Reward each storm cleared

Every storm’s finish bonus has been the same deal: ten points for each second under 100. A fast fifth storm paid no more than a fast first one, although the player had to survive four storms to reach it. In this lesson each storm’s bonus is multiplied by the storm’s number: ×1 for the first, up to ×5 for the fifth.

The Z80 has no multiply instruction. Here we multiply the way it is usually done for small numbers: by adding the same thing several times.

Space launches; O/P steer; hold Space for 2X. One hit ends the voyage. At the result R retries from the first storm and Q returns to the title.

Cross a storm and read the interlude: the bonus line now says what it is multiplied by.

What you should see

The interludes read FINISH BONUS 0800 X1, then X2 and so on, and the score jumps further each time. The title’s bonus line now reads FAST FINISH: BONUS X STORM.

Multiplying by adding

Here is where storm_cleared adds the bonus:

finish_bonus:
 ld (finish_points),a
 ; Storm n's bonus counts n times: add it once for each storm so far.
 ld e,a
 ld d,0
 ld a,(storm)
 inc a
 ld b,a
 ld hl,(score)
bonus_add:
 add hl,de
 djnz bonus_add
 ld (score),hl
 ld hl,(elapsed)

The bonus goes into DE as before. Then storm, which counts from 0, plus 1 goes into B: 1 for the first storm, 5 for the fifth. The loop at bonus_add adds DE to HL, and djnz counts B down and jumps back until it reaches zero. So the bonus is added B times: the bonus times the storm’s number.

That is how unit 14 found an event’s address, with five add hl,de instructions in a row, and how unit 29 found a course with two. There the multiplier was fixed and the additions were written out. Here it changes with the storm, so a loop does the counting.

Repeated addition is only sensible because the multiplier is small. The largest bonus is 99 tens, for a storm under a second, and five additions of it make 495 tens: unit 30’s two-byte score has room to spare.

Naming the multiplier

A bonus that changes from storm to storm needs explaining on screen. bonus_line writes the storm’s digit into the line before it prints it:

bonus_line:
 ld a,(storm)
 add a,'1'
 ld (bonus_storm),a
 ld a,(finish_points)
 ld hl,bonus_digits
 call decimal3
 ld bc,$0C05
 ld de,bonus_text
 jp text

storm plus the character code of '1' gives '1' to '5', the trick hud uses for STORM 3/5. The text it lands in is three pieces with labels, so the program can write into the middle of it:

bonus_digits: defb "0000"
bonus_times: defb " X"
bonus_storm: defb "1",0

bonus_text, the digits and bonus_times have no zero byte between them, so text prints straight through all three and stops at the zero after bonus_storm.

One more change keeps the line honest. start_storm now sets finish_points to 0 when a storm begins. Before this, a ship lost in the third storm showed the second storm’s bonus on the result screen with the third storm’s number beside it. Now it shows FINISH BONUS 0000 X3: nothing earned in the storm it was lost in.

How much more

On the route the checks fly, with Space held, the storms and their bonuses come out like this:

The route and the stars are the same as in unit 30, so the only difference is the multiplied bonuses: 801 tens more than before. The voyage that ended at 05140 now ends at 13150.

Experiments

Keep a working copy before each change, then press Revert when you have finished.

Then make the bonus count for the storms still to come instead: ld a,STORMS and sub the storm number, so the first storm counts five times and the last once. Which voyage does that reward?

The score now rewards getting further. The storms themselves are still alike: the same speeds, the same density, the same colours. The next lesson makes each storm harder than the last.