feat: add EppoPrecomputedClient

[leoromanovsky]

Motivation

The precomputed client provides a new way to use Eppo feature flags where all assignments are computed server-side for a specific subject. This eliminates client-side evaluation overhead and provides instant lookups.

Changes

Client (EppoPrecomputedClient):

Assignment Methods:

• getStringAssignment(), getBooleanAssignment(), getIntegerAssignment(), getNumericAssignment(), getJSONAssignment()
• getBanditAction() - Returns variation + recommended action

Initialization:

• Builder pattern with extensive configuration options
• Supports offline mode with initial configuration bytes
• Loads from disk cache on startup (races with network for speed on slow devices)
• Fetches fresh configuration from edge endpoint

Polling:

• Background polling with configurable interval and jitter
• Polling starts after initial fetch completes (not during)
• Pause/resume/stop lifecycle methods

Logging:

• Assignment and bandit logging with deduplication caches
• Decodes Base64-encoded extra logging fields
• Includes SDK metadata (version, language, obfuscation flag)

Error Handling:

• Graceful mode (default): returns defaults on errors
• Non-graceful mode: throws exceptions

Tests (EppoPrecomputedClientTest):

• Builder validation (missing API key, subject key)
• All flag type assignments with mock data
• Assignment logging and cache deduplication
• Bandit actions
• Type mismatch handling
• Offline mode
• Integration with sdk-test-data files

Decisions

• Singleton pattern: Matches existing EppoClient API
• Subject key required at init: Precomputed assignments are subject-specific
• Polling after fetch: Prevents concurrent fetches during initialization
• Null safety in bandit decoding: Handles malformed responses gracefully
• No API key in logs: Redacted for security
• In-memory update on disk failure: Ensures client works even with storage issues

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PR Stack

This PR is part of the precomputed client feature, split for easier review:

main
├── precomputed/1-base-cache-file
│   └─┬─ precomputed/3-storage-layer ⬅ (base)
│     │   └── 👉 precomputed/4-client (this PR)
│     │       └── precomputed/5-example-app
│     │
└── precomputed/2-dtos-and-utils ──────┘

Why this structure: The client is the largest piece (~1000 lines) but depends on all the infrastructure built in previous PRs. Reviewing it separately allows focus on the client logic without being distracted by DTO definitions or storage implementation.

Merge order: PRs 1, 2, and 3 must be merged first.

[Copilot]

Pull request overview

This PR introduces EppoPrecomputedClient, a new client for Eppo feature flags that receives precomputed assignments from the server rather than performing client-side evaluation. This eliminates evaluation overhead and provides instant flag lookups.

Changes:

• Implements EppoPrecomputedClient with builder pattern initialization, assignment methods for all flag types (string, boolean, integer, numeric, JSON), bandit action support, and configurable polling
• Adds comprehensive test suite covering offline mode, assignment logging, cache deduplication, type mismatches, and integration with sdk-test-data files
• Supports both online (fetch + poll) and offline modes, graceful/non-graceful error handling, and disk cache persistence with network racing for fast initialization

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 8 comments.

File	Description
EppoPrecomputedClient.java	Core client implementation with assignment retrieval, HTTP fetching, polling lifecycle, logging, and builder-based initialization
EppoPrecomputedClientTest.java	Test suite covering builder validation, flag assignments, logging, caching, and sdk-test-data integration tests

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[sameerank]

A nit: would clean up these comments that are stating what is self-evident in the assertions

[sameerank]

Oh interesting, this metaData differs from iOS (which has obfuscated, sdkName, and sdkVersion).

But looks consistent with Android's EppoClient

android-sdk/eppo/src/androidTest/java/cloud/eppo/android/EppoClientTest.java

Lines 1043 to 1045 in 3209265

	expectedMeta.put("obfuscated", "true");
	expectedMeta.put("sdkLanguage", "android");
	expectedMeta.put("sdkLibVersion", BuildConfig.EPPO_VERSION);