-- File: QueueOut.mesa - last edit:
  -- MAS  May 19, 1980  6:22 PM
  -- HGM  July 15, 1979  4:53 PM

-- Copyright  Xerox Corporation 1979, 1980

DIRECTORY
  StatsDefs: FROM "StatsDefs" USING [StatIncr],
  CommUtilDefs: FROM "CommUtilDefs" USING [GetReturnFrame],
  DriverDefs: FROM "DriverDefs" USING [
    doDebug, doSee, doStats, Glitch,
    freeQueueLock, firstBuffer, freeQueue, freeQueueNotEmpty],
  PupDefs: FROM "PupDefs",  -- EXPORTs
  BufferDefs: FROM "BufferDefs" USING [Buffer, PupBuffer, Dequeue, buffersToReserve];

QueueOut: MONITOR LOCKS DriverDefs.freeQueueLock
  IMPORTS StatsDefs, CommUtilDefs, DriverDefs, BufferDefs
  EXPORTS DriverDefs, PupDefs, BufferDefs
  SHARES BufferDefs =
BEGIN OPEN DriverDefs, BufferDefs;

FreeQueueNotInitialized: PUBLIC ERROR = CODE;
QueueScrambled: PUBLIC ERROR = CODE;

GetFreePupBuffer: PUBLIC PROCEDURE RETURNS [b: PupBuffer] =
  BEGIN
  b _ LOOPHOLE[GetFreeBuffer[]];
  IF doDebug OR doSee THEN b.type _ pup;
  b.pupType _ data;
  IF doDebug THEN b.debug _ CommUtilDefs.GetReturnFrame[].accesslink;
  END;

GetFreeBuffer: PUBLIC ENTRY PROCEDURE RETURNS [b: Buffer] =
  BEGIN ENABLE UNWIND => NULL;
  IF doDebug AND firstBuffer=NIL THEN Glitch[FreeQueueNotInitialized];
  IF doStats AND ~freeQueue.length>buffersToReserve THEN
    StatsDefs.StatIncr[statBufferWaits];
  UNTIL freeQueue.length>buffersToReserve DO WAIT freeQueueNotEmpty; ENDLOOP;
  b_Dequeue[@freeQueue];
  IF doDebug AND b=NIL THEN Glitch[QueueScrambled];
  IF doDebug OR doSee THEN b.type _ raw;
  IF doDebug THEN b.debug _ CommUtilDefs.GetReturnFrame[].accesslink;
  END;

-- Get free buffer, but don't wait if there is none
-- NB: These will return the last buffer, use with caution

MaybeGetFreePupBuffer: PUBLIC PROCEDURE RETURNS [ b: PupBuffer ] =
  BEGIN
  b _ LOOPHOLE[MaybeGetFreeBuffer[]];
  IF b#NIL THEN
    BEGIN
    IF doDebug OR doSee THEN b.type _ pup;
    b.pupType _ data;
    END;
  END;


MaybeGetFreeBuffer: PUBLIC ENTRY PROCEDURE RETURNS [ b: Buffer ] =
  BEGIN
  IF doDebug AND firstBuffer=NIL THEN Glitch[FreeQueueNotInitialized];
  IF doStats AND freeQueue.length=0 THEN StatsDefs.StatIncr[statBufferWaits];
  b _ Dequeue[@freeQueue];
  IF b#NIL THEN
    BEGIN
    IF doDebug OR doSee THEN b.type _ raw;
    IF doDebug THEN b.debug _ CommUtilDefs.GetReturnFrame[].accesslink;
    END;
  END;

-- initialization
END.  -- QueueOut(2048)