Rednet Radio

math.randomseed(os.time())
local currentFrequency = 0

local chars = {
    "a",
    "b",
    "c",
    "d",
    "e",
    "f",
    "g",
    "h",
    "i",
    "j",
    "k",
    "l",
    "m",
    "n",
    "o",
    "p",
    "q",
    "r",
    "s",
    "t",
    "u",
    "v",
    "w",
    "x",
    "y",
    "z",
    " ",
    "_",
    "#",
    "@",
    "!",
    "*",
    "^",
    "1",
    "2",
    "3",
    "4",
    "5",
    "6",
    "7",
    "8",
    "9",
    "0",
    "+",
    "-",
    "~",
    "`",
    "%",
    "$",
    "{",
    "}",
    "\\",
    "/",
    ">",
    "<",
    ";",
    ":",
}


function run()
    while true do
        local id, msg, distance = rednet.receive()
        msg = textutils.unserialize(msg)
        if type(msg) == "table" then
            local freq = msg[1]
            local data = msg[2]
            local offset = math.abs(currentFrequency-freq)
            --print("Got a message from "..id..", frequency "..freq.." data "..data.." freqOffset "..offset.." our freq is "..currentFrequency)
            if offset < 10 then
                local doScramble = true
                if distance <= 100 then
                    if offset == 1 then
                        if math.random(1,10) < 5 then
                            doScramble = false
                        end
                    elseif offset == 2 then
                        if math.random(1,100) < 25 then
                            doScramble = false
                        end
                    end
                end
                local scrambleAmt = 0
                if doScramble then
                    scrambleAmt = math.ceil(#data * (offset/10))
                end
                if distance > 100 then
                    scrambleAmt = scrambleAmt + math.ceil(#data * (distance-100 / 100))
                end
                if scrambleAmt < #data then
                    --print("Scrambling "..scrambleAmt.." characters")
                    local scrambledPositions = {}
                    for i=1, #data do
                        scrambledPositions[i] = string.sub(data,i, i)
                    end
                    if doScramble then
                        for i=1, scrambleAmt do
                            local scramblePos = math.random(1, #data)
                            if scrambledPositions[scramblePos] == string.sub(data, scramblePos, scramblePos) then
                                if math.random(1,2) == 2 then
                                    scrambledPositions[scramblePos] = string.upper(chars[math.random(1, #chars)])
                                else
                                    scrambledPositions[scramblePos] = chars[math.random(1, #chars)]
                                end
                            end
                        end
                    end
                    local newData = ""
                    for i=1, #scrambledPositions do
                        newData = newData..scrambledPositions[i]
                    end
                    --print("Received radio transmission from "..id.." data: "..newData)
                    os.queueEvent("radio_received", id, newData, distance, freq)
                end
            end
        end
    end
end

function receive(timeout)
    local timer = -1
    if timeout then
        timer = os.startTimer(timeout)
    end
    while true do
        local event, p1, p2, p3, p4 = os.pullEvent()
        if event == "timer" and p1 == timer then
            return
        elseif event == "radio_received" then
            return p1, p2, p3, p4
        end
    end
end

function transmit(message)
    rednet.broadcast(textutils.serialize({currentFrequency, message}))
end

function setFrequency(newFreq)
    currentFrequency = newFreq
end

function getFrequency()
    return currentFrequency
end