A guard who waits for your keypress is easy to outwit: do nothing. Our guard needs a schedule of its own.
Continue from Take the file home. For now, touching the guard causes capture. Facing is visible, but distant sight has not been added.
Replace lines 100, 140, 370, 470, 7000.
Add lines 120, 130, 150, 230, 240, 400, 410, 430, 440, 455, 460, 2050, 3300, 4010, 7010, 7020.
There are no lines to delete. Enter the changed lines below; their numbers replace existing lines or insert new ones.
100 LET x=3: LET y=16
120 LET gx=7: LET gy=5: LET gd=1
130 LET got=0: LET outcome=0: LET beat=0: LET steps=0
140 GO SUB 1000: GO SUB 3300: GO SUB 2500
150 LET tick=PEEK 23672
230 LET elapsed=PEEK 23672-tick: IF elapsed<0 THEN LET elapsed=elapsed+256
240 IF elapsed<8 THEN GO TO 200
370 IF x=gx AND y=gy THEN LET outcome=1: GO TO 4000
400 LET beat=beat+1: IF beat<2 THEN GO TO 470
410 LET beat=0: LET r=gy: LET c=gx
430 IF (gd=1 AND gx=24) OR (gd=2 AND gy=14) OR (gd=3 AND gx=7) OR (gd=4 AND gy=5) THEN LET gd=gd+1: IF gd=5 THEN LET gd=1
440 LET gx=gx+a(gd): LET gy=gy+b(gd): GO SUB 2000
455 GO SUB 3300
460 IF x=gx AND y=gy THEN LET outcome=1: GO TO 4000
470 LET tick=PEEK 23672: GO TO 200
2050 IF r=gy AND c=gx THEN LET z$=g$(gd): LET colour=6
3300 LET r=gy: LET c=gx: GO SUB 2000: RETURN
4010 IF outcome=1 THEN PRINT AT 1,1; INK 2; BRIGHT 1;"CAUGHT! Keep clear of the guard."
7000 RESTORE 7500: FOR j=0 TO 31: READ a: POKE USR "a"+j,a: NEXT j
7010 DIM a(4): DIM b(4): LET a(1)=1: LET b(2)=1: LET a(3)=-1: LET b(4)=-1
7020 LET g$=">v<"+CHR$ 94: RETURNComplete program at this stage
10 GO SUB 7000: GO SUB 7100
100 LET x=3: LET y=16
120 LET gx=7: LET gy=5: LET gd=1
130 LET got=0: LET outcome=0: LET beat=0: LET steps=0
140 GO SUB 1000: GO SUB 3300: GO SUB 2500
150 LET tick=PEEK 23672
200 LET k$=INKEY$
210 IF k$="q" OR k$="Q" THEN GO TO 9000
220 IF k$="r" OR k$="R" THEN GO SUB 8100: GO TO 100
230 LET elapsed=PEEK 23672-tick: IF elapsed<0 THEN LET elapsed=elapsed+256
240 IF elapsed<8 THEN GO TO 200
250 LET k$=INKEY$
260 LET dx=0: LET dy=0
270 IF k$="i" OR k$="I" THEN LET dy=-1
280 IF k$="k" OR k$="K" THEN LET dy=1
290 IF k$="j" OR k$="J" THEN LET dx=-1
300 IF k$="l" OR k$="L" THEN LET dx=1
310 LET nx=x+dx: LET ny=y+dy
320 IF m$(ny,nx)=CHR$ 145 THEN GO TO 370
330 IF nx=x AND ny=y THEN GO TO 370
340 LET r=y: LET c=x: LET x=nx: LET y=ny: GO SUB 2000
350 LET steps=steps+1: LET r=y: LET c=x: GO SUB 2000
370 IF x=gx AND y=gy THEN LET outcome=1: GO TO 4000
380 IF x=27 AND y=3 AND got=0 THEN LET got=1: GO SUB 2500
390 IF x=3 AND y=16 AND got=1 THEN LET outcome=2: GO TO 4000
400 LET beat=beat+1: IF beat<2 THEN GO TO 470
410 LET beat=0: LET r=gy: LET c=gx
430 IF (gd=1 AND gx=24) OR (gd=2 AND gy=14) OR (gd=3 AND gx=7) OR (gd=4 AND gy=5) THEN LET gd=gd+1: IF gd=5 THEN LET gd=1
440 LET gx=gx+a(gd): LET gy=gy+b(gd): GO SUB 2000
455 GO SUB 3300
460 IF x=gx AND y=gy THEN LET outcome=1: GO TO 4000
470 LET tick=PEEK 23672: GO TO 200
1000 BORDER 0: PAPER 0: INK 7: BRIGHT 0: CLS
1010 PRINT AT 0,1; INK 5; BRIGHT 1;"NIGHT PATROL";AT 0,20;"THE ARCHIVE"
1020 FOR r=1 TO 18: PRINT AT r+1,1; INK 1; BRIGHT 1;m$(r): NEXT r
1022 LET r=3: LET c=27: GO SUB 2000
1024 LET r=y: LET c=x: GO SUB 2000
1030 PRINT AT 20,1;"I up J left K down L right"
1040 PRINT AT 21,1;"R retry Q quit";
1050 RETURN
2000 LET z$=" ": LET colour=7: LET shade=0
2010 IF m$(r,c)=CHR$ 145 THEN LET z$=CHR$ 145: LET colour=1
2030 IF r=3 AND c=27 AND got=0 THEN LET z$=CHR$ 146: LET colour=5
2040 IF r=16 AND c=3 THEN LET z$=CHR$ 147: LET colour=4
2050 IF r=gy AND c=gx THEN LET z$=g$(gd): LET colour=6
2060 IF r=y AND c=x THEN LET z$=CHR$ 144: LET colour=7
2080 PRINT AT r+1,c; PAPER shade; INK colour; BRIGHT 1;z$: RETURN
2500 PRINT AT 1,1; INK 5;"TAKE THE FILE. RETURN TO EXIT. "
2510 IF got=1 THEN PRINT AT 1,1; INK 4;"FILE TAKEN. GET BACK TO EXIT. "
2520 RETURN
3300 LET r=gy: LET c=gx: GO SUB 2000: RETURN
4000 LET r=y: LET c=x: GO SUB 2000
4010 IF outcome=1 THEN PRINT AT 1,1; INK 2; BRIGHT 1;"CAUGHT! Keep clear of the guard."
4020 IF outcome=2 THEN PRINT AT 1,1; INK 4; BRIGHT 1;"FILE RECOVERED. CLEAN GETAWAY! "
4030 PRINT AT 20,1;"R tries again. Q quits. "
4040 GO SUB 8000
4050 IF k$="r" OR k$="R" THEN GO SUB 8100: GO TO 100
4060 IF k$="q" OR k$="Q" THEN GO TO 9000
4070 GO TO 4040
7000 RESTORE 7500: FOR j=0 TO 31: READ a: POKE USR "a"+j,a: NEXT j
7010 DIM a(4): DIM b(4): LET a(1)=1: LET b(2)=1: LET a(3)=-1: LET b(4)=-1
7020 LET g$=">v<"+CHR$ 94: RETURN
7100 DIM m$(18,30): RESTORE 7200
7110 FOR r=1 TO 18: READ m$(r): FOR c=1 TO 30
7120 IF m$(r,c)="#" THEN LET m$(r,c)=CHR$ 145
7130 NEXT c: NEXT r: RETURN
7200 DATA "##############################"
7210 DATA "##############################"
7220 DATA "########### ############# ###"
7230 DATA "##### ####### # ###"
7240 DATA "##### ###"
7250 DATA "##### ###### # #####"
7260 DATA "###### ######### ##### ######"
7270 DATA "###### ############ ######"
7280 DATA "###### ############ ######"
7290 DATA "###### ############## ######"
7300 DATA "#### ################ ####"
7310 DATA "#### #### ########## ####"
7320 DATA "##### ####### #####"
7330 DATA "##### #####"
7340 DATA "##### ###### # #####"
7350 DATA "## ######### ############"
7360 DATA "##############################"
7370 DATA "##############################"
7500 DATA 24,24,0,60,90,24,36,66
7510 DATA 255,128,128,255,8,8,8,255
7520 DATA 126,66,90,66,90,66,126,0
7530 DATA 126,66,66,74,66,66,126,0
8000 GO SUB 8100
8010 LET k$=INKEY$: IF k$="" THEN GO TO 8010
8020 RETURN
8100 IF INKEY$<>"" THEN GO TO 8100
8110 RETURN
9000 PAPER 0: INK 7: BRIGHT 0: PRINT AT 21,0;"Finished. RUN to try again. ";: STOPRUN and stay at the entrance. Watch the guard make a full circuit without pressing anything. It walks between (7,5), (24,5), (24,14) and (7,14). Try meeting it in a corridor; the capture result should freeze play.
Direction is a pair of changes
gd is the guard’s direction, numbered clockwise. The arrays a() and b() hold its horizontal and vertical changes. DIM initially fills their entries with zero; line 7010 sets the four nonzero values.
| gd | Facing | a(gd) | b(gd) |
|---|---|---|---|
| 1 | East | 1 | 0 |
| 2 | South | 0 | 1 |
| 3 | West | −1 | 0 |
| 4 | North | 0 | −1 |
The matching character in g$ draws an arrow. CHR$ 94 supplies the upward caret. At a corner, line 430 advances the direction and wraps five back to one. Line 440 then adds the chosen offsets to gx and gy. The route’s coordinates are part of this map’s design; this guard does not search for paths.
Give both actors a turn
The main loop reads input even while waiting. When an update is due, it tries the player’s move, checks contact, then handles the file and entrance. beat counts those updates. Every second update resets beat and advances the guard. A second contact check catches the guard arriving on the player.
No key is required to increase beat. A blocked move also consumes an update. At a result, execution leaves this loop, freezing both actors. Reaching home safely with the file ends the game before another guard move.
Read a clock byte
PEEK reads a byte from memory. Address 23672 is the low byte of the ROM’s frame counter; on this PAL target it advances fifty times a second and wraps from 255 to zero. tick remembers its value after an update. Subtracting tick gives elapsed frames; if the result is negative, adding 256 handles a wrap. For example, 3 − 252 + 256 is seven frames.
Line 240 waits until at least eight frames have elapsed. This is a minimum wait after the preceding work, not a fixed frame rate: drawing and BASIC calculations take time too. Reading only one byte is sufficient for this short wait, not for measuring arbitrary long pauses.
If the player presses into a wall, should the guard stop?
Show the explanation
No. The wall rejects the proposed position, but execution continues through the contact and objective checks to beat. The world keeps moving while the player makes no progress.
Let the guard reach you, then hold a direction at the result. Neither actor should move. Release it, retry, and check that the guard returns to its opening position and heading. You can still complete the file’s round trip; only contact is dangerous in this version.
Sources
Steven Vickers, edited by Robin Bradbeer, ZX Spectrum BASIC Programming, second edition (Sinclair Research, 1983), chapter 12, chapter 18, chapter 25.
Run it here
This unit's program, built for you.
- Program
- night-patrol.tap
- Machine
- ZX Spectrum 48K (PAL)