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Crystal Lang Bisect

Provides helpers for dealing with sorted Arrays. It uses binary search to reduce the number of comparisons.

Usage

There are two primary functions that you need to know about Bisect.insort and Bisect.bisect.

Bisect.insort adds a new element to the Array, but keeps the Array sorted:

require "bisect"
a = [1, 2, 4]
Bisect.insort(a, 3)
a == [1, 2, 3, 4]

Bisect.bisect gives you the index at which the element would have been inserted:

require "bisect"
a = ['a', 'b', 'd']
Bisect.bisect(a, 'c') == 2

If there are equal elements in the Array then insort will insert the element after the last equal element. Similarly bisect will return the index one higher than the last equal element. If you'd like to add new elements before equal elements, use insort_left and bisect_left. If you need to be explicit then insort_right and bisect_right are aliases for insort and bisect.

The above methods are useful for finding insertion points but can be tricky or awkward to use for common searching tasks. Hence the following methods focus on searching for specific values:

Bisect.find_gt(a, x) return left most value greater than x otherwise nil if no value greater than x exists:

scores = [33, 70, 77, 89, 89, 90, 99, 100]
Bisect.find_gt(scores, 65)  #=> 70
Bisect.find_gt(scores, 70)  #=> 77
Bisect.find_gt(scores, 101) #=> nil

Bisect.find_ge(a, x) returns left most value greater than or equal to x otherwise nil if no value greater than or equal to x exists:

Bisect.find_ge(scores, 65)  #=> 70
Bisect.find_ge(scores, 70)  #=> 70
Bisect.find_ge(scores, 101) #=> nil

Bisect.find_lt returns rightmost value less than x otherwise nil if no value less than x exists:

Bisect.find_lt(scores, 65)  #=> 33
Bisect.find_lt(scores, 70)  #=> 33
Bisect.find_lt(scores, 32)  #=> nil

Bisect.find_le returns rightmost value less than or equal to x otherwise nil if no value less than or equal to x exists:

Bisect.find_le(scores, 65)  #=> 33
Bisect.find_le(scores, 70)  #=> 70
Bisect.find_le(scores, 32)  #=> nil

Core ext

These methods are also available directly on Arrays

require "bisect/ext"
a = [1, 2, 4]
a.insort(3)
a == [1, 2, 3, 4]

scores = [33, 70, 77, 89, 89, 90, 99, 100]
scores.find_gt(70) #=> 77
scores.find_ge(70) #=> 70
scores.find_lt(70) #=> 33
scores.find_le(70) #=> 70