## Tuesday, October 02, 2012

### Five Player Go First Dice

In a previous post I proposed a challenge to come up with a set of go first dice for five players. I thought my prize would be safe since this seems like a difficult unsolved problem. Paul Vanetti has successfully won the challenge with the following solution:
• 0, 1, 2, 3, 4, 5, 6, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 259, 260, 261, 262, 263, 264, 265
• 7, 20, 21, 22, 35, 36, 37, 50, 83, 96, 97, 98, 111, 112, 113, 126, 139, 152, 153, 154, 167, 168, 169, 182, 215, 228, 229, 230, 243, 244, 245, 258
• 8, 12, 13, 14, 15, 19, 23, 27, 28, 29, 30, 34, 38, 42, 43, 44, 45, 49, 84, 88, 89, 90, 91, 95, 99, 103, 104, 105, 106, 110, 114, 118, 119, 120, 121, 125, 140, 144, 145, 146, 147, 151, 155, 159, 160, 161, 162, 166, 170, 174, 175, 176, 177, 181, 216, 220, 221, 222, 223, 227, 231, 235, 236, 237, 238, 242, 246, 250, 251, 252, 253, 257
• 9, 11, 16, 18, 24, 26, 31, 33, 39, 41, 46, 48, 85, 87, 92, 94, 100, 102, 107, 109, 115, 117, 122, 124, 141, 143, 148, 150, 156, 158, 163, 165, 171, 173, 178, 180, 217, 219, 224, 226, 232, 234, 239, 241, 247, 249, 254, 256
• 10, 17, 25, 32, 40, 47, 86, 93, 101, 108, 116, 123, 142, 149, 157, 164, 172, 179, 218, 225, 233, 240, 248, 255
These dice have the following number of faces:
• 90
• 32
• 72
• 48
• 24
When I described the problem in my original blog post I did not specify any constraint specifying that the dice need to have the same number of faces. This constraint is unnecessary because:

Conjecture: Given any valid set of N go first dice with number of faces x1, x2, ..., xN, it is always possible to generate a new set of go first dice with exactly LCM(x1, x2, ..., xN) faces per die.

This can be done using the following algorithm:

Suppose you have go first dice with the following labels:
• a1, a2, ..., aQ
• b1, b2, ..., bR
• ...
• y1, y2, ..., yZ
Let m = LCM(Q, R, ..., Z). The following should be a set of go first dice with m faces:
• m*a1, m*a1 + 1, ..., m*a1 + (m/Q) - 1, m*a2, m*a2 + 1, ..., m*a2 + (m/Q) - 1, m*aQ, m*aQ + 1, ..., m*aQ + (m/Q) - 1
• m*b1, m*b1 + 1, ..., m*b1 + (m/R) - 1, m*b2, m*b2 + 1, ..., m*b2 + (m/R) - 1, m*bR, m*bR + 1, ..., m*bR + (m/R) - 1
• ...
• m*y1, m*y1 + 1, ..., m*y1 + (m/Z) - 1, m*y2, m*y2 + 1, ..., m*y2 + (m/Z) - 1, m*yZ, m*yZ + 1, ..., m*yZ + (m/Z) - 1
Example of how to apply the algorithm to a simple set of dice (which are not go first dice):
• 1,2,4
• 3,5
LCM(3,2) = 6, so the merged dice are:
• 6*1, 6*1 + 1, 6*2, 6*2 + 1, 6*4, 6*4 + 1
• 6*3, 6*3 + 1, 6*3 + 2, 6*5, 6*5 + 1, 6*5 + 2
Which becomes:
• 6, 7, 12, 13, 24, 25
• 18, 19, 20, 30, 31, 32
Empty regions can be removed to form:
• 1, 2, 3, 4, 8, 9
• 5, 6, 7, 10, 11, 12
Since LCM(90, 32, 72, 48, 24) = 1440, Paul's solution can be used to generate a set of 5 go first dice with 1440 faces. Paul says he found a technique to use a solution for N-1 to generate N go first dice (I do not know how this is done). His technique can be used to quickly generate solutions for any N. The solutions generated by his algorithm are not optimal in terms of minimizing the number of faces on the dice.