Walkers that turn at walls and at each other, the way Mario's Goombas do

Two walkers of our own between two walls, with the square from the solid lesson beside them. Play it and watch them meet, turn, walk to the walls and back. The program is 415 instructions in 1,129 bytes, built and checked the same way.

Play it here Change it on the desk The cartridge, to play anywhere that plays NES cartridges: walkers.nes

On the screen

A walker needs only two rules

The Goombas in Super Mario Bros. move by two simple rules. They step half a pixel a frame, one pixel every second frame, and when one walks into a wall it turns round: for one frame it is a pixel inside the wall, then it is pushed back out and walks the other way. When two meet, both turn on the same frame, already overlapping a little, because the box the game checks is smaller than the picture. Our walkers keep the same pace, look the level up at their leading edge the way the square does, and turn as soon as they touch, so they are never seen inside a wall or overlapping.

	this cartridge	Super Mario Bros.
pixels a step, and frames between steps	1, every 2	1, every 2
turns at walls seen	4	3
a frame spent a pixel inside the wall	no	yes
meetings seen, both turning on the same frame	3	1
pixels the two overlap when they turn	0	7

As written

The part of the program this lesson is about. The whole program is a plain text file: walkers.s

;; The walkers, on every second frame: each takes its step, and if its
;; leading edge is then in a wall it goes back and turns. Then, if the one
;; on the left walks right and the one on the right walks left and they
;; touch (16 apart), both turn.
walkers:
    LDA $44
    EOR #$01
    STA $44
    BEQ resting
    LDX #$00
    JSR stride
    LDX #$01
    JSR stride
    LDA $42
    CMP #$01
    BNE resting
    LDA $43
    CMP #$FF
    BNE resting
    SEC
    LDA $41
    SBC $40
    CMP #$11
    BCS resting
    LDA #$FF
    STA $42
    LDA #$01
    STA $43
resting:
    RTS
;; Walker X: a step, and the block at its leading edge (its right edge
;; walking right, its left walking left), at its feet's height less one.
stride:
    CLC
    LDA $40,X
    ADC $42,X
    STA $40,X
    LDY $42,X
    CPY #$01
    BNE leftward
    CLC
    ADC #$0F
leftward:
    STA $20
    LDA #$CF
    STA $21
    STX $26
    JSR solid
    LDX $26
    CMP #$00
    BEQ strode
    SEC
    LDA $40,X
    SBC $42,X
    STA $40,X
    LDA #$00
    SEC
    SBC $42,X
    STA $42,X

Before this one: Ground, a block and a wall, solid the way Mario's are

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