stream/mts: adding discontinuity.go which contains discontinuity repairer

stream/mts/discontinuity.go

@@ -0,0 +1,99 @@
+/*
+NAME
+  discontinuity.go
+
+DESCRIPTION
+  discontinuity.go provides functionality for detecting discontinuities in
+	mpegts and accounting for using the discontinuity indicator in the adaptation
+	field.
+
+AUTHOR
+  Saxon A. Nelson-Milton <saxon@ausocean.org>
+
+LICENSE
+  discontinuity.go is Copyright (C) 2017-2019 the Australian Ocean Lab (AusOcean)
+
+  It is free software: you can redistribute it and/or modify them
+  under the terms of the GNU General Public License as published by the
+  Free Software Foundation, either version 3 of the License, or (at your
+  option) any later version.
+
+  It is distributed in the hope that it will be useful, but WITHOUT
+  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+  FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
+ for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with revid in gpl.txt.  If not, see [GNU licenses](http://www.gnu.org/licenses).
+*/
+
+package mts
+
+import "github.com/Comcast/gots/packet"
+
+// discontinuityRepairer provides function to detect discontinuities in mpegts
+// and set the discontinuity indicator as appropriate.
+type DiscontinuityRepairer struct {
+	expCC uint8
+}
+
+// NewDiscontinuityRepairer returns a pointer to a new discontinuityRepairer
+func NewDiscontinuityRepairer() *DiscontinuityRepairer {
+	return &DiscontinuityRepairer{expCC: 16}
+}
+
+func (dr *DiscontinuityRepairer) Failed() {
+	dr.decExpectedCC()
+}
+
+// Repair takes a slice of mpegts
+func (dr *DiscontinuityRepairer) Repair(d []byte) error {
+	var pkt [PacketSize]byte
+	copy(pkt[:], d[:PacketSize])
+	p := (*packet.Packet)(&pkt)
+	if p.PID() != PatPid {
+		panic("Clip to repair must have PAT first")
+	}
+	cc := p.ContinuityCounter()
+	expect, exists := dr.expectedCC()
+	dr.incExpectedCC()
+	if !exists {
+		dr.setExpectedCC(uint8(cc))
+	} else if cc != int(expect) {
+		if packet.ContainsAdaptationField(p) {
+			(*packet.AdaptationField)(p).SetDiscontinuity(true)
+		} else {
+			err := addAdaptationField(&pkt, DiscontinuityIndicator(true))
+			if err != nil {
+				return err
+			}
+		}
+		dr.setExpectedCC(uint8(cc))
+		copy(d[:PacketSize], pkt[:])
+	}
+	return nil
+}
+
+// expectedCC returns the expected cc for the given pid. If the cc hasn't been
+// used yet, then 16 and false is returned.
+func (dr *DiscontinuityRepairer) expectedCC() (byte, bool) {
+	if dr.expCC == 16 {
+		return 16, false
+	}
+	return dr.expCC, true
+}
+
+// incExpectedCC increments the expected cc in dr's cc map for the given pid.
+func (dr *DiscontinuityRepairer) incExpectedCC() {
+	dr.expCC = (dr.expCC + 1) & 0xf
+}
+
+// decExpectedCC decrements the expected cc in dr's cc map for the given pid.
+func (dr *DiscontinuityRepairer) decExpectedCC() {
+	dr.expCC = (dr.expCC - 1) & 0xf
+}
+
+// setExpectedCC sets the expected cc in dr's cc map for the given pid, and cc.
+func (dr *DiscontinuityRepairer) setExpectedCC(cc uint8) {
+	dr.expCC = cc
+}