Integer to String Conversion (2014)

This compares several techniques for converting integers to decimal strings with an eye toward practical performance. It starts from the common snprintf approach, moves to a naive method that produces reversed digits by repeated division by 10 and then reverses the buffer, and then to an improved in-place approach that writes digits backward to avoid the reversal step. A handy buffer-size trick uses 3 * sizeof(int) to guarantee enough space for MAX_INT plus terminator. Replacing a table lookup with an arithmetic addition turned out slower; using a small lookup table for single digits yields about a 9% improvement over that change.

A faster method from Facebook uses a 200-character table containing the pairs "00" through "99" so the loop extracts two digits per division, halving division count and writing two bytes per iteration. That two-digit lookup produces roughly a 31% speedup over the single-digit-table variant in the author’s benchmarks on a Core i7-2640M 64-bit system; snprintf is much slower. Using a constant digit-length is consistently faster than a computed digits10() on that machine. The two-digit-table trick appears in multiple places historically, but unclear licensing for the Facebook code prevented direct reuse in the author’s project.