exercise: AllYourBase
version: 2
plan: 23
imports: '&convert-base'
tests: |
  
  for @($c-data<cases>) -> $case {
    if $case<expected> ~~ Array:D { test }
    else {
      given $case<description> {
        when 'empty list' { test [] }
        when /base|digit/ { throws-like {call-convert-base}, Exception, $_ }
        when /zero/ {
          when 'leading zeros' { test [4,2] }
          default { test [0] }
        }
        flunk "$_; not tested" if %*ENV<EXERCISM>; # To ensure that no canonical-data cases are missed.
      }
    }
  
    sub test (Array:D $expected = $case<expected>) {
      is-deeply call-convert-base, $expected, $case<description>
    }
  
    sub call-convert-base { convert-base(|$case<input_base input_digits output_base>) }
  }

unit: module
example: |
  class X::AllYourBase::InvalidBase is Exception {
    has $.payload;
    method message {"$!payload is not a valid base."}
  }
  
  class X::AllYourBase::InvalidDigit is Exception {
    has %.payload;
    method message {"%!payload<digit> is not a valid digit for base %!payload<base>."}
  }
  
  sub convert-base (Int:D $input-base, @input-digits, Int:D $output-base --> Array:D) is export {
    for $input-base, $output-base {
      X::AllYourBase::InvalidBase.new(payload => $_).throw if $_ < 2;
    }
    from-decimal($output-base, (to-decimal $input-base, @input-digits));
  }
  
  sub to-decimal ($input-base, @input-digits) {
    return [].Slip if !@input-digits;
    my $elems = @input-digits.elems;
    for @input-digits {
      if $_ == 0 { $elems-- }
      else { last }
    }
    my $dec = 0;
    loop (my $i = 0; $i < $elems; $i++) {
      if @input-digits.reverse[$i] < 0 || @input-digits.reverse[$i] >= $input-base {
        X::AllYourBase::InvalidDigit.new( :payload(base => $input-base, digit => @input-digits.reverse[$i]) ).throw;
      }
      $dec += @input-digits.reverse[$i] * $input-base ** $i;
    }
    return $dec;
  }
  
  sub from-decimal ($output-base, $dec) {
    my @output-digits;
    my $num = $dec;
    while $num >= $output-base {
      unshift @output-digits, $num % $output-base;
      $num div= $output-base;
    }
    unshift @output-digits, $num;
    return @output-digits;
  }