package main

import (
	"os"
	"fmt"
	"strings"
	"regexp/syntax"
	"unicode/utf8"
)

// Maximum number of repeats if unterminated (ex: *, +)
var MaxRepeats = 10

// syntax.OpNoMatch
func opNoMatch(pattern *syntax.Regexp) []string {
	return nil
}

// syntax.OpEmptyMatch, syntax.OpBeginLine, syntax.OpEndLine, syntax.OpBeginText, syntax.OpEndText
func opEmpty(pattern *syntax.Regexp) []string {
	return []string{""}
}

// syntax.OpLiteral
func opLiteral(pattern *syntax.Regexp) []string {
	// Literal is returned as-is, just 1 value
	return []string{string(pattern.Rune)}
}

// syntax.OpCharClass
func opCharClass(pattern *syntax.Regexp) []string {
	// pattern.Rune is already de-dupped
	var lits []string
	for idx := 0; idx < len(pattern.Rune); idx += 2 {
		for r := pattern.Rune[idx]; r <= pattern.Rune[idx+1]; r++ {
			lits = append(lits, string(r))
		}
	}
	return lits
}

// syntax.OpAnyCharNotNL
func opAnyCharNotNL(pattern *syntax.Regexp) []string {
	lits := make([]string, utf8.MaxRune - 1)
	idx := 0
	for r := 0; r < utf8.MaxRune; idx++ {
		if rune(r) != rune('\n') {
			lits[idx] = string(rune(idx))
			idx++
		}
	}
	return lits
}

// syntax.OpAnyChar
func opAnyChar(pattern *syntax.Regexp) []string {
	lits := make([]string, utf8.MaxRune)
	for idx := 0; idx < int(utf8.MaxRune); idx++ {
		lits[idx] = string(rune(idx))
	}
	return lits
}

// syntax.OpCapture
func opCapture(pattern *syntax.Regexp) ([]string, error) {
	return expand(pattern.Sub[0])
}

// syntax.OpStar
func opStar(pattern *syntax.Regexp) ([]string, error) {
	pattern.Min = 0
	pattern.Max = MaxRepeats
	return opRepeat(pattern)
}

// syntax.OpPlus
func opPlus(pattern *syntax.Regexp) ([]string, error) {
	pattern.Min = 1
	pattern.Max = MaxRepeats
	return opRepeat(pattern)
}

// syntax.OpQuest
func opQuest(pattern *syntax.Regexp) ([]string, error) {
	lits, err := expand(pattern.Sub[0])
	if err != nil {
		return nil, err
	}
	return append(lits, ""), nil
}

// syntax.OpRepeat
func opRepeat(pattern *syntax.Regexp) ([]string, error) {
	lits, err := expand(pattern.Sub[0])
	if err != nil {
		return nil, err
	}
	if pattern.Max == -1 {
		pattern.Max = MaxRepeats
	}
	rlits := make([]string, len(lits) * (pattern.Max - pattern.Min))
	idx := 0
	for repeat := pattern.Min; repeat < pattern.Max; repeat++ {
		if repeat == 0 {
			rlits[idx] = ""
			idx++
			rlits = rlits[:len(rlits) - len(lits) + 1]
			continue
		}
		for _, lit := range lits {
			rlits[idx] = strings.Repeat(lit, repeat)
			idx++
		}
	}
	return rlits, nil
}

// syntax.OpConcat
func opConcat(pattern *syntax.Regexp) ([]string, error) {
	// Ensure unique values
	uniq := make(map[string]struct{})
	// recursively concat
	var recur func(prefixes []string, patterns []*syntax.Regexp) error
	recur = func(prefixes []string, patterns []*syntax.Regexp) error {
		lits, err := expand(patterns[0])
		if err != nil {
			return err
		}
		swap := make([]string, len(lits) * len(prefixes))
		idx := 0
		for _, prefix := range prefixes {
			for _, lit := range lits {
				swap[idx] = prefix + lit
				idx++
			}
		}
		if len(patterns) <= 1 {
			for _, lit := range swap {
				uniq[lit] = struct{}{}
			}
			return nil
		} else {
			return recur(swap, patterns[1:])
		}
	}
	if err := recur([]string{""}, pattern.Sub); err != nil {
		return nil, err
	}
	lits := make([]string, len(uniq))
	idx := 0
	for lit := range uniq {
		lits[idx] = lit
		idx++
	}
	return lits, nil
}

// syntax.OpAlternate
func opAlternate(pattern *syntax.Regexp) ([]string, error) {
	// Ensure unique values
	uniq := make(map[string]struct{})
	for _, sub := range pattern.Sub {
		// Each alternate stands on it's own
		lits, err := expand(sub)
		if err != nil {
			return nil, err
		}
		for _, lit := range lits {
			uniq[lit] = struct{}{}
		}
	}
	lits := make([]string, len(uniq))
	idx := 0
	for lit := range uniq {
		lits[idx] = lit
		idx++
	}
	return lits, nil
}

// expand recursively expands the possible values for a regular expression
func expand(pattern *syntax.Regexp) ([]string, error) {
	switch pattern.Op {
	case syntax.OpNoMatch:
		return opNoMatch(pattern), nil
	case syntax.OpEmptyMatch, syntax.OpBeginLine, syntax.OpEndLine, syntax.OpBeginText, syntax.OpEndText:
		return opEmpty(pattern), nil
	case syntax.OpLiteral:
		return opLiteral(pattern), nil
	case syntax.OpCharClass:
		return opCharClass(pattern), nil
	case syntax.OpAnyCharNotNL:
		return opAnyCharNotNL(pattern), nil
	case syntax.OpAnyChar:
		return opAnyChar(pattern), nil
	case syntax.OpCapture:
		return opCapture(pattern)
	case syntax.OpStar:
		return opStar(pattern)
	case syntax.OpPlus:
		return opPlus(pattern)
	case syntax.OpQuest:
		return opQuest(pattern)
	case syntax.OpRepeat:
		return opRepeat(pattern)
	case syntax.OpConcat:
		return opConcat(pattern)
	case syntax.OpAlternate:
		return opAlternate(pattern)
	default:
		return nil, fmt.Errorf("unhandled pattern operation %s", pattern.Op)
	}
}

// Entry Point
func main() {
	if len(os.Args) < 2 {
		fmt.Printf("usage: %s <regexp>", os.Args[0])
	}
	exp, err := syntax.Parse(os.Args[1], syntax.POSIX)
	if err != nil {
		fmt.Fprintf(os.Stderr, "failed to parse regexp: %s", err)
		os.Exit(1)
	}
	// Operate on the simplified expression as it's faster
	matches, err := expand(exp.Simplify())
	if err != nil {
		fmt.Fprintf(os.Stderr, "failed to expand regexp: %s", err)
		os.Exit(1)
	}
	for _, match := range matches {
		fmt.Println(match)
	}
}