package main

import "fmt"

// Builtins is a handy holder for our various default words
type Builtins struct{}

// ErrExit is a special sentinel value to cease computation and quit
var ErrExit = fmt.Errorf("exit requested")

// Add sums the top two numbers on the stack and pushes the result
func (b *Builtins) Add(s *Stack) func() error {
	return func() error {
		r1, err := s.Pop()
		if err != nil {
			return err
		}
		r2, err := s.Pop()
		if err != nil {
			return err
		}
		s.Push(r1 + r2)
		return nil
	}
}

// Sub performs NOS - TOS and pushes the result
func (b *Builtins) Sub(s *Stack) func() error {
	return func() error {
		r1, err := s.Pop()
		if err != nil {
			return err
		}
		r2, err := s.Pop()
		if err != nil {
			return err
		}
		s.Push(r2 - r1)
		return nil
	}
}

// Mul multiplies the two numbers on the top of the stack and pushes the result
func (b *Builtins) Mul(s *Stack) func() error {
	return func() error {
		r1, err := s.Pop()
		if err != nil {
			return err
		}
		r2, err := s.Pop()
		if err != nil {
			return err
		}
		s.Push(r2 * r1)
		return nil
	}
}

// Div performs NOS/TOS and pushes the (integer!) result
func (b *Builtins) Div(s *Stack) func() error {
	return func() error {
		r1, err := s.Pop()
		if err != nil {
			return err
		}
		r2, err := s.Pop()
		if err != nil {
			return err
		}
		s.Push(r2 / r1)
		return nil
	}
}

// Print pops the stack and outputs it to stdout
func (b *Builtins) Print(s *Stack) func() error {
	return func() error {
		r1, err := s.Pop()
		if err != nil {
			return err
		}
		fmt.Print(r1, " ")
		return nil
	}
}

// Dup pops the stack, then pushes two copies onto the stack
func (b *Builtins) Dup(s *Stack) func() error {
	return func() error {
		r1, err := s.Pop()
		if err != nil {
			return err
		}
		s.Push(r1)
		s.Push(r1)
		return nil
	}
}

// Swap inverts the order of TOS and NOS
func (b *Builtins) Swap(s *Stack) func() error {
	return func() error {
		r1, err := s.Pop()
		if err != nil {
			return err
		}
		r2, err := s.Pop()
		if err != nil {
			return err
		}
		s.Push(r1)
		s.Push(r2)
		return nil
	}
}

// Over duplicates NOS to TOS, resulting in NOS TOS NOS
func (b *Builtins) Over(s *Stack) func() error {
	return func() error {
		r1, err := s.Pop()
		if err != nil {
			return err
		}
		r2, err := s.Pop()
		if err != nil {
			return err
		}
		s.Push(r2)
		s.Push(r1)
		s.Push(r2)
		return nil
	}
}

// Drop simply discards TOS
func (b *Builtins) Drop(s *Stack) func() error {
	return func() error {
		_, err := s.Pop()
		return err
	}
}

// Rot cycles the first three items on the stack: TOS 1 2 3 -> TOS 3 1 2
func (b *Builtins) Rot(s *Stack) func() error {
	return func() error {
		r1, err := s.Pop()
		if err != nil {
			return err
		}
		r2, err := s.Pop()
		if err != nil {
			return err
		}
		r3, err := s.Pop()
		if err != nil {
			return err
		}
		s.Push(r2)
		s.Push(r1)
		s.Push(r3)
		return err
	}
}

// Words outputs a list of all known words in the dictionary
func (b *Builtins) Words(d Dictionary) func() error {
	return func() error {
		for n := range d {
			fmt.Printf("%s ", n)
		}
		return nil
	}
}

// Emit outputs the UTF-8 rune for the int on the top of the stack
func (b *Builtins) Emit(s *Stack) func() error {
	return func() error {
		i, err := s.Pop()
		if err != nil {
			return err
		}
		fmt.Print(string(rune(i)) + " ")
		return nil
	}
}

// CR prints a newline to standard out
func (b *Builtins) CR() func() error {
	return func() error {
		fmt.Print("\n")
		return nil
	}
}

// Quit exits the repl
func (b *Builtins) Quit() func() error {
	return func() error {
		return ErrExit
	}
}

// Debug prints the stack without modifying it
func (b *Builtins) Debug(s *Stack) func() error {
	return func() error {
		fmt.Print(s.values, " ")
		return nil
	}
}