# Emit . and stop as soon as "condition" is true.
def until(condition; next):
  def u: if condition then . else (next|u) end;
  u;

def url_decode:
  # The helper function converts the input string written in the given
  # "base" to an integer
  def to_i(base):
    explode
    | reverse
    | map(if 65 <= . and . <= 90 then . + 32  else . end)   # downcase
    | map(if . > 96  then . - 87 else . - 48 end)  # "a" ~ 97 => 10 ~ 87
    | reduce .[] as $c
        # base: [power, ans]
        ([1,0]; (.[0] * base) as $b | [$b, .[1] + (.[0] * $c)]) | .[1];

  .  as $in
  | length as $length
  | [0, ""]  # i, answer
  | until ( .[0] >= $length;
      .[0] as $i
      |  if $in[$i:$i+1] == "%"
         then [ $i + 3, .[1] + ([$in[$i+1:$i+3] | to_i(16)] | implode) ]
         else [ $i + 1, .[1] + $in[$i:$i+1] ]
         end)
  | .[1];  # answer