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reference: add information on the cost of hashing

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Merge to v6.0
This commit is contained in:
Matthew Flatt 2013-11-27 08:09:25 -07:00
parent 8514662b83
commit bb5429a039
1 changed files with 11 additions and 0 deletions
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pkgs/racket-pkgs/racket-doc/scribblings/reference/hashes.scrbl
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@ -32,6 +32,17 @@ access and update. Since @math{N} is limited by the address space so
that @math{log N} is limited to less than 30 or 62 (depending on the that @math{log N} is limited to less than 30 or 62 (depending on the
platform), @math{log N} can be treated reasonably as a constant.} platform), @math{log N} can be treated reasonably as a constant.}
For @racket[equal?]-based hashing, the built-in hash functions on
@tech{strings}, @tech{pairs}, @tech{lists}, @tech{vectors},
@tech{prefab} or transparent @tech{structures}, @|etc|, take time
proportional to the size of the value. The hash code for a compound
data structure, such as a list or vector, depends on hashing each item
of the container, but the depth of such recursive hashing is
limited (to avoid potential problems with cyclic data). For a
non-@tech{list} @tech{pair}, both @racket[car] and @racket[cdr]
hashing is treated as a deeper hash, but the @racket[cdr] of a
@tech{list} is treated as having the same hashing depth as the list.
A hash table can be used as a two-valued @tech{sequence} (see A hash table can be used as a two-valued @tech{sequence} (see
@secref["sequences"]). The keys and values of the hash table serve as @secref["sequences"]). The keys and values of the hash table serve as
elements of the sequence (i.e., each element is a key and its elements of the sequence (i.e., each element is a key and its