[Bug-gnubg] Is eval-4ply and 0-ply-rollout-truncated-4ply the same?

[Christian Anthon]

On gammonu Michael Depreli asked,

Is truncate at 4 the same as a 4-ply eval assuming you roll out at 0-ply ?

I thought that the answer was an obvious yes, and Raccoon did some
tests showing that the results where almost the same, but not quite.

Jordan Lampe seems to think differently:

>No.
>
>First of all, it makes a difference whether we are looking at a
checker play or a cube decision,
>but let's suppose it's a cube decision just to make things easier.
Let's further make things
>easier by assuming there are only two die rolls (1 and 2) and only
two possible moves (A and B) at each step.
>
>A 2-ply evaluation looks at all of the following 16 positions: 1A1A,
1A1B, 1A2A,
> 1A2B, 1B1A, 1B1B, 1B2A, 1B2B 2A1A, 2A1B, 2A2A, 2A2B, 2B1A, 2B1B, 2B2A, 2B2B
>
>A 0-ply, trucated at 2-rollout, on the roll sequence 1 followed by 2 does this:
>first it looks at 1A and 1B and decides which is better. Assume 1A
looks better,
>then it looks at 1A2A and 1A2B and decides which is better.
>
>So, in particular, the truncated rollout NEVER looks at 1B1A, 1B1B,
1B2A or 1B2B, no matter what dice it rolls.

This seems horrible to me. Anybody know what is actually going on or
do I have to go look at the code? :)

Christian.