Fixed incorrect call for .map on a hash

[Kaakati]

Motivation / Background

This Pull Request has been created because There was a mistake in the method implementation where coder.map.key? is called instead of directly using coder.key?("value") The error was causing method 'key?' for #<Enumerator: {}:map> indicates that coder.map returns an enumerator, which does not have a key?

Used a constant for coder args as an empty hash, this is better for optimizing memory usage and ensuring immutability, especially in scenarios where the method might be called frequently without arguments.

Detail

This Pull Request changes:

@value = coder["value"] if coder.map.key?("value")
# with
@value = coder["value"] if coder.key?("value")

set default value for coder argument in each of the following methods:

init_with(coder = EMPTY_HASH)
encode_with(coder = EMPTY_HASH)

Updated the following line to reduce memory allocation.

UNINITIALIZED_ORIGINAL_VALUE = Object.new.freeze

Added a value check in initialize_dup(other)

def initialize_dup(other)
  # Using `defined?(@value)` ensures that it only proceed if `@value` is actually set
  # Ensures no unnecessary memory allocation by checking if `@value` is set and duplicable.
  if defined?(@value) && @value&.duplicable?
    @value = @value.dup
  end
end

Additional information

Checklist

Before submitting the PR make sure the following are checked:

• [*] This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• [*] Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• [*] Tests are added or updated if you fix a bug or add a feature.
• [*] CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[rafaelfranca]

Can you please explain why this code was failing, or write a test showing the problem?

[rafaelfranca]

This method should never be called without an argument, so why this change?

[rafaelfranca]

Not sure I get his. Why wouldn't @value be set?

[rafaelfranca]

This mapper can't be an enumerable. It is always an hash. See https://github.com/ruby/psych/blob/master/lib/psych/coder.rb#L14

[rafaelfranca]

The more I look at those changes, the more I think they are wrong. Perhaps you are miss using init_with and encode_with, that is the only reason why you would not get the right object to those methods.

For example, the YAML::Coder doesn't have a key? method, so this patch can't work. All the tests failing shows that.