docs: initial comparison document

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: Ic50cc37d50c50ef279f65e87011d252f6a6a6964

docs/COMPARISON.md

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+# FC vs Hydra
+
+FC is not exactly a competitor, but a direct replacement to Hydra with a limited
+set of features that are prioritized. While FC hopes to be a complete
+replacement, not everything can be considered in-scope for a team of FOSS
+developers without funding. _Should_ you create any issues, your feature may or
+may not be implemented depending on our own needs. Please keep that in mind.
+
+Below document contains a "feature matrix" of concepts and features that we've
+decided to think about. Not all of them will be fully implemented (i.e., we are
+a-okay with being less powerful in some regards) but FC _does_ aim to be better
+than Hydra in the long term, through different means; reliability, UX and
+performance.
+
+## Executive Summary
+
+FC currently implements more or less 50% of Hydra's core features, and has
+various improvements over Hydra's architecture. As of writing, some gaps (such
+as the plugin architecture, VCS diversity and notification integrations) remain.
+
+### Strengths
+
+- Modern Rust codebase with better error handling
+- Simpler, more maintainable architecture (5 Rust crates vs Perl & C++ monolith)
+- Better API-first design with proper REST structure
+- User management with argon2 password hashing and granular RBAC
+- Cleaner database schema (16 tables vs 70+)
+- Better Nix Flake support from day one
+- Improved & tested declarative jobsets
+
+### Weaknesses
+
+- Limited VCS support (Git only in FC vs 6 types in Hydra)
+- No plugin architecture for extensibility
+- Missing several notification integrations (Slack, BitBucket, etc.)
+- No declarative project specification (coming soon)
+- No coverage/build metrics collection
+
+TODO: add a better comparison matrix