tic-tac-toe-minimax.apl

make_game ← {¯2 + {?⍵ ⍵⍴3}⍵}
sum_hv ← {g←⍵ ⋄ (+/¨) ¨ ({⍵⌷g}¨⍳≢g) ({⍵⌷⍉g}¨⍳≢g)}
sum_dlr ← {g←⍵ ⋄ y←≢g ⋄ (+/) ¨ ({⍵ (y-⍵-1)⌷g}¨⍳y) ({⍵ ⍵⌷g}¨⍳y)}
won_hv ← +/¨ 1 ¯1 × { t←≢⍵ ⋄ ∨/ t (¯1×t)∘.=sum_hv ⍵}
won_dg ← 1 ¯1 × {t←≢⍵ ⋄ ∨/ t (¯1×t)∘.=sum_dlr ⍵}
idx_empt_map ← {(⍵=0)×({{⍵∘.,⍵}⍳≢⍵} ⍵)}
idx_empt ← {(,idx_empt_map ⍵)~⊂0 0}
at ← {(g r c)←⍵ ⋄ (r c⌷g)←⍺ ⋄ g}
emul_emp ← {g←⍵ ⋄ ⍺ {(r c)←⍵ ⋄ ⍺ at g r c} ¨idx_empt ⍵}
won_fl ← { { ({⌈/⊃⌈/⍵}⍵) ({⌊/⊃⌊/⍵}⍵) } (won_hv ⍵) (won_dg ⍵)}
won_unorm ← {r←won_fl ⍵ ⋄ 1=⌈/r:1 ⋄ ⌊/r}
won ← {g←⍵ ⋄ ⌊/1 (⌈/¯1 ({won_unorm ⍵}g))}
pt ← {(g dp mdp)←⍵ ⋄ r←won g ⋄ (mdp-dp)×r}
minmax←{
    (g max dp mdp)←⍵
    0=≢idx_empt g:0
    dp≥mdp:0
    0≠won g:pt g dp mdp
    not←max≢1
    p←(max+1)⌷¯1 1
    nxt_gen←p emul_emp g
    flat← (∊ {minmax ⍵ not (dp+1) mdp}¨nxt_gen)
    1=max:⌈/¯1 (⌈/flat)
    ⌊/1 (⌊/flat)
}
⍝ let's make a random mid-game and run this
game ← make_game 3
{'X_O'[⍵+2]}game
minmax game 1 0 100