Easy way to generate 8 piece sliding puzzle so that it is always solveable?

Starting from the final configuration, just have a routine that finds the possible pieces that could be moved to the hole position, and pick one at random to move there. Do this in a loop a few dozen times, and you end up with a scrambled puzzle that's guaranteed to be solvable since it's just "playing" the game, backward.

[I know the question is old, but Google still sends people here.]

My suggestion is to start with the solved puzzle, then write code to scramble it. Start at the empty location, randomly select a connected piece, slide that piece, repeat 20 or 30 times.

Since the end result was achieved using legal moves, you know that it is solvable if the player reverses those moves. However, randomness means less predictability on how hard it will be, and 30 scrambling moves may actually be solvable in only 3.

So there's still a lot to think about in how you handle the scrambling, and you'd probably want a flag on each block so your scramble code doesn't select the same tile to move twice in a row.

The Brute force method might be the best approach in this case (and for my mini game too :) ) but if you want a truly random start position, this might be a good article to read!

It's too late? @RyanZimmerman87

 public class Puzzle
 {
     public int[] puzzle = {1, 2, 3,
                            4, 5, 6,
                            7, 8, -1};
 
     private int[,] connections = { { 1, 3, -1, -1 }, { 0, 2, 4, -1 }, { 1, 5, -1, -1 },
                                   { 0, 4, 6, -1 }, { 1, 3, 5, 7 }, { 2, 4, 8, -1 }, 
                                   { 3, 7, -1, -1 }, { 4, 6, 8, -1 }, { 5, 7, -1, -1 } };
 
     public void Generate(int iterations)
     {
         for(int a = 0; a < iterations; a++)
         {
             Next();
         }
     }
 
     public void Next()
     {
         for (int x = 0; x < puzzle.Length; x++)
         {
             if (puzzle[x] == -1)
             {
                 List<int> possibleConnections = new List<int>();
 
                 for (int m = 0; m < 4; m++)
                 {
                     if (connections[x, m] != -1)
                     {
                         possibleConnections.Add(connections[x, m]);
                     }
                 }
                 int target = RandomValueFromArray(possibleConnections.ToArray());
                 int targetValue = puzzle[target];
                 puzzle[target] = puzzle[x];
                 puzzle[x] = targetValue;
                 return;
             }
         }
     }
 
     public T RandomValueFromArray<T>(T[] array)
     {
         return array[Random.Range(0, array.Length)];
     }
 
 }