src/main.rs

   1 use std::io;
   2 use std::io::Write;
   3 use regex::Regex;
   4 use ansi_term::Colour;
   5 use ansi_term::Style;
   6
   7 const ILLEGAL_EXP : &'static str = "Illegal Expression";
   8 const ERR_PARSING_NOT_MATCH : &'static str = "Error parsing expression! \
   9                                  Must be a number or operator (+,-,* or /)";
  10 const ERR_POSTFIX_INCOMPLETE : &'static str = "Postfix expression incomplete!";
  11 const ERR_FLUSHING : &'static str = "Error flushing!";
  12 const ERR_READING_LINE : &'static str = "Error reading line!";
  13 const HELP_TEXT : [&str ; 4] =
  14                                 ["Type an expression in postfix style to evaluate.",
  15                                  "Example: 4 5 + 12 -",
  16                                  "Supported operators: +, -, *, /",
  17                                  "Type q, Q to quit"
  18                                  ];
  19
  20 const RESULT : &'static str = "Result";
  21 const ERROR : &'static str = "Error";
  22 const ERROR_HELP : &'static str = "Type ? or h or H for help";
  23
  24 // Describe an operator - one of add, subtract, multiply or divide
  25 enum Operator {
  26         ADD,
  27         SUB,
  28         MUL,
  29         DIV,
  30 }
  31
  32 // structure to hold an expression in the stack
  33 struct Expression {
  34         value : f32,
  35 }
  36
  37 // function to compute a result by popping the stack and
  38 // pushing back the result
  39 fn get_result (t : &mut Vec<Expression>, op : Operator) -> Result<f32,String> {
  40         // pop the stack for last operand
  41         // if nothing - panic with error
  42         let n1 = t.pop ();
  43         if n1.is_none () {
  44                 return Err (ILLEGAL_EXP.to_string());
  45         }
  46         // pop the stack for the first operand
  47         // if nothing - panic with error
  48         let n2 = t.pop ();
  49         if n2.is_none () {
  50                 return Err (ILLEGAL_EXP.to_string());
  51         }
  52
  53         let num1 = n1.unwrap().value;
  54         let num2 = n2.unwrap().value;
  55
  56         // depending on the operation, set the result
  57         match op {
  58                 Operator::ADD => {
  59                                                         t.push (Expression{value: num2 + num1});
  60                                                         Ok (num2 + num1)
  61                                                  },
  62                 Operator::SUB => {
  63                                                         t.push (Expression{value: num2 - num1});
  64                                                         Ok (num2 + num1)
  65                                                  },
  66                 Operator::MUL => {
  67                                                         t.push (Expression{value: num2 * num1});
  68                                                         Ok (num2 * num1)
  69                                                  },
  70                 Operator::DIV => {
  71                                                         t.push (Expression{value: num2 / num1});
  72                                                         Ok (num2 / num1)
  73                                                  },
  74         }
  75 }
  76
  77 // evaluation function
  78 fn evaluate (expr : &str, match_num : &regex::Regex) -> Result<f32,String> {
  79         let mut ops = Vec::<Expression>::new ();
  80         // tokenize the individual words by splitting at whitespace
  81         let words = expr.split_whitespace ();
  82
  83         // iterate over the words
  84         for word in words {
  85                 match word {
  86                 // if the word matches one of +, -, *, / then push it on the stack
  87                 // and immediately evaluate the expression by popping the operator
  88                 // and the last two operands and push the result back on to the stack
  89                 "+" => {
  90                                 let m = get_result (&mut ops, Operator::ADD);
  91                                 if ! m.is_ok () {
  92                                         return Err (m.unwrap_err().to_string());
  93                                 }
  94                            },
  95                 "-" => {
  96                                 let m = get_result (&mut ops, Operator::SUB);
  97                                 if ! m.is_ok () {
  98                                         return Err (m.unwrap_err().to_string());
  99                                 }
 100                            },
 101                 "*" => {
 102                                 let m = get_result (&mut ops, Operator::MUL);
 103                                 if ! m.is_ok () {
 104                                         return Err (m.unwrap_err().to_string());
 105                                 }
 106                            },
 107                 "/" => {
 108                                 let m = get_result (&mut ops, Operator::DIV);
 109                                 if ! m.is_ok () {
 110                                         return Err (m.unwrap_err().to_string());
 111                                 }
 112                            },
 113                         // if word matches a number, push it on to the stack
 114                 _ =>  if match_num.is_match (word) {
 115                                 let num = word.parse ().unwrap ();
 116                                 ops.push (Expression { value: num });
 117                           }
 118                         // if word doesn't match either operator or number then panic.
 119                         else {
 120                                 return Err (ERR_PARSING_NOT_MATCH.to_string());
 121                         }
 122                 }
 123         }
 124
 125         if ops.len () > 1 {
 126         // if the stack has more than one value, it means that the postfix
 127         // expression is not complete - so display the stack status
 128                 return Err (ERR_POSTFIX_INCOMPLETE.to_string());
 129         } else {
 130         // stack has only one item which is the result so display it
 131                 let res = ops[0].value;
 132                 return Ok (res);
 133         }
 134 }
 135
 136 fn main() {
 137         // get a line from input and evaluate it
 138         let mut expr = String::new ();
 139         // regular expression to match a number
 140         let match_num = Regex::new (r"^\d+?\.*?\d*?$").unwrap ();
 141         // loop until a blank line is received
 142         loop {
 143                 expr.clear ();
 144                 print!("{}", Style::new().bold().paint("evpf>"));
 145                 io::stdout().flush ().expect (ERR_FLUSHING);
 146                 // read a line of text
 147                 io::stdin().read_line (&mut expr).expect (ERR_READING_LINE);
 148                 // trim the text
 149                 let expr = expr.trim ();
 150
 151                 if expr == "q" || expr == "Q" {
 152                 // quit if the expression is q or Qs
 153                         break;
 154                 } else if expr == "?" || expr == "h" || expr == "H" {
 155                 // display help text
 156                         for text in HELP_TEXT.iter() {
 157                                 println! ("{}", Colour::Cyan.paint(*text));
 158                         }
 159
 160                         continue;
 161                 } else if expr == "" {
 162                 // continue without proceeding
 163                         continue;
 164                 }
 165
 166                 // Evaluate result
 167                 let res = evaluate (&expr, &match_num);
 168
 169                 // if Result is OK then print the result in green
 170                 if res.is_ok () {
 171                         let restxt = format! ("{}: {}", RESULT,
 172                                                                                         res.unwrap());
 173                         println! ("{}", Colour::Green.paint (restxt));
 174                 } else {
 175                 // print the error in purple
 176                         let errtxt = format! ("{}: {}", ERROR,
 177                                                                                         res.unwrap_err());
 178                         eprintln! ("{}", Colour::Purple.paint (errtxt));
 179                         eprintln! ("{}", Colour::Purple.paint (ERROR_HELP));
 180                 }
 181         }
 182 }