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Lu Pan (@uvdn7) profile picture

Lu Pan

@uvdn7

Working on distributed systems at Facebook. “uvdn7” is “lupan” rotated by 180.

 Boston
uvdn7.github.io
Joined March 2010
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Photo by Lu Pan (@uvdn7)Photo by Lu Pan (@uvdn7)Photo by Lu Pan (@uvdn7)Photo by Lu Pan (@uvdn7)Photo by Lu Pan (@uvdn7)Photo by Lu Pan (@uvdn7)Photo by Lu Pan (@uvdn7)Photo by Lu Pan (@uvdn7)Photo by Lu Pan (@uvdn7)Photo by Lu Pan (@uvdn7)
A really cool small piece of integer division code in `folly::BucketedTimeSeries` – uvdn7.github.io/bucketed-tim….

Integer Division in Bucketed Time Series

This post is about a cool piece of integer math code in folly::BucketedTimeSeries.

uvdn7.github.io
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I wrote this post on the mental model for C++ coro in March, and found myself referring back to it often. Let me just post it uvdn7.github.io/cpp-coro/.

A Mental Model for C++ Coroutine

C++ coroutine is not a library that is ready to go (e.g. std::vector). It is not even a trait (think of Rust’s Future trait) that library writers or users can implement (or the compiler generates for...

uvdn7.github.io
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Guice configuration is like makefile, and injection is like dynamic linking, but all in Java.
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OK I have my answer – I think you should switch.
shachaf (@shachaf) profile pictureshachaf@shachaf5 Jan 2025
My sister sent me this puzzle. It's so good!
You are playing the Monty Hall problem. However, you secretly know one of the goats is the former pet of an eccentric billionaire who lost it and is willing to pay an enormous amount for its return, way more than the car is worth. You really want that goat. The host is unaware of this. After you pick your door, as is traditional, the host opens one door, which he knows doesn't have the car. He reveals a goat, which you can tell is the ordinary goat and not the secretly valuable one. The host offers to let you switch doors. Should you?

ALT You are playing the Monty Hall problem. However, you secretly know one of the goats is the former pet of an eccentric billionaire who lost it and is willing to pay an enormous amount for its return, way more than the car is worth. You really want that goat. The host is unaware of this. After you pick your door, as is traditional, the host opens one door, which he knows doesn't have the car. He reveals a goat, which you can tell is the ordinary goat and not the secretly valuable one. The host offers to let you switch doors. Should you?

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Say there are 100 doors with 98 regular goats, 1 special goat and 1 car. You pick on door, then the host opens 98 doors with 98 goats behind them. These 98 goats happen to be all regular goats. Would you switch the door to maximize the probability of getting the special goat?
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[new blog post] Access a function's return value in C++ scope guard blog.the-pans.com/return-val…

Access a function’s return value in C++ scope guard

It is best to avoid accessing a function’s return value in a scope guard or we need to be really careful about the return value’s lifetime.

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[new blog post] When `__cxa_pure_virtual` is just a different flavor of SEGFAULT blog.the-pans.com/pure_virtu…

When __cxa_pure_virtual is just a different flavor of SEGFAULT

A program crashed during shutdown, with message pure virtual method called which came from __cxa_pure_virtual – where a pure virtual function points to, in its vtable. Its implementation involves...

uvdn7.github.io
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The gnu bash manual is great. It's well-structured and concise. Just wonderful. Lots of effort must went into it. gnu.org/software/bash/manual…
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The list of talks of NEDB Day 2024 is out! bu-disc.github.io/nedbday/20… I saw "Why Machine Learning for Automatically Optimizing Databases Doesn't Work" and was thinking Andy Pavlo would have something to say about it, then I realized that he's the one giving the talk! #NEDBDAY2024
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Not only the book Infinite Powers is fascinating (and I can’t put it down), I can feel @stevenstrogatz’s passion and love of the subject from the pages. The book is a wonderful gift. I am grateful.
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`async && closure && requires_explicit_memory_management && !borrow_checker` is really challenging. Borrow checker is really good to have especially when writing async code with closure! #rust
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[new blog post] Rust's early vs. late lifetime binding blog.the-pans.com/rusts-earl…

Rust’s early vs. late lifetime binding

Lifetime in many ways are similar to types but also different. The goal of this post is to explain my mental model of rust’s lifetime binding works, when it takes place, and a trick to force late...

uvdn7.github.io
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It appears that the rust compiler tries really hard to infer how _and_ what to capture when it comes to closures. Do you think the following program compiles? #rust
Image from Lu Pan (@uvdn7)
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It does indeed compile in the playground play.rust-lang.org/?version=…. But it doesn't on Compiler Explorer (godbolt.org/z/WjozhxGoW), or on my local machine. All tested on version 1.76.
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users.rust-lang.org/t/captur… is the answer. The actual capture algorithm is described here github.com/.../closure-doc/s…...

Capture the deref'ed value in closure

I think what we discovered here is that not only the compiler would infer how to capture the variables (and consequently the trait of the closure) - with or without the move keyword - to make the...

users.rust-lang.org
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Why a standalone redirect e.g. `2>/dev/null` gets interpreted as a `cat` command in zsh? The consequence is that then the shell will wait for the cat command which is waiting on stdin ... bash simply does nothing in this case.
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It looks like every value expression in rust behaves like xvalue in C++ – it has (or can have) an identity (so it can be borrowed) and can be moved from.
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`getcontext/setcontext` are not system calls but why there are getcontext(2)/setcontext(2) entries man7.org/linux/man-pages/man…? `man getcontext.2` also exists although it shows the content for getcontext(3).
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How come gcc has `extern inline` with inline definition in the header without running into ODR (xmmintrin.h)?
Image from Lu Pan (@uvdn7)
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@davidtgoldblatt helped me! It is because of the __gnu__inline__ attribute which changed the behavior.
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If the function is declared extern, then this definition of the function is used only for inlining. In no case is the function compiled as a standalone function, not even if you take its address explicitly. gcc.gnu.org/onlinedocs/gcc-6…
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