Re: [StrongED] Time stamps

[Fred Graute <fjgraute@xxxxxxxxx>]

In message <da221be656.gavin@xxxxxxxxxxxxxxx>
          Gavin Wraith <gavin@xxxxxxxxxxxxxxx> wrote:

> In message <e92c0ae656.fjgraute@xxxxxxxxx>
>           Fred Graute <fjgraute@xxxxxxxxx> wrote:
> 
> > I've done something similar (in Lua) to update my local copy of the
> > StrongED website. As far as I can see it's working fine. How are you
> > comparing the time stamps?
> 
> I am using RO 5.23 on a Raspberry Pi3. So 'disk' is an SD card.
> My timestamp-compare code is slightly Byzantine. Function ok
> should return true if either fout (the target) does not exist or if its
> time stamp is earlier than that of fin (the source):
> 
> local funny = " has a funny type"
> local ok = \ (fin, fout)
>    local r0,r1,r2,r3 = sys (8,23,fout..'\0') -- OS_File
>    if r0 == 0 then => true end -- if (fout non-existent)
>    assert (r0 == 1, fout..funny)
>    local s0,s1,s2,s3 = sys (8,23, fin..'\0')
>    assert (s0 == 1, fin..funny)
>    r3 & = 0xff
>    s3 & = 0xff
>    if s3 > r3 then => true end -- if (5-th byte compared)
>    if s3 < r3 then => end -- if
>    local r2h = (r2 & 0xff000000) >> 6
>    local s2h = (s2 & 0xff000000) >> 6
>    if s2h > r2h then => true end -- if (4-th byte compared)
>    if s2h < r2h then => end -- if
>    r2 & = 0x00ffffff
>    s2 & = 0x00ffffff
>    => (s2 > r2) -- (bottom three bytes compared)
>    end -- function
> 
> It is probably not optimal, and I may be being excessively cautious
> about avoiding signed comparisons.

No, being careful with signed comparisons is good. Below is the code I
use to check if a rebuild is required, which is equally cautious.


function getfileinfo(file)
  riscos.$[wimpblk] = file.."\x00"
  local r0,r1,r2,r3 = riscos.sys("OS_File",23,wimpblk)
  -- make sure 32 bit values are returned, RiscLua 6 sign extends to 64 bit
  return r0,r2 & 0x000000FF,r3 & 0xFFFFFFFF 
end -- function

function comparedatestamps(sourcefile,targetfile)
  local sourcetype,sourcedatehi,sourcedatelo = getfileinfo(sourcefile)
  local targettype,targetdatehi,targetdatelo = getfileinfo(targetfile)

  -- times are unsigned but lua treats integers as signed
  sourcedatehi = ((sourcedatehi << 8) | (sourcedatelo >> 24))
  targetdatehi = ((targetdatehi << 8) | (targetdatelo >> 24))
  sourcedatelo = (sourcedatelo & 0x00FFFFFF)
  targetdatelo = (targetdatelo & 0x00FFFFFF)

  local rebuild = (targettype == 0)
  --rebuild = true

  if not rebuild then
    if (sourcedatehi) == (targetdatehi) then
      rebuild = (sourcedatelo) > (targetdatelo)
    else
      rebuild = (sourcedatehi) > (targetdatehi)
    end -- if
  end -- if
  if rebuild then
    print(sourcetype,sourcedatehi,sourcedatelo,sourcefile)
    print(targettype,targetdatehi,targetdatelo,targetfile)
    print()
  end -- if
  return rebuild
end -- function


Cheers,
Fred.

-- 
StrongED Developer
http://www.stronged.iconbar.com/

-- 
To unsubscribe send a mail to StrongED+unsubscribe@xxxxxxxxxxx
List archives and instructions at
http://diy.Torrens.org/RO/StrongED/index.html