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path: root/go/forth/forth.go
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package forth

import (
	"errors"
	"strconv"
	"strings"
)

var (
	ErrEOL     = errors.New("end of line")
	ErrStack   = errors.New("stack underflow")
	ErrZero    = errors.New("division by zero")
	ErrWord    = errors.New("name cannot be a number")
	ErrUnknown = errors.New("unknown word")
)

type stack []int

func (s *stack) push(n ...int) error {
	*s = append(*s, n...)
	return nil
}

func (s *stack) pop() (int, error) {
	depth := len(*s)
	if depth < 1 {
		return 0, ErrStack
	}
	tos := (*s)[depth-1]
	*s = (*s)[:depth-1]
	return tos, nil
}

func (s *stack) pop2() (int, int, error) {
	tos, err := s.pop() // top on stack
	if err != nil {
		return 0, 0, err
	}
	nos, err := s.pop() // next on stack
	if err != nil {
		return 0, 0, err
	}
	return tos, nos, nil
}

func (s *stack) values() []int {
	return []int(*s)
}

type word func(*stack) error

type dictionary map[string][]word

func (d dictionary) find(name string) ([]word, bool) {
	words, ok := d[strings.ToLower(name)]
	return words, ok
}

func (d dictionary) add(name string, words ...word) {
	d[strings.ToLower(name)] = words
}

func add(s *stack) error {
	tos, nos, err := s.pop2()
	if err != nil {
		return err
	}
	return s.push(nos + tos)
}

func sub(s *stack) error {
	tos, nos, err := s.pop2()
	if err != nil {
		return err
	}
	return s.push(nos - tos)
}

func mul(s *stack) error {
	tos, nos, err := s.pop2()
	if err != nil {
		return err
	}
	return s.push(nos * tos)
}

func div(s *stack) error {
	tos, nos, err := s.pop2()
	if err != nil {
		return err
	}
	if tos == 0 {
		return ErrZero
	}
	return s.push(nos / tos)
}

func dup(s *stack) error {
	tos, err := s.pop()
	if err != nil {
		return err
	}
	return s.push(tos, tos)
}

func drop(s *stack) error {
	_, err := s.pop()
	return err
}

func swap(s *stack) error {
	tos, nos, err := s.pop2()
	if err != nil {
		return err
	}
	return s.push(tos, nos)
}

func over(s *stack) error {
	tos, nos, err := s.pop2()
	if err != nil {
		return err
	}
	return s.push(nos, tos, nos)
}

func literal(n int) word {
	return func(s *stack) error {
		return s.push(n)
	}
}

func atoi(s string) (int, bool) {
	n, err := strconv.Atoi(s)
	return n, err == nil
}

func colon(dict dictionary, l *lexer) error {
	if l.atEOL() {
		return ErrEOL
	}
	name := l.next()
	if _, ok := atoi(name); ok {
		return ErrWord
	}
	w, err := compile(dict, l)
	if err != nil {
		return err
	}
	dict.add(name, w...)
	return nil
}

func compile(dict dictionary, l *lexer) ([]word, error) {
	var words []word
	for !l.atEOL() {
		v := l.next()
		// colon operator
		if v == ":" {
			if err := colon(dict, l); err != nil {
				return nil, err
			}
			continue
		}
		if v == ";" {
			break
		}
		// lookup dictionary first
		if w, ok := dict.find(v); ok {
			words = append(words, w...)
			continue
		}
		// try to parse literal
		if n, ok := atoi(v); ok {
			words = append(words, literal(n))
			continue
		}
		return nil, ErrUnknown
	}
	return words, nil
}

type lexer []string

func newLexer(line string) lexer {
	return strings.Fields(line)
}

func (l *lexer) atEOL() bool {
	return len(*l) == 0
}

func (l *lexer) next() (s string) {
	s, *l = (*l)[0], (*l)[1:]
	return s
}

func Forth(v []string) ([]int, error) {
	dict := make(dictionary)
	dict.add("+", add)
	dict.add("-", sub)
	dict.add("*", mul)
	dict.add("/", div)
	dict.add("dup", dup)
	dict.add("drop", drop)
	dict.add("swap", swap)
	dict.add("over", over)
	s := new(stack)
	for _, line := range v {
		// compile
		l := newLexer(line)
		words, err := compile(dict, &l)
		if err != nil {
			return nil, err
		}
		// execute
		for _, w := range words {
			if err := w(s); err != nil {
				return nil, err
			}
		}
	}
	return s.values(), nil
}