source: code/trunk/ring.go@ 309

package soju

import (
        "fmt"

        "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
        cur    uint64
}

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

// Cur returns the current history sequence number.
func (r *Ring) Cur() uint64 {
        return r.cur
}

// NewConsumer creates a new ring buffer consumer.
//
// The consumer will get messages starting from the specified history sequence
// number (see Ring.Cur).
func (r *Ring) NewConsumer(seq uint64) *RingConsumer {
        return &RingConsumer{ring: r, cur: seq}
}

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

// 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 {
        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
}