316: Mainline

Next, note that you can arrange for recursivePowerOfTwo to hand over a little less work to powerOfTwo by performing one of the multiplications by 2 in recursivePowerOfTwo itself, and subtracting 1 from powerOfTwo's argument:

Integer method definition • instance 
recursivePowerOfTwo 
  self = 0 
    ifTrue: [^ 1] 
    ifFalse: [^ 2 * (self - 1) powerOfTwo] 
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Transcript show: 4 recursivePowerOfTwo printString; cr  
Transcript 
16 

Clearly, recursivePowerOfTwo must work as long as one of the following two situations holds:

• The value of self is 0; in this situation, the recursivePowerOfTwo answers 1.
• The value of self is not 0, but the powerOfTwo method is able to compute the power of 2 that is 1 less than the value of self.