1 // NFAs and Thompson's construction |
1 // NFAs in Scala based on sets of partial functions |
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3 // helper function for recording time |
3 // type abbreviation for partial functions |
4 def time_needed[T](i: Int, code: => T) = { |
4 type :=>[A, B] = PartialFunction[A, B] |
5 val start = System.nanoTime() |
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6 for (j <- 1 to i) code |
6 case class NFA[A, C](starts: Set[A], // starting states |
7 val end = System.nanoTime() |
7 delta: Set[(A, C) :=> A], // transitions |
8 (end - start)/(i * 1.0e9) |
8 fins: A => Boolean) { // final states |
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9 |
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10 // given a state and a character, what is the set of next states? |
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11 // if there is none => empty set |
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12 def next(q: A, c: C) : Set[A] = |
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13 delta.flatMap(_.lift.apply(q, c)) |
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14 |
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15 def nexts(qs: Set[A], c: C) : Set[A] = |
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16 qs.flatMap(next(_, c)) |
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17 |
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18 def deltas(qs: Set[A], s: List[C]) : Set[A] = s match { |
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19 case Nil => qs |
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20 case c::cs => deltas(nexts(qs, c), cs) |
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21 } |
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22 |
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23 def accepts(s: List[C]) : Boolean = |
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24 deltas(starts, s).exists(fins) |
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25 } |
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26 |
11 |
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12 // state nodes |
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13 abstract class State |
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14 type States = Set[State] |
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15 |
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16 case class IntState(i: Int) extends State |
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17 |
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18 object NewState { |
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19 var counter = 0 |
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20 |
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21 def apply() : IntState = { |
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22 counter += 1; |
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23 new IntState(counter - 1) |
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24 } |
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25 } |
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26 |
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27 |
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28 case class NFA(states: States, |
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29 start: State, |
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30 delta: (State, Char) => States, |
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31 eps: State => States, |
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32 fins: States) { |
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33 |
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34 def epsclosure(qs: States) : States = { |
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35 val ps = qs ++ qs.flatMap(eps(_)) |
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36 if (qs == ps) ps else epsclosure(ps) |
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37 } |
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38 |
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39 def deltas(qs: States, s: List[Char]) : States = s match { |
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40 case Nil => epsclosure(qs) |
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41 case c::cs => deltas(epsclosure(epsclosure(qs).flatMap (delta (_, c))), cs) |
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42 } |
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43 |
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44 def accepts(s: String) : Boolean = |
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45 deltas(Set(start), s.toList) exists (fins contains (_)) |
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46 } |
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47 |
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48 // A small example NFA from the lectures |
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49 val Q0 = NewState() |
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50 val Q1 = NewState() |
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51 val Q2 = NewState() |
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52 |
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53 val delta : (State, Char) => States = { |
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54 case (Q0, 'a') => Set(Q0) |
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55 case (Q1, 'a') => Set(Q1) |
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56 case (Q2, 'b') => Set(Q2) |
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57 case (_, _) => Set () |
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58 } |
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59 |
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60 val eps : State => States = { |
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61 case Q0 => Set(Q1, Q2) |
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62 case _ => Set() |
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63 } |
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64 |
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65 val NFA1 = NFA(Set(Q0, Q1, Q2), Q0, delta, eps, Set(Q2)) |
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66 |
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67 NFA1.accepts("aa") |
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68 NFA1.accepts("aaaaa") |
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69 NFA1.accepts("aaaaabbb") |
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70 NFA1.accepts("aaaaabbbaaa") |
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71 NFA1.accepts("ac") |
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72 |
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73 |
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74 // explicit construction of some NFAs; used in |
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75 // Thompson's construction |
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76 |
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77 // NFA that does not accept any string |
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78 def NFA_NULL() : NFA = { |
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79 val Q = NewState() |
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80 NFA(Set(Q), Q, { case (_, _) => Set() }, { case _ => Set() }, Set()) |
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81 } |
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82 |
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83 // NFA that accepts the empty string |
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84 def NFA_EMPTY() : NFA = { |
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85 val Q = NewState() |
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86 NFA(Set(Q), Q, { case (_, _) => Set() }, { case _ => Set() }, Set(Q)) |
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87 } |
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88 |
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89 // NFA that accepts the string "c" |
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90 def NFA_CHAR(c: Char) : NFA = { |
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91 val Q1 = NewState() |
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92 val Q2 = NewState() |
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93 NFA(Set(Q1, Q2), |
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94 Q1, |
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95 { case (Q1, d) if (c == d) => Set(Q2) |
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96 case (_, _) => Set() }, |
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97 { case _ => Set() }, |
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98 Set(Q2)) |
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99 } |
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100 |
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101 // alternative of two NFAs |
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102 def NFA_ALT(nfa1: NFA, nfa2: NFA) : NFA = { |
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103 val Q = NewState() |
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104 NFA(Set(Q) ++ nfa1.states ++ nfa2.states, |
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105 Q, |
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106 { case (q, c) if (nfa1.states contains q) => nfa1.delta(q, c) |
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107 case (q, c) if (nfa2.states contains q) => nfa2.delta(q, c) |
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108 case (_, _) => Set() }, |
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109 { case Q => Set(nfa1.start, nfa2.start) |
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110 case q if (nfa1.states contains q) => nfa1.eps(q) |
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111 case q if (nfa2.states contains q) => nfa2.eps(q) |
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112 case _ => Set() }, |
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113 nfa1.fins ++ nfa2.fins) |
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114 } |
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115 |
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116 // sequence of two NFAs |
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117 def NFA_SEQ(nfa1: NFA, nfa2: NFA) : NFA = { |
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118 NFA(nfa1.states ++ nfa2.states, |
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119 nfa1.start, |
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120 { case (q, c) if (nfa1.states contains q) => nfa1.delta(q, c) |
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121 case (q, c) if (nfa2.states contains q) => nfa2.delta(q, c) |
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122 case (_, _) => Set() }, |
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123 { case q if (nfa1.fins contains q) => nfa1.eps(q) ++ Set(nfa2.start) |
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124 case q if (nfa1.states contains q) => nfa1.eps(q) |
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125 case q if (nfa2.states contains q) => nfa2.eps(q) |
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126 case _ => Set() }, |
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127 nfa2.fins) |
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128 } |
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129 |
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130 // star of an NFA |
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131 def NFA_STAR(nfa: NFA) : NFA = { |
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132 val Q = NewState() |
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133 NFA(Set(Q) ++ nfa.states, |
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134 Q, |
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135 nfa.delta, |
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136 { case Q => Set(nfa.start) |
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137 case q if (nfa.fins contains q) => nfa.eps(q) ++ Set(Q) |
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138 case q if (nfa.states contains q) => nfa.eps(q) |
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139 case _ => Set() }, |
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140 Set(Q)) |
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141 } |
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142 |
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143 |
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144 // regular expressions used for Thompson's construction |
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145 abstract class Rexp |
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146 |
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147 case object NULL extends Rexp |
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148 case object EMPTY extends Rexp |
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149 case class CHAR(c: Char) extends Rexp |
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150 case class ALT(r1: Rexp, r2: Rexp) extends Rexp |
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151 case class SEQ(r1: Rexp, r2: Rexp) extends Rexp |
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152 case class STAR(r: Rexp) extends Rexp |
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153 |
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154 // some convenience for typing in regular expressions |
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155 def charlist2rexp(s : List[Char]) : Rexp = s match { |
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156 case Nil => EMPTY |
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157 case c::Nil => CHAR(c) |
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158 case c::s => SEQ(CHAR(c), charlist2rexp(s)) |
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159 } |
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160 implicit def string2rexp(s : String) : Rexp = charlist2rexp(s.toList) |
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161 |
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162 |
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163 |
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164 def thompson (r: Rexp) : NFA = r match { |
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165 case NULL => NFA_NULL() |
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166 case EMPTY => NFA_EMPTY() |
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167 case CHAR(c) => NFA_CHAR(c) |
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168 case ALT(r1, r2) => NFA_ALT(thompson(r1), thompson(r2)) |
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169 case SEQ(r1, r2) => NFA_SEQ(thompson(r1), thompson(r2)) |
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170 case STAR(r1) => NFA_STAR(thompson(r1)) |
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171 } |
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172 |
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173 // some examples for Thompson's |
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174 val A = thompson(CHAR('a')) |
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175 |
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176 println(A.accepts("a")) |
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177 println(A.accepts("c")) |
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178 println(A.accepts("aa")) |
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179 |
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180 val B = thompson(ALT("ab","ac")) |
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181 |
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182 println(B.accepts("ab")) |
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183 println(B.accepts("ac")) |
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184 println(B.accepts("bb")) |
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185 println(B.accepts("aa")) |
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186 |
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187 val C = thompson(STAR("ab")) |
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188 |
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189 println(C.accepts("")) |
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190 println(C.accepts("a")) |
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191 println(C.accepts("ababab")) |
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192 println(C.accepts("ab")) |
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193 println(C.accepts("ac")) |
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194 println(C.accepts("bb")) |
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195 println(C.accepts("aa")) |
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196 |
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197 // regular expression matcher using Thompson's |
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198 def matcher(r: Rexp, s: String) : Boolean = thompson(r).accepts(s) |
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199 |
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200 |
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201 //optional |
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202 def OPT(r: Rexp) = ALT(r, EMPTY) |
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203 |
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204 //n-times |
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205 def NTIMES(r: Rexp, n: Int) : Rexp = n match { |
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206 case 0 => EMPTY |
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207 case 1 => r |
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208 case n => SEQ(r, NTIMES(r, n - 1)) |
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209 } |
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210 |
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211 // evil regular exproession |
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212 def EVIL(n: Int) = SEQ(NTIMES(OPT("a"), n), NTIMES("a", n)) |
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213 |
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214 // test harness for the matcher |
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215 for (i <- 0 to 100 by 5) { |
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216 println(i + ": " + "%.5f".format(time_needed(1, matcher(EVIL(i), "a" * i)))) |
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217 } |
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218 |
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219 |
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220 // regular expression matching via search and backtracking |
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221 def accepts2(nfa: NFA, s: String) : Boolean = { |
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222 |
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223 def search(q: State, s: List[Char]) : Boolean = s match { |
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224 case Nil => nfa.fins contains (q) |
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225 case c::cs => |
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226 (nfa.delta(q, c) exists (search(_, cs))) || |
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227 (nfa.eps(q) exists (search(_, c::cs))) |
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228 } |
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229 |
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230 search(nfa.start, s.toList) |
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231 } |
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232 |
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233 def matcher2(r: Rexp, s: String) : Boolean = accepts2(thompson(r), s) |
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234 |
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235 // test harness for the backtracking matcher |
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236 for (i <- 0 to 20 by 1) { |
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237 println(i + ": " + "%.5f".format(time_needed(1, matcher2(EVIL(i), "a" * i)))) |
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238 } |
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240 |
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242 |
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