#!/usr/bin/env bash
# vim: set ft=ruby:

# This file executes as a bash script, which turns around and executes Ruby via
# the line below. The -x argument to Ruby makes it discard everything before
# the second "!ruby" shebang. This allows us to work on Linux, where the
# shebang can only have one argument so we can't directly say
# "#!/usr/bin/env ruby --disable-gems". Thanks for that, Linux.
#
# If this seems confusing, don't worry. You can treat it as a normal Ruby file
# starting with the "!ruby" shebang below.

exec /usr/bin/env ruby --disable-gems -x "$0" $*
#!ruby

if RUBY_VERSION < '1.9.3'
  abort "error: Selecta requires Ruby 1.9.3 or higher."
end

require "optparse"
require "io/console"

KEY_CTRL_N = ?\C-n
KEY_CTRL_P = ?\C-p
KEY_CTRL_U = ?\C-u
KEY_CTRL_H = ?\C-h
KEY_CTRL_W = ?\C-w
KEY_CTRL_J = ?\C-j
KEY_CTRL_M = ?\C-m
KEY_DELETE = 127.chr # Equivalent to ?\C-?
KEY_ESC    = ?\C-[

class Selecta
  VERSION = [0, 0, 2]

  def main
    # We have to parse options before setting up the screen or trying to read
    # the input in case the user did '-h', an invalid option, etc. and we need
    # to terminate.
    options = Configuration.parse_options(ARGV)

    search = Screen.with_screen do |screen, tty|
      config = Configuration.from_inputs($stdin.readlines, options, screen.height)
      run_in_screen(config, screen, tty)
    end

    if search.selection == Search::NoSelection
      exit(1)
    else
      puts search.selection
    end
  rescue ScreenValidator::NotATTY
    $stderr.puts(
      "Can't get a working TTY. Selecta requires an ANSI-compatible terminal.")
    exit(1)
  end

  def run_in_screen(config, screen, tty)
    search = Search.blank(config)

    # We emit the number of lines we'll use later so we don't clobber whatever
    # was already on the screen.
    config.visible_choices.times { tty.puts }
    begin
      search = ui_event_loop(search, screen, tty)
    ensure
      # Always move the cursor to the bottom so the next program doesn't draw
      # over whatever we left on the screen.
      screen.move_cursor(screen.height - 1, 0)
    end
    search
  end

  # Use the search and screen to process user actions until they quit.
  def ui_event_loop(search, screen, tty)
    while !search.done?
      Renderer.render!(search, screen)
      search = handle_key(search, tty.get_char)
    end
    search
  end

  # On each keystroke, generate a new search object
  def handle_key(search, key)
    case key

    when KEY_CTRL_N then search.down
    when KEY_CTRL_P then search.up

    when KEY_CTRL_U then search.clear_query
    when KEY_CTRL_W then search.delete_word
    when KEY_CTRL_H, KEY_DELETE then search.backspace

    when ?\r, KEY_CTRL_J, KEY_CTRL_M then search.done

    when KEY_ESC then search.abort

    when /[[:print:]]/ then search.append_search_string(key.chr)

    else search
    end
  end
end

class Configuration < Struct.new(:visible_choices, :initial_search, :choices)
  def initialize(visible_choices, initialize, choices)
    # Constructor is defined to force argument presence; otherwise Struct
    # defaults missing arguments to nil
    super
  end

  def self.from_inputs(choices, options, screen_height=21)
    # Shrink the number of visible choices if the screen is too small
    visible_choices = [20, screen_height - 1].min

    choices = massage_choices(choices)
    Configuration.new(visible_choices, options.fetch(:search), choices)
  end

  def self.default_options
    parse_options([])
  end

  def self.parse_options(argv)
    options = {}

    parser = OptionParser.new do |opts|
      opts.banner = "Usage: #{$PROGRAM_NAME} [options]"

      opts.on_tail("-h", "--help", "Show this message") do |v|
        puts opts
        exit
      end

      opts.on_tail("--version", "Show version") do
        puts Selecta::VERSION.join('.')
        exit
      end

      options[:search] = ""
      opts.on("-s", "--search SEARCH", "Specify an initial search string") do |search|
        options[:search] = search
      end
    end

    begin
      parser.parse!(argv)
    rescue OptionParser::InvalidOption => e
      $stderr.puts e
      $stderr.puts parser
      exit 1
    end

    options
  end

  def self.massage_choices(choices)
    choices.map do |choice|
      # Encoding to UTF-8 with `:invalid => :replace` isn't good enough; it
      # still leaves some invalid characters. For example, this string will fail:
      #
      # echo "девуш\xD0:" | selecta
      #
      # Round-tripping through UTF-16, with `:invalid => :replace` as well,
      # fixes this. I don't understand why. I found it via:
      #
      # http://stackoverflow.com/questions/2982677/ruby-1-9-invalid-byte-sequence-in-utf-8
      if choice.valid_encoding?
        choice
      else
        utf16 = choice.encode('UTF-16', 'UTF-8', :invalid => :replace, :replace => '')
        utf16.encode('UTF-8', 'UTF-16')
      end.strip
    end
  end
end

class Search
  attr_reader :choices, :index, :query, :config, :matches

  def initialize(vars)
    @config = vars.fetch(:config)
    @choices = vars.fetch(:choices)
    @index = vars.fetch(:index)
    @query = vars.fetch(:query)
    @done = vars.fetch(:done)
    @aborted = vars.fetch(:aborted)

    # Lazily compute matches if there aren't any
    @matches = vars.fetch(:matches) { compute_matches }
    @vars = vars.merge(:matches => @matches)
  end

  def self.blank(config)
    new(:config => config,
        :choices => config.choices,
        :index => 0,
        :query => config.initial_search,
        :done => false,
        :aborted => false)
  end

  # Construct a new Search by merging in a hash of changes.
  def merge(changes)
    vars = @vars.merge(changes)

    # If the query changed, throw away the old matches so that new ones will be
    # computed.
    matches_are_stale = vars.fetch(:query) != @query
    if matches_are_stale
      vars = vars.reject { |key| key == :matches }
    end

    Search.new(vars)
  end

  def done?
    @done || @aborted
  end

  def selection
    if @aborted
      NoSelection
    else
      matches.fetch(@index) { NoSelection }
    end
  end

  def down
    index = (@index + 1) % max_visible_choices
    merge(:index => index)
  end

  def up
    index = (@index - 1) % max_visible_choices
    merge(:index => index)
  end

  def max_visible_choices
    [@config.visible_choices, choices.count].min
  end

  def append_search_string(string)
    merge(:index => 0,
          :query => @query + string)
  end

  def backspace
    merge(:index => 0,
          :query => @query[0...-1])
  end

  def clear_query
    merge(:index => 0,
          :query => "")
  end

  def delete_word
    merge(:index => 0,
          :query => @query.sub(/[^ ]* *$/, ""))
  end

  def done
    merge(:done => true)
  end

  def abort
    merge(:aborted => true)
  end

  private

  def compute_matches
    @choices.map do |choice|
      [choice, Score.score(choice, @query)]
    end.select do |choice, score|
      score > 0.0
    end.sort_by do |choice, score|
      -score
    end.map do |choice, score|
      choice
    end
  end

  class NoSelection; end
end

class Score
  class << self
    def score(choice, query)
      return 1.0 if query.length == 0
      return 0.0 if choice.length == 0

      choice = choice.downcase
      query = query.downcase

      match_length = compute_match_length(choice, query.each_char.to_a)
      return 0.0 unless match_length

      # Penalize longer matches.
      score = query.length.to_f / match_length.to_f

      # Normalize vs. the length of the choice, penalizing longer strings.
      score / choice.length
    end

    # Find the length of the shortest substring matching the given characters.
    def compute_match_length(string, chars)
      first_char, *rest = chars
      first_indexes = find_char_in_string(string, first_char)

      first_indexes.map do |first_index|
        last_index = find_end_of_match(string, rest, first_index)
        if last_index
          last_index - first_index + 1
        else
          nil
        end
      end.compact.min
    end

    # Find all occurrences of the character in the string, returning their indexes.
    def find_char_in_string(string, char)
      index = 0
      indexes = []
      while index
        index = string.index(char, index)
        if index
          indexes << index
          index += 1
        end
      end
      indexes
    end

    # Find each of the characters in the string, moving strictly left to right.
    def find_end_of_match(string, chars, first_index)
      last_index = first_index
      chars.each do |this_char|
        index = string.index(this_char, last_index + 1)
        return nil unless index
        last_index = index
      end
      last_index
    end
  end
end

class Renderer < Struct.new(:search)
  def self.render!(search, screen)
    rendered = Renderer.new(search).render
    start_line = screen.height - search.config.visible_choices - 1
    screen.with_cursor_hidden do
      screen.write_lines(start_line, rendered.choices)
      screen.move_cursor(start_line, rendered.search_line.length)
    end
  end

  def render
    index, matches = search.index, search.matches
    search_line = "> " + search.query
    unless matches.empty?
      selection_line = Text[:inverse, matches.fetch(index), :reset]
      matches = replace_array_element(matches, index, selection_line)
    end
    matches = correct_match_count(matches)
    lines = [search_line] + matches
    Rendered.new(lines, search_line)
  end

  def correct_match_count(matches)
    limited = matches[0, search.config.visible_choices]
    padded = limited + [""] * (search.config.visible_choices - limited.length)
    padded
  end

  class Rendered < Struct.new(:choices, :search_line)
  end

  private

  def replace_array_element(array, index, new_value)
    array = array.dup
    array[index] = new_value
    array
  end
end

class Screen
  def self.with_screen
    TTY.with_tty do |tty|
      screen = self.new(tty)
      screen.configure_tty
      begin
        ScreenValidator.raise_unless_screen_is_valid(screen.height)
        yield screen, tty
      ensure
        screen.restore_tty
        tty.puts
      end
    end
  end

  attr_reader :tty, :ansi

  def initialize(tty)
    @tty = tty
    @ansi = ANSI.new(tty.out_file)
    @original_stty_state = tty.stty("-g")
  end


  def configure_tty
    # -echo: terminal doesn't echo typed characters back to the terminal
    # -icanon: terminal doesn't  interpret special characters (like backspace)
    tty.stty("-echo -icanon")
  end

  def restore_tty
    tty.stty("#{@original_stty_state}")
  end

  def suspend
    restore_tty
    begin
      yield
      configure_tty
    rescue
      restore_tty
    end
  end

  def with_cursor_hidden(&block)
    ansi.hide_cursor!
    begin
      block.call
    ensure
      ansi.show_cursor!
    end
  end

  def height
    size[0]
  end

  def width
    size[1]
  end

  def size
    height, width = tty.winsize
    [height, width]
  end

  def move_cursor(line, column)
    ansi.setpos!(line, column)
  end

  def write_line(line, text)
    write(line, 0, text)
  end

  def write_lines(line, texts)
    texts.each_with_index do |text, index|
      write(line + index, 0, text)
    end
  end

  def write(line, column, text)
    # Discard writes outside the main screen area
    write_unrestricted(line, column, text) if line < height
  end

  def write_unrestricted(line, column, text)
    text = Text[:default, text] unless text.is_a? Text
    write_text_object(line, column, text)
  end

  def write_text_object(line, column, text)
    # Blank the line before drawing to it
    ansi.setpos!(line, 0)
    ansi.addstr!(" " * width)

    text.components.each do |component|
      if component.is_a? String
        ansi.setpos!(line, column)
        # Don't draw off the edge of the screen.
        #   - width - 1 is the last column we have (zero-indexed)
        #   - subtract the current column from that to get the number of
        #   columns we have left.
        chars_to_draw = [0, width - 1 - column].max
        component = component[0..chars_to_draw]
        ansi.addstr!(component)
        column += component.length
      elsif component == :inverse
        ansi.inverse!
      elsif component == :reset
        ansi.reset!
      else
        if component =~ /_/
          fg, bg = component.to_s.split(/_/).map(&:to_sym)
        else
          fg, bg = component, :default
        end
        ansi.color!(fg, bg)
      end
    end
  end
end

class ScreenValidator
  def self.raise_unless_screen_is_valid(screen_height)
    raise NotATTY if screen_height == 0
  end

  class NotATTY < RuntimeError; end
end

class Text
  attr_reader :components

  def self.[](*args)
    new(args)
  end

  def initialize(components)
    @components = components
  end

  def ==(other)
    components == other.components
  end

  def +(other)
    Text[*(components + other.components)]
  end
end

class ANSI
  ESC = 27.chr

  attr_reader :file

  def initialize(file)
    @file = file
  end

  def escape(sequence)
    ESC + "[" + sequence
  end

  def clear
    escape "2J"
  end

  def hide_cursor
    escape "?25l"
  end

  def show_cursor
    escape "?25h"
  end

  def setpos(line, column)
    escape "#{line + 1};#{column + 1}H"
  end

  def addstr(str)
    str
  end

  def color(fg, bg=:default)
    fg_codes = {
      :black => 30,
      :red => 31,
      :green => 32,
      :yellow => 33,
      :blue => 34,
      :magenta => 35,
      :cyan => 36,
      :white => 37,
      :default => 39,
    }
    bg_codes = {
      :black => 40,
      :red => 41,
      :green => 42,
      :yellow => 43,
      :blue => 44,
      :magenta => 45,
      :cyan => 46,
      :white => 47,
      :default => 49,
    }
    fg_code = fg_codes.fetch(fg)
    bg_code = bg_codes.fetch(bg)
    escape "#{fg_code};#{bg_code}m"
  end

  def inverse
    escape("7m")
  end

  def reset
    escape("0m")
  end

  def clear!(*args); write clear(*args); end
  def setpos!(*args); write setpos(*args); end
  def addstr!(*args); write addstr(*args); end
  def color!(*args); write color(*args); end
  def inverse!(*args); write inverse(*args); end
  def reset!(*args); write reset(*args); end
  def hide_cursor!(*args); write hide_cursor(*args); end
  def show_cursor!(*args); write show_cursor(*args); end

  def write(bytes)
    file.write(bytes)
  end
end

class TTY < Struct.new(:in_file, :out_file)
  def self.with_tty(&block)
    File.open("/dev/tty", "r") do |in_file|
      File.open("/dev/tty", "w") do |out_file|
        tty = TTY.new(in_file, out_file)
        block.call(tty)
      end
    end
  end

  def get_char
    in_file.getc
  end

  def puts
    out_file.puts
  end

  def winsize
    out_file.winsize
  end

  def stty(args)
    command("stty #{args}")
  end

  private

  # Run a command with the TTY as stdin, capturing the output via a pipe
  def command(command)
    IO.pipe do |read_io, write_io|
      pid = Process.spawn(command, :in => "/dev/tty", :out => write_io)
      Process.wait(pid)
      raise "Command failed: #{command.inspect}" unless $?.success?
      write_io.close
      read_io.read
    end
  end
end

if $0 == __FILE__
  begin
    Selecta.new.main
  rescue Interrupt
    exit(1)
  end
end