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// Copyright 2013, 2014 MongoDB, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// A thread-safe fixed-capacity queue. Deletes last item on overflow
package queue
type Queue struct {
items chan interface{}
onForcedDequeue func(interface{})
}
func New(capacity int, onForcedDequeue func(interface{})) *Queue {
return &Queue{
make(chan interface{}, capacity),
onForcedDequeue,
}
}
func (q *Queue) Cap() int {
return cap(q.items)
}
func (q *Queue) Dequeue() (interface{}, error) {
select {
case item := <-q.items:
return item, nil
default:
return nil, UnderflowError{}
}
}
func (q *Queue) Enqueue(item interface{}) {
for {
select {
case q.items <- item:
return
default:
// q.items must be full
// force a dequeue in order to make room
select {
case item := <-q.items:
if q.onForcedDequeue != nil {
q.onForcedDequeue(item)
}
default:
// wow, we went from full to empty very fast. let's start over
}
}
}
}
func (q *Queue) IsEmpty() bool {
return q.Len() == 0
}
func (q *Queue) IsFull() bool {
return q.Len() == q.Cap()
}
func (q *Queue) Len() int {
return len(q.items)
}
type UnderflowError struct {
}
func (UnderflowError) Error() string {
return "Queue underflowed"
}
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