Day11_2.aplf

 Day11_2←{
     o←↑⍎¨¨snl ⍵   ⍝ Octopus energy levels
     F←{a←0≠⍵ ⋄ f←10≤⍵×a ⋄ r←(a∧~f)×{+/,⍵}⌺3 3⊢f ⋄ 0@(⍸f)⊢r+⍵ ⋄ a f r ⍵}⍣≡
     s←0
     S←{s+←1 ⋄ F 1+⍵}
     _←S⍣{∧/,0=⍵}⊢o
     ¯1+s
 }