# Which is faster? select case vs. lookup

June 15, 2010

So I am working on Walter's head moves which are working quite nicely. There are about 100 different moves stored in EEPROMS and can be cued-up and "played" via a number I have assigned to each move. I.e. I know that numbers 1-10 are all just sorta looking around moves. Move 47 is look quickly down and to the left, then a tilt. Etc. Etc. This leads me to the need to play some of the headmoves back at random. Not just at random, but I want one of the random picks to be part of a catagory of moves. Therefor I have a pack of "fwd looking around", a pack of "looking to the right", a pack of "looking to the left" etc. These numbers when put together, could almost be considered a string or array.

Also in the code, I have one byte (b19) clicking by... I have a inc b19 in many places of the code, mostly in the main loops. This variable is allowed to overflow and does nothing but count up, overflow and count up again. This is my randomizing number. Now, I know there is a random command but it sucks. Even if you seed it or bounce it off the timer, etc, it always has a pattern. With my "always changing variable" approch, I have very good, random results.

Now I might want to send a code to say "play a random fwd head move" and right now, I do this:

**ranfwd:****select case b19****case 1 to 23****let b13=1****case 24 to 46****let b13=3****case 47 to 69****let b13=5****case 70 to 92****let b13=7****case 93 to 115****let b13=9****case 116 to 138****let b13=11****case 139 to 161****let b13=13****case 162 to 184****let b13=15****case 185 to 208****let b13=17****case 209 to 230****let b13=19****case >230****let b13=21****endselect****'sertxd ("Ranfwd b13 ",#b13,CR,LF)**

return

This returns b13 as a randomly chosen number between 1 and 21 (only odds --not important) and that number is used to call up a move. Also, this method allows me to change probabilty and favor or unfavor one or many moves.

Now I have been thinking of putting these numbers into a lookup table, converting b19 down to a number that could be used with the lookup command and getting my b13 that way. It seems that the lookup table would be faster (and for sure, cleaner to see written-out) but would it really be faster with the extra math at the begining? Is there a rule for calculating speed in these cases?

Did one single thing make sense at all in this whole post?

## Your lookup table function

Your lookup table function

couldbe made faster than the big case statement you've currently got, but the math will need to be fairly simple.Something along the lines of:

b19 = b13/22

b19 = 2*b19+1 MAX 21

would give you 0-255 -> 1-21, odd numbers only, evenly distributed.

## math fu

input b19 holds all values 0-255 inclusive, evenly distributed

black box holds eleven hidden values: 0-10 inclusive, evenly distributed

output b13 holds eleven values: 1-21 inclusive, odd numbers only, evenly distributed

abracadabra algebra:

b13 = b19 * 11 / 256 ' now ranges 0-10 inclusive

b13 = b13 * 2 + 1 ' now ranges 1 -21, odd, inclusive

Your code example ignores b19 = 0. Is this on purpose? If it's important, write:

b13 = b19 min 1 - 1 * 11 / 255 ' now ranges 0-10 inclusive

b13 = b13 * 2 + 1 ' now ranges 1 -21, odd, inclusive

Note the lack of parentheses in this picaxe basic math. the position of the "min" operator is crucial, as we learned in your previous post. On your X2 mcu however, parentheses are welcome.

## Benchmarks will set you free

And the winner is: TABLE LOOKUPS!

The lookup routine uses ca .37 ms per lookup.

The case/select uses ca 7.5 ms per lookup.

The math routines use on avg 2.9 ms per lookup.

Here's the full output of my test program (running on non-modified 28x2). Times are in increments of 262 ms. Each lookup is performed for every input value ranging 1-254. Times fifty.

timer3=0

timer3=10

store table

baseline empty routine times 50

timer3=37

ctc case/select routine times 50

timer3=396

ctc table lookup routine times 50timer3=55rik simple mathfu routine times 50

timer3=173

rik complicated mathfu routine times 50

timer3=173

telefox simplest mathfu routine times 50

timer3=184

and that concludes the lotto numbers for today, stay tuned for Ran & Stimpy after the break

## Well, provided you've got

Well, provided you've got the memory spare to store 255 bytes of lookup table then we have a clear winner =)

Solid testing there rik!

## Memory to spare?

I'm running the x2's, sir! I got 4 times the room I used to have!

--Thanks rik, you da man.... I've got some recoding to do!