challenge-081/richard-park/apl/CommonBaseString.aplf


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 CommonBaseString←{
     FindBaseStrings←{
         ⍺←1
         ⍺=≢⍵:⍬                                   ⍝ Reached end of string
         bs←⍺↑⍵                                   ⍝ Base String candidate
         bs←⍺{(bs←⍺↑⍵)≡,↑∪⍵⊂⍨(⍺↑1)⍴⍨≢⍵:bs ⋄ ⍬}⍵   ⍝ Substring is a base string, else ⍬
         ⍬≡bs:(⍺+1)∇ ⍵                            ⍝ If no base string, increment substring length and try again
         (⊂⍬)~⍨bs,⍥⊆(2×⍺)∇ ⍵                      ⍝ Return base string, and check superstrings (,⍨bs)
     }
     ⍺∪⍥FindBaseStrings ⍵   ⍝ Common base strings of ⍺ and ⍵
 }