-- msg_snap.lua -- read-only frame side channel + on-demand screen snapshots.
--
-- No breakpoints and no firmware memory writes: this only reads the frame
-- counter and takes MAME screenshots when the host creates the trigger file.
--
--   MSG_TRIGGER=<path>  file the host writes; content: "<prefix> <count> <stride>"
--   MSG_DIR=<dir>       where PNGs go
--
-- The host writes "<prefix> 1 0" for one immediate frame, or e.g.
-- "<prefix> 8 5" for eight snapshots five frames apart.  The file is removed
-- when consumed.  Snapshots render through the emulated palette.
local machine = manager.machine
local dir = os.getenv("MSG_DIR") or "."
local trigger = os.getenv("MSG_TRIGGER") or (dir .. "/trigger")
local screen = nil
for _, s in pairs(machine.screens) do screen = s; break end

local frame = 0
local job = nil

local function consume_trigger()
    local fh = io.open(trigger, "r")
    if not fh then return end
    local text = fh:read("*a") or ""
    fh:close()
    os.remove(trigger)
    local prefix, count, stride = text:match("(%S+)%s+(%d+)%s+(%d+)")
    if not prefix then
        print("msg_snap: bad trigger " .. text)
        return
    end
    job = { prefix = prefix, left = tonumber(count), stride = tonumber(stride), next = frame }
    print(string.format("msg_snap: frame %d job %s x%d every %d", frame,
                        prefix, job.left, job.stride))
end

emu.register_frame_done(function()
    frame = frame + 1
    consume_trigger()
    if job and frame >= job.next and screen then
        local out = string.format("%s/%s_frame%05d.png", dir, job.prefix, frame)
        local ok, err = pcall(function() return screen:snapshot(out) end)
        print(string.format("msg_snap: frame %d %s ok=%s err=%s", frame,
                            out, tostring(ok), tostring(err)))
        job.left = job.left - 1
        job.next = frame + job.stride
        if job.left <= 0 then job = nil end
    end
    if frame % 500 == 0 then
        local cpu = machine.devices[':maincpu']
        print(string.format("msg_snap: frame %d PC=%08X", frame,
                            cpu.state['PC'].value))
    end
end, "vt1200-msg-snap")

if machine.debugger then machine.debugger.execution_state = 'run' end