Woo hoo! Log base 2 for LI's is something I've been wanting for a long time.

On Fri, Feb 8, 2019 at 4:25 PM commits@source.squeak.org wrote:

Nicolas Cellier uploaded a new version of Kernel to project The Inbox: http://source.squeak.org/inbox/Kernel-nice.1213.mcz

==================== Summary ====================

Name: Kernel-nice.1213 Author: nice Time: 8 February 2019, 11:24:45.054968 pm UUID: d7a472c9-49dd-4b1d-b73a-13b9c7c8d4c5 Ancestors: Kernel-tonyg.1212

Provide a logarithm in base 2: log2. This (x log2) has an additional property that (x log: 2) has not: it is exact for exact powers of 2.

While implementing LogarithmicInterval, I recently wanted to use log: 2, and this was getting as bad as in Python. See https://stackoverflow.com/questions/931995/inaccurate-logarithm-in-python/54...

I don't know if this feature is really worth it, but whether it ends up integrated or not, at least it demonstrates how easy it is to patch Squeak.

=============== Diff against Kernel-tonyg.1212 ===============

Item was added:

• ----- Method: Float>>log2 (in category 'mathematical functions') -----
• log2

•   "Answer the base 2 logarithm of the receiver.
  

•   Arrange to answer exact result in case of exact power of 2."
  

•   ^ self significand ln / Ln2 + self exponent!
  
  

Item was added:

• ----- Method: Fraction>>log2 (in category 'mathematical functions') -----
• log2

•   "This function is defined because super log might overflow."
  

•   | res |
  

•   self <= 0 ifTrue: [DomainError signal: 'log2 is only defined for x > 0'].
  

•   "Test self < 1 before converting to float in order to avoid precision loss due to gradual underflow."
  

•   numerator < denominator ifTrue: [^self reciprocal log2 negated].
  

•   res := super log2.
  

•   res isFinite ifTrue: [^res].
  

•   ^numerator log2 - denominator log2!
  
  

Item was added:

• ----- Method: Integer>>log2 (in category 'mathematical functions') -----
• log2

•   self > 0 ifTrue: [^super log2].
  

•   ^DomainError signal: 'log2 is only defined for x > 0'!
  
  

Item was added:

• ----- Method: LargeNegativeInteger>>log2 (in category 'mathematical functions') -----
• log2

•   ^DomainError signal: 'log2 is only defined for x > 0'!
  
  

Item was added:

• ----- Method: LargePositiveInteger>>log2 (in category 'mathematical functions') -----
• log2

•   "This function is defined because super log2 might overflow."
  

•   | res h |
  

•   res := super log2.
  

•   res isFinite ifTrue: [^res].
  

•   h := self highBit.
  

•   ^h + (self / (1 << h)) asFloat log2!
  
  

Item was added:

• ----- Method: Number>>log2 (in category 'mathematical functions') -----
• log2

•   "Answer the base-2 log of the receiver."
  

•   ^self asFloat log2!
  
  

Item was changed: ----- Method: Number>>log: (in category 'mathematical functions') ----- log: aNumber "Answer the log base aNumber of the receiver."

•   aNumber = 2 ifTrue: [^self log2].
    ^self ln / aNumber ln!
  
  

Item was added:

• ----- Method: ScaledDecimal>>log2 (in category 'mathematical functions') -----
• log2

•   "Unlike super, avoid Float overflow/underflow"
  

•   ^self asFraction log2!