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I will first start with a scenario, I have to apply some adjustments to a particular value. These adjustments are either compound or non-compounded and they can either be added or subtracted to the value. They are executed in the order they are stored (they are stored in a database with a sequence number starting at 1).

The properties of these adjustments are stored in a database, so once I get them, I have access to the value, whether it is compounded or not and whether it should be added or subtracted from the base.

In all of the scenarios, I have 2 objects. The first object is to hold the adjustments which I read in from the database. The second object which I happen to call Divisor holds the collection of values depending on whether the adjustment is compounded, non-compounded, and should be subtracted or added. This will become more clear in my example.

Algorithm:

loop through each adjustment    
  if adjustment is compounded    
    if compound adjustment is to be added  
      assign 1 + (amount/100) to the divisor value  
    else if compound adjustment is to be subtracted  
      assign 1 - (amount/100) to the divisor value    
  else if adjustment is non-compounded  
    assign amount/100 to the divisor value  

Once I have all the divisors, I loop through each divisor's value to construct the equation and apply it to my base value, so:

keep track of the calculated value in a variable called value initialized to 1.  

loop through each divisor  
  if value is compound    
    value = value * divisor value  
  else if value is non-compound    
    if divisor is to be added    
      value = value + divisor value  
    else if divisor is to be subtracted  
      value = value - divisor value    

adjustedTotal = total * value

My first scenario involves a mixture of compounded and non-compounded percentages and adding and removing them from the base value:

Base Value: 100

Adjustments:

+3% compounded  
-6% compounded  
+10% non-compounded  
-8%  non-compounded  

If I apply the above algorithm to the above adjustments and the above base value, I get 98.82 as my adjustedTotal. If I take 98.82 as total and apply the adjustments, but divide 98.82 by value, I get back to 100, so it appears that this works.

My second scenario is where I have all compound values and they are all added. If I use 100 as the base and apply a +10% and +7% compound adjustment, I get 117.7 which I believe is correct. Now if I take 117.7 and apply a -10% and -7% adjustment, I don't get back to 100, I get 140.62 if I divide the 117.7 by the value and 98.51 if I multiple 117.7 by the value.

My third scenario is where I have all non-compounded values and they are all added. If I use 100 as the base and apply a +10% and +7% non-compounded adjustment, I get 117 which I believe is correct. If I take 117 and apply a -10% and +7% non-compound adjustment, I get 97.11.

There are other scenarios which I won't bother with right here, but they include:

All Compound, Some Add, Some Subtract
All Non-Compound, Some Add, Some Subtract

In a nutshell, basically, I am looking for an algorithm where I can cover all the bases, but as you can see, it breaks in some scenarios and breaks in other scenarios. If I fix one scenario, I end up breaking another.

My setup is this, given a base value and a set of adjustments. If I apply these adjustments, I should get the correct value and if I reverse these adjustments, I should get back my original value.

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