package main

import (
	"bufio"
	"bytes"
	"errors"
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"strings"
)

// FALSE is a flag for, what else, false
var FALSE = 0

// TRUE is a flag for, what else, true.
// It's -1 because historically, FORTHs would define TRUE as "all bits set to 1 in a cell",
// and interpreting that literally flips the sign flag in a signed integer.
// More generally, TRUE is defined as "not FALSE" for most purposes, and any non-zero integer will work.
var TRUE = -1

// 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")

// Colon sets the COMPILE/IMMEDIATE flag to COMPILE
func (b *Builtins) Colon(c *Context) func(string) error {
	return func(_ string) error {
		c.Flags.SetFlag("Immediate", false)
		return nil
	}
}

// Semicolon sets the COMPILE/IMMEDIATE flag back to IMMEDIATE and adds the defintion to the dictionary
func (b *Builtins) Semicolon(c *Context) func(string) error {
	return func(_ string) error {
		c.Dictionary.AddWord(c.Words[0], Word{Name: c.Words[0], Source: c.Words[1:]})
		c.Words = []string{}
		c.Flags.SetFlag("Immediate", true)
		return nil
	}
}

// OpenComment puts the parser into an "ignore" mode
func (b *Builtins) OpenComment(c *Context) func(string) error {
	return func(next string) error {
		c.Flags.SetFlag("StashImmediate", c.Flags.GetFlag("Immediate"))
		for i := 0; i < len(next); i = i + 1 {
			switch next[i] {
			case ')':
				j, _ := c.RStack.Pop()
				c.RStack.Push(j + i - 1) // push the end-point onto the stack
				return nil
			default:
				continue
			}
		}
		return nil
	}
}

// EOLComment discards the rest of the parsed line
func (b *Builtins) EOLComment(c *Context) func(string) error {
	return func(next string) error {
		j, _ := c.RStack.Pop()
		c.RStack.Push(j + len(next)) // push the end-point onto the stack
		return nil
	}
}

// CloseComment resumes parsing by consuming itself and resetting the immediate flag
func (b *Builtins) CloseComment(c *Context) func(string) error {
	return func(_ string) error {
		c.Flags.SetFlag("Immediate", c.Flags.GetFlag("StashImmediate"))
		return nil
	}
}

// OpenQuote consumes text until its closing pair to output
func (b *Builtins) OpenQuote(out io.Writer, c *Context, close byte) func(string) error {
	if out == nil {
		out = os.Stdout
	}
	return func(next string) error {
		c.Flags.SetFlag("StashImmediate", c.Flags.GetFlag("Immediate"))
		w := []byte{}
		for i := 0; i < len(next); i = i + 1 {
			switch next[i] {
			case close:
				fmt.Fprint(out, string(w))
				j, _ := c.RStack.Pop()
				c.RStack.Push(j + i - 1) // push the end-point onto the stack
				return nil
			default:
				w = append(w, next[i])
				continue
			}
		}
		return nil
	}
}

// CloseQuote consumes itself.
func (b *Builtins) CloseQuote(c *Context) func(string) error {
	return func(next string) error {
		c.Flags.SetFlag("Immediate", c.Flags.GetFlag("StashImmediate"))
		return nil
	}
}

// StringBuffer consumes text until its closing byte, then puts that value into the context's stringbuffer
func (b *Builtins) StringBuffer(c *Context, close byte) func(string) error {
	return func(next string) error {
		var buff bytes.Buffer
		err := b.OpenQuote(&buff, c, '"')(next)
		if err != nil {
			return err
		}
		s, err := ioutil.ReadAll(&buff)
		c.StringBuffer = string(s)
		return err
	}
}

// Eq compares TOS and NOS and puts -1 on the stack if they're equal, 0 otherwise.
func (b *Builtins) Eq(s *Stack) func(string) error {
	return func(_ string) error {
		tos, err := s.Pop()
		if err != nil {
			return err
		}
		nos, err := s.Pop()
		if err != nil {
			return err
		}
		if tos == nos {
			s.Push(TRUE)
			return nil
		}
		s.Push(FALSE)
		return nil
	}
}

// NEq compares TOS and NOS and puts -1 on the stack if they're not equal, 0 otherwise.
func (b *Builtins) NEq(s *Stack) func(string) error {
	return func(_ string) error {
		tos, err := s.Pop()
		if err != nil {
			return err
		}
		nos, err := s.Pop()
		if err != nil {
			return err
		}
		if tos != nos {
			s.Push(TRUE)
			return nil
		}
		s.Push(FALSE)
		return nil
	}
}

// Lt compares TOS and NOS and puts -1 on the stack if TOS is less than NOS, 0 otherwise.
func (b *Builtins) Lt(s *Stack) func(string) error {
	return func(_ string) error {
		tos, err := s.Pop()
		if err != nil {
			return err
		}
		nos, err := s.Pop()
		if err != nil {
			return err
		}
		if nos < tos {
			s.Push(TRUE)
			return nil
		}
		s.Push(FALSE)
		return nil
	}
}

// Gt compares TOS and NOS and puts -1 on the stack if TOS is greater than NOS, 0 otherwise.
func (b *Builtins) Gt(s *Stack) func(string) error {
	return func(_ string) error {
		tos, err := s.Pop()
		if err != nil {
			return err
		}
		nos, err := s.Pop()
		if err != nil {
			return err
		}
		if nos > tos {
			s.Push(TRUE)
			return nil
		}
		s.Push(FALSE)
		return nil
	}
}

// LtEq compares TOS and NOS and puts -1 on the stack if TOS is less than or equal to NOS, 0 otherwise.
func (b *Builtins) LtEq(s *Stack) func(string) error {
	return func(_ string) error {
		tos, err := s.Pop()
		if err != nil {
			return err
		}
		nos, err := s.Pop()
		if err != nil {
			return err
		}
		if nos <= tos {
			s.Push(TRUE)
			return nil
		}
		s.Push(FALSE)
		return nil
	}
}

// GtEq compares TOS and NOS and puts -1 on the stack if TOS is greater than or equal to NOS, 0 otherwise.
func (b *Builtins) GtEq(s *Stack) func(string) error {
	return func(_ string) error {
		tos, err := s.Pop()
		if err != nil {
			return err
		}
		nos, err := s.Pop()
		if err != nil {
			return err
		}
		if nos >= tos {
			s.Push(TRUE)
			return nil
		}
		s.Push(FALSE)
		return nil
	}
}

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

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

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

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

// Mod performs NOS%TOS and pushes the (integer!) modulo result
func (b *Builtins) Mod(s *Stack) func(string) error {
	return func(_ string) error {
		tos, err := s.Pop()
		if err != nil {
			return err
		}
		nos, err := s.Pop()
		if err != nil {
			return err
		}
		s.Push(nos % tos)
		return nil
	}
}

// Print pops the stack and outputs it to the writer with a trailing space (defaults to stdout)
func (b *Builtins) Print(out io.Writer, s *Stack) func(string) error {
	if out == nil {
		out = os.Stdout
	}
	return func(_ string) error {
		tos, err := s.Pop()
		if err != nil {
			return err
		}
		fmt.Fprint(out, tos, " ")
		return nil
	}
}

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

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

// Over duplicates NOS to TOS, resulting in NOS TOS NOS
func (b *Builtins) Over(s *Stack) func(string) error {
	return func(_ string) error {
		nos, err := s.Pick(1)
		if err != nil {
			return err
		}
		s.Push(nos)
		return nil
	}
}

// Drop simply discards TOS
func (b *Builtins) Drop(s *Stack) func(string) error {
	return func(_ string) 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(string) error {
	return func(_ string) error {
		tos, err := s.Pop()
		if err != nil {
			return err
		}
		nos, err := s.Pop()
		if err != nil {
			return err
		}
		p3, err := s.Pop()
		if err != nil {
			return err
		}
		s.Push(nos)
		s.Push(tos)
		s.Push(p3)
		return err
	}
}

// Pick duplicates TOS from within the stack to the top
func (b *Builtins) Pick(s *Stack) func(string) error {
	return func(_ string) error {
		tos, err := s.Pop()
		if err != nil {
			return err
		}
		swp, err := s.Pick(tos)
		if err != nil {
			return err
		}
		s.Push(swp)
		return nil
	}
}

// Words outputs a list of all known words in the dictionary
func (b *Builtins) Words(out io.Writer, d Dictionary) func(string) error {
	if out == nil {
		out = os.Stdout
	}
	return func(_ string) error {
		for n := range d {
			fmt.Fprintf(out, "%s ", n)
		}
		return nil
	}
}

// Flags outputs a list of all flags
func (b *Builtins) Flags(out io.Writer, c *Context) func(string) error {
	if out == nil {
		out = os.Stdout
	}
	return func(_ string) error {
		for n := range c.Flags {
			fmt.Fprintf(out, "%s %v\n", n, c.Flags.GetFlag(n))
		}
		return nil
	}
}

// Emit outputs the UTF-8 rune for the int on the top of the stack
func (b *Builtins) Emit(out io.Writer, s *Stack) func(string) error {
	if out == nil {
		out = os.Stdout
	}
	return func(_ string) error {
		tos, err := s.Pop()
		if err != nil {
			return err
		}
		fmt.Fprint(out, string(rune(tos))+" ")
		return nil
	}
}

// ToR pops from the stack to the return stack
func (b *Builtins) ToR(s *Stack, r *Stack) func(string) error {
	return func(_ string) error {
		tos, err := s.Pop()
		if err != nil {
			return err
		}
		r.Push(tos)
		return nil
	}
}

// RFrom pops from the return stack to the stack
func (b *Builtins) RFrom(s *Stack, r *Stack) func(string) error {
	return func(_ string) error {
		tors, err := r.Pop()
		if err != nil {
			return err
		}
		s.Push(tors)
		return nil
	}
}

// RFetch copies from the return stack to the stack
func (b *Builtins) RFetch(s *Stack, r *Stack) func(string) error {
	return func(_ string) error {
		tors, err := r.Pop()
		if err != nil {
			return err
		}
		r.Push(tors)
		s.Push(tors)
		return nil
	}
}

// If checks if TOS != 0 and executes any following statements if it is. If TOS == 0, skip execution until ELSE or THEN.
// Nested IFs and ELSE/THENs inside "non-executing" blocks still manipulate the if stack, in order to properly balance
// IFs and THENs. i.e. `0 IF 0 IF 42 EMIT THEN 42 EMIT THEN` should not print anything, but a naive "go until you see any THEN"
// would output "* ".
func (b *Builtins) If(s *Stack, i *Stack) func(string) error {
	return func(_ string) error {
		// check to see the current IF state. If we're inside a non-executing branch,
		// add our own non-execution state, AND tell ELSE to skip its work to boot.
		itop, err := i.Pick(0)
		if err != nil && !errors.Is(err, ErrUnderflow) {
			return err
		} else if err == nil && itop == 0 {
			i.Push(-1)
			return nil
		}

		tos, err := s.Pop()
		if err != nil {
			return err
		}
		// TOS was FALSE, don't execute this branch
		if tos == FALSE {
			i.Push(0)
			return nil
		}
		// TOS wasn't 0, execute until we see ELSE/THEN
		i.Push(1)
		return nil
	}
}

// Else executes a separate piece of code if the IF stack
func (b *Builtins) Else(i *Stack) func(string) error {
	return func(_ string) error {
		itop, err := i.Pop()
		if err != nil {
			return err
		}
		// don't execute until we see our THEN
		if itop == -1 || itop == 1 {
			i.Push(0)
		}
		// we weren't running code, but now we can until THEN
		if itop == 0 {
			i.Push(1)
		}
		return nil
	}
}

// Then ends an If/Else block
func (b *Builtins) Then(i *Stack) func(string) error {
	return func(_ string) error {
		// Pop off the existing IF/ELSE state and return
		_, err := i.Pop()
		return err
	}
}

// Do sets up a loop by marking its own location as the return code.
// It also puts TOS and NOS onto the return stack to use as loop control variables.
func (b *Builtins) Do(s *Stack, r *Stack) func(string) error {
	return func(_ string) error {
		tors, err := r.Pop()
		if err != nil {
			return err
		}
		tos, err := s.Pop()
		if err != nil {
			return err
		}
		nos, err := s.Pop()
		if err != nil {
			return err
		}
		r.Push(nos)
		r.Push(tos)
		r.Push(tors)
		r.Push(tors)
		return nil
	}
}

// Loop closes a loop by removing its own location from the stack and letting the one that Do inserted get returned.
// If it's reached the end of the loop (checking the loop-control variables _underneath_ the return vars),
// it continues, otherwise it re-adds the original location of the Do and returns.
func (b *Builtins) Loop(s *Stack, r *Stack) func(string) error {
	return func(_ string) error {
		tors, err := r.Pop()
		if err != nil {
			return err
		}
		nors, err := r.Pop()
		if err != nil {
			return err
		}
		counter, err := r.Pop()
		if err != nil {
			return err
		}
		target, err := r.Pop()
		if err != nil {
			return err
		}
		counter = counter + 1
		if counter == target {
			r.Push(tors)
		} else {
			r.Push(target)
			r.Push(counter)
			r.Push(nors)
			r.Push(nors)
		}
		return nil
	}
}

// I puts the current value of the loop counter on the top of the stack
func (b *Builtins) I(s *Stack, r *Stack) func(string) error {
	return func(_ string) error {
		counter, err := r.Pick(2)
		if err != nil {
			return err
		}
		s.Push(counter)
		return nil
	}
}

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

// Debug prints the stack without modifying it
func (b *Builtins) Debug(out io.Writer, s *Stack) func(string) error {
	if out == nil {
		out = os.Stdout
	}
	return func(_ string) error {
		fmt.Fprint(out, s.values, " ")
		return nil
	}
}

// See prints the defintion of a word
func (b *Builtins) See(out io.Writer, r *Stack, d Dictionary) func(string) error {
	if out == nil {
		out = os.Stdout
	}
	return func(next string) error {
		w := []byte{}
		// start at 1, not 0, because a ' ' is the character at 0.
		for i := 1; i < len(next); i = i + 1 {
			switch next[i] {
			case ' ':
				word, err := d.GetWord(string(w))
				if err != nil {
					return err
				}
				if word.Impl != nil {
					fmt.Fprintf(out, "%s [INTERNAL]\n", string(w))
				} else {
					fmt.Fprintf(out, ": %s %s ;\n", string(w), strings.Join(word.Source, " "))
				}
				j, err := r.Pop()
				r.Push(j + i)
				return err
			default:
				w = append(w, next[i])
				continue
			}
		}
		return nil
	}
}

// Depth puts the current count of stack items on the stacks
func (b *Builtins) Depth(s *Stack) func(string) error {
	return func(_ string) error {
		s.Push(len(s.values))
		return nil
	}
}

// Variable adds a new word to the dictionary that returns a pointer to memory
func (b *Builtins) Variable(c *Context) func(string) error {
	return func(next string) error {
		w := []byte{}
		for i := 1; i < len(next); i = i + 1 {
			switch next[i] {
			case ' ':
				next := c.Memory.NextFreeAddress()
				if next == 0 {
					// don't use the 0 cell, since we can't distinguish that from the uninitialized field
					next = 1
				}
				c.Dictionary.AddWord(string(w), Word{Name: string(w), Variable: next})
				j, _ := c.RStack.Pop()
				c.RStack.Push(j + i - 1) // push the end-point onto the stack
				return c.Memory.Write(next, []int{0})
			default:
				w = append(w, next[i])
				continue
			}
		}
		return nil
	}
}

// Constant adds a new word to the dictionary that puts a value on the stack
func (b *Builtins) Constant(c *Context) func(string) error {
	return func(next string) error {
		w := []byte{}
		for i := 1; i < len(next); i = i + 1 {
			switch next[i] {
			case ' ':
				v, err := c.Stack.Pop()
				if err != nil {
					return err
				}
				c.Dictionary.AddWord(string(w), Word{
					Name: string(w),
					Impl: func(_ string) error {
						c.Stack.Push(v)
						return nil
					},
				})
				j, _ := c.RStack.Pop()
				c.RStack.Push(j + i - 1) // push the end-point onto the stack
				return nil
			default:
				w = append(w, next[i])
				continue
			}
		}
		return nil
	}
}

// Store places a value in memory
func (b *Builtins) Store(c *Context) func(string) error {
	return func(_ string) error {
		addr, err := c.Stack.Pop()
		if err != nil {
			return err
		}
		val, err := c.Stack.Pop()
		if err != nil {
			return err
		}
		return c.Memory.Write(addr, []int{val})
	}
}

// Fetch returns a value from memory and puts it on the stack
func (b *Builtins) Fetch(c *Context) func(string) error {
	return func(_ string) error {
		addr, err := c.Stack.Pop()
		if err != nil {
			return err
		}
		res := c.Memory.Read(addr, 1)
		c.Stack.Push(res[0])
		return nil
	}
}

// Load loads a library into the current working environment
func (b *Builtins) Load(c *Context) func(string) error {
	return func(_ string) error {
		filename := strings.TrimSpace(c.StringBuffer)
		if filename == "" {
			return fmt.Errorf(`stringbuffer empty; try S" filename.prsp" LOAD`)
		}

		// store stacks
		s, r, i := c.Stack, c.RStack, c.IfStack
		c.Stack = &Stack{values: []int{}}
		c.RStack = &Stack{values: []int{}}
		c.IfStack = &Stack{values: []int{}}

		f, err := os.Open(filename)
		if err != nil {
			return err
		}
		reader := bufio.NewReader(f)
		scanner := bufio.NewScanner(reader)

		for {
			scan := scanner.Scan()
			line := strings.TrimSpace(scanner.Text())
			err = c.Eval(line + " ") // append a space to make sure we always close out our parse loop
			if err != nil && !errors.Is(err, ErrExit) {
				return fmt.Errorf("error in library evaluation: %v", err)
			}
			if scan == false {
				break
			}
		}
		// restore stacks
		c.Stack = s
		c.RStack = r
		c.IfStack = i
		return nil
	}
}