Viewing profile — hairtuq

The mapping after we have the leading 1's count can be done in 3 instructions (in 32-bit math) on either x86 or ARM: t = 0x2020c6 >> x return (t > 27

If you want to optimize it a little more, you can combine isLess(s, 0x20) and isChar('"') into isLess(s ^ (kOnes * 0x02), 0x21) (this works since '"' is 0x22).

Thanks, fixed.

The article leaves out optimizing is_leap_year. Here is an interesting version that is correct (in the Proleptic Gregorian calendar) for 0 ((year * 1073750999) & 3221352463) How th…

Here is a cute variant that doesn't need lzcnt nor tables, but only works up to 99,999: (((x + 393206) & (x + 524188)) ^ ((x + 916504) & (x + 514288))) >> 17 This is for integer lo…

The author wonders: > In theory at least, the compiler can see that rule only has 256 values and create a reduced version of ca1d_rule_apply for each value. Whether it actually doe…

Similarly, the eexpect table can be done in 3 instructions with with t = 0x3c783023f + i; return ((t >> (t & 63)) & 0xf0808088.

I wrote a solver for Atomix ( https://github.com/falk-hueffner/atomixer ), a similar game. Technically, the only difference is that in Atomix the solution location is not predefine…

On a related note, almost any task around shuffling or shifting bits can be improved with xor. For example the straightforward way to remove bit i is mask = -1 > 1) & mask); but wi…

It seems you're shifting the goal posts now. The fact that consensus criticisms are not as harsh as you'd like doesn't mean that Israel is "totally off limits". Discussions and cri…

I don't see that. For example, it has been the official position of the German government for at least 20 years to strongly criticize the Israeli settlements. There's regular criti…

I wrote an ingredient parser that is basically just one giant regular expression: https://github.com/falk-hueffner/metric-cooking/blob/master/... The use case is a bit different (t…

For the quite similar puzzle Atomix, it also seems like the branching factor would be much higher for backward search because upper bounds are weaker, but you can show that on aver…

You don't necessarily need a wider type (which might be slower to work with), you can just calculate (n|1) * ((n+1)/2). Clang does something much more complicated, probably because…

Slightly simpler: c = x ^ y; return popcount(((c >> 1) | 0x8080808080808080) - c) & 0x8080808080808080);

Actually, e is the inverse of the xor, so (e & c1) is guaranteed to be c1, and you still need popcount(c1 & ((e & c2) + c3)).

The name SWAR is sometimes used ( https://en.wikipedia.org/wiki/SWAR ). Indeed, even 8-bit fields can be added in parallel, using the fact that ^ is like a + that does not produce …

There is some work on automatically generating data structure implementations from high-level specifications: https://cozy.uwplse.org/

Since the shift count is implicitly modulo 32, you don't even need the subtraction: bool isvowel(char c) { return (1u

I wrote a browser extension that does this (although it will of course occasionally get something wrong): https://github.com/falk-hueffner/metric-cooking

Here's a version that at least for 64 bit might be faster: bool isPerfect(uint64_t x) { int ctz = __builtin_ctzll(x); return ((0x40051056 >> ctz) & 1) && ((x >> ctz) + 1) == (uint6…

Red-black tree presentations are often more complicated than necessary, mostly because of (premature?) optimizations. A simple presentation can be found here: https://www.cs.tufts.…

But it's not only impossible to decide halting faster than the program would run, but impossible at all, a much stronger statement.

That's very unlikely to happen, since finding a counterexample or proving there isn't one is much easier than finding a program candidate, so it'll usually time out in that phase.

I wrote a superoptimizer using z3 here: https://github.com/falk-hueffner/sematrope . The idea is to have variables that encode the program and a giant if-then-else statement that r…