package main

import (
	"strings"
	"fmt"
	"main/subex"
)

type parser struct {
	tokenStream chan Token
	rewinds []Token
}
func (p *parser) next() Token {
	var token Token
	if len(p.rewinds) == 0 {
		token = <- p.tokenStream
	} else {
		token = p.rewinds[len(p.rewinds)-1]
		p.rewinds = p.rewinds[:len(p.rewinds)-1]
	}
	if token.typ == TokenErr {
		fmt.Println(token)
		panic("Lexing error")
	}
	return token
}
func (p *parser) rewind(token Token) {
	p.rewinds = append(p.rewinds, token)
}
func (p *parser) peek() Token {
	token := p.next()
	p.rewind(token)
	return token
}

var segmentTokens map[TokenType]bool = map[TokenType]bool {
	TokenHash: true,
	TokenAt: true,
	TokenDot: true,
	TokenLBrack: true,
}

func (p *parser) parseSubex() subex.SubexState {
	delim := p.next()
	if delim.typ != TokenSubstituteDelimiter {
		panic("Missing substitute delimiter")
	}
	subexProgramToken := p.next()
	if subexProgramToken.typ != TokenSubex {
		panic("Missing subex from substitution")
	}
	reader := subex.NewStringRuneReader(subexProgramToken.val)
	subexAST := subex.Parse(reader)
	subex := subex.CompileTransducer(subexAST)
	delim = p.next()
	if delim.typ != TokenSubstituteDelimiter {
		panic("Missing end substitute delimiter")
	}
	return subex
}

func (p *parser) parseBasicCommand(commandChar rune) Command {
	switch commandChar {
		case 'p':
			return PrintValueCommand{}
		case 'd':
			return DeleteAllCommand{}
		case 'n':
			return NextCommand{}
		case 'N':
			return AppendNextCommand{}
		case 's':
			subex := p.parseSubex()
			var next Command
			token := p.peek()
			switch token.typ {
				case TokenEOF, TokenRBrace:
					next = NoopCommand{}
				default:
					next = p.parseCommand()
			}
			return SubstituteCommand {
				subex: subex,
				next: next,
			}
		case 'o':
			return NoopCommand{}
		case 'x':
			return SwapXRegCommand{}
		case 'X':
			return AppendXRegCommand{}
		case 'k':
			return SwapPathCommand{}
		case 'K':
			return AppendPathCommand{}
		default:
			panic("Invalid command")
	}
}

func (p *parser) parseCommand() Command {
	token := p.next()
	switch token.typ {
		case TokenLBrace:
			commands := p.parseCommands()
			if p.next().typ != TokenRBrace {
				panic("Missing matching }")
			}
			return SequenceCommand {commands}
		case TokenCommand:
			commandChar, _, err := strings.NewReader(token.val).ReadRune()
			if err != nil {
				panic("Error reading a command character!?")
			}
			return p.parseBasicCommand(commandChar)
		default:
			panic("Invalid token, expected command")
	}
}

func (p *parser) parseCommands() []Command {
	var commands []Command
	for {
		nextToken := p.peek()
		if nextToken.typ == TokenEOF || nextToken.typ == TokenRBrace {
			return commands
		}
		commands = append(commands, p.parseCommand())
		endToken := p.peek()
		if endToken.typ == TokenEOF || endToken.typ == TokenRBrace {
			return commands
		}
	}
}

func Parse(tokens chan Token) []Command {
	p := parser {
		tokenStream: tokens,
	}
	return p.parseCommands()
}