feat(fractional): support nested JSON Logic expressions as bucket variant names

[aepfli]

Summary

Implements flagd#1676 for the Rust core, mirroring the Java work in java-sdk-contrib#1737.

Allow arbitrary JSON Logic expressions as the variant name inside a fractional bucket definition:

{"fractional": [
  {"var": "email"},
  [{"if": [{"in": [{"var": "locale"}, ["us", "ca"]]}, "red", "grey"]}, 50],
  ["blue", 50]
]}

Nested fractional inside fractional is also supported:

{"fractional": [
  {"var": "email"},
  [{"fractional": [{"var": "tier"}, ["red", 50], ["blue", 50]]}, 50],
  [{"fractional": [{"var": "tier"}, ["green", 50], ["yellow", 50]]}, 50]
]}

Investigation

The operator already recurses into array elements via evaluator.evaluate(), so nested expressions worked at runtime. The only blocker was the JSON Schema enforcing "type": "string" on bucket variant names in strict mode, preventing configs from loading at all.

Changes

schemas/targeting.json

fractionalWeightArg first element changed from {"type": "string"} to {"oneOf": [{"type": "string"}, {"\": "#/definitions/anyRule"}]}, accepting any JSON Logic expression as a variant name.

src/operators/mod.rs

mod fractional → pub mod fractional to expose fractional::fractional for direct use in integration tests.

tests/integration_tests.rs

Five new tests:

• test_nested_if_in_bucket_variant_name — {"if": ...} resolves differently per locale context
• test_nested_fractional_inside_fractional — nested {"fractional": ...} inside outer buckets
• test_nested_var_in_bucket_variant_name — {"var": "preferred_theme"} pulled from context
• test_fractional_distribution_uniformity — statistical uniformity across the hash space (mirrors java-sdk-contrib#1740)
• test_fractional_boundary_hashes_do_not_panic — boundary keys don't panic (mirrors Java PR #1740)

Closes #30

[gemini-code-assist]

Summary of Changes

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This pull request introduces significant enhancements to the fractional targeting mechanism by enabling the use of nested JSON Logic expressions as bucket variant names. This change allows for highly dynamic and context-aware feature flag evaluations, aligning the Rust core with capabilities already present in other SDKs. The update primarily involves a schema modification to permit these expressions and is thoroughly validated with new integration tests covering various complex scenarios, including nested if and fractional rules, context variable usage, and statistical distribution integrity.

Highlights

• Schema Update for Fractional Targeting: The schemas/targeting.json file was updated to allow JSON Logic expressions as variant names within fractional bucket definitions, enabling more dynamic and context-aware targeting configurations.
• Module Visibility Adjustment: The fractional module in src/operators/mod.rs was made public to facilitate direct testing and integration.
• Comprehensive Integration Testing: Five new integration tests were added to validate nested JSON Logic expressions, ensure statistical uniformity of fractional distribution, and confirm robust handling of boundary hash values, mirroring work in other SDKs.

Changelog

• schemas/targeting.json
  • Updated the fractionalWeightArg definition to accept either a string or any JSON Logic rule (#/definitions/anyRule) for the variant name, thereby supporting dynamic variant resolution.
• src/operators/mod.rs
  • Changed the mod fractional; declaration to pub mod fractional; to expose the module for direct use in integration tests.
• tests/integration_tests.rs
  • Added test_nested_if_in_bucket_variant_name to verify that if expressions in variant names resolve correctly based on context.
  • Included test_nested_fractional_inside_fractional to confirm proper evaluation of nested fractional rules.
  • Implemented test_nested_var_in_bucket_variant_name to ensure context variables can be used as variant names.
  • Added test_fractional_distribution_uniformity to statistically validate the even distribution of fractional assignments.
  • Introduced test_fractional_boundary_hashes_do_not_panic to confirm robust handling of edge-case hash values without panics.

Activity

• No specific activity (comments, reviews, or progress updates) has been recorded for this pull request beyond its initial creation and description.

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[gemini-code-assist]

Code Review

This pull request correctly implements support for nested JSON Logic expressions as variant names in fractional bucketing. The schema change is appropriate, and the new integration tests provide good coverage for various nesting scenarios, including if, var, and nested fractional operators. The addition of statistical and boundary tests is also a great improvement. I have one suggestion to improve the performance of a new statistical test to prevent potential CI timeouts.