Implement class Evaluation Context

[NeaguGeorgiana23]

This PR

Implements the EvaluationContext class and its associated Builder. The EvaluationContext is immutable and represents a container for contextual information used during flag evaluation, such as a targeting key and custom attributes.

• Includes a static merge method that combines multiple contexts. Here, the attributes are merged with higher precedence overwrites and the targeting key is resolved by selecting the first valid key found in order of precedence (highest to lowest).

• Builder Pattern was used to make the EvaluationContext immutable

Related Issues

Fixes #55

[gemini-code-assist]

Summary of Changes

Hello @NeaguGeorgiana23, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces the foundational EvaluationContext class, a critical component for managing contextual information during feature flag evaluation. By leveraging the Builder pattern, it ensures that context objects are immutable and consistently constructed. The implementation also includes a sophisticated merge mechanism, allowing for the composition of multiple contexts while respecting defined precedence rules for both attributes and targeting keys. This enhancement provides a robust and predictable way to handle dynamic evaluation environments.

Highlights

• New Class Implementation: Introduced the EvaluationContext class and its associated Builder pattern for creating immutable context objects.
• Immutability and Builder Pattern: The EvaluationContext is designed to be immutable, with its construction strictly managed through a Builder to ensure consistent state.
• Contextual Data Storage: The EvaluationContext serves as a container for a targeting key and custom attributes, essential for flag evaluation.
• Context Merging Logic: A static merge method has been implemented to combine multiple EvaluationContext instances. This method handles attribute merging with higher precedence overwriting lower, and resolves the targeting key by selecting the first valid key based on precedence.
• Comprehensive Unit Tests: A new test file, evaluation_context_test.cpp, has been added with extensive unit tests covering the EvaluationContext's functionality, including attribute handling, targeting key management, and the complex merge logic.
• Build System Updates: The openfeature/BUILD and test/BUILD files have been updated to include the new evaluation_context.cpp source file and the evaluation_context_test target, respectively.

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[gemini-code-assist]

Code Review

This pull request implements the EvaluationContext class and its builder, which is a great addition. The implementation is solid, but I have a few suggestions to improve performance, API safety, and design clarity. Specifically, I've recommended using heterogeneous lookup for the attributes map to avoid unnecessary string allocations. I've also suggested a change to how a missing targeting_key is represented, using std::nullopt instead of an empty string to better align with the concept of an "undefined" value. Finally, I've pointed out a potential API safety issue with storing const char* in attributes and suggested standardizing on std::string.

[gemini-code-assist]

Using an empty string to represent a missing targeting key can be ambiguous. The OpenFeature specification says the key should be "undefined" if not set. Using std::optional throughout (from builder to the context object) by passing targeting_key_ directly to the constructor would better represent this "undefined" state as std::nullopt. This makes the implementation cleaner and less ambiguous. The corresponding test DefaultBuilderCreatesEmptyContext will need to be updated to expect std::nullopt.

Suggested change

	return EvaluationContext(targeting_key_.value_or(""), attributes_);
	return EvaluationContext(targeting_key_, attributes_);

[oxddr]

Please have a look at suggestions from Gemini and apply whenever they make sense.