/*
 * Copyright (c) 2024 Emma Tebibyte <emma@tebibyte.media>
 * SPDX-License-Identifier: AGPL-3.0-or-later
 *
 * This program is free software: you can redistribute it and/or modify it under
 * the terms of the GNU Affero General Public License as published by the Free
 * Software Foundation, either version 3 of the License, or (at your option) any
 * later version.
 * 
 * This program is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
 * FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
 * details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program. If not, see https://www.gnu.org/licenses/.
 *
 * This file incorporates work covered by the following copyright and permission
 * notice:
 *
 *     MIT License
 * 
 *     Copyright (c) 2022 Lilly Cham
 * 
 *     Permission is hereby granted, free of charge, to any person obtaining a
 *     copy of this software and associated documentation files 
 *     (the "Software"), to deal in the Software without restriction, including
 *     without limitation the rights
 *     to use, copy, modify, merge, publish, distribute, sublicense, and/or
 *     sell copies of the Software, and to permit persons to whom the Software
 *     is furnished to do so, subject to the following conditions:
 *
 *     The above copyright notice and this permission notice shall be included
 *     in all copies or substantial portions of the Software.
 * 
 *     THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
 *     OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 *     MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
 *     NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
 *     DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
 *     OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
 *     USE OR OTHER DEALINGS IN THE SOFTWARE.
 */

use std::{
	collections::VecDeque,
	env::args,
	fmt,
	io::stdin,
	process::ExitCode,
};

use CalcType::{ Add, Divide, Modulo, Multiply, Power, Subtract, Val };

extern crate sysexits;

use sysexits::EX_DATAERR;

#[derive(Clone, Copy, PartialEq, PartialOrd, Debug)]
// enum CalcType is a type containing operations used in the calculator.
enum CalcType {
	Add,
	Subtract,
	Multiply,
	Divide,
	Power,
	Modulo,
	Val(f64),
}

#[derive(Debug, Clone)]
struct EvaluationError { pub message: String }

impl fmt::Display for EvaluationError {
	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
		write!(f, "{}", self.message)
	}
}

// I’m no math nerd but I want the highest possible approximation of 0.9
// repeating and it seems this can give it to me
const PRECISION_MOD: f64 = 0.9 + f64::EPSILON * 100.0;

// str_to_calc_type converts a string to an optional `CalcType`.
fn str_to_calc_type(string: &str) -> Option<CalcType> {
	let as_int = string.parse::<f64>();
	let result = match as_int {
		Ok(x) => Some(Val(x)),
		Err(_) => None,
	};

	if result.is_some() { return result; }

	match string {
		"+" => Some(Add),
		"-" => Some(Subtract),
		"*" => Some(Multiply),
		"/" => Some(Divide),
		"^" => Some(Power),
		"%" => Some(Modulo),
		_ => None,
	}
}

fn eval(
	input: &str,
	initial_stack: VecDeque<f64>,
) -> Result<VecDeque<f64>, EvaluationError> {
	let mut stack = initial_stack;

	if input.is_empty() {
		stack.clear();
		return Ok(stack);
	}

	// Split the input into tokens.
	let toks = input.split(' ').collect::<Vec<&str>>();
	let mut ops: VecDeque<CalcType> = VecDeque::new();

	for tok in &toks {
		let x: CalcType = match str_to_calc_type(tok) {
			Some(x) => x,
			None => {
				return Err(EvaluationError {
					message: format!("Invalid token: {}", tok),
				})
			}
		};

		match x {
			Val(x_) => stack.push_back(x_),
			_ => ops.push_back(x),
		}
	}

	for op in &ops {
		match op {
			Val(_) => {
				return Err(EvaluationError {
					message: "Unexpected value in the operator stack."
						.to_string()
				})
			},
			_ => {
				let x = &stack.pop_back().ok_or(EvaluationError {
					message: "Stack is empty.".to_string(),
				})?;

				let y = &stack.pop_back().ok_or(EvaluationError {
					message: "Stack is empty.".to_string(),
				})?;

				match op {
					Add => &stack.push_back(y + x),
					Subtract => &stack.push_back(y - x),
					Multiply => &stack.push_back(y * x),
					Divide => &stack.push_back(y / x),
					Power => &stack.push_back(x.powf(*y)),
					Modulo => &stack.push_back(y % x),
					_ => &{},
				};
			},
		};
	}

	Ok(stack)
}

// Round a float to the given precision level
fn round_precise(value: &f64, precision: usize) -> f64 {
	let multiplier = 10_f64.powi(precision as i32);
	(value * multiplier).round() / multiplier
}

fn main() -> ExitCode {
	let argv = args().collect::<Vec<String>>();
	let mut stack = VecDeque::new();
	let mut buf = String::new();
	// Set floating-point precision for correcting rounding errors based on
	// machine epsilon
	let precision = (-f64::EPSILON.log10() * PRECISION_MOD).ceil() as usize;
	
	if argv.get(1).is_none() {
		while let Ok(_) = stdin().read_line(&mut buf) {
			match eval(&buf.trim(), stack) {
				Ok(s) => {
					stack = s.clone();

					let val = match stack.iter().last() {
						Some(v) => v,
						None => break,
					};

					println!("{}", round_precise(val, precision).to_string());
					buf.clear();
				},
				Err(err) => {
					eprintln!("{}: {}", argv[0], err.message);
					return ExitCode::from(EX_DATAERR as u8);
				},
			};
		}
	} else {
		let input = argv
			.iter()
			.skip(1)
			.map(|x| x.to_owned())
			.collect::<Vec<String>>()
			.join(" ");

		match eval(&input, stack) {
			Ok(s) => {
				stack = s.clone();

				let val = match stack.iter().last() {
					Some(v) => v,
					None => return ExitCode::from(0),
				};

				println!("{}", round_precise(val, precision).to_string())
			},
			Err(err) => {
				eprintln!("{}: {}", argv[0], err.message);
				return ExitCode::from(EX_DATAERR as u8);
			},
		};
	}
	ExitCode::from(0)
}