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further clarify performance guarantee of immutable hash tables

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svn: r16620
This commit is contained in:
Matthew Flatt 2009-11-08 19:06:57 +00:00
parent 94917e2447
commit 7cce5c98a3
1 changed files with 11 additions and 6 deletions
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17
collects/scribblings/reference/hashes.scrbl
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@ -21,11 +21,16 @@ A @deftech{hash table} (or simply @deftech{hash}) maps each of its
keys to a single value. For a given hash table, keys are equivalent
via @scheme[equal?], @scheme[eqv?], or @scheme[eq?], and keys are
retained either strongly or weakly (see @secref["weakbox"]). A hash
table is also either mutable or immutable. Immutable tables support
constant-time access and update, just like mutable hash tables; the
constant on immutable operations is usually larger, but the
functional nature of immutable hash tables can pay off in certain
algorithms.
table is also either mutable or immutable. Immutable hash tables
support effectively constant-time access and update, just like mutable
hash tables; the constant on immutable operations is usually larger,
but the functional nature of immutable hash tables can pay off in
certain algorithms.
@margin-note{Immutable hash tables actually provide @math{O(log N)}
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
platform), @math{log N} can be treated reasonably as a constant.}
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
@ -51,7 +56,7 @@ a table-specific semaphore as needed. Three caveats apply, however:
@item{If a thread is terminated while applying @scheme[hash-ref],
@scheme[hash-set!], @scheme[hash-remove!], @scheme[hash-ref!],
or @scheme[has-update!] to a hash table that
or @scheme[hash-update!] to a hash table that
uses @scheme[equal?] or @scheme[eqv?] key comparisons, all current
and future operations on the hash table may block indefinitely.}