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Copy pathdeque.go
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312 lines (278 loc) · 8.32 KB
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package deque
// minCapacity is the smallest capacity that deque may have.
// Must be power of 2 for bitwise modulus: x % n == x & (n - 1).
const minCapacity = 16
// Deque represents a single instance of the deque data structure.
type Deque[T any] struct {
buf []T
head int
tail int
count int
minCap int
}
// New creates a new Deque, optionally setting the current and minimum capacity
// when non-zero values are given for these.
//
// To create a Deque with capacity to store 2048 items without resizing, and
// that will not resize below space for 32 items when removing items:
// d := deque.New(2048, 32)
//
// To create a Deque that has not yet allocated memory, but after it does will
// never resize to have space for less than 64 items:
// d := deque.New(0, 64)
//
// Note that any values supplied here are rounded up to the nearest power of 2.
func New[T any](size ...int) *Deque[T] {
var capacity, minimum int
if len(size) >= 1 {
capacity = size[0]
if len(size) >= 2 {
minimum = size[1]
}
}
minCap := minCapacity
for minCap < minimum {
minCap <<= 1
}
var buf []T
if capacity != 0 {
bufSize := minCap
for bufSize < capacity {
bufSize <<= 1
}
buf = make([]T, bufSize)
}
return &Deque[T]{
buf: buf,
minCap: minCap,
}
}
// Cap returns the current capacity of the Deque.
func (q *Deque[T]) Cap() int {
if q == nil {
return 0
}
return len(q.buf)
}
// Len returns the number of elements currently stored in the queue.
func (q *Deque[T]) Len() int {
if q == nil {
return 0
}
return q.count
}
// PushBack appends an element to the back of the queue. Implements FIFO when
// elements are removed with PopFront(), and LIFO when elements are removed
// with PopBack().
func (q *Deque[T]) PushBack(elem T) {
q.growIfFull()
q.buf[q.tail] = elem
// Calculate new tail position.
q.tail = q.next(q.tail)
q.count++
}
// PushFront prepends an element to the front of the queue.
func (q *Deque[T]) PushFront(elem T) {
q.growIfFull()
// Calculate new head position.
q.head = q.prev(q.head)
q.buf[q.head] = elem
q.count++
}
// PopFront removes and returns the element from the front of the queue.
// Implements FIFO when used with PushBack(). If the queue is empty, the call
// panics.
func (q *Deque[T]) PopFront() T {
if q.count <= 0 {
panic("deque: PopFront() called on empty queue")
}
ret := q.buf[q.head]
var empty T
q.buf[q.head] = empty
// Calculate new head position.
q.head = q.next(q.head)
q.count--
q.shrinkIfExcess()
return ret
}
// PopBack removes and returns the element from the back of the queue.
// Implements LIFO when used with PushBack(). If the queue is empty, the call
// panics.
func (q *Deque[T]) PopBack() T {
if q.count <= 0 {
panic("deque: PopBack() called on empty queue")
}
// Calculate new tail position
q.tail = q.prev(q.tail)
// Remove value at tail.
ret := q.buf[q.tail]
var empty T
q.buf[q.tail] = empty
q.count--
q.shrinkIfExcess()
return ret
}
// Front returns the element at the front of the queue. This is the element
// that would be returned by PopFront(). This call panics if the queue is
// empty.
func (q *Deque[T]) Front() T {
if q.count <= 0 {
panic("deque: Front() called when empty")
}
return q.buf[q.head]
}
// Back returns the element at the back of the queue. This is the element
// that would be returned by PopBack(). This call panics if the queue is
// empty.
func (q *Deque[T]) Back() T {
if q.count <= 0 {
panic("deque: Back() called when empty")
}
return q.buf[q.prev(q.tail)]
}
// At returns the element at index i in the queue without removing the element
// from the queue. This method accepts only non-negative index values. At(0)
// refers to the first element and is the same as Front(). At(Len()-1) refers
// to the last element and is the same as Back(). If the index is invalid, the
// call panics.
//
// The purpose of At is to allow Deque to serve as a more general purpose
// circular buffer, where items are only added to and removed from the ends of
// the deque, but may be read from any place within the deque. Consider the
// case of a fixed-size circular log buffer: A new entry is pushed onto one end
// and when full the oldest is popped from the other end. All the log entries
// in the buffer must be readable without altering the buffer contents.
func (q *Deque[T]) At(i int) T {
if i < 0 || i >= q.count {
panic("deque: At() called with index out of range")
}
// bitwise modulus
return q.buf[(q.head+i)&(len(q.buf)-1)]
}
// Set puts the element at index i in the queue. Set shares the same purpose
// than At() but perform the opposite operation. The index i is the same
// index defined by At(). If the index is invalid, the call panics.
func (q *Deque[T]) Set(i int, elem T) {
if i < 0 || i >= q.count {
panic("deque: Set() called with index out of range")
}
// bitwise modulus
q.buf[(q.head+i)&(len(q.buf)-1)] = elem
}
// Clear removes all elements from the queue, but retains the current capacity.
// This is useful when repeatedly reusing the queue at high frequency to avoid
// GC during reuse. The queue will not be resized smaller as long as items are
// only added. Only when items are removed is the queue subject to getting
// resized smaller.
func (q *Deque[T]) Clear() {
// bitwise modulus
modBits := len(q.buf) - 1
for h := q.head; h != q.tail; h = (h + 1) & modBits {
var empty T
q.buf[h] = empty
}
q.head = 0
q.tail = 0
q.count = 0
}
// Rotate rotates the deque n steps front-to-back. If n is negative, rotates
// back-to-front. Having Deque provide Rotate() avoids resizing that could
// happen if implementing rotation using only Pop and Push methods.
func (q *Deque[T]) Rotate(n int) {
if q.Len() <= 1 {
return
}
// Rotating a multiple of q.count is same as no rotation.
n %= q.count
if n == 0 {
return
}
modBits := len(q.buf) - 1
// If no empty space in buffer, only move head and tail indexes.
if q.head == q.tail {
// Calculate new head and tail using bitwise modulus.
q.head = (q.head + n) & modBits
q.tail = (q.tail + n) & modBits
return
}
if n < 0 {
// Rotate back to front.
for ; n < 0; n++ {
// Calculate new head and tail using bitwise modulus.
q.head = (q.head - 1) & modBits
q.tail = (q.tail - 1) & modBits
// Put tail value at head and remove value at tail.
q.buf[q.head] = q.buf[q.tail]
var empty T
q.buf[q.tail] = empty
}
return
}
// Rotate front to back.
for ; n > 0; n-- {
// Put head value at tail and remove value at head.
q.buf[q.tail] = q.buf[q.head]
var empty T
q.buf[q.head] = empty
// Calculate new head and tail using bitwise modulus.
q.head = (q.head + 1) & modBits
q.tail = (q.tail + 1) & modBits
}
}
// SetMinCapacity sets a minimum capacity of 2^minCapacityExp. If the value of
// the minimum capacity is less than or equal to the minimum allowed, then
// capacity is set to the minimum allowed. This may be called at anytime to
// set a new minimum capacity.
//
// Setting a larger minimum capacity may be used to prevent resizing when the
// number of stored items changes frequently across a wide range.
func (q *Deque[T]) SetMinCapacity(minCapacityExp uint) {
if 1<<minCapacityExp > minCapacity {
q.minCap = 1 << minCapacityExp
} else {
q.minCap = minCapacity
}
}
// prev returns the previous buffer position wrapping around buffer.
func (q *Deque[T]) prev(i int) int {
return (i - 1) & (len(q.buf) - 1) // bitwise modulus
}
// next returns the next buffer position wrapping around buffer.
func (q *Deque[T]) next(i int) int {
return (i + 1) & (len(q.buf) - 1) // bitwise modulus
}
// growIfFull resizes up if the buffer is full.
func (q *Deque[T]) growIfFull() {
if q.count != len(q.buf) {
return
}
if len(q.buf) == 0 {
if q.minCap == 0 {
q.minCap = minCapacity
}
q.buf = make([]T, q.minCap)
return
}
q.resize()
}
// shrinkIfExcess resize down if the buffer 1/4 full.
func (q *Deque[T]) shrinkIfExcess() {
if len(q.buf) > q.minCap && (q.count<<2) == len(q.buf) {
q.resize()
}
}
// resize resizes the deque to fit exactly twice its current contents. This is
// used to grow the queue when it is full, and also to shrink it when it is
// only a quarter full.
func (q *Deque[T]) resize() {
newBuf := make([]T, q.count<<1)
if q.tail > q.head {
copy(newBuf, q.buf[q.head:q.tail])
} else {
n := copy(newBuf, q.buf[q.head:])
copy(newBuf[n:], q.buf[:q.tail])
}
q.head = 0
q.tail = q.count
q.buf = newBuf
}