source: code/trunk/ring.go@ 185

package soju

import (
        "fmt"
        "sync"

        "gopkg.in/irc.v3"
)

// Ring implements a single producer, multiple consumer ring buffer. The ring
// buffer size is fixed. The ring buffer is stored in memory.
type Ring struct {
        buffer []*irc.Message
        cap    uint64

        lock      sync.Mutex
        cur       uint64
        consumers []*RingConsumer
}

// NewRing creates a new ring buffer.
func NewRing(capacity int) *Ring {
        return &Ring{
                buffer: make([]*irc.Message, capacity),
                cap:    uint64(capacity),
        }
}

// Produce appends a new message to the ring buffer.
func (r *Ring) Produce(msg *irc.Message) {
        r.lock.Lock()
        defer r.lock.Unlock()

        i := int(r.cur % r.cap)
        r.buffer[i] = msg
        r.cur++

        for _, consumer := range r.consumers {
                select {
                case consumer.ch <- struct{}{}:
                        // This space is intentionally left blank
                default:
                        // The channel already has a pending item
                }
        }
}

// NewConsumer creates a new ring buffer consumer.
//
// If seq is nil, the consumer will get messages starting from the last
// producer message. If seq is non-nil, the consumer will get messages starting
// from the specified history sequence number (see RingConsumer.Close).
//
// The returned channel yields a value each time the consumer has a new message
// available. Consume should be called to drain the consumer.
//
// The consumer can only be used from a single goroutine.
func (r *Ring) NewConsumer(seq *uint64) (*RingConsumer, <-chan struct{}) {
        consumer := &RingConsumer{
                ring: r,
                ch:   make(chan struct{}, 1),
        }

        r.lock.Lock()
        if seq != nil {
                consumer.cur = *seq
        } else {
                consumer.cur = r.cur
        }
        if consumer.diff() > 0 {
                consumer.ch <- struct{}{}
        }
        r.consumers = append(r.consumers, consumer)
        r.lock.Unlock()

        return consumer, consumer.ch
}

// RingConsumer is a ring buffer consumer.
type RingConsumer struct {
        ring   *Ring
        cur    uint64
        ch     chan struct{}
        closed bool
}

// diff returns the number of pending messages. It assumes the Ring is locked.
func (rc *RingConsumer) diff() uint64 {
        if rc.cur > rc.ring.cur {
                panic(fmt.Sprintf("soju: consumer cursor (%v) greater than producer cursor (%v)", rc.cur, rc.ring.cur))
        }
        return rc.ring.cur - rc.cur
}

// Peek returns the next pending message if any without consuming it. A nil
// message is returned if no message is available.
func (rc *RingConsumer) Peek() *irc.Message {
        if rc.closed {
                panic("soju: RingConsumer.Peek called after Close")
        }

        rc.ring.lock.Lock()
        defer rc.ring.lock.Unlock()

        diff := rc.diff()
        if diff == 0 {
                return nil
        }
        if diff > rc.ring.cap {
                // Consumer drops diff - cap entries
                rc.cur = rc.ring.cur - rc.ring.cap
        }
        i := int(rc.cur % rc.ring.cap)
        msg := rc.ring.buffer[i]
        if msg == nil {
                panic(fmt.Sprintf("soju: unexpected nil ring buffer entry at index %v", i))
        }
        return msg
}

// Consume consumes and returns the next pending message. A nil message is
// returned if no message is available.
func (rc *RingConsumer) Consume() *irc.Message {
        msg := rc.Peek()
        if msg != nil {
                rc.cur++
        }
        return msg
}

// Close stops consuming messages. The consumer channel will be closed. The
// current history sequence number is returned. It can be provided later as an
// argument to Ring.NewConsumer to resume the message stream.
func (rc *RingConsumer) Close() uint64 {
        rc.ring.lock.Lock()
        for i := range rc.ring.consumers {
                if rc.ring.consumers[i] == rc {
                        rc.ring.consumers = append(rc.ring.consumers[:i], rc.ring.consumers[i+1:]...)
                        break
                }
        }
        rc.ring.lock.Unlock()

        close(rc.ch)
        rc.closed = true
        return rc.cur
}