As with the sample array from the docs, passing an equality returns inconsistent results:
[0, 4, 7, 10, 12].bsearch{ |x| x == 4} # => nil
[0, 4, 7, 10, 12].bsearch{ |x| x == 7} # => 7
[0, 4, 7, 10, 12].bsearch{ |x| x == 10} # => nil
[0, 4, 7, 10, 12].bsearch{ |x| x == 12} # => 12
[0, 4, 7, 10, 12].bsearch{ |x| x == 0} # => nil
As the docs show, you can get the correct result with >=; however you also get:
[0, 4, 7, 10, 12].bsearch {|x| x >= 6 } #=> 7
which is not what you want when you're looking specifically for 6. How do you get it to return consistent results?
According to the docs, to use bsearch, your array must be sorted with respect to the block.
Looking at your block results mapped out, [false, false, true, false, false] is not sorted.
Your block is currently set up to use find-minimum mode, because it returns true/false. Instead, try using find-any mode, with a block that returns -1/0/1.
[0, 4, 7, 10, 12].bsearch {|x| 7 <=> x } # => 7
[0, 4, 7, 10, 12].bsearch {|x| 6 <=> x } # => nil
In this case, we use the "comparator" or "spaceship" operator (<=>), which returns -1/0/1.