6502 Substring Finding Algorithm

/*
Copyright-free! For any use!

KickAssembler Substring Matching Subroutine.
Finds the index of the first occurence of a substring (can be any encoding, just make sure both the
string and substring are the same encoding!)

Both strings MUST have a final NULL ($00 byte) character.

Store the address of the string to search at $fb
Store the address of the substring at $fd

$c000 - $c002 are used as temp

A & Y registers will be used.

A <- boolean (0 or 1) on whether a match was found

X <- index of the first occurence of the substring (if found, 0 if not found

Y <- 0

__$fb - $fc, $c000 - $c002 and Y are set to zero!__
*/

find_substr: {
    .label stri = $3b
    .label subi = $3c
    .label finalindex = $2a
    .label stradd = $fb
    .label subadd = $fd
    lda #0 // index of the searched string
    sta stri
    lda #0 // index of the substring
    sta subi
mainloop:
    ldy stri
    lda (stradd),y
    ldy subi
    cmp (subadd),y

    beq letter_match
    bne letter_mismatch
letter_match:
    // letter matches, increment substring index
    inc subi
    jmp check
letter_mismatch:
    inc finalindex
    lda #0
    sta subi
check:
    ldy subi // check substring first, to prevent substring matches at the end of the
    // searched string from coming back false
    lda (subadd),y
    beq endsubreached // return true

    ldy stri
    lda (stradd),y
    beq endstrreached // return false

    inc stri
    jmp mainloop
endsubreached:
    lda #1 // substring found
    ldx finalindex
    jmp end
endstrreached:
    lda #0
    ldx #0
    jmp end
end:
    ldy #0
    sty stradd
    sty subadd
    sty finalindex
    sty stri
    sty subi
    rts
}