Add Longest Palindromic Substring in Pascal (TheRenegadeCoder#4839)

Author: pascalecu

archive/p/pascal/longest_palindromic_substring.pas

@@ -0,0 +1,108 @@
+program LongestPalindromicSubstring;
+
+{$mode objfpc}{$H+}
+
+uses
+   SysUtils;
+
+procedure ShowUsage;
+begin
+   Writeln('Usage: please provide a string that contains at least one palindrome');
+   Halt(1);
+end;
+
+function PreprocessString(const s: string): string;
+var
+   builder: TStringBuilder;
+   ch: char;
+begin
+   builder := TStringBuilder.Create;
+   builder.Append('#');
+   for ch in s do
+   begin
+      builder.Append(ch);
+      builder.Append('#');
+   end;
+   Result := builder.ToString;
+end;
+
+function LongestPalindromicSubstring(const s: string): string;
+var
+   Processed: string;
+   PalindromeRadii: array of integer;
+   Center, RightBoundary, i, Mirror, LeftExpand, RightExpand, ProcessedLength: integer;
+   MaxPalindromeLength, MaxCenterIndex, OriginalStartIndex: integer;
+
+   function Min(A, B: integer): integer; inline;
+   begin
+      Result := B;
+      if A < B then
+         Result := A;
+   end;
+
+begin
+   if s = '' then
+      Exit('');
+
+   Processed := PreprocessString(s);
+   ProcessedLength := Length(Processed);
+   SetLength(PalindromeRadii, ProcessedLength);
+
+   Center := 0;
+   RightBoundary := 0;
+   MaxPalindromeLength := 0;
+   MaxCenterIndex := 0;
+
+   for i := 0 to ProcessedLength - 1 do
+   begin
+      Mirror := 2 * Center - i;
+      if i < RightBoundary then
+         PalindromeRadii[i] := Min(RightBoundary - i, PalindromeRadii[Mirror]);
+
+      LeftExpand := i + (1 + PalindromeRadii[i]);
+      RightExpand := i - (1 + PalindromeRadii[i]);
+
+      while (LeftExpand < ProcessedLength) and (RightExpand >= 0) and
+         (Processed[LeftExpand + 1] = Processed[RightExpand + 1]) do
+      begin
+         Inc(PalindromeRadii[i]);
+         Inc(LeftExpand);
+         Dec(RightExpand);
+      end;
+
+      if i + PalindromeRadii[i] > RightBoundary then
+      begin
+         Center := i;
+         RightBoundary := i + PalindromeRadii[i];
+      end;
+
+      if PalindromeRadii[i] > MaxPalindromeLength then
+      begin
+         MaxPalindromeLength := PalindromeRadii[i];
+         MaxCenterIndex := i;
+      end;
+   end;
+
+   OriginalStartIndex := ((MaxCenterIndex - MaxPalindromeLength) div 2) + 1;
+   Result := Copy(s, OriginalStartIndex, MaxPalindromeLength);
+end;
+
+var
+   Input: string;
+   LongestPalindrome: string;
+begin
+   if ParamCount <> 1 then
+      ShowUsage;
+
+   Input := ParamStr(1);
+
+   if Trim(Input) = '' then
+      ShowUsage;
+
+   LongestPalindrome := LongestPalindromicSubstring(Input);
+
+   if LongestPalindrome.Length = 1 then
+      ShowUsage;
+
+   Writeln(LongestPalindrome);
+end.