Fill a Connected Grid Region on Spectrum | Pattern Library

A paint fill can also discover a connected room or reveal matching puzzle cells.

Spectrum shows the working diagnostic row 00 09 03 01 09 09 00 00. — A completed native caller displays 00 09 03 01 09 09 00 00. The public field order below identifies these bytes; the capture is a state diagnostic, not a recording of a finished game.

Use it in a project

The caller supplies a 4×4 row-major byte grid and SEED 0–15. Replace its four-connected region with REPLACEMENT and return CHANGED. Equal source/replacement is a valid no-op. The implementation repeatedly scans for source-colour cells touching the growing replacement region, but tracks membership separately in scratch so pre-existing replacement-colour cells cannot leak into it.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: SEED, REPLACEMENT, CHANGED, VALID, C0, C1, C2, C3, C4, C5, C6, C7, C8, C9, C10, C11, C12, C13, C14, C15. They occupy 20 contiguous bytes. Also reserve the private workspace: T0, T1, T2, T3, T4, T5, T6, T7, EXPR_SAVE, V0, V1, V2, V3, V4, V5, V6, V7, V8, V9, V10, V11, V12, V13, V14, V15 (25 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.

Call bounded_flood_fill_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 bounded_flood_fill_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	C0–C15
Supply before service	SEED and REPLACEMENT
Read after service	CHANGED and VALID

A separate membership map distinguishes cells reached from the seed from cells that happened to have the replacement colour already. Repeated scans grow that membership until a scan adds nothing; only then are colours changed.

Reusable initialisation and service

component.inc

; SPDX-License-Identifier: MIT
; Fill a Connected Grid Region. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
bounded_flood_fill_init:
    ld a,0
    ld (SEED),a
    ld (REPLACEMENT),a
    ld (CHANGED),a
    ld (VALID),a
    ld (C0),a
    ld (C1),a
    ld (C2),a
    ld (C3),a
    ld (C4),a
    ld (C5),a
    ld (C6),a
    ld (C7),a
    ld (C8),a
    ld (C9),a
    ld (C10),a
    ld (C11),a
    ld (C12),a
    ld (C13),a
    ld (C14),a
    ld (C15),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
    ld (EXPR_SAVE),a
    ld (V0),a
    ld (V1),a
    ld (V2),a
    ld (V3),a
    ld (V4),a
    ld (V5),a
    ld (V6),a
    ld (V7),a
    ld (V8),a
    ld (V9),a
    ld (V10),a
    ld (V11),a
    ld (V12),a
    ld (V13),a
    ld (V14),a
    ld (V15),a
    ret
bounded_flood_fill_step:
; VALID = 0
    ld a,0
    ld (VALID),a
; CHANGED = 0
    ld a,0
    ld (CHANGED),a
; if SEED >= 16:
    ld a,(SEED)
    cp 16
    jp nc,bounded_flood_fill_L1
    jp bounded_flood_fill_L2
bounded_flood_fill_L1:
; return
    ret
    jp bounded_flood_fill_L3
bounded_flood_fill_L2:
bounded_flood_fill_L3:
; VALID = 1
    ld a,1
    ld (VALID),a
; T0 = C[SEED]
    ld a,(SEED)
    ld e,a
    ld d,0
    ld hl,C0
    add hl,de
    ld a,(hl)
    ld (T0),a
; if T0 == REPLACEMENT:
    ld a,(T0)
    ld b,a
    ld a,(REPLACEMENT)
    ld c,a
    ld a,b
    cp c
    jp z,bounded_flood_fill_L4
    jp bounded_flood_fill_L5
bounded_flood_fill_L4:
; return
    ret
    jp bounded_flood_fill_L6
bounded_flood_fill_L5:
bounded_flood_fill_L6:
; T1 = 0
    ld a,0
    ld (T1),a
; while T1 < 16:
bounded_flood_fill_L7:
    ld a,(T1)
    cp 16
    jp c,bounded_flood_fill_L8
    jp bounded_flood_fill_L9
bounded_flood_fill_L8:
; V[T1] = 0
    ld a,0
    ld c,a
    ld a,(T1)
    ld e,a
    ld d,0
    ld hl,V0
    add hl,de
    ld (hl),c
; T1 = T1 + 1
    ld a,(T1)
    add a,1
    ld (T1),a
    jp bounded_flood_fill_L7
bounded_flood_fill_L9:
; V[SEED] = 1
    ld a,1
    ld c,a
    ld a,(SEED)
    ld e,a
    ld d,0
    ld hl,V0
    add hl,de
    ld (hl),c
; T2 = 1
    ld a,1
    ld (T2),a
; while T2 != 0:
bounded_flood_fill_L10:
    ld a,(T2)
    cp 0
    jp nz,bounded_flood_fill_L11
    jp bounded_flood_fill_L12
bounded_flood_fill_L11:
; T2 = 0
    ld a,0
    ld (T2),a
; T1 = 0
    ld a,0
    ld (T1),a
; while T1 < 16:
bounded_flood_fill_L13:
    ld a,(T1)
    cp 16
    jp c,bounded_flood_fill_L14
    jp bounded_flood_fill_L15
bounded_flood_fill_L14:
; T3 = V[T1]
    ld a,(T1)
    ld e,a
    ld d,0
    ld hl,V0
    add hl,de
    ld a,(hl)
    ld (T3),a
; if T3 == 0:
    ld a,(T3)
    cp 0
    jp z,bounded_flood_fill_L16
    jp bounded_flood_fill_L17
bounded_flood_fill_L16:
; T3 = C[T1]
    ld a,(T1)
    ld e,a
    ld d,0
    ld hl,C0
    add hl,de
    ld a,(hl)
    ld (T3),a
; if T3 == T0:
    ld a,(T3)
    ld b,a
    ld a,(T0)
    ld c,a
    ld a,b
    cp c
    jp z,bounded_flood_fill_L19
    jp bounded_flood_fill_L20
bounded_flood_fill_L19:
; T4 = 0
    ld a,0
    ld (T4),a
; T5 = T1 & 3
    ld a,(T1)
    and 3
    ld (T5),a
; if T5 != 0:
    ld a,(T5)
    cp 0
    jp nz,bounded_flood_fill_L22
    jp bounded_flood_fill_L23
bounded_flood_fill_L22:
; T6 = T1 - 1
    ld a,(T1)
    sub 1
    ld (T6),a
; T4 = V[T6]
    ld a,(T6)
    ld e,a
    ld d,0
    ld hl,V0
    add hl,de
    ld a,(hl)
    ld (T4),a
    jp bounded_flood_fill_L24
bounded_flood_fill_L23:
bounded_flood_fill_L24:
; if T5 != 3:
    ld a,(T5)
    cp 3
    jp nz,bounded_flood_fill_L25
    jp bounded_flood_fill_L26
bounded_flood_fill_L25:
; T6 = T1 + 1
    ld a,(T1)
    add a,1
    ld (T6),a
; T7 = V[T6]
    ld a,(T6)
    ld e,a
    ld d,0
    ld hl,V0
    add hl,de
    ld a,(hl)
    ld (T7),a
; T4 = T4 | T7
    ld a,(T4)
    ld b,a
    ld a,(T7)
    ld c,a
    ld a,b
    or c
    ld (T4),a
    jp bounded_flood_fill_L27
bounded_flood_fill_L26:
bounded_flood_fill_L27:
; if T1 >= 4:
    ld a,(T1)
    cp 4
    jp nc,bounded_flood_fill_L28
    jp bounded_flood_fill_L29
bounded_flood_fill_L28:
; T6 = T1 - 4
    ld a,(T1)
    sub 4
    ld (T6),a
; T7 = V[T6]
    ld a,(T6)
    ld e,a
    ld d,0
    ld hl,V0
    add hl,de
    ld a,(hl)
    ld (T7),a
; T4 = T4 | T7
    ld a,(T4)
    ld b,a
    ld a,(T7)
    ld c,a
    ld a,b
    or c
    ld (T4),a
    jp bounded_flood_fill_L30
bounded_flood_fill_L29:
bounded_flood_fill_L30:
; if T1 < 12:
    ld a,(T1)
    cp 12
    jp c,bounded_flood_fill_L31
    jp bounded_flood_fill_L32
bounded_flood_fill_L31:
; T6 = T1 + 4
    ld a,(T1)
    add a,4
    ld (T6),a
; T7 = V[T6]
    ld a,(T6)
    ld e,a
    ld d,0
    ld hl,V0
    add hl,de
    ld a,(hl)
    ld (T7),a
; T4 = T4 | T7
    ld a,(T4)
    ld b,a
    ld a,(T7)
    ld c,a
    ld a,b
    or c
    ld (T4),a
    jp bounded_flood_fill_L33
bounded_flood_fill_L32:
bounded_flood_fill_L33:
; if T4 != 0:
    ld a,(T4)
    cp 0
    jp nz,bounded_flood_fill_L34
    jp bounded_flood_fill_L35
bounded_flood_fill_L34:
; V[T1] = 1
    ld a,1
    ld c,a
    ld a,(T1)
    ld e,a
    ld d,0
    ld hl,V0
    add hl,de
    ld (hl),c
; T2 = 1
    ld a,1
    ld (T2),a
    jp bounded_flood_fill_L36
bounded_flood_fill_L35:
bounded_flood_fill_L36:
    jp bounded_flood_fill_L21
bounded_flood_fill_L20:
bounded_flood_fill_L21:
    jp bounded_flood_fill_L18
bounded_flood_fill_L17:
bounded_flood_fill_L18:
; T1 = T1 + 1
    ld a,(T1)
    add a,1
    ld (T1),a
    jp bounded_flood_fill_L13
bounded_flood_fill_L15:
    jp bounded_flood_fill_L10
bounded_flood_fill_L12:
; T1 = 0
    ld a,0
    ld (T1),a
; while T1 < 16:
bounded_flood_fill_L37:
    ld a,(T1)
    cp 16
    jp c,bounded_flood_fill_L38
    jp bounded_flood_fill_L39
bounded_flood_fill_L38:
; T3 = V[T1]
    ld a,(T1)
    ld e,a
    ld d,0
    ld hl,V0
    add hl,de
    ld a,(hl)
    ld (T3),a
; if T3 != 0:
    ld a,(T3)
    cp 0
    jp nz,bounded_flood_fill_L40
    jp bounded_flood_fill_L41
bounded_flood_fill_L40:
; C[T1] = REPLACEMENT
    ld a,(REPLACEMENT)
    ld c,a
    ld a,(T1)
    ld e,a
    ld d,0
    ld hl,C0
    add hl,de
    ld (hl),c
; CHANGED = CHANGED + 1
    ld a,(CHANGED)
    add a,1
    ld (CHANGED),a
    jp bounded_flood_fill_L42
bounded_flood_fill_L41:
bounded_flood_fill_L42:
; T1 = T1 + 1
    ld a,(T1)
    add a,1
    ld (T1),a
    jp bounded_flood_fill_L37
bounded_flood_fill_L39:
    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. Scope or rename routine and field labels and allocate a separate block for each independent instance. 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

demo.asm

    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:
SEED: defb 0
REPLACEMENT: defb 0
CHANGED: defb 0
VALID: defb 0
C0: defb 0
C1: defb 0
C2: defb 0
C3: defb 0
C4: defb 0
C5: defb 0
C6: defb 0
C7: defb 0
C8: defb 0
C9: defb 0
C10: defb 0
C11: defb 0
C12: defb 0
C13: defb 0
C14: defb 0
C15: 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
EXPR_SAVE: defb 0
V0: defb 0
V1: defb 0
V2: defb 0
V3: defb 0
V4: defb 0
V5: defb 0
V6: defb 0
V7: defb 0
V8: defb 0
V9: defb 0
V10: defb 0
V11: defb 0
V12: defb 0
V13: defb 0
V14: defb 0
V15: 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

A client retaining state between calls

exercise.inc, excerpt

preview_rules:
    if CUSTOM
; SEED = 15
    ld a,15
    ld (SEED),a
; REPLACEMENT = 9
    ld a,9
    ld (REPLACEMENT),a
; C0 = 1
    ld a,1
    ld (C0),a
; C1 = 1
    ld a,1
    ld (C1),a
; C4 = 1
    ld a,1
    ld (C4),a
; C5 = 2
    ld a,2
    ld (C5),a
; C15 = 1
    ld a,1
    ld (C15),a
    if BROKEN
; SEED = 15
    ld a,15
    ld (SEED),a
    endif
    call checked_step
    call emit_state
    ret
    else
; SEED = 0
    ld a,0
    ld (SEED),a
; REPLACEMENT = 9
    ld a,9
    ld (REPLACEMENT),a
; C0 = 1
    ld a,1
    ld (C0),a
; C1 = 1
    ld a,1
    ld (C1),a
; C4 = 1
    ld a,1
    ld (C4),a
; C5 = 2
    ld a,2
    ld (C5),a
; C15 = 1
    ld a,1
    ld (C15),a
    if BROKEN
; SEED = 15
    ld a,15
    ld (SEED),a
    endif
    call checked_step
    call emit_state
    endif
    ret

Customise it

Seed the disconnected bottom-right cell instead of the upper-left region. 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

custom.asm

    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:
SEED: defb 0
REPLACEMENT: defb 0
CHANGED: defb 0
VALID: defb 0
C0: defb 0
C1: defb 0
C2: defb 0
C3: defb 0
C4: defb 0
C5: defb 0
C6: defb 0
C7: defb 0
C8: defb 0
C9: defb 0
C10: defb 0
C11: defb 0
C12: defb 0
C13: defb 0
C14: defb 0
C15: 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
EXPR_SAVE: defb 0
V0: defb 0
V1: defb 0
V2: defb 0
V3: defb 0
V4: defb 0
V5: defb 0
V6: defb 0
V7: defb 0
V8: defb 0
V9: defb 0
V10: defb 0
V11: defb 0
V12: defb 0
V13: defb 0
V14: defb 0
V15: 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

The custom diagnostic row is 0F 09 01 01 01 01 00 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller always supplies SEED=15. It changes the disconnected bottom-right region instead of the selected upper-left region. Convert the selected cell to a checked row-major index, then pass that index unchanged.

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

wrong.asm

    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:
SEED: defb 0
REPLACEMENT: defb 0
CHANGED: defb 0
VALID: defb 0
C0: defb 0
C1: defb 0
C2: defb 0
C3: defb 0
C4: defb 0
C5: defb 0
C6: defb 0
C7: defb 0
C8: defb 0
C9: defb 0
C10: defb 0
C11: defb 0
C12: defb 0
C13: defb 0
C14: defb 0
C15: 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
EXPR_SAVE: defb 0
V0: defb 0
V1: defb 0
V2: defb 0
V3: defb 0
V4: defb 0
V5: defb 0
V6: defb 0
V7: defb 0
V8: defb 0
V9: defb 0
V10: defb 0
V11: defb 0
V12: defb 0
V13: defb 0
V14: defb 0
V15: 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

Spectrum shows the deliberate error row 0F 09 01 01 01 01 00 00. — The wrong client completes with 0F 09 01 01 01 01 00 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 677 code bytes; public state is 20 bytes and private scratch is 25 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.

Repeated full scans deliberately favour a small fixed workspace over a queue. At most fifteen growth scans can add cells, followed by a terminating scan. Larger or frame-budgeted maps need a different implementation.

The fixed 16-cell scan trades a queue for sixteen scratch membership bytes. It may scan repeatedly; larger maps should use an explicit bounded queue or span fill and budget work across frames. Diagonal contact is not connectivity. It is not a complete navigation solver.

Execution evidence

Four variants run on 48K PAL ROM tape load and USR, using Asm198x 0.0.72, Emu198x 0.44.2. Each executes 50 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

Tile-map queries: The fill changes a connected region in storage; a separate map query or redraw path can consume that result.

The Craft · Maths for Games · Source and native evidence.