package main

import (
	"strings"
	"strconv"
	"fmt"
)

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,
	TokenLParen: true,
}

func (p *parser) parsePathPatternFilter(minPower int) PathFilterAST {
	var lhs PathFilterAST
	token := p.next()
	switch token.typ {
		case TokenHash:
			stringIndex := p.next()
			if stringIndex.typ != TokenPatternStringIndex {
				panic("Expected string index after # in pattern")
			}
			lhs = StringSegmentPathFilterAST{stringIndex.val}
		case TokenAt:
			intIndex := p.next()
			if intIndex.typ != TokenPatternIntegerIndex {
				panic("Expected integer index after @ in pattern")
			}
			index, err := strconv.Atoi(intIndex.val)
			if err != nil {
				panic("Expected integer index after @ in pattern")
			}
			lhs = IntegerSegmentPathFilterAST{index}
		case TokenDot:
			lhs = AnySegmentPathFilterAST{}
		case TokenLParen:
			lhs = p.parsePathPatternFilter(0)
			if p.next().typ != TokenRParen {
				panic("Expected )")
			}
		default:
			panic("Expected path pattern filter segment")
	}
	loop: for {
		token = p.next()
		switch {
			case token.typ == TokenAst && 10 >= minPower:
				lhs = RepeatPathFilterAST {lhs}
			case token.typ == TokenQuestion && 10 >= minPower:
				lhs = OrPathFilterAST{lhs, NonePathFilterAST{}}
			case token.typ == TokenBar && 0 >= minPower:
				lhs = OrPathFilterAST{lhs, p.parsePathPatternFilter(1)}
			case segmentTokens[token.typ] && 2 >= minPower:
				p.rewind(token)
				lhs = SequencePathFilterAST {lhs, p.parsePathPatternFilter(3)}
			default:
				p.rewind(token)
				break loop
		}
	}
	return lhs
}

// TODO: should only return a single filter
func (p *parser) parseFilter() []Filter {
	var filters []Filter
	token := p.next()
	switch token.typ {
		case TokenHash, TokenAt, TokenDot:
			p.rewind(token)
			filterAst := p.parsePathPatternFilter(0)
			filters = append(filters, compilePathFilterAST(filterAst))
			token = p.next()
	}
	if len(filters) == 0 {
		panic("Missing filter")
	}
	p.rewind(token)
	return filters
}

func (p *parser) parseBasicCommand(commandChar rune) Command {
	switch commandChar {
		case 'p':
			return PrintValueCommand{}
		case 'd':
			return DeleteAllCommand{}
		default:
			panic("Invalid command")
	}
}

func (p *parser) parseCommand() Command {
	token := p.next()
	switch token.typ {
		case TokenHash, TokenAt, TokenDot:
			p.rewind(token)
			filters := p.parseFilter()
			command := p.parseCommand()
			for _, filter := range filters {
				command = FilteredCommand {
					filter: filter,
					command: command,
				}
			}
			return command
		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())
		semicolon := p.next()
		if semicolon.typ == TokenEOF || semicolon.typ == TokenRBrace {
			return commands
		}
		if semicolon.typ != TokenSemicolon {
			panic("Expected ; after command")
		}
	}
}

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