uxn.lua

-- an implementation of the Uxn VM in Lua, written by @thacuber2a03
-- compatible with 5.1 and JIT
-- thanks to @soxfox42 for helping on stack issues with the keep flag
-- and other things

local unpack = table.unpack or unpack

local ram, dev = {}, {}
local wst, rst = {kidx=1,idx=1}, {kidx=1,idx=1}

local function wrap(n, m)
	while n > m do n = n - m end
	while n < 1 do n = n + m end
	return n
end

local function idxwrap(i)  return wrap(i, 0x100  ) end
local function addrwrap(a) return wrap(a, 0x10000) end

local function push(stk, v, t)
	if stk.kidx then stk.idx, stk.kidx = stk.kidx, nil end
	if t then
		stk[stk.idx] = (v >> 8) & 0xff
		stk.idx = idxwrap(stk.idx + 1)
	end
	stk[stk.idx] = v & 0xff
	stk.idx = idxwrap(stk.idx + 1)
end

local function pop(stk, t)
	stk.idx = idxwrap(stk.idx - 1)
	local v = stk[stk.idx] & 0xff
	if t then
		stk.idx = idxwrap(stk.idx - 1)
		v = ((stk[stk.idx] & 0xff) << 8) | v
	end
	return v
end

local function popn(s, t, n)
	local r = {}
	for i=1, n do r[#r+1] = pop(s,t) end
	return unpack(r)
end

local function pushn(s, t, ...)
	for _, n in ipairs{...} do push(s, t, n) end
end

local pc

local instrs = {
[0x01] = function(s, t) push(s,t, pop(s,t)+1)                                end, -- INC
[0x02] = function(s, t) pop(s,t)                                             end, -- POP
[0x03] = function(s, t) local a,b=popn(s,t,2)   push (s,t, a)                end, -- NIP
[0x04] = function(s, t) local a,b=popn(s,t,2)   pushn(s,t, a,b)              end, -- SWP
[0x05] = function(s, t) local a,b,c=popn(s,t,3) pushn(s,t, c,a,b)            end, -- ROT
[0x18] = function(s, t) local a,b=popn(s,t,2)   push(s,t,a+b)                end, -- ADD
[0x19] = function(s, t) local a,b=popn(s,t,2)   push(s,t,a-b)                end, -- SUB
[0x1a] = function(s, t) local a,b=popn(s,t,2)   push(s,t,a*b)                end, -- MUL
[0x1b] = function(s, t) local a,b=popn(s,t,2)   push(s,t,b==0 and 0 or a/b)  end, -- DIV
[0x1c] = function(s, t) local a,b=popn(s,t,2)   push(s,t,b&a)                end, -- AND
[0x1d] = function(s, t) local a,b=popn(s,t,2)   push(s,t,b|a)                end, -- ORA
[0x1e] = function(s, t) local a,b=popn(s,t,2)   push(s,t,b~a)                end, -- EOR
[0x1f] = function(s, t) local a,b=popn(s,t,2)   push(s,t,b>>(a&0xf)<<(a>>4)) end, -- SFT
}

local function jmi()
	local off = ram[pc] << 8 | ram[pc+1]
	pc = idxwrap(pc + 2 + off)
end

local function eval(rv)
	assert(ram[1], "no program loaded")
	pc = addrwrap(rv)
	while true do
		local byt = ram[addrwrap(pc)]
		local ins = byt & 0x1f
		local flg = byt & 0xe0
		pc = pc + 1

		local s = wst
		if (flg & 0x80) ~= 0 then s = rst end
		local t = (flg & 0x40) ~= 0
		local k = (flg & 0x20) ~= 0

		if ins == 0x00 then
			if k then -- LIT[2r]
				for i=1, t and 1 or 2 do
					push(s, false, ram[pc]); pc = pc + 1
				end
			elseif byt == 0x40 then jmi()
			elseif byt == 0x20 then -- JCI
				if pop(wst, false) ~= 0 then jmi() else pc = pc + 2 end
			elseif byt == 0x60 then
				push(rst, true, pc+2); jmi() -- JSI
			else return true end -- BRK
		else
			if k then s.kidx = s.idx end
			instrs[ins](s, t)
			s.kidx = nil
		end
	end
end

local function load(prg)
	for i=1, 0x10000 do ram[i] = 0 end
	for i=1, 0x100   do dev[i] = 0 end
	for i, b in ipairs(prg) do ram[0x100+i] = b end
end

return {
	load = load,
	eval = eval,
	ram = ram,
	dev = dev,
}