Macro for shimming 2-byte charmaps

2byte_charmaps.asm

    NEWCHARMAP "Low byte"
    charmap "A", $12

    NEWCHARMAP "High byte"
    charmap "A", $34


    ; ...


text: MACRO
    PUSHC ; Preserve the outer charmap

NTH = 0
    REPT _NARG
NTH = NTH + 1
I = 1
LEN = STRLEN(\1)
        ; We actually don't know how many bytes the string will output, but we know an upper bound
        REPT LEN
            IF I != LEN ; This will skip all remaining iterations once all bytes have been output

                setcharmap "Low byte"

                ; Determine length of first charmap mapping
                ; (heuristically, but what can I do...)
CHAR_BYTES = 0
                REPT 16 ; FIXME: assuming max mapping length is 16 chars
CHAR_BYTES = CHAR_BYTES + 1
                    IF !DEF(UNIT)
UNIT equs STRSUB(\1, I, CHAR_BYTES)
                        IF "{UNIT}" != LOW(\1)
                            PURGE UNIT
                        ENDC
                    ENDC
                ENDR

                ; If failed to determine length, we have a problem
                IF !DEF(UNIT)
                    FAIL "Failed to write \"\1\": couldn't determine {d:NTH}th mapping"
                ENDC

                ; Write low byte
                db "{UNIT}"

                ; Write high byte
                setcharmap "High byte"
                static_assert "{UNIT}" < 256, "\"{UNIT}\" doesn't have a high byte mapping?"
                db "{UNIT}"

I = I + CHAR_BYTES
            ENDC
        ENDR
        SHIFT
    ENDR
    POPC
ENDM