use std::io;
use std::io::Write;
use regex::Regex;
use colored::*;
use std::env;

const ILLEGAL_EXP : &'static str = "Illegal Expression";
const ERR_PARSING_NOT_MATCH : &'static str = "Error parsing expression! \
				 Must be a number or operator (+,-,* or /)";
const ERR_POSTFIX_INCOMPLETE : &'static str = "Postfix expression incomplete!";
const ERR_FLUSHING : &'static str = "Error flushing!";
const ERR_READING_LINE : &'static str = "Error reading line!";
const HELP_TEXT : [&str ; 4] = 
				["Type a postfix expression to evaluate.",
				 "Example: 4 5 + 12 -",
				 "Supported operators: +, -, *, /",
				 "Type q, Q to quit"
				 ];

const ERROR : &'static str = "Error";
const ERROR_HELP : &'static str = "Type ? or h or H for help";

// Describe an operator - one of add, subtract, multiply or divide
enum Operator { 
	ADD,
	SUB,
	MUL,
	DIV,
}

// structure to hold an expression in the stack
struct Expression {
	value : f32,
}

// function to compute a result by popping the stack and 
// pushing back the result
fn get_result (t : &mut Vec<Expression>, op : Operator) -> Result<f32,String> {
	// pop the stack for last operand 
	// if nothing - panic with error
	let n1 = t.pop ();
	if n1.is_none () { 
		return Err (ILLEGAL_EXP.to_string());
	}
	// pop the stack for the first operand
	// if nothing - panic with error
	let n2 = t.pop ();
	if n2.is_none () {
		return Err (ILLEGAL_EXP.to_string());
	}
	
	let num1 = n1.unwrap().value;
	let num2 = n2.unwrap().value;
	
	// depending on the operation, set the result
	match op {
		Operator::ADD => {				
							t.push (Expression{value: num2 + num1});
						 	Ok (num2 + num1)
						 },
		Operator::SUB => {				
							t.push (Expression{value: num2 - num1});
						 	Ok (num2 + num1)
						 },
		Operator::MUL => {				
							t.push (Expression{value: num2 * num1});
						 	Ok (num2 * num1)
						 },
		Operator::DIV => {				
							t.push (Expression{value: num2 / num1});
						 	Ok (num2 / num1)
						 },
	}
}

// evaluation function
fn evaluate (expr : &str, match_num : &regex::Regex) -> Result<f32,String> {
	let mut ops = Vec::<Expression>::new ();
	// tokenize the individual words by splitting at whitespace
	let words = expr.split_whitespace ();

	// iterate over the words
	for word in words {
		match word {
		// if the word matches one of +, -, *, / then push it on the stack
		// and immediately evaluate the expression by popping the operator 
		// and the last two operands and push the result back on to the stack		
		"+" => {
			  	let m = get_result (&mut ops, Operator::ADD);
			  	if ! m.is_ok () {
					return Err (m.unwrap_err().to_string());
				}
			   },
		"-" => {
			  	let m = get_result (&mut ops, Operator::SUB);
			  	if ! m.is_ok () {
			  		return Err (m.unwrap_err().to_string());
		  		}
	  		   },
		"*" => {
			  	let m = get_result (&mut ops, Operator::MUL);
 			  	if ! m.is_ok () {
					return Err (m.unwrap_err().to_string());
			  	}
			   },
		"/" => {
			  	let m = get_result (&mut ops, Operator::DIV); 
			  	if ! m.is_ok () {
					return Err (m.unwrap_err().to_string());
			  	}
			   },
			// if word matches a number, push it on to the stack
		_ =>  if match_num.is_match (word) {
				let num = word.parse ().unwrap ();
				ops.push (Expression { value: num });
			  }	
			// if word doesn't match either operator or number then panic. 			
			else {
				return Err (ERR_PARSING_NOT_MATCH.to_string());
			}
		}
	}
	
	if ops.len () > 1 {
	// if the stack has more than one value, it means that the postfix
	// expression is not complete - so display the stack status
		return Err (ERR_POSTFIX_INCOMPLETE.to_string());
	} else { 
	// stack has only one item which is the result so display it
		let res = ops[0].value;
		return Ok (res);
	}
}

// Single command mode - command line arguments mode - evaluate the expression
// given in the command line and quit
fn run_command_line (args : &Vec<String>, match_num : &regex::Regex) {
	let mut expr = String::new ();
	let mut i = 0;
	// create the expression string to evaluate
	for arg in args.iter() {
		if i > 0 {
			expr.push_str (&arg);
			expr.push_str (" ");
		}
		i += 1;
	}
	// evaluate the result 
	let res = evaluate (&expr, &match_num);
	// if Result is OK then print the result in green
	if res.is_ok () {
		let restxt = format! ("{}", res.unwrap());
		println! ("{}", restxt.green ());
	} else {
	// print the error in purple
		let errtxt = format! ("{}: {}", ERROR, 
										res.unwrap_err());
		eprintln! ("{}", errtxt.purple ());
	}
}

// Interactive mode - display the prompt and evaluate expressions entered into
// the prompt - until user quits
fn run_interactive_mode (match_num : &regex::Regex) {
	// get a line from input and evaluate it 
	let mut expr = String::new ();

	// loop until a blank line is received	
	loop {
		expr.clear ();	
		print!("{}", "evpf>".bold() );
		io::stdout().flush ().expect (ERR_FLUSHING);
		// read a line of text
		io::stdin().read_line (&mut expr).expect (ERR_READING_LINE);
		// trim the text
		let expr = expr.trim ();

		if expr == "q" || expr == "Q" {
		// quit if the expression is q or Qs 
			break;
		} else if expr == "?" || expr == "h" || expr == "H" {
		// display help text
			for text in HELP_TEXT.iter() {
				println! ("{}", text.cyan() );
			}

			continue;
		} else if expr == "" {
		// continue without proceeding
			continue;
		}
		
		// Evaluate result
		let res = evaluate (&expr, &match_num);
		
		// if Result is OK then print the result in green
		if res.is_ok () {
			let restxt = format! ("{}", res.unwrap());
			println! ("{}", restxt.green ());
		} else {
		// print the error in purple
			let errtxt = format! ("{}: {}", ERROR, 
											res.unwrap_err());
			eprintln! ("{}", errtxt.purple());
			eprintln! ("{}", ERROR_HELP.purple());
		}
	}
}

fn main() {
	// collect the command line arguments - if any 
	let args : Vec<String> = env::args().collect ();
	// regular expression to match a number 
	let match_num = Regex::new (r"^\d+?\.*?\d*?$").unwrap ();
	
	if args.len () > 1 {
	// if arguments are provided run in command line mode - i.e. print the 
	// result and exit	
		run_command_line (&args, &match_num);
		
	} else {
		// if arguments are not provided run in interactive mode - 
		// display a prompt and get the expression 
		// repeat until the user quits
		run_interactive_mode (&match_num);

	}
}