[Issue 15381] New: Allow hashes to use only lowest bits

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=15381

          Issue ID: 15381
           Summary: Allow hashes to use only lowest bits
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: performance, spec
          Severity: enhancement
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: verylonglogin.reg at gmail.com

It's a common situation that hash tables and other algorithms use only some
bits of a hash so all hash bits must equally represent the object (currently
not documented, filled as Issue 15380).

It may be useful to require an algorithm working with hashes cut only highest
bits (as current Druntime's associative array implementation does). This allows
e.g. unique indexed elements use its index as a hash (if I'm not missing
something regarding probabilities of usage patterns):

  size_t toHash() { return myIndex; } // Will be allowed

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