This is a binary search implementation that returns the last element found.

CRuby API reference

The behavior is very similar to that of the existing Array#bsearch and Array#bsearch_index methods, except Find-minimum mode is replaced by a Find-maximum mode.

In Find-maximum mode, the last element/index for which the block evaluates to true is returned, as opposed to the first.

Analogously to the conditions in the block for the normal bsearch and bsearch_index methods, in bsearch_last and bsearch_last_index the block needs to satisfy the following in the Find-maximum mode:

The block is such that all true-evaluating elements precede all false-evaluating elements. This is the exact opposite of the condition for bsearch.

Examples:

a = [0, 4, 7, 10, 12]
a.bsearch_last {|x| x<=4 } # => 4
a.bsearch_last {|x| x<=6 } # => 4
a.bsearch_last {|x| x<=-1 } # => nil
a.bsearch_last {|x| x>=100 } # => nil

The Find-any mode behaves the same as in the Array#bsearch methods.

Run rake build. This takes the steps:

• create Makefile with ruby extconf.rb
• create .so file with make
• the .so file can be imported into Ruby with require bsearch_last.so (normally the extension is not needed, Ruby checks these files automatically, but here there's another file with the same name and the rb extension, which takes precedence).

require 'bsearch_last'

[1, 1, 2, 2, 3, 4].bsearch_last_index {|x| x <= 2 } # => 3

Run rake. This tests the Ruby and C versions of the functions.

rake bench

The C version is about 30% faster than the Ruby one for the somewhat long array used in the benchmark.