And as long as we're complaining about compute-heavy problems, 2016 day 5 is the worst; it took 3 hours and 11 minutes to complete in m4 on my hardware (computing md5 in python involves calling into a library which in turn probably uses pre-compiled C code rather than native python for the core computations, but computing it natively in m4 is just painfully slow). Source: about 3 years ago

So I coded up something that I did not see mentioned anywhere in the megathread. Instead of storing just the recipe number in each array location, with an array entry for every recipe, I stored a tuple of recipe number and next array position. For a0-a22, the next number is pre-computed. But for a23 onwards, the next array position is exactly one larger - that is, the rest of my array stores ONLY the recipes... Source: about 3 years ago

So my next round did what many of the megathread solutions did: for each pair of input lines, determine if they are similar. This cuts the complexity from exponential O(2^n) to quadratic O(n^2), and runtime was down to 1.3s (a 300x speedup over regex abuse). Note that due to the use of substr, I risk inadvertent macro expansion if slicing a lower-case line forms a macro name such as 'nl', so I had to further... Source: about 3 years ago

Wow, that really made a difference! I got another ~250x speedup (best-case run from ~18.5s to 71ms) with this tweak. Instead of performing a full matrix multiply (O(1) work per iteration regardless of active rows, and O(log n) iterations), I take advantage of the sparseness of the initial few iterations. That is, I start out with only one row active; (however, by iteration 50 all 92 rows are active), as well as... Source: about 3 years ago

Or course, only after I had written my entire solution did I notice in the megathread that avoiding division when computing math modulo M31 (0x7fffffff) is already a known optimization. But I thought it was pretty cool that I was able to derive that optimization from something more general, and that my solution works without any 64-bit math or explicit division operators. Source: about 3 years ago

Check out Skyflow. They built a privacy vault to specifically address this issue. Source: over 1 year ago