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225 lines
6.2 KiB
Go
225 lines
6.2 KiB
Go
![]() |
package deque
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// minCapacity is the smallest capacity that deque may have.
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// Must be power of 2 for bitwise modulus: x % n == x & (n - 1).
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const minCapacity = 16
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// Deque represents a single instance of the deque data structure.
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type Deque struct {
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buf []interface{}
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head int
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tail int
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count int
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}
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// Len returns the number of elements currently stored in the queue.
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func (q *Deque) Len() int {
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return q.count
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}
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// PushBack appends an element to the back of the queue. Implements FIFO when
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// elements are removed with PopFront(), and LIFO when elements are removed
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// with PopBack().
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func (q *Deque) PushBack(elem interface{}) {
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q.growIfFull()
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q.buf[q.tail] = elem
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// Calculate new tail position.
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q.tail = q.next(q.tail)
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q.count++
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}
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// PushFront prepends an element to the front of the queue.
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func (q *Deque) PushFront(elem interface{}) {
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q.growIfFull()
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// Calculate new head position.
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q.head = q.prev(q.head)
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q.buf[q.head] = elem
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q.count++
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}
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// PopFront removes and returns the element from the front of the queue.
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// Implements FIFO when used with PushBack(). If the queue is empty, the call
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// panics.
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func (q *Deque) PopFront() interface{} {
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if q.count <= 0 {
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panic("deque: PopFront() called on empty queue")
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}
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ret := q.buf[q.head]
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q.buf[q.head] = nil
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// Calculate new head position.
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q.head = q.next(q.head)
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q.count--
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q.shrinkIfExcess()
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return ret
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}
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// PopBack removes and returns the element from the back of the queue.
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// Implements LIFO when used with PushBack(). If the queue is empty, the call
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// panics.
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func (q *Deque) PopBack() interface{} {
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if q.count <= 0 {
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panic("deque: PopBack() called on empty queue")
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}
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// Calculate new tail position
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q.tail = q.prev(q.tail)
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// Remove value at tail.
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ret := q.buf[q.tail]
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q.buf[q.tail] = nil
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q.count--
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q.shrinkIfExcess()
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return ret
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}
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// Front returns the element at the front of the queue. This is the element
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// that would be returned by PopFront(). This call panics if the queue is
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// empty.
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func (q *Deque) Front() interface{} {
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if q.count <= 0 {
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panic("deque: Front() called when empty")
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}
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return q.buf[q.head]
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}
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// Back returns the element at the back of the queue. This is the element
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// that would be returned by PopBack(). This call panics if the queue is
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// empty.
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func (q *Deque) Back() interface{} {
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if q.count <= 0 {
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panic("deque: Back() called when empty")
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}
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return q.buf[q.prev(q.tail)]
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}
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// At returns the element at index i in the queue without removing the element
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// from the queue. This method accepts only non-negative index values. At(0)
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// refers to the first element and is the same as Front(). At(Len()-1) refers
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// to the last element and is the same as Back(). If the index is invalid, the
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// call panics.
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//
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// The purpose of At is to allow Deque to serve as a more general purpose
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// circular buffer, where items are only added to and removed from the the ends
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// of the deque, but may be read from any place within the deque. Consider the
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// case of a fixed-size circular log buffer: A new entry is pushed onto one end
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// and when full the oldest is popped from the other end. All the log entries
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// in the buffer must be readable without altering the buffer contents.
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func (q *Deque) At(i int) interface{} {
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if i < 0 || i >= q.count {
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panic("deque: At() called with index out of range")
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}
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// bitwise modulus
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return q.buf[(q.head+i)&(len(q.buf)-1)]
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}
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// Clear removes all elements from the queue, but retains the current capacity.
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// This is useful when repeatedly reusing the queue at high frequency to avoid
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// GC during reuse. The queue will not be resized smaller as long as items are
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// only added. Only when items are removed is the queue subject to getting
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// resized smaller.
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func (q *Deque) Clear() {
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// bitwise modulus
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modBits := len(q.buf) - 1
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for h := q.head; h != q.tail; h = (h + 1) & modBits {
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q.buf[h] = nil
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}
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q.head = 0
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q.tail = 0
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q.count = 0
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}
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// Rotate rotates the deque n steps front-to-back. If n is negative, rotates
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// back-to-front. Having Deque provide Rotate() avoids resizing that could
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// happen if implementing rotation using only Pop and Push methods.
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func (q *Deque) Rotate(n int) {
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if q.count <= 1 {
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return
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}
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// Rotating a multiple of q.count is same as no rotation.
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n %= q.count
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if n == 0 {
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return
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}
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modBits := len(q.buf) - 1
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// If no empty space in buffer, only move head and tail indexes.
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if q.head == q.tail {
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// Calculate new head and tail using bitwise modulus.
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q.head = (q.head + n) & modBits
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q.tail = (q.tail + n) & modBits
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return
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}
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if n < 0 {
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// Rotate back to front.
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for ; n < 0; n++ {
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// Calculate new head and tail using bitwise modulus.
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q.head = (q.head - 1) & modBits
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q.tail = (q.tail - 1) & modBits
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// Put tail value at head and remove value at tail.
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q.buf[q.head] = q.buf[q.tail]
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q.buf[q.tail] = nil
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}
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return
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}
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// Rotate front to back.
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for ; n > 0; n-- {
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// Put head value at tail and remove value at head.
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q.buf[q.tail] = q.buf[q.head]
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q.buf[q.head] = nil
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// Calculate new head and tail using bitwise modulus.
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q.head = (q.head + 1) & modBits
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q.tail = (q.tail + 1) & modBits
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}
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}
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// prev returns the previous buffer position wrapping around buffer.
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func (q *Deque) prev(i int) int {
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return (i - 1) & (len(q.buf) - 1) // bitwise modulus
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}
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// next returns the next buffer position wrapping around buffer.
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func (q *Deque) next(i int) int {
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return (i + 1) & (len(q.buf) - 1) // bitwise modulus
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}
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// growIfFull resizes up if the buffer is full.
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func (q *Deque) growIfFull() {
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if len(q.buf) == 0 {
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q.buf = make([]interface{}, minCapacity)
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return
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}
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if q.count == len(q.buf) {
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q.resize()
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}
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}
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// shrinkIfExcess resize down if the buffer 1/4 full.
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func (q *Deque) shrinkIfExcess() {
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if len(q.buf) > minCapacity && (q.count<<2) == len(q.buf) {
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q.resize()
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}
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}
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// resize resizes the deque to fit exactly twice its current contents. This is
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// used to grow the queue when it is full, and also to shrink it when it is
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// only a quarter full.
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func (q *Deque) resize() {
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newBuf := make([]interface{}, q.count<<1)
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if q.tail > q.head {
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copy(newBuf, q.buf[q.head:q.tail])
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} else {
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n := copy(newBuf, q.buf[q.head:])
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copy(newBuf[n:], q.buf[:q.tail])
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}
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q.head = 0
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q.tail = q.count
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q.buf = newBuf
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}
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