Anthropic's C compiler also had an incredible test suite available, and was trained on compiler codebases. It might have been built "from scratch" but compilers are incredibly well-trodden ground.
And to contrast, Cursor's browser explicitly reviewed Servo's architecture before setting out, and still wound up like that.
The difference between using general knowledge and summaries and doing code-for-code rewrites cannot be understated. When creating `wedeo`, despite DIRECTLY reviewing FFmpeg's code, Claude would frequently make mistakes that would have to be fixed. I would constantly have it go and refer back to the ground truth FFmpeg source implementation. It did eventually work, but only after several rounds of reviews, and often dead-end investigations. Being able to directly inspect (and in some cases modify for extra debug output) FFmpeg at any given time was invaluable, and I doubt any rewrite would succeed without having that.
As for optimization, that seems to be more of a question of effort than whether it’s possible. I was able to take down the performance gap on Rust vs C (without Assembly) from 10x to 1.5x through detailed profiling and iterative improvements with Claude.
It also looks like the Anthropic C compiler was built from scratch. By contrast, `wedeo` was directly based on FFmpeg’s existing code. Going by spec and test suite only would have taken a lot longer, and the quality would have been significantly lower.