host

--By xXxMoNkEyMaNxXx
local type,next=type,next
local tonumber,tostring=tonumber,tostring

local concat=table.concat
local remove=table.remove

local abs=math.abs
local floor=math.floor
local rand=math.random

local tick=os.clock

math.randomseed(os.time())
math.random()

local arg={...}

if peripheral then
for _,side in next,{"left","right","front","back","bottom","top"} do
    if peripheral.getType(side)=="modem" then
        rednet.open(side)
    end
end
end

local nans={tostring(1/0),tostring(-1/0),tostring(-(0/0)),tostring(0/0)}
local function isnan(x)
    local s=tostring(x)
    for i,v in next,nans do
        if s==v then
            return i
        end
    end
end

local function nonenan(t)
    for i=1,#t do
        if isnan(t[i]) then
            return false,i
        end
    end
    return true
end

local function GPS(e0,e1,e2,e3,e4)
    local x0,y0,z0,t0=e0[1],e0[2],e0[3],e0[4]
    local x1,y1,z1,t1=e1[1],e1[2],e1[3],e1[4]
    local x2,y2,z2,t2=e2[1],e2[2],e2[3],e2[4]
    local x3,y3,z3,t3=e3[1],e3[2],e3[3],e3[4]
    local x4,y4,z4,t4=e4[1],e4[2],e4[3],e4[4]

    --I left brackets in some because it's easier to see the grouping
    local s1=x1-x0
    local s3=y1-y0
    local s7=z1-z0
    local s10=t1-t0
    local s2=(x2-x0)/s1
    local s5=(x3-x0)/s1
    local s9=(x4-x0)/s1
    local s6=(y2-y0)-s2*s3
    local s13=(z2-z0)-s2*s7
    local s16=(t2-t0)-s2*s10
    local s11=((y3-y0)-s5*s3)/s6
    local s17=((z3-z0)-s5*s7)-s11*s13
    local s21=((t3-t0)-s5*s10)-s11*s16
    local s15=((y4-y0)-s9*s3)/s6
    local s20=(((z4-z0)-s9*s7)-s15*s13)/s17
    local s4=x0^2+y0^2+z0^2-t0^2
    local s12=((x1^2+y1^2+z1^2-t1^2)-s4)/2
    local s14=((x2^2+y2^2+z2^2-t2^2)-s4)/2-s2*s12
    local s19=((x3^2+y3^2+z3^2-t3^2)-s4)/2-s5*s12-s11*s14

    --Calculators weren't good enough for this!  I solved it by hand ;D
    local t=-(((x4^2+y4^2+z4^2-t4^2)-s4)/2-s9*s12-s15*s14-s20*s19)/((((t4-t0)-s9*s10)-s15*s16)-s20*s21)
    local z=(s21*t+s19)/s17
    local y=(s16*t-s13*z+s14)/s6
    local x=(s10*t-s3*y-s7*z+s12)/s1
    return {floor(x+0.5),floor(y+0.5),floor(z+0.5),t}
end

local rec=rednet.receive
local broadcast=rednet.broadcast
local unserialize=textutils.unserialize
local function receive(timeout)
    local id,msg,d=rec(timeout)
    local ret=msg and unserialize(msg) or {}
    ret.senderId=id
    ret.message=msg
    ret.distance=d
    return ret
end

local function locate(timeout,period)
    timeout=timeout or 10
    period=period or 1
    local t0=tick()
    broadcast("PING")
    local p={}
    local sat={}
    repeat
        local data=receive(period)
        if #data==4 then
            local e={}
            for i=1,4 do
                local v=data[i]
                if type(v)=="number" then
                    e[i]=v
                else
                    break
                end
            end
            if #e==4 then
                sat[#sat+1]=e
            end
        elseif #data==3 and type(data.distance)=="number" then
            local e={}
            for i=1,3 do
                local v=data[i]
                if type(v)=="number" then
                    e[i]=v
                else
                    break
                end
            end
            if #e==3 then
                e[4]=-data.distance
                sat[#sat+1]=e
            end
        else
            local jesus=true
            while #sat>=5 do
                local chosen={}
                for i=1,5 do
                    chosen[i]=remove(sat,rand(#sat))
                end
                p=GPS(unpack(chosen))
                if #p==4 and nonenan(p) and abs(p[4])<0.001 then
                    jesus=false
                    break
                end
            end
            if jesus then
                print'Not enough hosts, needs more.'
                broadcast'PING'
            end
        end
    until #p==4 and nonenan(p) and abs(p[4])<0.001 or tick()-t0>timeout
    return p
end

local p
if #arg==3 then
    local x,y,z=unpack(arg)
    if tonumber(x) and tonumber(y) and tonumber(z) then
        p={tonumber(x),tonumber(y),tonumber(z)}
    else
        error'Invalid coordinates'
    end
elseif #arg==1 and type(arg[1])=="string" and arg[1]~="here" then
    local x,y,z=arg[1]:match'(.+),(.+),(.+)'
    if tonumber(x) and tonumber(y) and tonumber(z) then
        p={tonumber(x),tonumber(y),tonumber(z)}
    end
else
    p=locate(1000,5)
end

local send=rednet.send
local receive=rednet.receive

local serialize=textutils.serialize
if p and #p>=3 and nonenan(p) then
    print(concat(p,","))
    local n=0
    while true do
        local id,msg,d=receive()
        if msg=="PING" then
            send(id,serialize{p[1],p[2],p[3],-d})
            n=n+1
            if n>1 then
                local x,y=term.getCursorPos()
                term.setCursorPos(1,y-1)
            end
            print(n.." GPS request"..(n==1 and "" or "s").." served.")
        end
    end
else
    error'Invalid/missing/indeterminate position.'
end