src/main.rs

   1 use regex::Regex;
   2 use colored::*;
   3 use std::env;
   4 use rustyline::Editor;
   5
   6 const ILLEGAL_EXP : &'static str = "Illegal Expression";
   7 const ERR_PARSING_NOT_MATCH : &'static str = "Error parsing expression! \
   8                                  Must be a number or operator (+,-,* or /)";
   9 const ERR_POSTFIX_INCOMPLETE : &'static str = "Postfix expression incomplete!";
  10 const HELP_TEXT : [&str ; 4] =
  11                                 ["Type a postfix expression to evaluate.",
  12                                  "Example: 4 5 + 12 -",
  13                                  "Supported operators: +, -, *, /",
  14                                  "Type q, Q to quit"
  15                                  ];
  16
  17 const ERROR : &'static str = "Error";
  18 const ERROR_HELP : &'static str = "Type ? or h or H for help";
  19
  20 // Describe an operator - one of add, subtract, multiply or divide
  21 enum Operator {
  22         ADD,
  23         SUB,
  24         MUL,
  25         DIV,
  26 }
  27
  28 // structure to hold an expression in the stack
  29 struct Expression {
  30         value : f32,
  31 }
  32
  33 // function to compute a result by popping the stack and
  34 // pushing back the result
  35 fn get_result (t : &mut Vec<Expression>, op : Operator) -> Result<f32,String> {
  36         // pop the stack for last operand
  37         // if nothing - panic with error
  38         let n1 = t.pop ();
  39         if n1.is_none () {
  40                 return Err (ILLEGAL_EXP.to_string());
  41         }
  42         // pop the stack for the first operand
  43         // if nothing - panic with error
  44         let n2 = t.pop ();
  45         if n2.is_none () {
  46                 return Err (ILLEGAL_EXP.to_string());
  47         }
  48
  49         let num1 = n1.unwrap().value;
  50         let num2 = n2.unwrap().value;
  51
  52         // depending on the operation, set the result
  53         match op {
  54                 Operator::ADD => {
  55                                                         t.push (Expression{value: num2 + num1});
  56                                                         Ok (num2 + num1)
  57                                                  },
  58                 Operator::SUB => {
  59                                                         t.push (Expression{value: num2 - num1});
  60                                                         Ok (num2 + num1)
  61                                                  },
  62                 Operator::MUL => {
  63                                                         t.push (Expression{value: num2 * num1});
  64                                                         Ok (num2 * num1)
  65                                                  },
  66                 Operator::DIV => {
  67                                                         t.push (Expression{value: num2 / num1});
  68                                                         Ok (num2 / num1)
  69                                                  },
  70         }
  71 }
  72
  73 // evaluation function
  74 fn evaluate (expr : &str, match_num : &regex::Regex) -> Result<f32,String> {
  75         let mut ops = Vec::<Expression>::new ();
  76         // tokenize the individual words by splitting at whitespace
  77         let words = expr.split_whitespace ();
  78
  79         // iterate over the words
  80         for word in words {
  81                 match word {
  82                 // if the word matches one of +, -, *, / then push it on the stack
  83                 // and immediately evaluate the expression by popping the operator
  84                 // and the last two operands and push the result back on to the stack
  85                 "+" => {
  86                                 let m = get_result (&mut ops, Operator::ADD);
  87                                 if ! m.is_ok () {
  88                                         return Err (m.unwrap_err().to_string());
  89                                 }
  90                            },
  91                 "-" => {
  92                                 let m = get_result (&mut ops, Operator::SUB);
  93                                 if ! m.is_ok () {
  94                                         return Err (m.unwrap_err().to_string());
  95                                 }
  96                            },
  97                 "*" => {
  98                                 let m = get_result (&mut ops, Operator::MUL);
  99                                 if ! m.is_ok () {
 100                                         return Err (m.unwrap_err().to_string());
 101                                 }
 102                            },
 103                 "/" => {
 104                                 let m = get_result (&mut ops, Operator::DIV);
 105                                 if ! m.is_ok () {
 106                                         return Err (m.unwrap_err().to_string());
 107                                 }
 108                            },
 109                         // if word matches a number, push it on to the stack
 110                 _ =>  if match_num.is_match (word) {
 111                                 let num = word.parse ().unwrap ();
 112                                 ops.push (Expression { value: num });
 113                           }
 114                         // if word doesn't match either operator or number then panic.
 115                         else {
 116                                 return Err (ERR_PARSING_NOT_MATCH.to_string());
 117                         }
 118                 }
 119         }
 120
 121         if ops.len () > 1 {
 122         // if the stack has more than one value, it means that the postfix
 123         // expression is not complete - so display the stack status
 124                 return Err (ERR_POSTFIX_INCOMPLETE.to_string());
 125         } else {
 126         // stack has only one item which is the result so display it
 127                 let res = ops[0].value;
 128                 return Ok (res);
 129         }
 130 }
 131
 132 // Single command mode - command line arguments mode - evaluate the expression
 133 // given in the command line and quit
 134 fn run_command_line (args : &Vec<String>, match_num : &regex::Regex) {
 135         let mut expr = String::new ();
 136         let mut i = 0;
 137         // create the expression string to evaluate
 138         for arg in args.iter() {
 139                 if i > 0 {
 140                         expr.push_str (&arg);
 141                         expr.push_str (" ");
 142                 }
 143                 i += 1;
 144         }
 145         // evaluate the result
 146         let res = evaluate (&expr, &match_num);
 147         // if Result is OK then print the result in green
 148         if res.is_ok () {
 149                 let restxt = format! ("{}", res.unwrap());
 150                 println! ("{}", restxt.green ());
 151         } else {
 152         // print the error in purple
 153                 let errtxt = format! ("{}: {}", ERROR,
 154                                                                                 res.unwrap_err());
 155                 eprintln! ("{}", errtxt.purple ());
 156         }
 157 }
 158
 159 // Interactive mode - display the prompt and evaluate expressions entered into
 160 // the prompt - until user quits
 161 fn run_interactive_mode (match_num : &regex::Regex) {
 162         // get a line from input and evaluate it
 163         let mut expr = String::new ();
 164
 165         let mut rl = Editor::<()>::new ();
 166         // loop until a blank line is received
 167         loop {
 168                 expr.clear ();
 169
 170                 let line = rl.readline ("evpf> ");
 171                 if line.is_err () {
 172                         break;
 173                 }
 174                 let hist = line.unwrap ();
 175                 rl.add_history_entry (&hist);
 176                 expr.push_str (&hist);
 177
 178                 if expr == "q" || expr == "Q" {
 179                 // quit if the expression is q or Qs
 180                         break;
 181                 } else if expr == "?" || expr == "h" || expr == "H" {
 182                 // display help text
 183                         for text in HELP_TEXT.iter() {
 184                                 println! ("{}", text.cyan() );
 185                         }
 186
 187                         continue;
 188                 } else if expr == "" {
 189                 // continue without proceeding
 190                         continue;
 191                 }
 192
 193                 // Evaluate result
 194                 let res = evaluate (&expr, &match_num);
 195
 196                 // if Result is OK then print the result in green
 197                 if res.is_ok () {
 198                         let restxt = format! ("{}", res.unwrap());
 199                         println! ("{}", restxt.green ());
 200                 } else {
 201                 // print the error in purple
 202                         let errtxt = format! ("{}: {}", ERROR,
 203                                                                                         res.unwrap_err());
 204                         eprintln! ("{}", errtxt.purple());
 205                         eprintln! ("{}", ERROR_HELP.purple());
 206                 }
 207         }
 208 }
 209
 210 fn main() {
 211         // collect the command line arguments - if any
 212         let args : Vec<String> = env::args().collect ();
 213         // regular expression to match a number
 214         let match_num = Regex::new (r"^\d+?\.*?\d*?$").unwrap ();
 215
 216         if args.len () > 1 {
 217         // if arguments are provided run in command line mode - i.e. print the
 218         // result and exit
 219                 run_command_line (&args, &match_num);
 220
 221         } else {
 222                 // if arguments are not provided run in interactive mode -
 223                 // display a prompt and get the expression
 224                 // repeat until the user quits
 225                 run_interactive_mode (&match_num);
 226
 227         }
 228 }