Fast Inverse Square Root — A Quake III Algorithm on HackerTube: recommended by 3 Hacker News commenters across 3 separate months, 2021–2022.
explainer · intermediate · evergreen
A walkthrough of the Quake III fast inverse square root, explaining where the constant 0x5f3759df comes from. It covers IEEE 754 floats, the bit hack that approximates a logarithm, and a Newton's method refinement.
HN recommends it as an approachable explanation that makes the famous algorithm finally make sense.
“This video is approachable and explains it pretty well”
4 Hacker News comments link this video. Each one links back to its thread.
Very interesting that this was posted as I literally JUST watched an even MORE interesting youtube upload about this very bit of code just last weekend. Here's the very fun video if anyone wants to take a look: ▶ this video
in “Copilot regurgitating Quake code, including sweary comments” · View on HN
Apparently the bit pattern of a float interpreted as an int is sorta-kinda it's own logarithm, which is why the famous fast inverse square root from quake 3 works: log(x^-½) = -½log(x) =~ -log(x) >> 1. This video is approachable and explains it pretty well: ▶ this video
in “Bit Patterns of Float” · View on HN
> The bits of a float as an integer literal are proportional to its log2 value (plus a constant), … I watched a video[0] a while back about that. Once that coincidence(?) was explained, Q_rsqrt finally made sense. [0]: ▶ this video (~20 minutes)
in “One does not simply calculate the absolute value” · View on HN
print("Hello World") /s But I've heard good things about the Fast Inverse Square Root Calculation algorithm in the Doom 3 source code. ▶ this video
in “Ask HN: What's a small example of some of the best code ever written?” · View on HN