1 | #include <stdlib.h>
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2 | #include <ctype.h>
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3 |
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4 | /*
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5 | grammar:
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6 |
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7 | Start = Expr ';'
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8 | Expr = Or | Or '?' Expr ':' Expr
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9 | Or = And | Or '||' And
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10 | And = Eq | And '&&' Eq
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11 | Eq = Rel | Eq '==' Rel | Eq '!=' Rel
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12 | Rel = Add | Rel '<=' Add | Rel '>=' Add | Rel '<' Add | Rel '>' Add
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13 | Add = Mul | Add '+' Mul | Add '-' Mul
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14 | Mul = Prim | Mul '*' Prim | Mul '/' Prim | Mul '%' Prim
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15 | Prim = '(' Expr ')' | '!' Prim | decimal | 'n'
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16 |
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17 | internals:
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18 |
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19 | recursive descent expression evaluator with stack depth limit.
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20 | for binary operators an operator-precedence parser is used.
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21 | eval* functions store the result of the parsed subexpression
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22 | and return a pointer to the next non-space character.
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23 | */
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24 |
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25 | struct st {
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26 | unsigned long r;
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27 | unsigned long n;
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28 | int op;
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29 | };
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30 |
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31 | static const char *skipspace(const char *s)
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32 | {
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33 | while (isspace(*s)) s++;
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34 | return s;
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35 | }
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36 |
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37 | static const char *evalexpr(struct st *st, const char *s, int d);
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38 |
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39 | static const char *evalprim(struct st *st, const char *s, int d)
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40 | {
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41 | char *e;
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42 | if (--d < 0) return "";
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43 | s = skipspace(s);
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44 | if (isdigit(*s)) {
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45 | st->r = strtoul(s, &e, 10);
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46 | if (e == s || st->r == -1) return "";
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47 | return skipspace(e);
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48 | }
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49 | if (*s == 'n') {
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50 | st->r = st->n;
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51 | return skipspace(s+1);
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52 | }
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53 | if (*s == '(') {
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54 | s = evalexpr(st, s+1, d);
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55 | if (*s != ')') return "";
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56 | return skipspace(s+1);
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57 | }
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58 | if (*s == '!') {
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59 | s = evalprim(st, s+1, d);
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60 | st->r = !st->r;
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61 | return s;
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62 | }
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63 | return "";
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64 | }
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65 |
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66 | static int binop(struct st *st, int op, unsigned long left)
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67 | {
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68 | unsigned long a = left, b = st->r;
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69 | switch (op) {
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70 | case 0: st->r = a||b; return 0;
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71 | case 1: st->r = a&&b; return 0;
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72 | case 2: st->r = a==b; return 0;
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73 | case 3: st->r = a!=b; return 0;
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74 | case 4: st->r = a>=b; return 0;
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75 | case 5: st->r = a<=b; return 0;
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76 | case 6: st->r = a>b; return 0;
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77 | case 7: st->r = a<b; return 0;
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78 | case 8: st->r = a+b; return 0;
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79 | case 9: st->r = a-b; return 0;
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80 | case 10: st->r = a*b; return 0;
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81 | case 11: if (b) {st->r = a%b; return 0;} return 1;
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82 | case 12: if (b) {st->r = a/b; return 0;} return 1;
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83 | }
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84 | return 1;
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85 | }
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86 |
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87 | static const char *parseop(struct st *st, const char *s)
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88 | {
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89 | static const char opch[11] = "|&=!><+-*%/";
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90 | static const char opch2[6] = "|&====";
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91 | int i;
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92 | for (i=0; i<11; i++)
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93 | if (*s == opch[i]) {
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94 | /* note: >,< are accepted with or without = */
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95 | if (i<6 && s[1] == opch2[i]) {
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96 | st->op = i;
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97 | return s+2;
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98 | }
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99 | if (i>=4) {
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100 | st->op = i+2;
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101 | return s+1;
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102 | }
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103 | break;
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104 | }
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105 | st->op = 13;
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106 | return s;
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107 | }
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108 |
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109 | static const char *evalbinop(struct st *st, const char *s, int minprec, int d)
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110 | {
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111 | static const char prec[14] = {1,2,3,3,4,4,4,4,5,5,6,6,6,0};
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112 | unsigned long left;
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113 | int op;
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114 | d--;
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115 | s = evalprim(st, s, d);
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116 | s = parseop(st, s);
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117 | for (;;) {
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118 | /*
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119 | st->r (left hand side value) and st->op are now set,
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120 | get the right hand side or back out if op has low prec,
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121 | if op was missing then prec[op]==0
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122 | */
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123 | op = st->op;
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124 | if (prec[op] <= minprec)
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125 | return s;
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126 | left = st->r;
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127 | s = evalbinop(st, s, prec[op], d);
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128 | if (binop(st, op, left))
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129 | return "";
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130 | }
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131 | }
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132 |
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133 | static const char *evalexpr(struct st *st, const char *s, int d)
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134 | {
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135 | unsigned long a, b;
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136 | if (--d < 0)
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137 | return "";
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138 | s = evalbinop(st, s, 0, d);
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139 | if (*s != '?')
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140 | return s;
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141 | a = st->r;
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142 | s = evalexpr(st, s+1, d);
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143 | if (*s != ':')
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144 | return "";
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145 | b = st->r;
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146 | s = evalexpr(st, s+1, d);
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147 | st->r = a ? b : st->r;
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148 | return s;
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149 | }
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150 |
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151 | unsigned long __pleval(const char *s, unsigned long n)
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152 | {
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153 | struct st st;
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154 | st.n = n;
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155 | s = evalexpr(&st, s, 100);
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156 | return *s == ';' ? st.r : -1;
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157 | }
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