Hard data structure and algorithm practice puzzle.
Given a base reflectance f0 as [r, g, b] and the cosine of the viewing angle, return the reflectance for each channel, rounded to 4 decimals: f0 + (1 - f0) x (1 - cos)^5. Looking straight on, you get f0. At a grazing angle, everything turns into a mirror. That's Schlick's shortcut for the Fresnel effect.
Solve it three ways: snap the jigsaw pieces together, fill in the blanks, or type it from scratch against the tests. This one is free.
Every line of the solution comes with a note explaining why it is there, and a step by step replay shows the code running. At level 3, where you type it yourself, you can ask for an AI mock interview on this problem that ends with a hiring manager's scorecard.
Then grow it into a real project with the Software Engineer button: this puzzle becomes the Frame Replay Log, a pipeline you can run and download. See every Software Engineer project.
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