"""Generates the CoCo cartridge viewer (Motorola 6809 machine code).

Runs from the Program Pak ROM at $C000 (the CoCo autostarts it).  Sets the VDG
graphics mode via PIA1 $FF22 and the SAM video registers (PMODE 3 and 4 are both
6144-byte SAM mode 6; only the $FF22 byte differs), copies the 6144-byte image
from cart ROM down to the video page at $0E00, then holds it.

display: forever (hold), key (poll the keyboard then reset), seconds (count 60 Hz
field-syncs then reset).  A Program Pak can't cleanly return to BASIC, so key and
seconds reset the machine (which re-displays the picture).
"""

from __future__ import annotations

from .mc6809 import Asm

CART_BASE = 0xC000
VIDEO = 0x0E00
SCREEN_BYTES = 6144
RATE = 60                        # NTSC field-syncs per second


def build(data_src: int, vdg: int = 0xF8, display: str = "forever",
          seconds: int = 0) -> bytes:
    a = Asm(CART_BASE)
    a.orcc(0x50)                 # mask IRQ + FIRQ; we run standalone

    a.lda_imm(vdg)
    a.sta_ext(0xFF22)            # VDG graphics mode (PMODE 3 = $E0/$E8, PMODE 4 = $F8)

    # SAM video mode 6 (V2=1, V1=1, V0=0). SAM regs toggle by address; data ignored.
    a.sta_ext(0xFFC5)            # V2 set
    a.sta_ext(0xFFC3)            # V1 set
    a.sta_ext(0xFFC0)            # V0 clear

    # SAM video offset = 7 ($0E00): F0,F1,F2 set; F3..F6 clear.
    a.sta_ext(0xFFC7); a.sta_ext(0xFFC9); a.sta_ext(0xFFCB)
    a.sta_ext(0xFFCC); a.sta_ext(0xFFCE); a.sta_ext(0xFFD0); a.sta_ext(0xFFD2)

    # copy SCREEN_BYTES from cart ROM (data_src) to the video page
    a.ldx_imm(data_src)
    a.ldu_imm(VIDEO)
    a.label("copy")
    a.lda_postinc("x")
    a.sta_postinc("u")
    a.cmpu_imm(VIDEO + SCREEN_BYTES)
    a.bne("copy")

    # ---- hold the picture ----
    if display == "key":
        a.clra()
        a.sta_ext(0xFF02)        # drive all keyboard columns low
        a.label("kwait")
        a.lda_ext(0xFF00)        # row sense
        a.ora_imm(0x80)          # ignore the joystick-compare bit 7
        a.cmpa_imm(0xFF)
        a.beq("kwait")           # all rows high -> no key, keep waiting
        a.jmp_ind(0xFFFE)        # reset
    elif display == "seconds":
        a.ldd_imm(max(1, min(0xFFFF, int(seconds) * RATE)))
        a.label("swait")
        a.lda_ext(0xFF03)        # PIA0 CB1 (field-sync) status
        a.bpl("swait")           # bit 7 clear -> no new field yet
        a.lda_ext(0xFF02)        # read PB data to clear the field-sync flag
        a.subd_imm(1)
        a.bne("swait")
        a.jmp_ind(0xFFFE)        # reset
    else:                        # forever
        a.label("hang")
        a.bra("hang")
    return a.resolve()