Talk:Happy numbers: Difference between revisions
→On caching (and laziness): But just doing one iteration is still better (and I'm now convinced it actually is).
(→On caching (and laziness): no permutations: a single "bunch of data" is enough) |
(→On caching (and laziness): But just doing one iteration is still better (and I'm now convinced it actually is).) |
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:: We don't need to store the permutations. Since, for the final computation, what really matters is how many of every digits there are...; e.g. 910: I don't need to store 190, 910, 109 (and eventually 019, which indeed with the "Bag" technics is considered different from 19); it is enough I say that a number containing one 0, one 1 and one 9 is happy. --[[User:ShinTakezou|ShinTakezou]] 10:50, 7 May 2009 (UTC)
::: However if instead of the "bunch of numbers" you instead store the sum of their squares (which is actually the only thing that matters), you still get more efficient (indeed, you get more efficient in two ways; you need less storage per entry, and you need less entries, because e.g. the cases "4 ones", "4 ones and a zero", "one two", "one two and ten zeroes", etc. all go into the same cache entry).
::: However, I now think it would be a saving anyway. The digit square sums tend to be much smaller than the original numbers; indeed, with a 32 bit unsigned int, the largest possible value after one step would be 657 (unless I mis-calculated). If storing the flags in single bits, you'd need just 83 bytes, and lookup would be strictly O(1). Moreover, if you don't have that number in the cache, you have to do this calculation anyway, so you don't lose much time in that case, so the only efficiency loss is for numbers already in the cache, but that's just one iteration (for 32 bit uint in the worst case, 18 divisions, 18 modulo operations, 9 multiplications and 17 additions, if I counted correctly). Moreover the needed memory then goes linear in the number of digits, so even with 64 bits you won't need too much memory. --[[User:Ce|Ce]] 13:30, 7 May 2009 (UTC)
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