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  1. #include <iostream>
  2. #include <string>
  3. #include <vector>
  4. #include <stack>
  5.  
  6. #include <cctype>
  7. #include <cassert>
  8.  
  9. struct parser_state {
  10. 	parser_state(const std::string& str, size_t pos):
  11. 		str(str), pos(pos) {}
  12.  
  13. 	bool end () const {
  14. 		return pos >= str.size();
  15. 	}
  16.  
  17. 	bool restore (bool matched) {
  18. 		if (!matched) pos = history.top();
  19. 		history.pop();
  20. 		return matched;
  21. 	}
  22.  
  23. 	parser_state& save () {
  24. 		history.push(pos);
  25. 		return *this;
  26. 	}
  27.  
  28. 	const std::string& str;
  29. 	std::stack <size_t> history;
  30. 	size_t pos;
  31. };
  32.  
  33. bool match_digit(parser_state& s) {
  34. 	if (s.end()) return false;
  35. 	return isdigit (s.str[s.pos++]);
  36. }
  37.  
  38. bool match_digits(parser_state& s) {
  39. 	bool matched = false;
  40. 	while (match_digit(s)) matched = true;
  41. 	return matched;
  42. }
  43.  
  44. bool match_symbol(parser_state& s, const char* options) {
  45. 	if (s.end()) return false;
  46. 	while (*options) {
  47. 		if (s.str[s.pos] == *options) break;
  48. 		options ++;
  49. 	}
  50. 	s.pos ++;
  51. 	return *options != '\0';
  52. }
  53.  
  54. bool match_frac(parser_state& s) {
  55. 	return match_symbol(s, ".") && match_digits(s);
  56. }
  57.  
  58. bool maybe_match_sign(parser_state& s) {
  59. 	s.restore(match_symbol(s.save(), "+-"));
  60. 	return true;
  61. }
  62.  
  63. bool match_exponent(parser_state& s) {
  64. 	return match_symbol(s, "Ee") && maybe_match_sign(s) && match_digits(s);
  65. }
  66.  
  67. bool match_mantissa(parser_state& s) {
  68. 	maybe_match_sign(s);
  69. 	const bool digits = s.restore(match_digits(s.save()));
  70. 	const bool frac = s.restore(match_frac(s.save()));
  71. 	if (frac) return true; // .000 or 000.000
  72. 	if (!digits) return false;
  73. 	s.restore(match_symbol(s.save(), "."));
  74. 	return true; // 000 or 000.
  75. }
  76.  
  77. bool match_number(parser_state& s) {
  78. 	const bool m = s.restore(match_mantissa(s.save()));
  79. 	const bool e = s.restore(match_exponent(s.save()));
  80. 	return (m || e)	&& s.end();
  81. }
  82.  
  83. bool match_number(const std::string& s) {
  84. 	parser_state state(s, 0);
  85. 	return match_number(state);
  86. }
  87.  
  88. int main() {
  89. 	std::vector<std::string> examples = {
  90. 		"0",
  91. 		"+0",
  92. 		"0.00",
  93. 		"0.",
  94. 		".0",
  95. 		".",
  96. 		"0.E1",
  97. 		"0.E+2",
  98. 		".0E+3",
  99. 		".0E+3 ",
  100. 		"0.0."
  101. 	};
  102. 	parser_state s("", 0);
  103. 	std::cout << (match_exponent(s)) << std::endl;
  104. 	for (const auto& s : examples) {
  105. 		std::cout << s << ": " << match_number(s) << std::endl;
  106. 	}
  107. 	return 0;
  108. }
Success #stdin #stdout 0s 4696KB
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stdin
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Standard input is empty
stdout
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0
0: 1
+0: 1
0.00: 0
0.: 1
.0: 1
.: 0
0.E1: 1
0.E+2: 1
.0E+3: 0
.0E+3 : 0
0.0.: 0
https://ideone.com/2mpH8O
language:
C++ (gcc 8.3)
created:
3 years ago
visibility:
secret

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