package main

import (
	"fmt"
	"main/subex"
	"strings"
	"unicode/utf8"
)

type parser struct {
	tokenStream chan Token
	rewinds []Token
	labels map[rune]int
}
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
}

func (p *parser) parseSubex() subex.SubexAST {
	delim := p.next()
	if delim.typ != TokenSubstituteDelimiter {
		panic("Missing substitute delimiter")
	}
	subexProgramToken := p.next()
	if subexProgramToken.typ != TokenSubex {
		panic("Missing subex from substitution")
	}
	var subexProgram string
	if delim.val == "=" || delim.val == "~" || delim.val == "\"" || delim.val == "`" || delim.val == "^" {
		subexProgram = delim.val + subexProgramToken.val + delim.val
	} else {
		subexProgram = subexProgramToken.val
	}
	reader := subex.NewStringRuneReader(subexProgram)
	subexAST := subex.Parse(reader)
	delim = p.next()
	if delim.typ != TokenSubstituteDelimiter {
		panic("Missing end substitute delimiter")
	}
	return subexAST
}

func (p *parser) parseBasicCommand(commands []Command, commandChar rune) []Command {
	switch commandChar {
		case 'p':
			return append(commands, PrintValueCommand{})
		case 'd':
			return append(commands, DeleteValueCommand{})
		case 'n':
			return append(commands, NextCommand{})
		case 'N':
			return append(commands, AppendNextCommand{})
		case 's':
			ast := p.parseSubex()
			subex := subex.CompileTransducer(ast)
			return append(commands, SubstituteValueCommand {subex}, JumpCommand {len(commands) + 3})
		case 'o':
			return append(commands, NoopCommand{})
		case 'x':
			return append(commands, SwapXRegCommand{})
		case 'X':
			return append(commands, AppendXRegCommand{})
		case 'y':
			return append(commands, SwapYRegCommand{})
		case 'Y':
			return append(commands, AppendYRegCommand{})
		case 'z':
			return append(commands, SwapZRegCommand{})
		case 'Z':
			return append(commands, AppendZRegCommand{})
		case ':':
			labelToken := p.next()
			if labelToken.typ != TokenLabel {
				panic("Missing branch label")
			}
			label, _ := utf8.DecodeRuneInString(labelToken.val)
			p.labels[label] = len(commands)
			return commands
		case 'b':
			labelToken := p.next()
			if labelToken.typ != TokenLabel {
				panic("Missing branch label")
			}
			label, _ := utf8.DecodeRuneInString(labelToken.val)
			return append(commands, BranchPlaceholderCommand {label})
		default:
			panic("Invalid command")
	}
}

func (p *parser) parseCommand(commands []Command) []Command {
	token := p.next()
	switch token.typ {
		case TokenLBrace:
			jumpToBlockCommand := &JumpCommand{0}
			commands = append(commands, JumpCommand {len(commands) + 2}, jumpToBlockCommand)
			commands = p.parseCommands(commands)
			if p.next().typ != TokenRBrace {
				panic("Missing matching }")
			}
			jumpToBlockCommand.destination = len(commands)
			return commands
		case TokenCommand:
			commandChar, _, err := strings.NewReader(token.val).ReadRune()
			if err != nil {
				panic("Error reading a command character!?")
			}
			return p.parseBasicCommand(commands, commandChar)
		default:
			panic("Invalid token, expected command")
	}
}

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

func Parse(tokens chan Token) []Command {
	p := parser {
		tokenStream: tokens,
		rewinds: nil,
		labels: make(map[rune]int),
	}
	program := p.parseCommands(nil)
	for i, command := range program {
		switch branch := command.(type) {
			case BranchPlaceholderCommand:
				destination, exists := p.labels[branch.label]
				if !exists {
					panic("Tried to branch to a label that doesn't exist")
				}
				program[i] = JumpCommand {destination}
		}
	}
	return program
}