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Class jcsp.lang.Alternative

java.lang.Object
   |
   +----jcsp.lang.Alternative

public class Alternative
extends Object

Description

The Alternative class enables a CSProcess to wait for data to become available from one or more Channels.

The select method performs the Channel selection function. The method returns the index of the Channel within the array which should be read from. The developer must then read from that Channel. There are several versions of this method which perform the same basic functionality but provide extra features such as timeouts and guards. The method will return -1 if the method completes without any Channels being ready to be read from.

NOTE: For those familiar with the OCCAM programming language the Alternative class is equivalent to the PRI ALT construct. Example

Simple Alt

 import jcsp.lang.*;
 public class AltExample1 implements CSProcess {
   private AltingChannelInput in1;
   private AltingChannelInput in2;
   private ChannelOutput out;
   public AltExample1(AltingChannelInput in1, AltingChannelInput in2, ChannelOutput out) {
     this.in1 = in1;
     this.in2 = in2;
     this.out = out;
   }
   public void run() {
     Alternative alt = new Alternative();
     AltingChannelInput[] altChans = {
       in1,
       in2
     };
     while (true) {
       int index = alt.select(altChans);
       altChans[index].read();
       out.write(new Integer(index))
     }
   }
 }

Alt with Timeout

 import jcsp.lang.*;
 public class AltExample2 implements CSProcess {
   private AltingChannelInput in1;
   private AltingChannelInput in2;
   private ChannelOutput out;
   public AltExample2(AltingChannelInput in1, AltingChannelInput in2, ChannelOutput out) {
     this.in1 = in1;
     this.in2 = in2;
     this.out = out;
   }
   public void run() {
     Alternative alt = new Alternative();
     AltingChannelInput[] altChans = {
       in1,
       in2
     };
     while (true) {
       switch (alt.select(altChans, 100)) {
         case 0:
           out.write(in1.read());
         break;
         case 1:
           out.write(in2.read());
         break;
         default:
           // do something like update the screen here
         break;
       }
     }
   }
 }

Alt with Skip

 import jcsp.lang.*;
 public class AltExample3 implements CSProcess {
   private AltingChannelInput in1;
   private AltingChannelInput in2;
   private ChannelOutput out;
   public AltExample3(AltingChannelInput in1, AltingChannelInput in2, ChannelOutput out) {
     this.in1 = in1;
     this.in2 = in2;
     this.out = out;
   }
   public void run() {
     Alternative alt = new Alternative();
     AltingChannelInput[] altChans = {
       in1,
       in2
     };
     boolean skip = false;
     while (true) {
       switch (alt.select(altChans, skip)) {
         case 0:
           skip = true;
           out.write(in1.read());
         break;
         case 1:
           skip = false;
           out.write(in2.read());
         break;
         default:
           // do something like update the screen here
         break;
       }
     }
   }
 }

Alt with Guards

 import jcsp.lang.*;
 public class AltExample4 implements CSProcess {
   private AltingChannelInput in1;
   private AltingChannelInput in2;
   private ChannelOutput out;
   public AltExample4(AltingChannelInput in1, AltingChannelInput in2, ChannelOutput out) {
     this.in1 = in1;
     this.in2 = in2;
     this.out = out;
   }
   public void run() {
     Alternative alt = new Alternative();
     AltingChannelInput[] altChans = {
       in1,
       in2
     };
     boolean[] altGuards = {
       false,
       false
     };
     boolean skip = false;
     int numRead = 0;
     int sum = 0;
     while (true) {
       altGuards[0] = (numRead < 10);
       altGuards[1] = (numRead = 10);
       switch (alt.select(altChans, altGuards)) {
         case 0:
           Integer i = (Integer)in1.read();
           sum += i.intValue();
           numRead++;
         break;
         case 1:
           in2.read();
           out.write(new Integer(sum));
           sum = 0;
           numRead = 0;
         break;
       }
     }
   }
 }

NOTE: There are also methods to perform Alt with Guards and Timeout and Alt with Guards and Skip

Author:: P.D.Austin

Alternative()

select(AltingChannelInput[]): Returns the index of one of the Channels which has data that can be read.
select(AltingChannelInput[], boolean): Returns the index of one of the Channels which has data that can be read.
select(AltingChannelInput[], boolean[]): Returns the index of one of the Channels which has data that can be read ignoring those Channels which have a false guard.
select(AltingChannelInput[], boolean[], boolean): Returns the index of one of the Channels which has data that can be read ignoring those Channels which have a false guard.
select(AltingChannelInput[], boolean[], long): Returns the index of one of the Channels which has data that can be read ignoring those Channels which have a false guard.
select(AltingChannelInput[], boolean[], long, int): Returns the index of one of the Channels which has data that can be read ignoring those Channels which have a false guard.
select(AltingChannelInput[], long): Returns the index of one of the Channels which has data that can be read.
select(AltingChannelInput[], long, int): Returns the index of one of the Channels which has data that can be read.

Alternative

 public Alternative()

select

 public synchronized int select(AltingChannelInput c[])

Returns the index of one of the Channels which has data that can be read. The method will block until one of the Channels becomes ready. The index of the Channel in the array will be returned.

Parameters:: c - the Channels to be selected from

select

 public synchronized int select(AltingChannelInput c[],
                                long msecs)

Returns the index of one of the Channels which has data that can be read. The method will block until either one of the Channels becomes ready or the timeout expires (negative or 0 timeouts will timeout immediately). If one of the Channels is ready the index of that Channel in the array will be returned otherwise -1 will be returned to indicate that no Channels are ready.

Parameters:: c - the Channels to be selected from; msecs - the time in milli-seconds to wait for a Channel to become ready

select

 public synchronized int select(AltingChannelInput c[],
                                long msecs,
                                int nsecs)

Parameters:: c - the Channels to be selected from; msecs - the time in milli-seconds to wait for a Channel to become ready; nsecs - the extra time above msecs to wait in nano-seconds

select

 public int select(AltingChannelInput c[],
                   boolean skip)

Returns the index of one of the Channels which has data that can be read. If the skip flag is true the method will not block if no Channels are ready otherwise it will block until one of the Channels is ready. If one of the Channels is ready the index of that Channel in the array will be returned otherwise -1 will be returned to indicate that no Channels are ready.

NOTE: This method enables the development of a polling programs which can consume extra processor time. Try to use one of the other select methods.

Parameters:: c - the Channels to be selected from; skip - A Boolean indicating whether the method should block until data is ready

select

 public synchronized int select(AltingChannelInput c[],
                                boolean guard[])

Returns the index of one of the Channels which has data that can be read ignoring those Channels which have a false guard. The method will block until one of the Channels becomes ready. The index of the Channel in the array will be returned.

NOTE: The c and guard arrays must be of the same size.

Parameters:: c - the Channels to be selected from; guard - the guards for the Channels

select

 public synchronized int select(AltingChannelInput c[],
                                boolean guard[],
                                long msecs)

Returns the index of one of the Channels which has data that can be read ignoring those Channels which have a false guard. The method will block until either one of the Channels becomes ready or the timeout expires (negative or 0 timeouts will timeout immediately). If one of the Channels is ready the index of that Channel in the array will be returned otherwise -1 will be returned to indicate that no Channels are ready.

NOTE: The c and guard arrays must be of the same size.

Parameters:: c - the Channels to be selected from; guard - the guards for the Channels; msecs - the time in milli-seconds to wait for a Channel to become ready

select

 public synchronized int select(AltingChannelInput c[],
                                boolean guard[],
                                long msecs,
                                int nsecs)

NOTE: The c and guard arrays must be of the same size.

Parameters:: c - the Channels to be selected from; guard - the guards for the Channels; msecs - the time in milli-seconds to wait for a Channel to become ready; nsecs - the extra time above msecs to wait in nano-seconds

select

 public int select(AltingChannelInput c[],
                   boolean guard[],
                   boolean skip)

Returns the index of one of the Channels which has data that can be read ignoring those Channels which have a false guard. If the skip flag is true the method will not block if no Channels are ready otherwise it will block until one of the Channels is ready. If one of the Channels is ready the index of that Channel in the array will be returned otherwise -1 will be returned to indicate that no Channels are ready.

NOTE: The c and guard arrays must be of the same size.

NOTE: This method enables the development of a polling programs which can consume extra processor time. Try to use one of the other select methods.

Parameters:: c - the Channels to be selected from; guard - the guards for the Channels; skip - A Boolean indicating whether the method should block until data is ready

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