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103 lines
2.7 KiB
103 lines
2.7 KiB
8 years ago
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/*
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Package queue provides a fast, ring-buffer queue based on the version suggested by Dariusz Górecki.
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Using this instead of other, simpler, queue implementations (slice+append or linked list) provides
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substantial memory and time benefits, and fewer GC pauses.
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The queue implemented here is as fast as it is for an additional reason: it is *not* thread-safe.
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*/
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package queue
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// minQueueLen is smallest capacity that queue may have.
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// Must be power of 2 for bitwise modulus: x % n == x & (n - 1).
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const minQueueLen = 16
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// Queue represents a single instance of the queue data structure.
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type Queue struct {
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buf []interface{}
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head, tail, count int
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}
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// New constructs and returns a new Queue.
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func New() *Queue {
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return &Queue{
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buf: make([]interface{}, minQueueLen),
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}
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}
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// Length returns the number of elements currently stored in the queue.
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func (q *Queue) Length() int {
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return q.count
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}
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// resizes the queue to fit exactly twice its current contents
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// this can result in shrinking if the queue is less than half-full
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func (q *Queue) 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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// Add puts an element on the end of the queue.
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func (q *Queue) Add(elem interface{}) {
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if q.count == len(q.buf) {
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q.resize()
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}
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q.buf[q.tail] = elem
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// bitwise modulus
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q.tail = (q.tail + 1) & (len(q.buf) - 1)
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q.count++
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}
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// Peek returns the element at the head of the queue. This call panics
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// if the queue is empty.
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func (q *Queue) Peek() interface{} {
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if q.count <= 0 {
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panic("queue: Peek() called on empty queue")
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}
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return q.buf[q.head]
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}
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// Get returns the element at index i in the queue. If the index is
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// invalid, the call will panic. This method accepts both positive and
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// negative index values. Index 0 refers to the first element, and
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// index -1 refers to the last.
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func (q *Queue) Get(i int) interface{} {
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// If indexing backwards, convert to positive index.
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if i < 0 {
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i += q.count
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}
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if i < 0 || i >= q.count {
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panic("queue: Get() 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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// Remove removes and returns the element from the front of the queue. If the
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// queue is empty, the call will panic.
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func (q *Queue) Remove() interface{} {
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if q.count <= 0 {
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panic("queue: Remove() 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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// bitwise modulus
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q.head = (q.head + 1) & (len(q.buf) - 1)
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q.count--
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// Resize down if buffer 1/4 full.
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if len(q.buf) > minQueueLen && (q.count<<2) == len(q.buf) {
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q.resize()
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}
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return ret
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}
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