[Kmymoney-devel] [Android Port] Help with computing the account balance

[Thomas Baumgart]

Eric,

On Friday 05 October 2012 13:48:20 you wrote:

> Hi Thomas:
> 
> Actually that was the function that I was looking at and referring to. I
> tried looking at it but I'll be honest, I am struggling trying to figure
> out just what exactly it is doing.
> 
> I see it takes in a String value currency, which I am assuming is the
> constant defined somewhere for the currency denomination the user is using,
> then it integer for precision and a boolean to for thousands. So I got all
> that down. :)
> 
> I am guessing that you are controlling the precision of
> accounts/transactions past two for things like stock prices and
> transactions right? So since I am not implementing this type I can safely
> "ignore" that, but need to keep it in the back of my mind for future
> reference.

Don't forget those currencies that are denominated in 1/1000 (some currencies 
used in the middle east are using that). Also, we know of currencies that 
don't have a fraction (the Hungarian Forint comes to mind).


> 
> It looks like the meat of the actual function starts with the on line 145:
> #if 1
> 
> and runs through line 161:
> mpz_class right = (valueRef() - mpq_class(left)) * denom;
> 
> I am lost looking at this stuff, the rest of the function looks like it is
> just string manipulation which is easy enough to replicate.

:)  It took me a while to write it as well ;)


> 
> What is a mpz_class? What is actually going on with lines 152, 154, 160 and
> 161?

Check http://gmplib.org/manual/C_002b_002b-Interface-
Integers.html#C_002b_002b-Interface-Integers for information about the 
mpz_class. It is basically an integer with arbitrary precision. mpq_class is a 
rational which can be thought of as two mpz_class members (nominator and 
denominator). Information for mpq can be found here: 
http://gmplib.org/manual/C_002b_002b-Interface-Rationals.html#C_002b_002b-
Interface-Rationals. AlkValue (and thus MyMoneyMoney) simply wrap the 
mpq_class value with a nice interface) and a bit of logic not found in libgmp.

'left' in the logic takes the part left of the decimal symbol, 'right' the 
part to the right (fraction).

Looking at

  value = 
static_cast<MyMoneyMoney>(convertDenominator(d)).valueRef().get_num();

For convertDenominator we find in alkvalue.h:

  /**
    * Returns the current value converted to the given @a denom (default is 
100
    * or two digits of precision). The rounding method used is controlled by
    * the @a how argument and defaults to @p RoundRound.
    */
  AlkValue convertDenominator(const int denom = 100, const RoundingMethod how 
= RoundRound) const;


The 'current value' is the value of the current object (not the variable 
'value' in the above assignment statement. The logic of this method is a bit 
tricky. Check the source in libalkimia and also the testcases for it contained 
in alkvaluetest.cpp in method

 void AlkValueTest::convertDenominator(void)

starting on line 426.

Maybe, I should drop an example here: Let's say, the 'current value' of the 
MyMoneyMoney object is "1/2" (or in the decimal world: 0.5). The nominator is 
1 and the denominator is 2. Calling convertDenominator(100) on this object 
will return a new AlkValue object with nominator 50 and denominator 100 (which 
still yields 0.5). Using 'valueRef().get_num()' returns the nominator as 
mpz_class object. Now the variable 'value' in the assignment contains '50' in 
our example, while d is 100.

Continuing in the MyMoneyMoney code, the assignment of 'left' is the next part 
of interest. value is divided by the converted denominator and 50/100 is 0 
(treated as an integers (mpz_class)) which is what we need left of the decimal 
symbol. 'right' will be assigned the result of "'current value' minus 'left' 
times the denomination we want":

   right = 0.5 - 0 * 100 = 50

The remainder - as you already noticed - is just a bit of string handling.

Have a nice weekend. If you still need more information, please just ask.

-- 

Regards

Thomas Baumgart

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