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Manacher's algorithm: insert # separators (length 2n+1) to unify odd/even cases, then maintain palindrome radii p[] alongside a running (center, right) pair to skip work via mirror reflection. Linear time. manacher "babadaba" = 7 (* witness: "abadaba", positions 1..7 *) Note: requires parenthesizing the if-expression on the rhs of <-: p.(i) <- (if pm < v then pm else v) Real OCaml parses bare `if` at <-rhs since the rhs is at expr level; our parser places <-rhs at binop level which doesn't include `if` / `match` / `let`. Workaround until we relax the binop RHS grammar. 149 baseline programs total.
33 lines
761 B
OCaml
33 lines
761 B
OCaml
let manacher s =
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let n = String.length s in
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let m = 2 * n + 1 in
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let t = Array.make m '#' in
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for i = 0 to n - 1 do
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t.(2 * i + 1) <- s.[i]
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done;
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let p = Array.make m 0 in
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let center = ref 0 and right = ref 0 in
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let max_p = ref 0 in
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for i = 0 to m - 1 do
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let mirror = 2 * !center - i in
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if i < !right then begin
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let v = !right - i in
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let pm = p.(mirror) in
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p.(i) <- (if pm < v then pm else v)
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end;
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while i + p.(i) + 1 < m && i - p.(i) - 1 >= 0
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&& t.(i + p.(i) + 1) = t.(i - p.(i) - 1) do
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p.(i) <- p.(i) + 1
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done;
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if i + p.(i) > !right then begin
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center := i;
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right := i + p.(i)
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end;
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if p.(i) > !max_p then max_p := p.(i)
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done;
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!max_p
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;;
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manacher "babadaba"
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