begin
    % show some Hailstone Sequence related information                       %
    % calculates the length of the sequence generated by n,                  %
    % if showFirstAndLast is true, the first and last 4 elements of the      %
    % sequence are stored in first and last                                  %
    % hs holds a cache of the upbHs previously calculated sequence lengths   %
    % if showFirstAndLast is false, the cache will be used                   %
    procedure hailstone ( integer value  n
                        ; integer array  first, last ( * )
                        ; integer result length
                        ; integer array  hs          ( * )
                        ; integer value  upbHs
                        ; logical value  showFirstAndLast
                        ) ;
    if not showFirstAndLast and n <= upbHs and hs( n ) not = 0 then begin
        % no need to store the start and end of the sequence and we already  %
        % know the length of the sequence for n                              %
        length := hs( n )
        end
    else begin
        % must calculate the sequence length                                 %
        integer sv;
        for i := 1 until 4 do first( i ) := last( i ) := 0;
        length := 0;
        sv     := n;
        if sv > 0 then begin
            while begin
                length := length + 1;
                if showFirstAndLast then begin
                    if length <= 4 then first( length ) := sv;
                    for lPos := 1 until 3 do last( lPos ) := last( lPos + 1 );
                    last( 4 ) := sv
                    end
                else if sv <= upbHs and hs( sv ) not = 0 then begin
                    % have a known value                                 %
                    length := ( length + hs( sv ) ) - 1;
                    sv     := 1
                end ;
                sv not = 1
            end do begin
                sv := if odd( sv ) then ( 3 * sv ) + 1 else sv div 2
            end while_sv_ne_1 ;
            if n < upbHs then hs( n ) := length
        end if_sv_gt_0
    end hailstone ;
    begin
        % test the hailstone procedure                                       %
        integer HS_CACHE_SIZE;
        HS_CACHE_SIZE := 100000;
        begin
            integer array first, last ( 1 :: 4 );
            integer       length, maxLength, maxNumber;
            integer array hs          ( 1 :: HS_CACHE_SIZE );
            for i := 1 until HS_CACHE_SIZE do hs( i ) := 0;
            hailstone( 27, first, last, length, hs, HS_CACHE_SIZE, true );
            write( i_w := 1, s_w := 0
                 , "27: length ", length, ", first: ["
                 , first( 1 ), " ", first( 2 ), " ", first( 3 ), " ", first( 4 )
                 , "] last: ["
                 , last( 1 ), " ", last( 2 ), " ", last( 3 ), " ", last( 4 )
                 , "]"
                 );
            maxNumber := 0;
            maxLength := 0;
            for n := 1 until 100000 do begin
                hailstone( n, first, last, length, hs, HS_CACHE_SIZE, false );
                if length > maxLength then begin
                    maxNumber := n;
                    maxLength := length
                end if_length_gt_maxLength
            end for_n ;
            write( i_w := 1, s_w := 1, "Maximum sequence length: ", maxLength, " for: ", maxNumber )
        end
    end
end.