BASIC Decompiler

local args = {...}

local function printUsage()
    print "Usage:"
    print " decompile [-l|-p] inputFile [outputFile]"
end

if #args == 0 then
    return printUsage()
end

local lineNumbers = args[1] == "-l" or args[2] == "-l"
local printOutput = args[1] == "-p" or args[2] == "-p"

for i = 1, (lineNumbers and 1 or 0) + (printOutput and 1 or 0) do
    table.remove(args, 1)
end

if #args == 0 then
    printError "Missing argument inputFile"
    return printUsage()
end

local inf, opf = unpack(args)
if inf:find "%." then
    opf = opf or inf:gsub("%.", "_decompiled.")
else
    opf = opf or inf .. "_decompiled"
end

local BC_HALT = 0x00
local BC_GOSUB = 0x01
local BC_ADD = 0x02
local BC_SUB = 0x03
local BC_VSET = 0x04
local BC_VGET = 0x05
local BC_ARGS = 0x06
local BC_MUL = 0x07
local BC_DIV = 0x08
local BC_PUSH = 0x09
local BC_POP = 0x0A
local BC_CALL = 0x0B
local BC_JUMP = 0x0C
local BC_RET = 0x0D
local BC_VFREE = 0x0E
local BC_VDEF = 0x0F
local BC_MOD = 0x10
local BC_POW = 0x11
local BC_CMP = 0x12
local BC_JPC = 0x13
local BC_STORE = 0x14
local BC_LOAD = 0x15
local BC_DUP = 0x16
local BC_CARR = 0x17
local BC_AND = 0x18
local BC_OR = 0x19

local bc_END_STRING = 0x0
local bc_START_STRING = 0x1
local bc_NEXT_BC = 0x2
local bc_NEXT_ARG = 0x3

local opToName = {
    [ BC_HALT ] = { "HALT", 0 };
    [ BC_GOSUB ] = { "GOSUB", 1 };
    [ BC_ADD ] = { "ADD", 0 };
    [ BC_SUB ] = { "SUB", 0 };
    [ BC_VSET ] = { "VARSET", 1 };
    [ BC_VGET ] = { "VARGET", 1 };
    [ BC_ARGS ] = { "ARGS", 1 };
    [ BC_MUL ] = { "MUL", 0 };
    [ BC_DIV ] = { "DIV", 0 };
    [ BC_PUSH ] = { "PUSH", 1 };
    [ BC_POP ] = { "POP", 0 };
    [ BC_CALL ] = { "CALL", 1 };
    [ BC_JUMP ] = { "JUMP", 1 };
    [ BC_RET ] = { "RETURN", 0 };
    [ BC_VFREE ] = { "VARFREE", 1 };
    [ BC_VDEF ] = { "VARDEF", 1 };
    [ BC_MOD ] = { "MOD", 0 };
    [ BC_POW ] = { "POW", 0 };
    [ BC_CMP ] = { "COMPARE", 1 };
    [ BC_JPC ] = { "CONDJMP", 2 };
    [ BC_STORE ] = { "STORE", 1 };
    [ BC_LOAD ] = { "LOAD", 1 };
    [ BC_DUP ] = { "DUP", 0 };
    [ BC_AND ] =  { "AND", 0 };
    [ BC_OR ] = { "OR", 0 };
    [ BC_CARR ] = { "CREATEARRAY", 0 };
}

local function readBytecodeFile( fn )
    local file = fs.open( fn, 'r' )
    local content = file.readAll()
    local t = {}

    file.close()

    for seg in content:gmatch "%S+" do
        t[#t + 1] = tonumber(seg, 16)
    end

    return t
end

local function makeBytecode( bc )
    local b = {}
    local check = false
    local i = 1
    local function fetch()
        local v = bc[ i ]
        i = i + 1
        return v
    end
    local function fetchString()
        local str = ""
        while true do
            local v = fetch()
            if v == bc_END_STRING then
                break
            else
                str = str .. string.char( v )
            end
        end
        return str
    end
    while i <= #bc do
        local tp = fetch()
        if tp == bc_NEXT_BC then
            local byte = fetch()
            local op = opToName[ byte ]
            local iter = op[ 2 ]
            b[ #b + 1 ] = byte
            for j=1, iter do
                local nx = fetch()
                if nx == bc_NEXT_ARG then
                    local x = fetch()
                    b[ #b + 1 ] = x
                elseif nx == bc_START_STRING then
                    local x = fetchString()
                    b[ #b + 1 ] = x
                else
                    error( "Expected Start of String or Start of Argument!", 0 )
                end
            end
        else
            print( bc[ i-1 ] )
            error( "Expected bytecode!", 0 )
        end
    end
    return b
end

local i = 1
local of = io.open(opf, "w")
local bc = makeBytecode(readBytecodeFile(inf))
while i <= #bc do
    local s = "UNKNOWN (" .. bc[i] .. ")"
    if opToName[bc[i]] then
        s = (lineNumbers and "%03d: " or ""):format(i) .. opToName[bc[i]][1]:lower()

        for j = 1, opToName[bc[i]][2] do
            i = i + 1
            s = s .. " " .. (type(bc[i]) == "string" and ("%q"):format(bc[i]) or bc[i])
        end
    end
    if printOutput then
        print(s)
    end
    of:write(s .. "\n")
    i = i + 1
end

of:close()