An enemy or homing projectile needs a target-selection rule that stays predictable when candidates tie.
Use it in a project
ACTIVE uses its low four bits for four candidates. Select the smallest Manhattan distance from unsigned byte ORIGIN_X/Y, breaking ties by lowest slot. Return a little-endian distance 0–510 and FOUND=1; no active targets returns SLOT=255, distance zero and FOUND=0. High ACTIVE bits are ignored.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: SLOT, DISTANCE_LO, DISTANCE_HI, FOUND, ACTIVE, ORIGIN_X, ORIGIN_Y, PX0, PX1, PX2, PX3, PY0, PY1, PY2, PY3. They occupy 15 contiguous bytes. Also reserve the private workspace: T0, T1, T2, T3, T4, T5, T6, T7 (8 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call nearest_target_init once at the intended reset boundary. It clears inputs as well as outputs and workspace. Set your starting state and configuration afterwards, then call nearest_target_step whenever the described event occurs. The helper reads no device, waits for no frame and installs no interrupt handler.
| Ownership | Fields |
|---|---|
| Retain between calls | none |
| Supply before service | ACTIVE, origin and candidate coordinates |
| Read after service | SLOT, DISTANCE_LO/HI and FOUND |
Compute two absolute coordinate differences and add them with a ninth bit. Replace the best candidate only for a strictly smaller distance, preserving the earlier slot on ties.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Select the Nearest Active Target. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
nearest_target_init:
ld a,0
ld (SLOT),a
ld (DISTANCE_LO),a
ld (DISTANCE_HI),a
ld (FOUND),a
ld (ACTIVE),a
ld (ORIGIN_X),a
ld (ORIGIN_Y),a
ld (PX0),a
ld (PX1),a
ld (PX2),a
ld (PX3),a
ld (PY0),a
ld (PY1),a
ld (PY2),a
ld (PY3),a
ld (T0),a
ld (T1),a
ld (T2),a
ld (T3),a
ld (T4),a
ld (T5),a
ld (T6),a
ld (T7),a
ret
nearest_target_step:
; SLOT = 255
ld a,255
ld (SLOT),a
; DISTANCE_LO = 0
ld a,0
ld (DISTANCE_LO),a
; DISTANCE_HI = 0
ld a,0
ld (DISTANCE_HI),a
; FOUND = 0
ld a,0
ld (FOUND),a
; T0 = 0
ld a,0
ld (T0),a
; T1 = 1
ld a,1
ld (T1),a
; while T0 < 4:
nearest_target_L1:
ld a,(T0)
cp 4
jp c,nearest_target_L2
jp nearest_target_L3
nearest_target_L2:
; T2 = ACTIVE & T1
ld a,(ACTIVE)
ld b,a
ld a,(T1)
ld c,a
ld a,b
and c
ld (T2),a
; if T2 != 0:
ld a,(T2)
cp 0
jp nz,nearest_target_L4
jp nearest_target_L5
nearest_target_L4:
; T2 = PX[T0]
ld a,(T0)
ld e,a
ld d,0
ld hl,PX0
add hl,de
ld a,(hl)
ld (T2),a
; if T2 >= ORIGIN_X:
ld a,(T2)
ld b,a
ld a,(ORIGIN_X)
ld c,a
ld a,b
cp c
jp nc,nearest_target_L7
jp nearest_target_L8
nearest_target_L7:
; T3 = T2 - ORIGIN_X
ld a,(T2)
ld b,a
ld a,(ORIGIN_X)
ld c,a
ld a,b
sub c
ld (T3),a
jp nearest_target_L9
nearest_target_L8:
; T3 = ORIGIN_X - T2
ld a,(ORIGIN_X)
ld b,a
ld a,(T2)
ld c,a
ld a,b
sub c
ld (T3),a
nearest_target_L9:
; T2 = PY[T0]
ld a,(T0)
ld e,a
ld d,0
ld hl,PY0
add hl,de
ld a,(hl)
ld (T2),a
; if T2 >= ORIGIN_Y:
ld a,(T2)
ld b,a
ld a,(ORIGIN_Y)
ld c,a
ld a,b
cp c
jp nc,nearest_target_L10
jp nearest_target_L11
nearest_target_L10:
; T4 = T2 - ORIGIN_Y
ld a,(T2)
ld b,a
ld a,(ORIGIN_Y)
ld c,a
ld a,b
sub c
ld (T4),a
jp nearest_target_L12
nearest_target_L11:
; T4 = ORIGIN_Y - T2
ld a,(ORIGIN_Y)
ld b,a
ld a,(T2)
ld c,a
ld a,b
sub c
ld (T4),a
nearest_target_L12:
; T5 = T3 + T4
ld a,(T3)
ld b,a
ld a,(T4)
ld c,a
ld a,b
add a,c
ld (T5),a
; T6 = 0
ld a,0
ld (T6),a
; if T5 < T3:
ld a,(T5)
ld b,a
ld a,(T3)
ld c,a
ld a,b
cp c
jp c,nearest_target_L13
jp nearest_target_L14
nearest_target_L13:
; T6 = 1
ld a,1
ld (T6),a
jp nearest_target_L15
nearest_target_L14:
nearest_target_L15:
; T7 = 0
ld a,0
ld (T7),a
; if FOUND == 0:
ld a,(FOUND)
cp 0
jp z,nearest_target_L16
jp nearest_target_L17
nearest_target_L16:
; T7 = 1
ld a,1
ld (T7),a
jp nearest_target_L18
nearest_target_L17:
nearest_target_L18:
; if T6 < DISTANCE_HI:
ld a,(T6)
ld b,a
ld a,(DISTANCE_HI)
ld c,a
ld a,b
cp c
jp c,nearest_target_L19
jp nearest_target_L20
nearest_target_L19:
; T7 = 1
ld a,1
ld (T7),a
jp nearest_target_L21
nearest_target_L20:
nearest_target_L21:
; if T6 == DISTANCE_HI:
ld a,(T6)
ld b,a
ld a,(DISTANCE_HI)
ld c,a
ld a,b
cp c
jp z,nearest_target_L22
jp nearest_target_L23
nearest_target_L22:
; if T5 < DISTANCE_LO:
ld a,(T5)
ld b,a
ld a,(DISTANCE_LO)
ld c,a
ld a,b
cp c
jp c,nearest_target_L25
jp nearest_target_L26
nearest_target_L25:
; T7 = 1
ld a,1
ld (T7),a
jp nearest_target_L27
nearest_target_L26:
nearest_target_L27:
jp nearest_target_L24
nearest_target_L23:
nearest_target_L24:
; if T7 != 0:
ld a,(T7)
cp 0
jp nz,nearest_target_L28
jp nearest_target_L29
nearest_target_L28:
; SLOT = T0
ld a,(T0)
ld (SLOT),a
; DISTANCE_LO = T5
ld a,(T5)
ld (DISTANCE_LO),a
; DISTANCE_HI = T6
ld a,(T6)
ld (DISTANCE_HI),a
; FOUND = 1
ld a,1
ld (FOUND),a
jp nearest_target_L30
nearest_target_L29:
nearest_target_L30:
jp nearest_target_L6
nearest_target_L5:
nearest_target_L6:
; T0 = T0 + 1
ld a,(T0)
add a,1
ld (T0),a
; T1 = T1 << 1
ld a,(T1)
sla a
ld (T1),a
jp nearest_target_L1
nearest_target_L3:
ret
component_end:Machine contract
AF/BC/DE/HL may change; IX/IY are preserved. A direct CALL needs two return-address stack bytes. The helper makes no nested calls and adds no other stack use. Its named scratch is private workspace, not a pointer, a ROM variable or shared zero-page space that the game may borrow concurrently. The samples use fixed symbol bindings. Scope or rename every label and binding, including the component_end measurement marker, and allocate a separate block for each independent instance. Different helpers can also use common field names, so combining them needs separate namespaces or explicitly renamed bindings. Give each block one mutation owner.
The 48K diagnostic boots through ROM tape loading and USR, disables interrupts and owns the screen. The helper performs no port or screen writes.
Adapt the caller
demo.asm supplies the bindings and include order. exercise.inc first runs table cases, then a separate client retaining state between calls. A game can call the public step entry directly and remove the diagnostic packet and display. Do not copy the diagnostic’s hardware takeover into an existing game loop.
Build with make ASM198X=/path/to/asm198x. The Makefile produces normal, custom, deliberately wrong and repaired programs. Tools must already be installed; the sample downloads nothing.
Bindings and include order
org $8000
CUSTOM equ 0
BROKEN equ 0
COYOTE_STEPS equ 4
BUFFER_STEPS equ 4
TEXT_CAP equ 8
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 7
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "data.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
SLOT: defb 0
DISTANCE_LO: defb 0
DISTANCE_HI: defb 0
FOUND: defb 0
ACTIVE: defb 0
ORIGIN_X: defb 0
ORIGIN_Y: defb 0
PX0: defb 0
PX1: defb 0
PX2: defb 0
PX3: defb 0
PY0: defb 0
PY1: defb 0
PY2: defb 0
PY3: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
T3: defb 0
T4: defb 0
T5: defb 0
T6: defb 0
T7: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
end startA client retaining state between calls
preview_rules:
if CUSTOM
; ACTIVE = 9
ld a,9
ld (ACTIVE),a
; ORIGIN_X = 255
ld a,255
ld (ORIGIN_X),a
; ORIGIN_Y = 255
ld a,255
ld (ORIGIN_Y),a
; PX0 = 0
ld a,0
ld (PX0),a
; PY0 = 0
ld a,0
ld (PY0),a
; PX3 = 254
ld a,254
ld (PX3),a
; PY3 = 255
ld a,255
ld (PY3),a
if BROKEN
; ACTIVE = 1
ld a,1
ld (ACTIVE),a
endif
call checked_step
call emit_state
ret
else
; ACTIVE = 3
ld a,3
ld (ACTIVE),a
; PX0 = 100
ld a,100
ld (PX0),a
; PY0 = 100
ld a,100
ld (PY0),a
; PX1 = 2
ld a,2
ld (PX1),a
; PY1 = 3
ld a,3
ld (PY1),a
if BROKEN
; ACTIVE = 1
ld a,1
ld (ACTIVE),a
endif
call checked_step
call emit_state
endif
retCustomise it
Select near the far corner of the coordinate range. The maintained custom caller changes its inputs or client sequence rather than hiding a different implementation behind the same label. Compare every state of the two clients, not only their final display.
Custom client selection
org $8000
CUSTOM equ 1
BROKEN equ 0
COYOTE_STEPS equ 2
BUFFER_STEPS equ 2
TEXT_CAP equ 4
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 49
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "data.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
SLOT: defb 0
DISTANCE_LO: defb 0
DISTANCE_HI: defb 0
FOUND: defb 0
ACTIVE: defb 0
ORIGIN_X: defb 0
ORIGIN_Y: defb 0
PX0: defb 0
PX1: defb 0
PX2: defb 0
PX3: defb 0
PY0: defb 0
PY1: defb 0
PY2: defb 0
PY3: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
T3: defb 0
T4: defb 0
T5: defb 0
T6: defb 0
T7: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
end startThe custom diagnostic row is 03 01 00 01 09 FF FF 00. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller only marks slot zero active, excluding a nearer candidate. Pass the actual occupancy mask and check FOUND before using SLOT.
The BROKEN branch lives in the sequential client, not the reusable helper. wrong.asm selects it; repaired.asm restores the normal call sequence. The repaired executable matches the normal program (for Spectrum, compare loaded CODE rather than the tape filename).
Select the deliberately wrong client
org $8000
CUSTOM equ 0
BROKEN equ 1
COYOTE_STEPS equ 4
BUFFER_STEPS equ 4
TEXT_CAP equ 8
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 7
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "data.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
SLOT: defb 0
DISTANCE_LO: defb 0
DISTANCE_HI: defb 0
FOUND: defb 0
ACTIVE: defb 0
ORIGIN_X: defb 0
ORIGIN_Y: defb 0
PX0: defb 0
PX1: defb 0
PX2: defb 0
PX3: defb 0
PY0: defb 0
PY1: defb 0
PY2: defb 0
PY3: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
T3: defb 0
T4: defb 0
T5: defb 0
T6: defb 0
T7: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
end start
Cost and alternatives
The measured helper extent is 448 code bytes; public state is 15 bytes and private scratch is 8 bytes. There are no immutable helper tables. Caller data, the test cases, return-address stack, alignment and display buffers cost additional space. These are assembled sizes, not cycle measurements.
Four candidates, no multiplication or square root. Squared Euclidean distance changes the selection and needs a wider sum.
Manhattan distance favours an axis-aligned metric, not circular Euclidean distance. It ignores walls and reachability.
Execution evidence
Four variants run on 48K PAL ROM tape load and USR, using Asm198x 0.0.72, Emu198x 0.44.2. Each executes 64 boundary/varied/error table cases, dirty initialisation and the separate retained-state client. An independent mathematical/container model checks every public byte after every call. This is bounded example coverage, not exhaustive correctness for every possible state.
The records retain loaded immutable code, completion, register sentinels, full packets, source/tool/firmware hashes and captures. The public gate validates these retained observations and rejects altered evidence; it does not run restricted firmware. No firmware is distributed. The eight displayed bytes are the first eight public fields, zero-padded when shorter; the packet includes every field.
Where it fits
Manhattan distance: The metric is part of the targeting behaviour, not merely an optimisation.