alias SigProc = Proc(Array(String), Int32, Array(String))

def fetch_decrypt_function(id = "CvFH_6DNRCY")
  document = YT_POOL.client &.get("/watch?v=#{id}&gl=US&hl=en&disable_polymer=1").body
  url = document.match(/src="(?<url>\/yts\/jsbin\/player_ias-[^\/]+\/en_US\/base.js)"/).not_nil!["url"]
  player = YT_POOL.client &.get(url).body

  function_name = player.match(/^(?<name>[^=]+)=function\(\w\){\w=\w\.split\(""\);[^\. ]+\.[^( ]+/m).not_nil!["name"]
  function_body = player.match(/^#{Regex.escape(function_name)}=function\(\w\){(?<body>[^}]+)}/m).not_nil!["body"]
  function_body = function_body.split(";")[1..-2]

  var_name = function_body[0][0, 2]
  var_body = player.delete("\n").match(/var #{Regex.escape(var_name)}={(?<body>(.*?))};/).not_nil!["body"]

  operations = {} of String => SigProc
  var_body.split("},").each do |operation|
    op_name = operation.match(/^[^:]+/).not_nil![0]
    op_body = operation.match(/\{[^}]+/).not_nil![0]

    case op_body
    when "{a.reverse()"
      operations[op_name] = ->(a : Array(String), b : Int32) { a.reverse }
    when "{a.splice(0,b)"
      operations[op_name] = ->(a : Array(String), b : Int32) { a.delete_at(0..(b - 1)); a }
    else
      operations[op_name] = ->(a : Array(String), b : Int32) { c = a[0]; a[0] = a[b % a.size]; a[b % a.size] = c; a }
    end
  end

  decrypt_function = [] of {SigProc, Int32}
  function_body.each do |function|
    function = function.lchop(var_name).delete("[].")

    op_name = function.match(/[^\(]+/).not_nil![0]
    value = function.match(/\(\w,(?<value>[\d]+)\)/).not_nil!["value"].to_i

    decrypt_function << {operations[op_name], value}
  end

  return decrypt_function
end

def decrypt_signature(fmt, op)
  return "" if !fmt["s"]? || !fmt["sp"]?

  sp = fmt["sp"]
  sig = fmt["s"].split("")
  op.each do |proc, value|
    sig = proc.call(sig, value)
  end

  return "&#{sp}=#{sig.join("")}"
end
