Lua seconds format questions - lua

Question

I have this function:
function SecondsFormat(X)
if X <= 0 then return "" end
local t ={}
local ndays = string.format("%02.f",math.floor(X / 86400))
if tonumber(ndays) > 0 then table.insert(t,ndays.."d ") end
local nHours = string.format("%02.f",math.floor((X/3600) -(ndays*24)))
if tonumber(nHours) > 0 then table.insert(t,nHours.."h ") end
local nMins = string.format("%02.f",math.floor((X/60) - (ndays * 1440) - (nHours*60)))
if tonumber(nMins) > 0 then table.insert(t,nMins.."m ") end
local nSecs = string.format("%02.f", math.fmod(X, 60));
if tonumber(nSecs) > 0 then table.insert(t,nSecs.."s") end
return table.concat(t)
end
I would like to add weeks and months to it but cant get my head around the month part to move on to the week part just because the days in a month aren't always the same so can anyone offer some help?
The second question is, is using a table to store the results the most efficient way of dealing with this given the function will be called every 3 seconds for up to 100 items (in a grid)?
Edit:
function ADownload.ETA(Size,Done,Tranrate) --all in bytes
if Size == nil then return "--" end
if Done == nil then return "--" end
if Tranrate == nil then return "--" end
local RemS = (Size - Done) / Tranrate
local RemS = tonumber(RemS)
if RemS <= 0 or RemS == nil or RemS > 63072000 then return "--" end
local date = os.date("%c",RemS)
if date == nil then return "--" end
local month, day, year, hour, minute, second = date:match("(%d+)/(%d+)/(%d+) (%d+): (%d+):(%d+)")
month = month - 1
day = day - 1
year = year - 70
if tonumber(year) > 0 then
return string.format("%dy %dm %dd %dh %dm %ds", year, month, day, hour, minute, second)
elseif tonumber(month) > 0 then
return string.format("%dm %dd %dh %dm %ds",month, day, hour, minute, second)
elseif tonumber(day) > 0 then
return string.format("%dd %dh %dm %ds",day, hour, minute, second)
elseif tonumber(hour) > 0 then
return string.format("%dh %dm %ds",hour, minute, second)
elseif tonumber(minute) > 0 then
return string.format("%dm %ds",minute, second)
else
return string.format("%ds",second)
end
end
I merged the function into the main function as I figured it would probably be quicker but I now have two questions:
1: I had to add
if date == nil then return "--" end
because it errors occasionally with date:match trying to compare with "nil" however os.date mentions nothing in the literature about returning nil as its a string or a table so although the extra line of code fixes the issue I'm wondering why that behaviour occurs as I'm sure I caught all the non events in the previous returns?
2: Sometimes I see functions written like myfunction(...) and I'm sure that just does away with the arguments and if so is there a one line of code that could do away with the first 3 "if" statements?

Answer 1

You can use the os.date function to get date values in a useable format. The '*t' formating parameter makes the returned date into a table instead of a string.
local t = os.date('*t')
print(t.year, t.month, t.day, t.hour, t.min, t.sec)
print(t.wday, t.yday)
os.data uses the current time by default, you can pass it an explicit time if you want (see the os.data docs for more info on this)
local t = os.date('*t', x)
As for table performance, I wouldn't worry about that. Not only is your function not called all that often, but table handling is much cheaper than other things you might be doing (calling os.date, lots of string formatting, etc)

Answer 2

Why not let Lua's os library do the hard work for you?
There is probably an easier (read: better) way to figure out the difference to 01/01/70, but here is a quick idea of how you could use it:
function SecondsFormat(X)
if X <= 0 then return "" end
local date = os.date("%c", X) -- will give something like "01/03/70 03:40:00"
local inPattern = "(%d+)/(%d+)/(%d+) (%d+):(%d+):(%d+)"
local outPattern = "%dy %dm %dd %dh %dm %ds"
local month, day, year, hour, minute, second = date:match(inPattern)
month = month - 1
day = day - 1
year = year - 70
return string.format(outPattern, year, month, day, hour, minute, second)
end
I think that this should also be a lot quicker than constructing the table and calling string.format multiple times - but you'd have to profile that.
EDIT: I ran a quick test with two functions that concatenate "abc", "def" and "ghi" using both methods. Inserting those strings into a table an concatenating took 14 seconds (for several million runs of course) and using a single string.format() took 6 seconds. This does not take into account, that your code calls string.format() anyway (multiple times) - nor the difference between you figuring out the values by division and I by pattern matching. Pattern matching is certainly slower, but I doubt that it outweighs the gains from not having a table - and it's certainly convenient to be able to leverage os.time(). The fastest way would probably be figuring out the month and day manually and then using a single string.format(). But again - you'd have to profile that.
EDIT2: missingno has a good point with using the "*t" option with os.date to give you the values separately in the first place. Again, this depends on whether you want to have a table for convenience vs. a string for storage or whatever reasons. Combining "*t" and a single string.format:
function SecondsFormat(X)
if X <= 0 then return "" end
local date = os.date("*t", X) -- will give you a table
local outPattern = "%dy %dm %dd %dh %dm %ds"
date.month = date.month - 1
date.day = date.day - 1
date.year = date.year - 70
return string.format(outPattern, date.year, date.month, date.day, date.hour, date.min, date.sec)
end