Implement our own RFC 6570 URI Template expansion and pattern matching to ensure symmetric behavior and eliminate dependency on third-party libraries with asymmetric implementations.

Background

Currently, Fedify relies on:

• url-template: For URI Template expansion (RFC 6570 compliant)
• uri-template-router: For pattern matching/parsing (non-standard, causes asymmetric behavior)

This leads to issues like #416 where expansion and parsing don't round-trip correctly, causing:

• Inconsistent encoding/decoding behavior
• Double-encoding problems in collections
• Unpredictable results for users

The asymmetric behavior is documented in uri-template-router issue #11.

Problem with current approach

The fundamental issue is that RFC 6570 only defines expansion behavior, not pattern matching. uri-template-router implements pattern matching as an extension, but this creates asymmetry:

Value → url-template (expand) → URL → uri-template-router (parse) → Different value

Proposed solution

Implement a custom RFC 6570 library that provides:

Core features

• Expansion: Full RFC 6570 compliance for all expression types
• Symmetric pattern matching: Custom implementation that guarantees round-trip consistency
• Type safety: Better TypeScript integration than current libraries

Expression types to support

• {var}: Simple string expansion
• {+var}: Reserved string expansion
• {/var}: Path segment expansion
• {?var}: Query parameter expansion
• {&var}: Query continuation expansion
• {#var}: Fragment expansion

Benefits

• Guaranteed symmetry: expand(parse(url)) === url and parse(expand(value)) === value
• Predictable behavior: Eliminates current inconsistencies
• Reduced dependencies: Remove url-template and uri-template-router
• Fedify-optimized: Can add ActivityPub-specific optimizations if needed

Implementation plan

Phase 1: RFC 6570 expansion

• Implement core expansion logic
• Support all expression types
• Comprehensive test suite against RFC 6570 examples
• Performance benchmarking vs url-template

Phase 2: Symmetric pattern matching

• Design pattern matching algorithm that mirrors expansion
• Implement parsing for all expression types
• Ensure round-trip consistency tests pass
• Handle edge cases and malformed inputs

Phase 3: Integration

• Replace url-template usage in Fedify
• Replace uri-template-router usage in Fedify
• Update tests and ensure no regressions
• Performance validation

Phase 4: Documentation

• Update RFC 6570 guide (Add URI Template syntax guide to documentation #417) to reference new implementation
• Document any Fedify-specific extensions or behaviors

Considerations

• Scope: This is a significant undertaking that essentially creates a new library
• Timeline: Should be treated as a long-term goal, not urgent
• Compatibility: Must maintain backward compatibility with existing Fedify APIs
• Testing: Requires extensive testing against RFC 6570 spec and real-world usage

Alternative approaches

• Contribute fixes to uri-template-router (but maintainer may not accept due to breaking changes)
• Find alternative libraries with symmetric behavior (research needed)
• Live with current limitations and document workarounds