[ruby-core:121053] [Ruby master Feature#21135] Feature request: Enumerable.compare_count

Issue #21135 has been updated by kddnewton (Kevin Newton).


We already have lazy enumerators, so I think your request can be simplified a bit by taking advantage of that and defining Enumerator::Lazy#length to return an object that knows how to compare itself to other stuff. Something like:

```ruby
class Enumerator
  class Lazy
    class Length
      def initialize(enum)
        @enum = enum
      end
    
      def force = @enum.to_a.length
      def <=>(value) = force <=> value
    
      def <(value) = value >= 0 && @enum.take(value).to_a.length < value
      def <=(value) = value >= 0 && @enum.take(value + 1).to_a.length <= value
      def >(value) = value <= 0 || @enum.take(value + 1).to_a.length > value
      def >=(value) = value <= 0 || @enum.take(value).to_a.length == value
      def ==(value) = value >= 0 && @enum.take(value + 1).to_a.length == value
      def !=(value) = value < 0 || @enum.take(value + 1).to_a.length != value
    end

    def length = Length.new(self)
  end
end
```

In your example, instead of `d.compare_count(op, x)` this would instead be `d.lazy.length.public_send(op, x)`, and instead of `d.compare_count(op, x){|p| Prime.prime?(p)}` it would instead be `d.lazy.select { |p| Prime.prime?(p) }.length.public_send(op, x)`.

I don't know if this is applicable enough to other projects that it would merit being in core, but I _do_ think this would be simpler than the requested method.

----------------------------------------
Feature #21135: Feature request: Enumerable.compare_count
https://bugs.ruby-lang.org/issues/21135#change-111968

* Author: Student (Nathan Zook)
* Status: Open
----------------------------------------
Enumerables at times can be quite expensive to complete expand.  If all is desired is to compare the count, we can terminate the enumeration once the result is known.

The functionality that I envision, if implemented in ruby, might look like this:

```ruby
module Enumerable
  def compare_count(operation, standard, &blk)
    case operation
    when :<
      compare_count_lt(standard, &blk)
    when :>=
      !compare_count_lt(standard, &blk)
    when :>
      compare_count_spaceship(standard, &blk) == 1
    when :<=
      compare_count_spaceship(standard, &blk) != 1
    when :==
      compare_count_spaceship(standard, &blk) == 0
    when :!=
      compare_count_spaceship(standard, &blk) != 0
    end
  end

  def compare_count_lt(standard, &blk)
    truncate_count(standard - 1, &blk) < standard
  end

  def compare_count_spaceship(standard, &blk)
    truncate_count(standard, &blk) <=> standard
  end

  def truncate_count(limit, &blk)
    return 0 if limit < 0
    found = 0
    each do |ele|
      found += 1 unless blk && !yield(ele)
      break if found > limit
    end
    found
  end
end

class Demo
  include Enumerable

  attr_reader :capacity
  attr_reader :cnt

  def initialize(capacity)
    @capacity = capacity
  end

  def each
    @cnt = 0
    while cnt < capacity
      @cnt += 1
      yield cnt
    end
  end
end

%i{< <= >= > == !=}.each do |op|
  d = Demo.new(3)
  (-1..5).each do |x|
    puts [op, x, d.compare_count(op, x), d].inspect
  end
end


require 'prime'

%i{< <= >= > == !=}.each do |op|
  d = Demo.new(6)
  (-1..5).each do |x|
    puts [op, x, d.compare_count(op, x){|p| Prime.prime?(p)} , d].inspect
  end
end

```

```
[:<, -1, false, #<Demo:0x034d567c @capacity=3>]
[:<, 0, false, #<Demo:0x034d567c @capacity=3>]
[:<, 1, false, #<Demo:0x034d567c @capacity=3, @cnt=1>]
[:<, 2, false, #<Demo:0x034d567c @capacity=3, @cnt=2>]
[:<, 3, false, #<Demo:0x034d567c @capacity=3, @cnt=3>]
[:<, 4, true, #<Demo:0x034d567c @capacity=3, @cnt=3>]
[:<, 5, true, #<Demo:0x034d567c @capacity=3, @cnt=3>]
[:<=, -1, false, #<Demo:0x034d512c @capacity=3>]
[:<=, 0, false, #<Demo:0x034d512c @capacity=3, @cnt=1>]
[:<=, 1, false, #<Demo:0x034d512c @capacity=3, @cnt=2>]
[:<=, 2, false, #<Demo:0x034d512c @capacity=3, @cnt=3>]
[:<=, 3, true, #<Demo:0x034d512c @capacity=3, @cnt=3>]
[:<=, 4, true, #<Demo:0x034d512c @capacity=3, @cnt=3>]
[:<=, 5, true, #<Demo:0x034d512c @capacity=3, @cnt=3>]
[:>=, -1, true, #<Demo:0x034d4d44 @capacity=3>]
[:>=, 0, true, #<Demo:0x034d4d44 @capacity=3>]
[:>=, 1, true, #<Demo:0x034d4d44 @capacity=3, @cnt=1>]
[:>=, 2, true, #<Demo:0x034d4d44 @capacity=3, @cnt=2>]
[:>=, 3, true, #<Demo:0x034d4d44 @capacity=3, @cnt=3>]
[:>=, 4, false, #<Demo:0x034d4d44 @capacity=3, @cnt=3>]
[:>=, 5, false, #<Demo:0x034d4d44 @capacity=3, @cnt=3>]
[:>, -1, true, #<Demo:0x034d4998 @capacity=3>]
[:>, 0, true, #<Demo:0x034d4998 @capacity=3, @cnt=1>]
[:>, 1, true, #<Demo:0x034d4998 @capacity=3, @cnt=2>]
[:>, 2, true, #<Demo:0x034d4998 @capacity=3, @cnt=3>]
[:>, 3, false, #<Demo:0x034d4998 @capacity=3, @cnt=3>]
[:>, 4, false, #<Demo:0x034d4998 @capacity=3, @cnt=3>]
[:>, 5, false, #<Demo:0x034d4998 @capacity=3, @cnt=3>]
[:==, -1, false, #<Demo:0x034d4600 @capacity=3>]
[:==, 0, false, #<Demo:0x034d4600 @capacity=3, @cnt=1>]
[:==, 1, false, #<Demo:0x034d4600 @capacity=3, @cnt=2>]
[:==, 2, false, #<Demo:0x034d4600 @capacity=3, @cnt=3>]
[:==, 3, true, #<Demo:0x034d4600 @capacity=3, @cnt=3>]
[:==, 4, false, #<Demo:0x034d4600 @capacity=3, @cnt=3>]
[:==, 5, false, #<Demo:0x034d4600 @capacity=3, @cnt=3>]
[:!=, -1, true, #<Demo:0x034d4268 @capacity=3>]
[:!=, 0, true, #<Demo:0x034d4268 @capacity=3, @cnt=1>]
[:!=, 1, true, #<Demo:0x034d4268 @capacity=3, @cnt=2>]
[:!=, 2, true, #<Demo:0x034d4268 @capacity=3, @cnt=3>]
[:!=, 3, false, #<Demo:0x034d4268 @capacity=3, @cnt=3>]
[:!=, 4, true, #<Demo:0x034d4268 @capacity=3, @cnt=3>]
[:!=, 5, true, #<Demo:0x034d4268 @capacity=3, @cnt=3>]
[:<, -1, false, #<Demo:0x00007f64147c37e0 @capacity=6>]
[:<, 0, false, #<Demo:0x00007f64147c37e0 @capacity=6>]
[:<, 1, false, #<Demo:0x00007f64147c37e0 @capacity=6, @cnt=2>]
[:<, 2, false, #<Demo:0x00007f64147c37e0 @capacity=6, @cnt=3>]
[:<, 3, false, #<Demo:0x00007f64147c37e0 @capacity=6, @cnt=5>]
[:<, 4, true, #<Demo:0x00007f64147c37e0 @capacity=6, @cnt=6>]
[:<, 5, true, #<Demo:0x00007f64147c37e0 @capacity=6, @cnt=6>]
[:<=, -1, false, #<Demo:0x00007f64147c21d8 @capacity=6>]
[:<=, 0, false, #<Demo:0x00007f64147c21d8 @capacity=6, @cnt=2>]
[:<=, 1, false, #<Demo:0x00007f64147c21d8 @capacity=6, @cnt=3>]
[:<=, 2, false, #<Demo:0x00007f64147c21d8 @capacity=6, @cnt=5>]
[:<=, 3, true, #<Demo:0x00007f64147c21d8 @capacity=6, @cnt=6>]
[:<=, 4, true, #<Demo:0x00007f64147c21d8 @capacity=6, @cnt=6>]
[:<=, 5, true, #<Demo:0x00007f64147c21d8 @capacity=6, @cnt=6>]
[:>=, -1, true, #<Demo:0x00007f64147c0860 @capacity=6>]
[:>=, 0, true, #<Demo:0x00007f64147c0860 @capacity=6>]
[:>=, 1, true, #<Demo:0x00007f64147c0860 @capacity=6, @cnt=2>]
[:>=, 2, true, #<Demo:0x00007f64147c0860 @capacity=6, @cnt=3>]
[:>=, 3, true, #<Demo:0x00007f64147c0860 @capacity=6, @cnt=5>]
[:>=, 4, false, #<Demo:0x00007f64147c0860 @capacity=6, @cnt=6>]
[:>=, 5, false, #<Demo:0x00007f64147c0860 @capacity=6, @cnt=6>]
[:>, -1, true, #<Demo:0x00007f64147df260 @capacity=6>]
[:>, 0, true, #<Demo:0x00007f64147df260 @capacity=6, @cnt=2>]
[:>, 1, true, #<Demo:0x00007f64147df260 @capacity=6, @cnt=3>]
[:>, 2, true, #<Demo:0x00007f64147df260 @capacity=6, @cnt=5>]
[:>, 3, false, #<Demo:0x00007f64147df260 @capacity=6, @cnt=6>]
[:>, 4, false, #<Demo:0x00007f64147df260 @capacity=6, @cnt=6>]
[:>, 5, false, #<Demo:0x00007f64147df260 @capacity=6, @cnt=6>]
[:==, -1, false, #<Demo:0x00007f64147dd898 @capacity=6>]
[:==, 0, false, #<Demo:0x00007f64147dd898 @capacity=6, @cnt=2>]
[:==, 1, false, #<Demo:0x00007f64147dd898 @capacity=6, @cnt=3>]
[:==, 2, false, #<Demo:0x00007f64147dd898 @capacity=6, @cnt=5>]
[:==, 3, true, #<Demo:0x00007f64147dd898 @capacity=6, @cnt=6>]
[:==, 4, false, #<Demo:0x00007f64147dd898 @capacity=6, @cnt=6>]
[:==, 5, false, #<Demo:0x00007f64147dd898 @capacity=6, @cnt=6>]
[:!=, -1, true, #<Demo:0x00007f64147dbf20 @capacity=6>]
[:!=, 0, true, #<Demo:0x00007f64147dbf20 @capacity=6, @cnt=2>]
[:!=, 1, true, #<Demo:0x00007f64147dbf20 @capacity=6, @cnt=3>]
[:!=, 2, true, #<Demo:0x00007f64147dbf20 @capacity=6, @cnt=5>]
[:!=, 3, false, #<Demo:0x00007f64147dbf20 @capacity=6, @cnt=6>]
[:!=, 4, true, #<Demo:0x00007f64147dbf20 @capacity=6, @cnt=6>]
[:!=, 5, true, #<Demo:0x00007f64147dbf20 @capacity=6, @cnt=6>]

```





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