ruby--ruby/doc/bsearch.rdoc

== Binary Searching

A few Ruby methods support binary searching in a collection:

Array#bsearch:: Returns an element selected via a binary search
                as determined by a given block.
Array#bsearch_index:: Returns the index of an element selected via a binary search
                      as determined by a given block.
Range#bsearch:: Returns an element selected via a binary search
                as determined by a given block.

Each of these methods returns an enumerator if no block is given.

Given a block, each of these methods returns an element (or element index) from +self+
as determined by a binary search.
The search finds an element of +self+ which meets
the given condition in <tt>O(log n)</tt> operations, where +n+ is the count of elements.
+self+ should be sorted, but this is not checked.

There are two search modes:

Find-minimum mode:: method +bsearch+ returns the first element for which
                    the block returns +true+;
                    the block must return +true+ or +false+.
Find-any mode:: method +bsearch+ some element, if any, for which
                the block returns zero.
                the block must return a numeric value.

The block should not mix the modes by sometimes returning +true+ or +false+
and other times returning a numeric value, but this is not checked.

<b>Find-Minimum Mode</b>

In find-minimum mode, the block must return +true+ or +false+.
The further requirement (though not checked) is that
there are no indexes +i+ and +j+ such that:

- <tt>0 <= i < j <= self.size</tt>.
- The block returns +true+ for <tt>self[i]</tt> and +false+ for <tt>self[j]</tt>.

Less formally: the block is such that all +false+-evaluating elements
precede all +true+-evaluating elements.

In find-minimum mode, method +bsearch+ returns the first element
for which the block returns +true+.

Examples:

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

  r = (0...a.size)
  r.bsearch {|i| a[i] >= 4 } #=> 1
  r.bsearch {|i| a[i] >= 6 } #=> 2
  r.bsearch {|i| a[i] >= 8 } #=> 3
  r.bsearch {|i| a[i] >= 100 } #=> nil
  r = (0.0...Float::INFINITY)
  r.bsearch {|x| Math.log(x) >= 0 } #=> 1.0

These blocks make sense in find-minimum mode:

  a = [0, 4, 7, 10, 12]
  a.map {|x| x >= 4 } # => [false, true, true, true, true]
  a.map {|x| x >= 6 } # => [false, false, true, true, true]
  a.map {|x| x >= -1 } # => [true, true, true, true, true]
  a.map {|x| x >= 100 } # => [false, false, false, false, false]

This would not make sense:

  a.map {|x| x == 7 } # => [false, false, true, false, false]

<b>Find-Any Mode</b>

In find-any mode, the block must return a numeric value.
The further requirement (though not checked) is that
there are no indexes +i+ and +j+ such that:

- <tt>0 <= i < j <= self.size</tt>.
- The block returns a negative value for <tt>self[i]</tt>
  and a positive value for <tt>self[j]</tt>.
- The block returns a negative value for <tt>self[i]</tt> and zero <tt>self[j]</tt>.
- The block returns zero for <tt>self[i]</tt> and a positive value for <tt>self[j]</tt>.

Less formally: the block is such that:

- All positive-evaluating elements precede all zero-evaluating elements.
- All positive-evaluating elements precede all negative-evaluating elements.
- All zero-evaluating elements precede all negative-evaluating elements.

In find-any mode, method +bsearch+ returns some element
for which the block returns zero, or +nil+ if no such element is found.

Examples:

  a = [0, 4, 7, 10, 12]
  a.bsearch {|element| 7 <=> element } # => 7
  a.bsearch {|element| -1 <=> element } # => nil
  a.bsearch {|element| 5 <=> element } # => nil
  a.bsearch {|element| 15 <=> element } # => nil

  a = [0, 100, 100, 100, 200]
  r = (0..4)
  r.bsearch {|i| 100 - a[i] } #=> 1, 2 or 3
  r.bsearch {|i| 300 - a[i] } #=> nil
  r.bsearch {|i|  50 - a[i] } #=> nil

These blocks make sense in find-any mode:

  a = [0, 4, 7, 10, 12]
  a.map {|element| 7 <=> element } # => [1, 1, 0, -1, -1]
  a.map {|element| -1 <=> element } # => [-1, -1, -1, -1, -1]
  a.map {|element| 5 <=> element } # => [1, 1, -1, -1, -1]
  a.map {|element| 15 <=> element } # => [1, 1, 1, 1, 1]

This would not make sense:

  a.map {|element| element <=> 7 } # => [-1, -1, 0, 1, 1]