Mathematica's pattern matching poorly optimized?

Question

I recently inquired about why PatternTest was causing a multitude of needless evaluations: PatternTest not optimized? Leonid replied that it is necessary for what

Answer 1

@the author of the first answer. As far as I know from reverse-engeneering and reading of available information, it may be due to different ways the patterns are checked. In fact - as they say - a special hash code is used for pattern matching. This hash (basically a FNV-1 round) makes it very easy to check for particular patterns related to the type of expression involved (matter of a few xor operations). The hashing algorithm cycles inside the expression and each subpart is xorred with the output of the previous one. Special xor values are used for each atom expression - machineInts, machineReals, bigNums, Rationals and so on. Hence, for example, _Integer is easy to check because the hash of any integer is formed with integer's xor value, so all we need to do is doing the inverse op and see if matches - i.e. if we get some particular value or something like that (sorry if I'm vague on actual implementation details. It's WIP). For general or uncommon patterns the check may not take advantage of this hash stuff and require something different.

@the OP Head[] simply acts on the internal expression, taking the value of the first pointer of the expression (expressions are implemented as arrays of pointers). So doing it is as easy as copying and printing a string - very very fast. The pattern matching engine is not even called in this case.