// Q: What's this?
// A: This is a mini "home-made" calculator. You may also regard it as a very elementary version of "interpreter".
import os

const numeric_char = [`0`, `1`, `2`, `3`, `4`, `5`, `6`, `7`, `8`, `9`, `.`, `e`, `E`]

// Convert expression to Reverse Polish Notation.
fn expr_to_rev_pol(expr string) ![]string {
    if expr == '' {
        return error('err: empty expression')
    }
    mut stack := []string{}
    mut rev_pol := []string{}
    mut pos := 0
    for pos < expr.len {
        mut end_pos := pos
        for end_pos < expr.len && expr[end_pos] in numeric_char {
            end_pos++
        }
        if end_pos > pos {
            stack << expr[pos..end_pos]
            pos = end_pos
        } else if end_pos == pos {
            op := expr[pos].ascii_str()
            match op {
                '(' {
                    stack << op
                }
                '*', '/' {
                    for stack.len > 0 && stack.last() !in ['(', '+', '-'] {
                        rev_pol << stack.last()
                        stack.delete(stack.len - 1)
                    }
                    stack << op
                }
                '+', '-' {
                    for stack.len > 0 && stack.last() != '(' {
                        rev_pol << stack.last()
                        stack.delete(stack.len - 1)
                    }
                    stack << op
                }
                ')' {
                    for stack.len > 0 && stack.last() != '(' {
                        rev_pol << stack.last()
                        stack.delete(stack.len - 1)
                    }
                    stack.delete(stack.len - 1)
                }
                else {
                    return error('err: invalid character `${op}`')
                }
            }
            pos++
        }
    }
    for stack.len > 0 {
        top := stack.last()
        rev_pol << top
        stack.delete(stack.len - 1)
    }
    return rev_pol
}

// Evaluate the result of Reverse Polish Notation.
fn eval_rev_pol(rev_pol []string) !f64 {
    mut stack := []f64{}
    for item in rev_pol {
        if is_num_string(item) {
            stack << item.f64()
        } else {
            if stack.len >= 2 {
                oprand_r := stack.last()
                stack.delete(stack.len - 1)
                oprand_l := stack.last()
                stack.delete(stack.len - 1)
                match item {
                    '+' {
                        stack << oprand_l + oprand_r
                    }
                    '-' {
                        stack << oprand_l - oprand_r
                    }
                    '*' {
                        stack << oprand_l * oprand_r
                    }
                    '/' {
                        if oprand_r == 0 {
                            return error('err: divide by zero')
                        }
                        stack << oprand_l / oprand_r
                    }
                    else {}
                }
            } else {
                return error('err: invalid expression')
            }
        }
    }
    return stack[0]
}

fn is_num_string(str string) bool {
    for c in str {
        if c !in numeric_char {
            return false
        }
    }
    return true
}

fn main() {
    println('Please enter the expression you want to calculate, e.g. 1e2+(3-2.5)*6/1.5 .')
    println("Enter 'exit' or 'EXIT' to quit.")
    mut expr_count := 0
    for {
        expr_count++
        expr := os.input_opt('[${expr_count}] ') or {
            println('')
            break
        }.trim_space()
        if expr in ['exit', 'EXIT'] {
            break
        }
        rev_pol := expr_to_rev_pol(expr) or {
            eprintln(err)
            continue
        }
        res := eval_rev_pol(rev_pol) or {
            eprintln(err)
            continue
        }
        println(res)
    }
}