Strings hash value memoized

[Jarrett Billingsley]

"bearophile" <bearophileHUGS at lycos.com> wrote in message 
news:g5a8h5$gmk$1 at digitalmars.com...
>I presume AAs are used often enough in D. If D 2.x strings are immutable, 
>then their hash function doesn't change, so it can be useful to store the 
>hash value inside the data structure of the string (Python works like 
>this), by default it can be initialized to a value like -1, so it's 
>computed only the first time it's actually needed, avoiding wasting time to 
>compute the hash value if it's not used.
>
> Having the hash value inside strings and other immutable data structures 
> allows quite faster AAs (and sets too, even if they are inside an external 
> library).
> Currently strings aren't special structures, they are just immutable 
> dynamical arrays of chars, so the has value (32 or 64 bit according to the 
> word size of the CPU, I presume) can't be stored in the dynamic array 
> record, so it may be stored in the -4/-8 bytes of the data block, but this 
> can't work for string slices. A possible solution is to create a special 
> string type that's just like the dynamic array struct, plus the hash 
> value, so this struct becomes bigger (12 instead of 8 bytes on 32 bit 
> CPUs), slowing down some operations a little.

Or, you store the precomputed hash in the structure (i.e. AA) that uses 
them.  Which is exactly what the default AA implementation does, actually.