end & (cb->size - 1)] = *elem;
    if (cbIsFull(cb)) // full, overwrite moves start pointer
        cb->start = cbIncr(cb, cb->start);
    cb->end = cbIncr(cb, cb->end);
}

void cbRead(CircularBuffer *cb, ElemType *elem) {
    *elem = cb->elems[cb->start & (cb->size - 1)];
    cb->start = cbIncr(cb, cb->start);
}