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Manacher's algorithm for finding the longest palindromic substring in Dyalog APL (⎕IO←0)
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| ∇ res←manacher str;S;c;r;radii;i;ir | |
| c r radii←0 0 (0↑⍨≢S←'|',∊'|',⍨⍪str) | |
| :for i :in ⍳≢S | |
| radii[i]←{r>i:radii[(2×c)-i]⌊r-i⋄0}⍬ | |
| :trap 3 ⍝ Catch index errors if the below goes out of bounds | |
| :while =/S[i(+,-)1+i⊃radii] | |
| radii[i]+←1 | |
| :endwhile | |
| :endtrap | |
| :if r<ir←i+i⊃radii | |
| c r←i ir | |
| :endif | |
| :endfor | |
| c←radii⍳r←⌈/radii | |
| res←S[c(+(⊢+∘⍳1+-)-)r]~'|' | |
| ∇ |
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Passable tradfn implementation of Manacher's algorithm (https://en.wikipedia.org/wiki/Longest_palindromic_substring)