Daniel Lemire is a software performance expert. He ranks among the top 2% of scientists globally (Stanford/Elsevier 2025) and is one of GitHub's top 1000 most followed developers.
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Though most strings online today follow the Unicode standard (e.g., using UTF-8), the Latin 1 standard is still in widespread inside some systems (such as browsers) as JavaScript strings are often stored as either Latin 1, UTF-8 or UTF-16 internally. Latin 1 captures the first 256 characters from the Unicode standard…
The ubiquitous IEEE floating-point standard defines two numbers to represent zero, the positive and the negative zeros. You also have the positive and negative infinity. If you compute the inverse of the positive zero, you get the positive infinity. If you compute the inverse of the negative zero, you get the negative…
When programming, we often have to 'escape' strings. A standard way to do it is to insert the backslash character (\) before some characters such as the double quote. For example, the string my title is "La vie" becomes my title is \"La vie\" A simple routine in C++ to escape a string might look … Continue reading…
Most C++ programmers rely on "STL" for their data structures. The most popular data structure is probably vector, which is just a dynamic array. The set and the map are other useful ones. The STL data structures are a minimalist design. You have relatively few methods. All of them allow you to compute the size ……
Modern processors are superscalar, meaning that they can execute many instructions at once. For example, some processors can retire four or six instructions per cycle. Furthermore, many of these processors can initiate instructions out-of-order: they can start working on instructions that appear much later in the…
In Python, the dict data structure is the conventional key-value structure. E.g., you might store a list of names as keys and have their phone numbers as values. Valentin Ignatev wrote this amusing post on X: It is indeed widely believed that, in the strict sense, the dict data structure and its companion, the set ……
In most systems, integers are stored using a fixed binary representation. It is common to store integers using 32-bit or 64-bit words. You sometimes need to convert it to a string. For example, the integer 12345 might need to be converted to the five characters '12345'. In an earlier blog post, I presented the simpler…
At a fundamental level, a programmer needs to manipulate bits. Modern processors operate over data by loading in 'registers' and not individual bits. Thus a programmer must know how to manipulate the bits within a register. Generally, we can do so while programming with 8-bit, 16-bit, 32-bit and 64-bit integers. For…
Most systems today rely on Unicode strings. However, we have two popular Unicode formats: UTF-8 and UTF-16. We often need to convert from one format to the other. For example, you might have a database formatted with UTF-16, but you need to produce JSON documents using UTF-8. This conversion is often called…
In C, the length of a string in marked by a 0 byte at the end of the string. Thus to determine the length of the string, one must scan it, looking for the 0 byte. Recent ARM processors have a powerful instruction set (SVE) that is well suited for such problems. It allows you … Continue reading Implementing ‘strlen’…
You sometimes feel the need to make all of your integers positive, without losing any information. That is, you want to map all of your integers from 'signed' integers (e.g., -1, 1, 3, -3) to 'unsigned integers' (e.g., 3,2,6,7). This could be useful if you have a fast function to compress integers that fails to ……
The C++ library has long been organized around stream classes, at least when it comes to reading and parsing strings. But streams can be surprisingly slow. For example, if you want to parse numbers, then this C++ routine is close to being the worst possible choice for performance: std::stringstream in(mystring)…
Our most important goal in writing software is that it be correct. The software must do what the programmer wants it to do. It must meet the needs of the user. In the business world, double-entry bookkeeping is the idea that transactions are recorded in at least two accounts (debit and credit). One of the … Continue…
Computers often need random numbers. Most times, random numbers are not actually random... in the sense that they are the output of a mathematical function that is purely deterministic. And it is not even entirely clear what "really random" would mean. It is not clear that we live in a randomized universe... it seems…
We often have to write code using permissive programming languages like C and C++. They tend to generate hard-to-debug problems that can crash your applications. Thankfully, many compilers offer "sanitizers". I discussed them in my post No more leaks with sanitize flags in gcc and clang. I strongly encourage the use…
Integers in programming languages have a valid range but arithmetic operations can result in values that exceed such ranges. For example, adding two large integers can result in an integer that cannot be represented in the integer type. We often refer to such error conditions as overflows. In a programming languages…
I my previous blog post, I documented how one might proceed to compute the number of digits of an integer quickly. E.g., given the integer 999, you want 3 but given the integer 1000, you want 4. It is effectively the integer logarithm in base 10. On computers, you can quickly compute the integer logarithm … Continue…