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This PR creates doc/bsearch.rdoc to provide common documentation for bsearch in Array and Range.
120 lines
4.2 KiB
Text
120 lines
4.2 KiB
Text
== Binary Searching
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A few Ruby methods support binary searching in a collection:
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Array#bsearch:: Returns an element selected via a binary search
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as determined by a given block.
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Array#bsearch_index:: Returns the index of an element selected via a binary search
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as determined by a given block.
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Range#bsearch:: Returns an element selected via a binary search
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as determined by a given block.
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Each of these methods returns an enumerator if no block is given.
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Given a block, each of these methods returns an element (or element index) from +self+
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as determined by a binary search.
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The search finds an element of +self+ which meets
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the given condition in <tt>O(log n)</tt> operations, where +n+ is the count of elements.
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+self+ should be sorted, but this is not checked.
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There are two search modes:
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Find-minimum mode:: method +bsearch+ returns the first element for which
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the block returns +true+;
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the block must return +true+ or +false+.
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Find-any mode:: method +bsearch+ some element, if any, for which
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the block returns zero.
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the block must return a numeric value.
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The block should not mix the modes by sometimes returning +true+ or +false+
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and other times returning a numeric value, but this is not checked.
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<b>Find-Minimum Mode</b>
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In find-minimum mode, the block must return +true+ or +false+.
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The further requirement (though not checked) is that
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there are no indexes +i+ and +j+ such that:
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- <tt>0 <= i < j <= self.size</tt>.
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- The block returns +true+ for <tt>self[i]</tt> and +false+ for <tt>self[j]</tt>.
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Less formally: the block is such that all +false+-evaluating elements
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precede all +true+-evaluating elements.
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In find-minimum mode, method +bsearch+ returns the first element
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for which the block returns +true+.
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Examples:
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a = [0, 4, 7, 10, 12]
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a.bsearch {|x| x >= 4 } # => 4
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a.bsearch {|x| x >= 6 } # => 7
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a.bsearch {|x| x >= -1 } # => 0
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a.bsearch {|x| x >= 100 } # => nil
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r = (0...a.size)
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r.bsearch {|i| a[i] >= 4 } #=> 1
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r.bsearch {|i| a[i] >= 6 } #=> 2
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r.bsearch {|i| a[i] >= 8 } #=> 3
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r.bsearch {|i| a[i] >= 100 } #=> nil
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r = (0.0...Float::INFINITY)
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r.bsearch {|x| Math.log(x) >= 0 } #=> 1.0
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These blocks make sense in find-minimum mode:
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a = [0, 4, 7, 10, 12]
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a.map {|x| x >= 4 } # => [false, true, true, true, true]
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a.map {|x| x >= 6 } # => [false, false, true, true, true]
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a.map {|x| x >= -1 } # => [true, true, true, true, true]
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a.map {|x| x >= 100 } # => [false, false, false, false, false]
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This would not make sense:
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a.map {|x| x == 7 } # => [false, false, true, false, false]
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<b>Find-Any Mode</b>
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In find-any mode, the block must return a numeric value.
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The further requirement (though not checked) is that
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there are no indexes +i+ and +j+ such that:
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- <tt>0 <= i < j <= self.size</tt>.
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- The block returns a negative value for <tt>self[i]</tt>
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and a positive value for <tt>self[j]</tt>.
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- The block returns a negative value for <tt>self[i]</tt> and zero <tt>self[j]</tt>.
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- The block returns zero for <tt>self[i]</tt> and a positive value for <tt>self[j]</tt>.
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Less formally: the block is such that:
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- All positive-evaluating elements precede all zero-evaluating elements.
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- All positive-evaluating elements precede all negative-evaluating elements.
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- All zero-evaluating elements precede all negative-evaluating elements.
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In find-any mode, method +bsearch+ returns some element
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for which the block returns zero, or +nil+ if no such element is found.
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Examples:
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a = [0, 4, 7, 10, 12]
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a.bsearch {|element| 7 <=> element } # => 7
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a.bsearch {|element| -1 <=> element } # => nil
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a.bsearch {|element| 5 <=> element } # => nil
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a.bsearch {|element| 15 <=> element } # => nil
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a = [0, 100, 100, 100, 200]
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r = (0..4)
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r.bsearch {|i| 100 - a[i] } #=> 1, 2 or 3
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r.bsearch {|i| 300 - a[i] } #=> nil
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r.bsearch {|i| 50 - a[i] } #=> nil
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These blocks make sense in find-any mode:
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a = [0, 4, 7, 10, 12]
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a.map {|element| 7 <=> element } # => [1, 1, 0, -1, -1]
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a.map {|element| -1 <=> element } # => [-1, -1, -1, -1, -1]
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a.map {|element| 5 <=> element } # => [1, 1, -1, -1, -1]
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a.map {|element| 15 <=> element } # => [1, 1, 1, 1, 1]
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This would not make sense:
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a.map {|element| element <=> 7 } # => [-1, -1, 0, 1, 1]
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