use num_traits::pow::Pow;
use std::collections::{ BTreeMap, HashMap };
// use std::env; // TODO: Get args from here
use std::io::{ self, BufRead };
use std::fmt;
use std::rc::Rc;
use rust_decimal::prelude::*;
use rust_decimal_macros::dec;

mod tests;


const HELP_TEXT: &str =
    "LITERALS\n\
     0-9     integers\n\
     _       toggle sign (or, as a command, negate the top item on the stack)\n\
     [...]   string\n\
     (...)   named command (see below)\n\
     \n\
     ARITHMETIC OPERATIONS\n\
     +       add the top two items on the stack\n\
     -       subtract the top item on the stack from the second item\n\
     *       multiply the top two items on the stack\n\
     /       divide the second item on the stack by the top item\n\
     %       take the modulus of the second item by the base of the top item\n\
     ^       raise the second item to the power of the top item\n\
     v       take the square root of the top item on the stack\n\
     \n\
     STACK OPERATIONS\n\
     d       duplicate the top item on the stack\n\
     c       clear the stack\n\
     ,       drop the top item from the stack\n\
     !       something semi-implemented\n\
     \n\
     REGISTER OPERATIONS\n\
     l       load from register (register name as next character)\n\
     s       store to register (register name as next character)\n\
     \n\
     CONTROL FLOW\n\
     x       evaluate string\n\
     \n\
     META\n\
     p       print the top item of the stack, leaving it in place\n\
     n       print the top item of the stack, popping it\n\
     q       quit (you can also ^D)\n\
     ?       help (this message)\n\
     \n\
     Good luck!\n\n";


#[derive(Debug,PartialEq,Eq,Clone)]
enum Value {
  Str(String),
  Num(Decimal),
}

impl fmt::Display for Value {
  fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    match self {
      Value::Str(s) => write!(f, "[{}]", s),
      Value::Num(d) => write!(f, "{}", d),
    }
  }
}

fn print_value(v: &Value) {
  match v {
    Value::Str(s) => print!("{}", s),
    Value::Num(d) => print!("{}", d),
  }
}


#[derive(Debug, PartialEq, Eq, PartialOrd, Ord)]
enum Mode {
  Integer,
  Decimal,
  Str,
  CommandChar,
  CommandNamed,
  RegisterChar,
  RegisterStr,
}


#[derive(Debug)]
enum Exit {
  WithMessage(String),
  Quit,
}

fn err_msg(msg: String) -> Exit {
  Exit::WithMessage(msg)
}


fn baseline_registers() -> HashMap<String, Value> {
  let kvs = [
    ("F->C", "1! 32- 5 9/*"),
    ("C->F", "1! 9 5/* 32+"),
    ("C->K", "1! 273+"),
    ("K->C", "1! 273-"),
    ("Km->mi", "1! 1.609344/"),
    ("mi->Km", "1! 1.609344*"),
  ].map(|kv: (&str, &str)| (String::from(kv.0), Value::Str(kv.1.into())));

  HashMap::from(kvs)
}


fn main() {
  let mut state = RPState {
    registers: baseline_registers(),
    stack: Vec::new(),
    mode: Mode::CommandChar,
    readtables: BTreeMap::from([
      (Mode::Integer, make_integer_readtable()),
      (Mode::Decimal, make_decimal_readtable()),
      (Mode::Str, make_string_readtable()),
      (Mode::CommandNamed, make_command_string_readtable()),
      (Mode::RegisterChar, make_register_character_readtable()),
      (Mode::RegisterStr, make_register_string_readtable()),
    ]),
    wip_str: String::from(""),
    // TODO: I don't think we need a return stack
    // But this the closest thing we have right now
    reg_command: String::from(""),
    eat_count: 0,
    num: dec!(0.0),
    is_num_negative: false,
    decimal_offset: dec!(1),
  };
  let stdin = io::stdin();
  for line in stdin.lock().lines() {
    match eval(&line.unwrap(), &mut state) {
      Ok(()) => {
        let mut first = true;
        print!("{}", "stack: {");
        for elem in state.stack.iter() {
          if !first { print!("{}", ", "); }
          print!("{}", elem);
          first = false;
        }
        println!("{}", "}");
      },
      Err(Exit::WithMessage(msg)) => println!("Error: {}", msg),
      Err(Exit::Quit) => break,
    }
  }
}


const RADIX: u32 = 10;
const INTEGER_RADIX: Decimal = dec!(10);

fn to_decimal(input: u32) -> Decimal {
  <u32 as Into<Decimal>>::into(input)
}

fn usize_to_decimal(input: usize) -> Decimal {
  <usize as Into<Decimal>>::into(input)
}


type ReadAction = Rc<dyn Fn(char, &mut RPState) -> Result<(), Exit>>;

struct ReadTable {
  action_map: BTreeMap<char, ReadAction>,
  default_action: ReadAction,
}

impl ReadTable {
  fn new(default_action: fn(char, &mut RPState) -> Result<(), Exit>)
    -> ReadTable
  {
    ReadTable {
      action_map: BTreeMap::new(),
      default_action: Rc::new(default_action),
    }
  }

  fn set_action(&mut self, c: char,
         action: fn(char, &mut RPState) -> Result<(), Exit>)
  {
    let _ = self.action_map.insert(c, Rc::new(action));
  }

  fn set_digit_action(&mut self,
                       action: fn(char, &mut RPState) -> Result<(), Exit>)
  {
    for range in ['0' ..= '9', 'a' ..= 'z', 'A' ..= 'Z'] {
      for c in range {
        if c.is_digit(RADIX) {
          let _ = self.action_map.insert(c, Rc::new(action.clone()));
        }
      }
    }
  }
}


struct RPState {
  registers: HashMap<String, Value>,
  stack: Vec<Value>,
  mode: Mode,
  readtables: BTreeMap<Mode, ReadTable>,
  reg_command: String,
  wip_str: String,
  eat_count: u32,
  is_num_negative: bool,
  num: Decimal,
  decimal_offset: Decimal
}


// TODO: Generalized shuffles
// [[a-aa\](shuf)]s[dup]
// [[ab-ba\](shuf)]s[swap]
// [[ab-b\](shuf)]s[nip]
//
// fn shuffle(_state: &mut RPState) -> Result<(), Exit> {
//    Err(err_msg("(shuf) not implemented yet!".into()))
// }

fn make_command_string_readtable() -> ReadTable {
  let mut result = ReadTable::new(move |c, state| {
    state.wip_str.push(c);

    Ok(())
  });

  result.set_action(')', move |_, state| {
    match state.wip_str.as_str() {
      // TODO:
      // (times)
      // (while)
      // (?) -- (cond ifTrue ifFalse)
      // [(?)x]s[if-else]
      // [[\](?)x]s[if]
      // (r?) -- show contents of registers
      // Scope out
      word => {
        if state.registers.contains_key(word) {
          match &state.registers[word].clone() {
            Value::Str(eval_body) => {
              state.wip_str.clear();
              state.mode = Mode::CommandChar;
              eval(&eval_body, state)
            }
            _ => {
              state.mode = Mode::CommandChar;
              Err(err_msg(format!(
                  "Unable to execute ({}) as a named word", word)))
            }
          }
        } else {
          state.mode = Mode::CommandChar;
          Err(err_msg(format!(
              "Unable to execute ({}) as a named word", word)))
        }
      }
    }
  });

  result
}


fn command_char(c: char, state: &mut RPState) -> Result<(), Exit> {
  if c.is_digit(RADIX) {
    state.mode = Mode::Integer;
    state.num = dec!(0);
    state.num += to_decimal(c.to_digit(10)
        .ok_or_else(|| err_msg(format!("{} isn't a digit", c)))?);
    Ok(())
  } else if c == '_' {
    state.mode = Mode::Integer;
    state.num = dec!(0);
    state.is_num_negative = true;
    Ok(())
  } else if c == ' ' {
    // do nothing
    Ok(())
  } else if c == '(' {
    state.mode = Mode::CommandNamed;
    state.wip_str.clear();
    state.eat_count = 0;
    Ok(())
  } else if c == '[' {
    state.mode = Mode::Str;
    state.wip_str.clear();
    state.eat_count = 0;
    Ok(())
  } else if let 'p' | 'n' | 'q' | 'l' | 's' | 'v' | 'x' | 'd' | ',' | 'c'
              | '!' | '?' = c
  {
    match c {
      '?' => {
        print!("{}", HELP_TEXT);
      },
      '!' => {
        if state.stack.is_empty() {
          return Err(err_msg("Data underflow!".into()));
        }
        match state.stack.pop() {
          Some(Value::Num(d)) => {
            if usize_to_decimal(state.stack.len()) < d {
              return Err(err_msg(format!(
                  "Stack depth should be at least: {}", d)));
            }
          }
          Some(Value::Str(_)) => return Err(err_msg(
              "Cannot assert a string as a stack depth".into())),
          None => return Err(err_msg("Data underflow!".into()))
        }
      },
      'q' => return Err(Exit::Quit),
      'c' => state.stack.clear(),
      'l' => {
        state.reg_command = "l".into();
        state.mode = Mode::RegisterChar;
      }
      'x' => {
        if state.stack.is_empty() {
          return Err(err_msg("Data underflow!".into()));
        }
        match state.stack.pop() {
          Some(Value::Str(s)) => {
            return eval(&s, state)
          }
          Some(v) => {
            return Err(err_msg(format!("Cannot eval {}", v)));
          }
          None => {
            return Err(err_msg("Data underflow!".into()));
          }
        }
      }
      's' => {
        state.reg_command = "s".into();
        state.mode = Mode::RegisterChar;
      }
      'd' => {
        if state.stack.is_empty() {
          return Err(err_msg("Data underflow!".into()));
        }
        let val = state.stack.pop().unwrap();
        state.stack.push(val.clone());
        state.stack.push(val.clone());
      }
      ',' => {
        if state.stack.is_empty() {
          return Err(err_msg("Data underflow!".into()));
        }
        state.stack.pop();
      }
      'v' => {
        if state.stack.is_empty() {
          return Err(err_msg("Data underflow!".into()));
        }
        match state.stack.pop() {
          Some(Value::Num(d)) => {
            match d.sqrt() {
              Some(n) => state.stack.push(Value::Num(n)),
              None => return Err(err_msg(
                  "Error attempting square root!".into())),
              }
          }
          Some(v) => {
            return Err(err_msg(format!("Invalid attempt to sqrt {}", v)));
          }
          None => {
            return Err(err_msg("Impossible data underflow!".into()));
          }
        }
      }
      'p' =>  {
        if state.stack.is_empty() {
          return Err(err_msg("Data underflow!".into()));
        }
        print_value(state.stack.last().unwrap());
      },
      'n' => {
        if state.stack.is_empty() {
          return Err(err_msg("Data underflow!".into()));
        }
        print_value(&state.stack.pop().unwrap());
      },
      _ => return Err(err_msg(format!(
          "{} is unimplemented, this shouldn't be reachable!", c))),
    }

    Ok(())
  } else if let '+' | '-' | '/' | '*' | '%' | '^' = c {
    if state.stack.len() < 2 {
      return Err(err_msg("Data underflow!".into()));
    }

    let a = state.stack.pop().ok_or_else(|| err_msg(
        "Data underflow!".into()))?;
    let b = state.stack.pop().ok_or_else(|| err_msg(
        "Data underflow!".into()))?;

    match (&a, &b) {
      (Value::Num(ia), Value::Num(ib)) => {
        let value = match c {
          '+' => Some(*ib + *ia),
          '-' => Some(*ib - *ia),
          '/' => Some(*ib / *ia),
          '*' => Some(*ib * *ia),
          '%' => Some(*ib % *ia),
          '^' => Some(Decimal::pow(*ib, *ia)),
          _ => None
        }.ok_or_else(|| err_msg("This should never happen".into()))?;

        state.stack.push(Value::Num(value));
      }
      _ =>  {
        state.stack.push(b);
        state.stack.push(a);
        return Err(err_msg(format!(
            "Invalid operation {} on non-numbers!", c)))
      }
    }
    Ok(())
  } else {
    panic!("{} isn't implemented yet!", c)
  }
}


fn finish_num(c: char, state: &mut RPState) -> Result<(), Exit> {
  // print!("finishing number, negative? {}", state.is_num_negative);
  if state.is_num_negative {
    state.num *= dec!(-1);
  }
  state.stack.push(Value::Num(state.num));
  state.mode = Mode::CommandChar;
  state.is_num_negative = false;
  command_char(c, state)
}


fn make_integer_readtable() -> ReadTable {
  let mut result = ReadTable::new(finish_num);

  result.set_action('.', move |_, state| {
    state.decimal_offset = dec!(1);
    state.mode = Mode::Decimal;

    Ok(())
  });

  result.set_action('_', move |_, state| {
    state.is_num_negative = true;

    Ok(())
  });

  result.set_digit_action(move |c: char, state: &mut RPState| {
    if c.is_digit(RADIX) {
      state.num *= INTEGER_RADIX;
      state.num += to_decimal(c.to_digit(10).unwrap());
    }

    Ok(())
  });

  result
}


fn make_decimal_readtable() -> ReadTable {
  let mut result = ReadTable::new(finish_num);

  result.set_digit_action(move |c: char, state: &mut RPState| {
    if c.is_digit(RADIX) {
      state.decimal_offset *= dec!(0.1);
      state.num += to_decimal(c.to_digit(10).unwrap())
                   * state.decimal_offset;
    }

    Ok(())
  });

  result
}


fn make_string_readtable() -> ReadTable {
  ReadTable::new(move |c, state| {
    if state.eat_count > 0 {
      state.eat_count -= 1;
      state.wip_str.push(c);
    } else if c == '\\' {
      state.eat_count = 1;
    } else if c != ']' {
      state.wip_str.push(c);
    } else if c == ']' {
      state.mode = Mode::CommandChar;
      state.stack.push(Value::Str(state.wip_str.clone()));
      state.wip_str.clear();
    } else {
      return Err(err_msg("Should Not Get Here!".into()))
    }

    Ok(())
  })
}


fn make_register_string_readtable() -> ReadTable {
  let mut result = ReadTable::new(move |c, state| {
    state.wip_str.push(c);

    Ok(())
  });

  result.set_action(']', move |_, state| {
    state.mode = Mode::CommandChar;

    let name = state.wip_str.clone();
    state.wip_str.clear();

    dispatch_register_command(&name, state)
  });

  result
}


fn make_register_character_readtable() -> ReadTable {
  let mut result = ReadTable::new(move |c, state| {
    state.mode = Mode::CommandChar;

    let name = String::from(c);

    dispatch_register_command(&name, state)
  });

  result.set_action('[', move |_, state| {
    state.mode = Mode::RegisterStr;
    Ok(())
  });

  result
}


fn dispatch_register_command(name: &str, state: &mut RPState)
  -> Result<(), Exit>
{
  match state.reg_command.as_str() {
    "l" => load_from_register(name, state),
    "s" => store_to_register(name, state),

    _ => {
      Err(err_msg(format!(
          "Unsupported register command {}", state.reg_command)))
    }
  }
}


fn store_to_register(name: &str, state: &mut RPState) -> Result<(), Exit> {
  if !state.stack.is_empty() {
    state.registers.insert(name.into(), state.stack.pop().unwrap());

    Ok(())
  } else {
    Err(err_msg(format!(
        "Data underflow attempting to store to register {}", name)))
  }
}


fn load_from_register(name: &str, state: &mut RPState) -> Result<(), Exit> {
  if state.registers.contains_key(name) {
    state.stack.push(state.registers[name].clone());

    Ok(())
  } else {
    Err(err_msg(format!(
        "Register {} is empty and was expected not to be", name)))
  }
}


fn eval(input: &str, state: &mut RPState) -> Result<(), Exit> {
  for (_cpos, c) in input.char_indices() {
    // TODO eventually this slightly weird logic will go away because
    // everything will be in a readtable
    let action = {
      if let Some(readtable) = state.readtables.get_mut(&state.mode) {
        if let Some(action) = readtable.action_map.get_mut(&c) {
          Some(Rc::clone(&action))
        } else {
          Some(Rc::clone(&readtable.default_action))
        }
      } else {
        None
      }
    };

    if let Some(action) = action {
      action(c, state)?;
    } else {
      match state.mode {
        Mode::CommandChar => command_char(c, state),
        _ => return Err(err_msg(
            "Mode has neither readtable nor hardcoded case".to_string())),
      }?;
    }
  }
  match state.mode {
    Mode::Integer | Mode::Decimal => {
      return finish_num(' ', state)
    },
    _ => { }
  };

  Ok(())
}