Multithreading Puzzles [closed] - multithreading

Question

As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, visit the help center for guidance.
Closed 9 years ago.
I'm trying to come up with some programming puzzles focused on multi-threading. Most of the problems I've been able to come up with, so far, have been pretty domain specific. Does anybody have any decent programming puzzles for developers attempting to learn the core concepts of multi-threading applications?

Answer 1

There are a number of topics covered at this link.
Multithreaded Programming with ThreadMentor : A Tutorial
Edit:
Here are some direct links to the problems listed at that link, along with their initial descriptions.
ThreadMentor : The Dining Philosopher's Problem
ThreadMentor : The Dining Philosopher's Problem: The Lefty-Righty Version
The dining philosophers problem is invented by E. W. Dijkstra. Imagine that five philosophers who spend their lives just thinking and easting. In the middle of the dining room is a circular table with five chairs. The table has a big plate of spaghetti. However, there are only five chopsticks available, as shown in the following figure. Each philosopher thinks. When he gets hungry, he sits down and picks up the two chopsticks that are closest to him. If a philosopher can pick up both chopsticks, he eats for a while. After a philosopher finishes eating, he puts down the chopsticks and starts to think.
ThreadMentor : The Cigarette Smoker's Problem
This problem is due to S. S. Patil in 1971. Suppose a cigarette requires three ingredients, tobacco, paper and match. There are three chain smokers. Each of them has only one ingredient with infinite supply. There is an agent who has infinite supply of all three ingredients. To make a cigarette, the smoker has tobacco (resp., paper and match) must have the other two ingredients paper and match (resp., tobacco and match, and tobacco and paper). The agent and smokers share a table. The agent randomly generates two ingredients and notifies the smoker who needs these two ingredients. Once the ingredients are taken from the table, the agent supplies another two. On the other hand, each smoker waits for the agent's notification. Once it is notified, the smoker picks up the ingredients, makes a cigarette, smokes for a while, and goes back to the table waiting for his next ingredients.
ThreadMentor : The Producer/Consumer (or Bounded-Buffer) Problem
Suppose we have a circular buffer with two pointers in and out to indicate the next available position for depositing data and the position that contains the next data to be retrieved. See the diagram below. There are two groups of threads, producers and consumers. Each producer deposits a data items into the in position and advances the pointer in, and each consumer retrieves the data item in position out and advances the pointer out.
ThreadMentor : The Roller Coaster Problem
Suppose there are n passengers and one roller coaster car. The passengers repeatedly wait to ride in the car, which can hold maximum C passengers, where C < n. However, the car can go around the track only when it is full. After finishes a ride, each passenger wanders around the amusement park before returning to the roller coaster for another ride. Due to safety reasons, the car only rides T times and then shot-off.
This one has additional constraints:
The car always rides with exactly C passengers;
No passengers will jump off the car while the car is running;
No passengers will jump on the car while the car is running;
No passengers will request another ride before they can get off the car.
ThreadMentor : The Bridge Problem
The description for this one relies on images. Here is a modified quote with image references removed.
Consider a narrow bridge that can only allow three vehicles in the same direction to cross at the same time. If there are three vehicles on the bridge, any incoming vehicle must wait until the bridge is clear.
When a vehicle exits the bridge, we have two cases to consider. Case 1, there are other vehicles on the bridge; and Case 2 the exiting vehicle is the last one on bridge. In the first case, one new vehicle in the same direction should be allowed to proceed.
Case 2 is more complicated and has two subcases. In this case, the exiting vehicle is the last vehicle on the bridge. If there are vehicles waiting in the opposite direction, one of them should be allowed to proceed. Or, if there is no vehicle waiting in the opposite direction, then let the waiting vehicle in the same direction to proceed.

Answer 2

The Dining Philosophers Problem, is the first one I think of.

Answer 3

You have a large tree structure in memory. Many threads need to search the structure. Occasionally, a thread will need to insert or remove something from the structure. How do you control access to the structure so that the program will run correctly (no two threads will stomp on each other while changing the structure) and efficiently (no threads are blocked when they don't have to be)?

Answer 4

The Sleeping Barber Problem springs to mind, as does the Cigarette Smokers Problem.

Answer 5

Dining philosophers is one...
unisex bathroom is another one

Answer 6

Perhaps you can use the simple problem of testing and setting a shared flag or accessing some kind of list resource in some kind of sequentially consistent manner?

Answer 7

The Producer-Consumer problem.

Answer 8

Here's the first problem I ever completed with multi-threading, back during my undergraduate studies.

Answer 9

An Elevator Simulator is pretty common.

Answer 10

Depending upon what you are doing with your multi-threading, this makes a difference.
You are in a bank. Customers arrive at an average rate of 1 every 2 minutes. Each customer is served, on average, in 2 minutes.
Which is the better solution to serving the customers? One common line, or one line for each teller?
Is your choice enough to guarantee some bound on the length of the line?
Answers: because of the markov property of customer arrival and actual service time per individual, the line will never know a bound. additionally, it's a good idea to make them wait in one common line, although this is not enough to overcome the boundless line.

Answer 11

Here's a parallel N-puzzle solver implemented in PARLANSE. The language has a LISP-like syntax but is really closer to C (scalars, structs, pointers, function calls), but unlike C has local scopes. The secret is in the parallel fork-grain operator (|| ... ) which executes all of its operands in parallel, as well as PARLANSE's ability to use exceptions to stop parent grains.
This solver delivers linear speedups on all the 4 and 8 way machines on which I have tried it.
(define Version `N-puzzle Solver V1.1~l
Copyright (C) 1998-2009 Semantic Designs; All Rights Reserved~l')
(define SolveParticularPuzzle ~t)
(define ManhattanHeuristic ~t) ; Manhattan is really fast
(define PrintTrace ~f)
(include `parmodule.par')
(define ScrambleCount 10000)
(define PuzzleSize `Length of side of N-puzzle' +4) ; at least 3!
(define PuzzleSizeMinus1 +3)
(define PuzzleArea `Area of puzzle (= (-- N))' +16) ; (= (* PuzzleSize PuzzleSize))
(define PuzzleAreaMinus1 +15)
(define BlankTile `Code for a blank tile' 0)
(define puzzlepieceT `Codes for nonblank tiles'
(sort natural (range 1 PuzzleArea)))
(define BoardPositionT integer) ; normally positive, but sometime we reach off the edge
(define ConfigurationT (array puzzlepieceT 0 PuzzleAreaMinus1))
(define HardPuzzle1 `Solution found of length 29:
2 1 5 6 2 3 7 11 10 6 2 3 7 11 10 14 13 9 8
12 13 9 5 1 2 6 5 1 0'
(lambda (function ConfigurationT void)
(make ConfigurationT 01 11 02 00
04 06 09 05
13 12 07 03
08 14 10 15)
)lambda
)define
(define HardPuzzle2 `Solution found of length 31:
0 4 5 6 10 9 5 1 2 3 7 6 10 9 5 1
2 3 7 6 5 1 2 6 1 0 14 13 9 5 4 0'
(lambda (function ConfigurationT void)
(make ConfigurationT 13 00 02 09
04 05 06 01
08 07 03 11
12 14 10 15)
)lambda
)define
(define HardPuzzle3 `Solution found of length 56:
1 2 6 7 3 2 6 10 14 15 11 10 9 5
4 8 12 13 9 10 6 5 1 0 4 8 12 13
14 10 6 7 11 10 9 13 14 15 11 10
6 5 4 8 9 10 6 5 1 0 4 8 9 5 4 0
Total solution time in seconds: 18-24 (on 8 processor machine)'
(lambda (function ConfigurationT void)
(make ConfigurationT 00 09 10 08
15 12 03 02
01 11 13 14
06 04 07 05)
)lambda
)define
(define HardPuzzle4 `Solution found of length 50:
4 5 1 0 4 8 12 13 9 5 1 0 4 5 6
10 14 13 9 8 4 5 6 2 1 5 9 10 14
13 12 8 9 10 11 15 14 13 9 10 11
7 3 2 1 5 9 8 4 0
Total solution time in seconds: 125 (on 8 processor machine)'
(lambda (function ConfigurationT void)
(make ConfigurationT 00 15 06 07
12 03 08 11
04 13 02 05
01 14 09 10)
)lambda
)define
(define HardPuzzle5
`Solution found of length 68:
3 7 11 10 6 2 3 7 6 5 9 8 4 5 1 0 4 5 9 13 14 15 11
7 6 5 1 2 6 5 9 8 12 13 14 10 6 5 4 8 12 13 14 15 11
10 9 5 1 0 4 8 12 13 9 5 4 8 9 13 14 15 11 7 3 2 1 0
Total solution time in seconds: 2790 (on 8 processor machine)'
(lambda (function ConfigurationT void)
(make ConfigurationT 15 09 00 14
10 11 12 08
03 02 13 07
01 06 05 04)
)lambda
)define
(define ParticularPuzzleToSolve HardPuzzle5)
(define PrintConfiguration
(action (procedure [Puzzle (reference ConfigurationT)])
(do [position BoardPositionT] +0 PuzzleAreaMinus1 +1
(;; (ifthenelse (<= Puzzle:position 9)
(;; (PAR:PutConsoleCharacter "0")(PAR:PutConsoleNatural Puzzle:position) );;
(PAR:PutConsoleNatural Puzzle:position)
)ifthenelse
(PAR:PutConsoleSpace)
(ifthen (== (modulo (coerce natural position) (coerce natural PuzzleSize))
(coerce natural PuzzleSizeMinus1)coerce )==
(PAR:PutConsoleNewline)
)ifthen
);;
)do
)action
)define
(define Solved? `Determines if puzzle is solved.'
(lambda (function boolean
[board (reference ConfigurationT)]
)function
(value (;; `Fast check for completed':
(ifthen (~= board:0 BlankTile)
(return ~f)
)ifthen
(do [position BoardPositionT] PuzzleAreaMinus1 +1 -1
(ifthen (~= board:position (coerce natural position))
(return ~f)
)ifthen
)do
);;
~t ; all pieces are in proper places
)value
)lambda
)define
(define ScoreT `Estimate of configuration distance from solution.
Zero means configuration is a solution.'
(sort natural (range 0 1000))) ; s/b (range 0 (* PuzzleArea PuzzleArea))
(define SolvedScore `The score of a goal position.' 0)
(define UnsolvableScore `An impossibly big score.' 12345678)
(define LowerBoundOnScore
(lambda (function ScoreT [Puzzle (reference ConfigurationT)])
(let (= [OutOfPlaceTiles ScoreT] 0)
(value
(compileifthenelse ManhattanHeuristic ; ~t for Out-of-place, ~f for Manhattan
(do [Row BoardPositionT] PuzzleSizeMinus1 +0 -1
(do [Column BoardPositionT] PuzzleSizeMinus1 +0 -1
(local (;; (= [position integer] (+ (* Row PuzzleSize)
Column))=
(= [tile puzzlepieceT] Puzzle:position)
);;
(ifthen (~= tile BlankTile) ; ignore BlankTile
(+= OutOfPlaceTiles
(+ (magnitude (- Row (coerce integer (// tile (coerce natural PuzzleSize)))))
(magnitude (- Column (coerce integer (modulo tile (coerce natural PuzzleSize)))))
)+ ; add Manhattan distance of tile from tile goal
)+=
)ifthen
)local
)do ; Column
)do ; Row
(do [position BoardPositionT] PuzzleAreaMinus1
+1 ; skipping zero effectively ignores BlankTile
+1
(ifthen (~= Puzzle:position (coerce natural position))
(+= OutOfPlaceTiles)
)ifthen
)do
)compileifthenelse
OutOfPlaceTiles ; the answer
)value
)let
)lambda
)define
(recursive PathElementT
(define PathElementT `A series of moves of the blank tile.'
(structure [Move BoardPositionT]
[Next (reference PathElementT)]
)structure
)define
)recursive
(define EmptyPath (void (reference PathElementT))void )define
(define ValuedPathT `A path and the score it acheives.'
(structure [Solved boolean]
[Score ScoreT]
[Path (reference PathElementT)])
)define
(define MakeMove `Applies a move to a configuration'
(lambda (function ConfigurationT
(structure [BlankTilePosition BoardPositionT]
[NewBlankPosition BoardPositionT]
[ConfigurationBeforeMove
(reference ConfigurationT)]
)structure )function
(let (= [ResultConfiguration ConfigurationT]
(# ConfigurationBeforeMove) )=
(value
(;;
(compileifthen PrintTrace
(;; (PAR:PutConsoleNatural BlankTilePosition)
(PAR:PutConsoleNatural NewBlankPosition)
);;
)compileifthen
(trust (== ConfigurationBeforeMove:BlankTilePosition
BlankTile))
(= ResultConfiguration:BlankTilePosition
ConfigurationBeforeMove:NewBlankPosition)
(= ResultConfiguration:NewBlankPosition BlankTile)
);;
ResultConfiguration
)value
)let
)lambda
)define
(define TopEdge? `Determines if a position is along top edge of puzzle.'
(lambda (function boolean BoardPositionT)
(< ? PuzzleSize)
)lambda
)define
(define BottomEdge? `Determines if a position is along bottom edge of puzzle.'
(lambda (function boolean BoardPositionT)
(>= ? (- PuzzleArea PuzzleSize))
)lambda
)define
(define LeftEdge? `Determines if a position is along left edge of puzzle.'
(lambda (function boolean BoardPositionT)
(== (modulo (coerce natural ?) (coerce natural PuzzleSize)) 0)==
)lambda
)define
(define RightEdge? `Determines if a position is along right edge of puzzle.'
(lambda (function boolean BoardPositionT)
(== (modulo (coerce natural ?) (coerce natural PuzzleSize))modulo
(coerce natural PuzzleSizeMinus1)coerce )==
)lambda
)define
(define Solved! (exception (lambda (function string (reference ValuedPathT))
`N-puzzle solution is:~l'
)lambda
)exception
)define
[SerialPrint semaphore]
[MaxMoves natural]
(define Npuzzle
(lambda (function ValuedPathT
[BlankTilePosition BoardPositionT]
[PreviousBlankTilePosition BoardPositionT]
[Puzzle ConfigurationT]
[MovesToHere natural]
)function
)lambda
)define
(define Npuzzle `Solves a puzzle and generates a sequence which is a solution.'
(lambda (function ValuedPathT
[BlankTilePosition BoardPositionT]
[PreviousBlankTilePosition BoardPositionT]
[Puzzle ConfigurationT]
[MovesToHere natural]
)function
(ifthenelse (value (compileifthen PrintTrace
(;; (PAR:PutConsole (. `In Npuzzle at depth '))
(PAR:PutConsoleNatural MovesToHere) (PAR:PutConsoleNewline)
(PrintConfiguration (. Puzzle))
);;
)compileifthen
(Solved? (. Puzzle)))
(make ValuedPathT ~t 0 EmptyPath)make ; the answer
(let (|| [valuedpath1 ValuedPathT]
[valuedpath2 ValuedPathT]
[valuedpath3 ValuedPathT]
[valuedpath4 ValuedPathT]
[Best ValuedPathT]
(= [EstimatedDistance natural]
(+ MovesToHere (LowerBoundOnScore (. Puzzle)))+ )=
)||
(ifthenelse (value (compileifthen PrintTrace
(;; (PAR:PutConsole (. `Inside LET EstimatedDistance= '))
(PAR:PutConsoleNatural EstimatedDistance) (PAR:PutConsoleNewline)
);;
)compileifthen
(> EstimatedDistance MaxMoves) )
(make ValuedPathT ~f EstimatedDistance EmptyPath) ; don't explore any further
(value
(;; (assert (& (<= +0 BlankTilePosition)
(< BlankTilePosition PuzzleArea) )& )assert
; (PAR:PutConsole (. `Solve subpuzzles: blank # '))(PAR:PutConsoleNatural BlankTilePosition)(PAR:PutConsoleNewline)
(try `Solve subpuzzles':
(|| ; replace this by (;; to see pure serial execution times
`Fork Right':
(local (|| (= [NewBlankTilePosition BoardPositionT]
(++ BlankTilePosition) )=
[ExtendedPath (reference PathElementT)]
)||
(ifthenelse (value (;; ; (PAR:PutConsole (. `Fork Right~l'))
);;
(&& (~= NewBlankTilePosition
PreviousBlankTilePosition )~=
(~ (RightEdge? BlankTilePosition))~
)&& )value
(;; (= valuedpath1
(Npuzzle NewBlankTilePosition
BlankTilePosition
(MakeMove BlankTilePosition
NewBlankTilePosition
(. Puzzle) )MakeMove
(++ MovesToHere)
)Npuzzle )=
(ifthen valuedpath1:Solved
(;; (+= valuedpath1:Score) ; since we added a move
(= ExtendedPath (new PathElementT))
(= (# ExtendedPath) (make PathElementT NewBlankTilePosition valuedpath1:Path) )=
(= valuedpath1:Path ExtendedPath)
(raise Solved! (. valuedpath1))
);;
)ifthen
);;
(= valuedpath1 (make ValuedPathT ~f UnsolvableScore EmptyPath))=
)ifthenelse
)local
`Fork Left':
(local (|| (= [NewBlankTilePosition BoardPositionT]
(-- BlankTilePosition) )=
[ExtendedPath (reference PathElementT)]
)||
(ifthenelse (value (;; ; (PAR:PutConsole (. `Fork Left~l'))
);;
(&& (~= NewBlankTilePosition
PreviousBlankTilePosition )~=
(~ (LeftEdge? BlankTilePosition))~
)&& )value
(;; (= valuedpath2
(Npuzzle NewBlankTilePosition
BlankTilePosition
(MakeMove BlankTilePosition
NewBlankTilePosition
(. Puzzle) )MakeMove
(++ MovesToHere)
)Npuzzle )=
(ifthen valuedpath2:Solved
(;; (+= valuedpath2:Score) ; since we added a move
(= ExtendedPath (new PathElementT))
(= (# ExtendedPath) (make PathElementT NewBlankTilePosition valuedpath2:Path) )=
(= valuedpath2:Path ExtendedPath)
(raise Solved! (. valuedpath2))
);;
)ifthen
);;
(= valuedpath2 (make ValuedPathT ~f UnsolvableScore EmptyPath))=
)ifthenelse
)local
`Fork Down':
(local (|| (= [NewBlankTilePosition BoardPositionT]
(- BlankTilePosition PuzzleSize) )=
[ExtendedPath (reference PathElementT)]
)||
(ifthenelse (value (;; ; (PAR:PutConsole (. `Fork Down~l'))
);;
(&& (~= NewBlankTilePosition
PreviousBlankTilePosition )~=
(~ (TopEdge? BlankTilePosition))~
)&& )value
(;; (= valuedpath3
(Npuzzle NewBlankTilePosition
BlankTilePosition
(MakeMove BlankTilePosition
NewBlankTilePosition
(. Puzzle) )MakeMove
(++ MovesToHere)
)Npuzzle )=
(ifthen valuedpath3:Solved
(;; (+= valuedpath3:Score) ; since we added a move
(= ExtendedPath (new PathElementT))
(= (# ExtendedPath) (make PathElementT NewBlankTilePosition valuedpath3:Path) )=
(= valuedpath3:Path ExtendedPath)
(raise Solved! (. valuedpath3))
);;
)ifthen
);;
(= valuedpath3 (make ValuedPathT ~f UnsolvableScore EmptyPath))=
)ifthenelse
)local
`Fork Up':
(local (|| (= [NewBlankTilePosition BoardPositionT]
(+ BlankTilePosition PuzzleSize) )=
[ExtendedPath (reference PathElementT)]
)||
(ifthenelse (value (;; ; (PAR:PutConsole (. `Fork Up~l'))
);;
(&& (~= NewBlankTilePosition
PreviousBlankTilePosition )~=
(~ (BottomEdge? BlankTilePosition))~
)&& )value
(;; (= valuedpath4
(Npuzzle NewBlankTilePosition
BlankTilePosition
(MakeMove BlankTilePosition
NewBlankTilePosition
(. Puzzle) )MakeMove
(++ MovesToHere)
)Npuzzle )=
(ifthen valuedpath4:Solved
(;; (+= valuedpath4:Score) ; since we added a move
(= ExtendedPath (new PathElementT))
(= (# ExtendedPath) (make PathElementT NewBlankTilePosition valuedpath4:Path) )=
(= valuedpath4:Path ExtendedPath)
(raise Solved! (. valuedpath4))
);;
)ifthen
);;
(= valuedpath4 (make ValuedPathT ~f UnsolvableScore EmptyPath))=
)ifthenelse
)local
) ; || or ;;
`Exception handler':
(;; ; (PAR:PutConsole (. `Exception handler~l'))
(ifthenelse (== (exception) Solved!)==
(;; (= Best (# (exceptionargument (reference ValuedPathT))))=
(acknowledge (;; );; )acknowledge
);;
(propagate) ; oops, something unexpected!
)ifthenelse
);;
`Success handler':
(;; ; (PAR:PutConsole (. `Success (no exception raised)!~l'))
`If we get here, no result is a solution,
and all results have leaf-estimated scores.'
(ifthenelse (< valuedpath1:Score valuedpath2:Score)
(= Best valuedpath1)
(= Best valuedpath2)
)ifthenelse
(ifthen (< valuedpath3:Score Best:Score)
(= Best valuedpath3) )ifthen
(ifthen (< valuedpath4:Score Best:Score)
(= Best valuedpath4) )ifthen
);;
)try
);;
Best ; the answer to return
)value
)ifthenelse
)let
)ifthenelse
)lambda
)define
[StartTimeMicroseconds natural]
(define ElapsedTimeSeconds
`Returns time in seconds rounded to nearest integer'
(lambda (function natural void)
(/ (- (+ (MicrosecondClock) 500000) StartTimeMicroseconds) 1000000)
)lambda
)define
(define main
(action (procedure void)
(local (|| [PuzzleToSolve ConfigurationT]
[BlankTilePosition BoardPositionT]
[Solution ValuedPathT]
[BlankLocation BoardPositionT]
[Neighbor BoardPositionT]
[PathScanP (reference PathElementT)]
[ElapsedTime natural]
)||
(;; (PAR:PutConsoleString Version)
(consume (addresource SerialPrint 1))
`Set PuzzleToSolve to Solved position':
(do [position BoardPositionT] +0 PuzzleAreaMinus1 +1
(= PuzzleToSolve:position (coerce puzzlepieceT position) )=
)do
(ifthenelse SolveParticularPuzzle
(;; (PAR:PutConsole (. `Hard puzzle...~l'))
(= PuzzleToSolve (ParticularPuzzleToSolve) )= );;
(;; `Scramble puzzle position'
(PAR:PutConsole (. `Random puzzle...~l'))
(= BlankLocation +0)
(do [i natural] 1 (modulo (MicrosecondClock)
ScrambleCount)modulo 1
(;; (= Neighbor BlankLocation)
(ifthenelse (== (PAR:GetRandomNat 2) 0)
(;; `Move Blank up or down'
(ifthenelse (== (PAR:GetRandomNat 2) 0)
(ifthen (~ (TopEdge? BlankLocation)) (-= Neighbor PuzzleSize))
(ifthen (~ (BottomEdge? BlankLocation)) (+= Neighbor PuzzleSize))
)ifthenelse
);;
(;; `Move Blank left or right'
(ifthenelse (== (PAR:GetRandomNat 2) 0)
(ifthen (~ (LeftEdge? BlankLocation)) (-= Neighbor))
(ifthen (~ (RightEdge? BlankLocation)) (+= Neighbor))
)ifthenelse
);;
)ifthenelse
; (PAR:PutConsoleNatural BlankLocation)(PAR:PutConsoleNatural Neighbor)(PAR:PutConsoleSpace)
(ifthen (~= BlankLocation Neighbor)
(= PuzzleToSolve
(MakeMove BlankLocation Neighbor (. PuzzleToSolve). )MakeMove )=
)ifthen
(= BlankLocation Neighbor)=
);;
)do
);;
)ifthenelse
(;; `Initialize solver'
(= Solution:Solved ~f)
(= Solution:Score 0)
(do FindBlankTile
[position BoardPositionT] +0 PuzzleAreaMinus1 +1
(ifthen (== PuzzleToSolve:position BlankTile)
(;; (= BlankTilePosition position)
(exitblock FindBlankTile)
);; )ifthen )do
);;
(PAR:PutConsole (. `~lInitial Configuration:~l'))
(PrintConfiguration (. PuzzleToSolve))
(PAR:PutConsole (. `Estimate of solution length: '))
(PAR:PutConsoleNatural (LowerBoundOnScore (. PuzzleToSolve)))
(PAR:PutConsoleNewline)
(= StartTimeMicroseconds (MicrosecondClock))
(while (~ Solution:Solved)
(;; (critical SerialPrint 1
(;; (PAR:PutConsole (. `*** Iteration to depth '))
(PAR:PutConsoleNatural Solution:Score)
(PAR:PutConsole (. ` ')) (PAR:PutConsoleNatural (ElapsedTimeSeconds)) (PAR:PutConsole (. ` Seconds'))
(PAR:PutConsoleNewline)
);;
)critical
(= MaxMoves Solution:Score)
(= Solution (Npuzzle BlankTilePosition BlankTilePosition PuzzleToSolve 0) )=
);;
)while
(= ElapsedTime (ElapsedTimeSeconds))
(critical SerialPrint 1
(;; (PAR:PutConsole (. `Solution found of length '))
(PAR:PutConsoleNatural Solution:Score) (PAR:PutConsole (. `: '))
(iterate (= PathScanP Solution:Path)
(~= PathScanP EmptyPath)
(= PathScanP PathScanP:Next)
(;; (PAR:PutConsoleNatural (coerce natural PathScanP:Move)) (PAR:PutConsoleSpace)
);;
)iterate
(PAR:PutConsoleNewline)
(PAR:PutConsole (. `Total solution time in seconds: ')) (PAR:PutConsoleNatural ElapsedTime) (PAR:PutConsoleNewline)
);;
)critical
);;
)local
)action
)define

Answer 12

The little book of semaphores which is freely available book has lots of synchronization puzzles. It includes almost all the puzzles cited in other answers. Solutions are provided for all the puzzles.