Christopher Haster 46e22b2a38 Adopted lfs_ctz_index implementation using popcount ...

This reduces the O(n^2logn) runtime to read a file to only O(nlog).
The extra O(n) did not touch the disk, so it isn't a problem until the
files become very large, but this solution comes with very little cost.

Long story short, you can find the block index + offset pair for a
CTZ linked-list with this series of formulas:

n' = floor(N / (B - 2w/8))
N' = (B - 2w/8)n' + (w/8)popcount(n')
off' = N - N'
n, off =
  n'-1, off'+B                if off' <  0
  n',   off'+(w/8)(ctz(n')+1) if off' >= 0

For the long story, you will need to see the updated DESIGN.md

2017-10-18 00:44:30 -05:00

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