1 | using System;
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2 | using System.Collections.Generic;
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3 | using System.Linq;
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4 | using System.Text;
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5 |
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6 | namespace HeuristicLab.Problems.GrammaticalOptimization {
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7 | public class PalindromeProblem : IProblem {
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8 | // length of the longest palindrome in the sentence
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9 | private const string grammarString = @"
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10 | G(S):
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11 | S -> T | TS
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12 | T -> a .. z
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13 | ";
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14 |
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15 | private readonly IGrammar grammar;
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16 | public PalindromeProblem() {
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17 | this.grammar = new Grammar(grammarString);
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18 | }
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19 |
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20 | public double BestKnownQuality(int maxLen) {
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21 | // the whole sentence is a palindrome
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22 | return maxLen;
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23 | }
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24 |
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25 | public IGrammar Grammar {
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26 | get { return grammar; }
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27 | }
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28 |
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29 | public double Evaluate(string sentence) {
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30 | return LongestPalindromicSubstring(sentence).Length;
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31 | }
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32 |
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33 |
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34 | private static string Preprocess(string s) {
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35 | var t = new char[2 * s.Length + 3];
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36 | t[0] = '$';
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37 | t[t.Length - 2] = '#';
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38 | t[t.Length - 1] = '@';
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39 | for (int i = 0; i < s.Length; i++) {
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40 | t[2 * (i) + 1] = '#';
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41 | t[2 * i + 2] = s[i];
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42 | }
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43 |
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44 | return new string(t);
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45 | }
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46 |
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47 | public static string LongestPalindromicSubstring(string str) {
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48 | var tLen = 2 * str.Length + 3;
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49 | var p = new int[tLen];
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50 | var t = Preprocess(str);
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51 |
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52 | var center = 0;
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53 | var right = 0;
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54 | for (int i = 1; i < t.Length - 1; i++) {
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55 | var mirror = 2 * center - i;
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56 | if (right > i) p[i] = Math.Min(right - i, p[mirror]);
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57 | while (t[i + (1 + p[i])] == t[i - (1 + p[i])]) {
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58 | p[i]++;
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59 | }
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60 | if (i + p[i] > right) {
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61 | center = i;
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62 | right = i + p[i];
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63 | }
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64 | }
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65 | var len = 0;
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66 | center = 0;
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67 | for (int i = 0; i < t.Length - 1; i++) {
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68 | if (p[i] > len) {
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69 | center = i;
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70 | len = p[i];
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71 | }
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72 | }
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73 |
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74 | var start = (center - 1 - len) / 2;
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75 | var result = new StringBuilder();
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76 | for (int i = 0; i < len; i++) {
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77 | result.Append(str[start + i]);
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78 | }
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79 | return result.ToString();
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80 | }
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81 |
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82 | public string CanonicalRepresentation(string terminalPhrase) {
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83 | throw new NotImplementedException();
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84 | return terminalPhrase;
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85 | }
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86 | }
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87 | }
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