#!/usr/bin/env ruby -w

require "strscan"

module Lox
  class Error < StandardError
    def initialize(line:, where: "", message:)
      @line, @where, @message = line, where, message
    end

    def to_s
      "[line #@line] Error#@where: #@message"
    end
  end

  def self.run_prompt
    loop do
      print "> "
      line = gets
      break if line.empty?
      begin
        run(line)
      rescue Error => e
        puts e.message
      end
    end
  end

  def self.run_file(io)
    run(io.read)
  rescue Error
    puts e.message
    exit 65
  end

  def self.run(src)
    Runner.new.run(src)
  end

  def self.error(line, msg)
    raise Error(line:, message:)
  end

  class Runner
    def initialize(scanner: Scanner.new)
      @scanner = scanner
    end

    def run(src)
      @scanner.scan(src).each do |token|
        puts token
      end
    end
  end

  class Scanner
    TOKENS = %w[
      (  LEFT_PAREN
      )  RIGHT_PAREN
      {  LEFT_BRACE
      }  RIGHT_BRACE
      ,  COMMA
      .  DOT
      -  MINUS
      +  PLUS
      ;  SEMICOLON
      *  STAR
      != BANG_EQUAL
      !  BANG
      == EQUAL_EQUAL
      =  EQUAL
      <= LESS_EQUAL
      <  LESS
      >= GREATER_EQUAL
      >  GREATER
      /  SLASH
    ].each_slice(2).to_h.transform_values(&:to_sym)
    TOKENS_RE = Regexp.union(TOKENS.keys)

    KEYWORDS = %w[
      and    AND
      class  CLASS
      else   ELSE
      false  FALSE
      for    FOR
      fun    FUN
      if     IF
      nil    NIL
      or     OR
      print  PRINT
      return RETURN
      super  SUPER
      this   THIS
      true   TRUE
      var    VAR
      while  WHILE
    ].each_slice(2).to_h.transform_values(&:to_sym)

    State = Struct.new(:ss, :tokens, :errors, :line) do
      def eos? = ss.eos?
      def scan(re) = ss.scan(re)
      def pos = ss.pos

      def add_token(type, text: nil, literal: nil)
        text ||= ss.matched
        self.tokens << Token.new(type, text, literal, line)
      end
    end

    def scan(src)
      state = State.new(StringScanner.new(src), [], [], 1)

      until state.eos?
        case
        when state.scan(/\/\/(?~\n)/)       # ignore line comment
        when state.scan(/\/\*/)
          scan_block_comment(state)
        when matched = state.scan(TOKENS_RE)
          state.add_token(TOKENS.fetch(matched))
        when state.scan(/[ \r\t]/)    # ignore whitespace
        when state.scan(/\n/)
          state.line += 1
        when state.scan(/"/)
          scan_str(state)
        when number = state.scan(/\d+(\.\d+)?/)
          state.add_token(:NUMBER, literal: number.to_f)
        when identifier = state.scan(/[a-zA-Z_]\w*/)
          type = KEYWORDS.fetch(identifier, :IDENTIFIER)
          state.add_token(type)
        else state.scan(/./) # keep scanning
          state.errors << Error.new(line: state.line, message: "Unexpected character.")
        end
      end

      state.add_token(:EOF, text: "")
      state.tokens
    end

    private

    def scan_str(state)
      text = ?"
      loop do
        case
        when state.scan(/"/)
          text << ?"
          state.add_token(:STRING, text:, literal: text[1..-2])
          return
        when state.scan(/\n/)
          text << ?\n
          state.line += 1
        when state.eos?
          state.errors << Error.new(line: state.line, message: "Unterminated string.")
          return
        when c = state.scan(/(?~"|\n)/)
          text << c
        else
          fail "unreachable!"
        end
      end
    end

    def scan_block_comment(state)
      loop do
        case
        when state.scan(/\/\*/)
          scan_block_comment(state)
        when state.scan(/\*\//)
          return
        when state.scan(/\n/)
          state.line += 1
        when state.eos?
          state.errors << Error.new(line: state.line, message: "Unterminated block comment.")
          return
        when state.scan(/./)
          # no-op
        else
          fail "unreachable!"
        end
      end
    end
  end

  Token = Struct.new(:type, :lexeme, :literal, :line) do
    def to_s
      "#{type} #{lexeme} #{literal}"
    end
  end
end

if __FILE__ == $0
  puts "Usage: #$0 [script]" or exit 64 if ARGV.length > 1

  if ARGV.empty?
    Lox.run_prompt
  else
    Lox.run_file(ARGF)
  end
end