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Comment by _lce0 | Hacker News Reader

but that's trival, isn't that already solved?

    function f(x) { return 28 + maybeLeap(x) + (...); }

I remember it's something related with "(x % 4) == 0" and "not ends_with(00)"

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OP is trying to express the day count as a pure math expression. maybeLeap() probably won't do here.

Except for every 400th year.

how about this?

        leap = function(x) { 
          if ((x % 4) != 0) {
            return 0;
          }
          return Number(
            (x % 100) != 0 || (x % 400) == 0
          );
        }

        function(x) { return 28 + leap(x) + (...) };

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