3bb120545f
--- but Gwyth's amazingly helpful review of chapters 1-11
pointed out a few problems that are more difficult to fix
and are still pending
450 lines
17 KiB
Racket
450 lines
17 KiB
Racket
#lang scribble/doc
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@(require "mz.ss")
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@title[#:tag "hashtables"]{Hash Tables}
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@(define (concurrency-caveat)
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@elemref['(caveat "concurrency")]{caveats concerning concurrent modification})
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@(define (mutable-key-caveat)
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@elemref['(caveat "mutable-keys")]{caveat concerning mutable keys})
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@(define (see-also-caveats)
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@t{See also the @concurrency-caveat[] and the @mutable-key-caveat[] above.})
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@(define (see-also-concurrency-caveat)
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@t{See also the @concurrency-caveat[] above.})
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@(define (see-also-mutable-key-caveat)
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@t{See also the @mutable-key-caveat[] above.})
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@guideintro["hash-tables"]{hash tables}
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A @deftech{hash table} (or simply @deftech{hash}) maps each of its
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keys to a single value. For a given hash table, keys are equivalent
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via @scheme[equal?], @scheme[eqv?], or @scheme[eq?], and keys are
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retained either strongly or weakly (see @secref["weakbox"]). A hash
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table is also either mutable or immutable. Immutable hash tables
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support effectively constant-time access and update, just like mutable
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hash tables; the constant on immutable operations is usually larger,
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but the functional nature of immutable hash tables can pay off in
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certain algorithms.
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@margin-note{Immutable hash tables actually provide @math{O(log N)}
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access and update. Since @math{N} is limited by the address space so
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that @math{log N} is limited to less than 30 or 62 (depending on the
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platform), @math{log N} can be treated reasonably as a constant.}
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A hash table can be used as a two-valued @tech{sequence} (see
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@secref["sequences"]). The keys and values of the hash table serve as
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elements of the sequence (i.e., each element is a key and its
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associated value). If a mapping is added to or removed from the hash
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table during iteration, then an iteration step may fail with
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@scheme[exn:fail:contract], or the iteration may skip or duplicate
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keys and values. See also @scheme[in-hash], @scheme[in-hash-keys],
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@scheme[in-hash-values], and @scheme[in-hash-pairs].
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Two hash tables cannot be @scheme[equal?] unless they use the same
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key-comparison procedure (@scheme[equal?], @scheme[eqv?], or
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@scheme[eq?]), both hold keys strongly or weakly, and have the same
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mutability.
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@elemtag['(caveat "concurrency")]{@bold{Caveats concerning concurrent
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modification:}} A mutable hash table can be manipulated with
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@scheme[hash-ref], @scheme[hash-set!], and @scheme[hash-remove!]
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concurrently by multiple threads, and the operations are protected by
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a table-specific semaphore as needed. Three caveats apply, however:
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@itemize[
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@item{If a thread is terminated while applying @scheme[hash-ref],
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@scheme[hash-set!], @scheme[hash-remove!], @scheme[hash-ref!],
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or @scheme[hash-update!] to a hash table that
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uses @scheme[equal?] or @scheme[eqv?] key comparisons, all current
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and future operations on the hash table may block indefinitely.}
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@item{The @scheme[hash-map] and @scheme[hash-for-each] procedures do
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not use the table's semaphore to guard the traversal as a whole.
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Changes by one thread to a hash table can affect the keys and values
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seen by another thread part-way through its traversal of the same
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hash table.}
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@item{The @scheme[hash-update!] and @scheme[hash-ref!] functions
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use a table's semaphore
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independently for the @scheme[hash-ref] and @scheme[hash-set!] parts
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of their functionality, which means that the update as a whole is not
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``atomic.''}
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]
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@elemtag['(caveat "mutable-keys")]{@bold{Caveat concerning mutable
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keys:}} If a key in an @scheme[equal?]-based hash table is mutated
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(e.g., a key string is modified with @scheme[string-set!]), then the
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hash table's behavior for insertion and lookup operations becomes
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unpredictable.
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@defproc[(hash? [v any/c]) boolean?]{
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Returns @scheme[#t] if @scheme[v] is a @tech{hash table}, @scheme[#f]
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otherwise.}
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@defproc[(hash-equal? [hash hash?]) boolean?]{
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Returns @scheme[#t] if @scheme[hash] compares keys with @scheme[equal?],
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@scheme[#f] if it compares with @scheme[eq?] or @scheme[eqv?].}
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@defproc[(hash-eqv? [hash hash?]) boolean?]{
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Returns @scheme[#t] if @scheme[hash] compares keys with @scheme[eqv?],
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@scheme[#f] if it compares with @scheme[equal?] or @scheme[eq?].}
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@defproc[(hash-eq? [hash hash?]) boolean?]{
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Returns @scheme[#t] if @scheme[hash] compares keys with @scheme[eq?],
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@scheme[#f] if it compares with @scheme[equal?] or @scheme[eqv?].}
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@defproc[(hash-weak? [hash hash?]) boolean?]{
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Returns @scheme[#t] if @scheme[hash] retains its keys weakly,
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@scheme[#f] if it retains keys strongly.}
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@deftogether[(
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@defproc[(hash [key any/c] [val any/c] ... ...) (and/c hash? hash-equal? immutable?)]
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@defproc[(hasheq [key any/c] [val any/c] ... ...) (and/c hash? hash-eq? immutable?)]
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@defproc[(hasheqv [key any/c] [val any/c] ... ...) (and/c hash? hash-eqv? immutable?)]
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)]{
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Creates an immutable hash table with each given @scheme[key] mapped to
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the following @scheme[val]; each @scheme[key] must have a @scheme[val],
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so the total number of arguments to @scheme[hash] must be even.
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The @scheme[hash] procedure creates a table where keys are compared
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with @scheme[equal?], @scheme[hasheq] procedure creates a table where
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keys are compared with @scheme[eq?], and @scheme[hasheqv] procedure
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creates a table where keys are compared with @scheme[eqv?].
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The @scheme[key] to @scheme[val] mappings are added to the table in
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the order that they appear in the argument list, so later mappings can
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hide earlier mappings if the @scheme[key]s are equal.}
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@deftogether[(
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@defproc[(make-hash [assocs (listof pair?) null]) (and/c hash? hash-equal?)]
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@defproc[(make-hasheqv [assocs (listof pair?) null]) (and/c hash? hash-eqv?)]
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@defproc[(make-hasheq [assocs (listof pair?) null]) (and/c hash? hash-eq?)]
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)]{
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Creates a mutable hash table that holds keys strongly.
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The @scheme[make-hash] procedure creates a table where keys are
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compared with @scheme[equal?], @scheme[make-hasheq] procedure creates
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a table where keys are compared with @scheme[eq?], and
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@scheme[make-hasheqv] procedure creates a table where keys are
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compared with @scheme[eqv?].
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The table is initialized with the content of @scheme[assocs]. In each
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element of @scheme[assocs], the @scheme[car] is a key, and the
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@scheme[cdr] is the corresponding value. The mappings are added to the
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table in the order that they appear in @scheme[assocs], so later
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mappings can hide earlier mappings.
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See also @scheme[make-custom-hash].}
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@deftogether[(
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@defproc[(make-weak-hash [assocs (listof pair?) null]) (and/c hash? hash-equal? hash-weak?)]
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@defproc[(make-weak-hasheqv [assocs (listof pair?) null]) (and/c hash? hash-eqv? hash-weak?)]
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@defproc[(make-weak-hasheq [assocs (listof pair?) null]) (and/c hash? hash-eq? hash-weak?)]
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)]{
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Like @scheme[make-hash], @scheme[make-hasheq], and
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@scheme[make-hasheqv], but creates a mutable hash table that holds
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keys weakly.}
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@deftogether[(
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@defproc[(make-immutable-hash [assocs (listof pair?)])
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(and/c hash? hash-equal? immutable?)]
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@defproc[(make-immutable-hasheqv [assocs (listof pair?)])
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(and/c hash? hash-eqv? immutable?)]
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@defproc[(make-immutable-hasheq [assocs (listof pair?)])
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(and/c hash? hash-eq? immutable?)]
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)]{
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Like @scheme[hash], @scheme[hasheq], and @scheme[hasheqv], but accepts
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the key--value mapping in association-list form like
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@scheme[make-hash], @scheme[make-hasheq], and @scheme[make-hasheqv].}
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@defproc[(hash-set! [hash (and/c hash? (not/c immutable?))]
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[key any/c]
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[v any/c]) void?]{
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Maps @scheme[key] to @scheme[v] in @scheme[hash], overwriting
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any existing mapping for @scheme[key].
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@see-also-caveats[]}
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@defproc[(hash-set*! [hash (and/c hash? (not/c immutable?))]
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[key any/c]
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[v any/c]
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...
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...) void?]{
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Maps each @scheme[key] to each @scheme[v] in @scheme[hash], overwriting
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any existing mapping for each @scheme[key]. Mappings are added from the left, so
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later mappings overwrite earlier mappings.
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@see-also-caveats[]}
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@defproc[(hash-set [hash (and/c hash? immutable?)]
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[key any/c]
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[v any/c])
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(and/c hash? immutable?)]{
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Functionally extends @scheme[hash] by mapping @scheme[key] to
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@scheme[v], overwriting any existing mapping for @scheme[key], and
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returning the extended hash table.
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@see-also-mutable-key-caveat[]}
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@defproc[(hash-set* [hash (and/c hash? immutable?)]
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[key any/c]
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[v any/c]
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...
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...)
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(and/c hash? immutable?)]{
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Functionally extends @scheme[hash] by mapping each @scheme[key] to
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@scheme[v], overwriting any existing mapping for each @scheme[key], and
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returning the extended hash table. Mappings are added from the left, so
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later mappings overwrite earlier mappings.
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@see-also-mutable-key-caveat[]}
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@defproc[(hash-ref [hash hash?]
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[key any/c]
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[failure-result any/c (lambda ()
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(raise (make-exn:fail:contract ....)))])
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any]{
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Returns the value for @scheme[key] in @scheme[hash]. If no value
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is found for @scheme[key], then @scheme[failure-result] determines the
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result:
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@itemize[
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@item{If @scheme[failure-result] is a procedure, it is called
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(through a tail call) with no arguments to produce the result.}
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@item{Otherwise, @scheme[failure-result] is returned as the result.}
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]
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@see-also-caveats[]}
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@defproc[(hash-ref! [hash hash?] [key any/c] [to-set any/c])
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any]{
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Returns the value for @scheme[key] in @scheme[hash]. If no value is
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found for @scheme[key], then @scheme[to-set] determines the result as
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in @scheme[hash-ref] (i.e., it is either a thunk that computes a value
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or a plain value), and this result is stored in @scheme[hash] for the
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@scheme[key]. (Note that if @scheme[to-set] is a thunk, it is not
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invoked in tail position.)
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@see-also-caveats[]}
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@defproc[(hash-has-key? [hash hash?] [key any/c])
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boolean?]{
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Returns @scheme[#t] if @scheme[hash] contains a value for the given
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@scheme[key], @scheme[#f] otherwise.}
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@defproc[(hash-update! [hash (and/c hash? (not/c immutable?))]
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[key any/c]
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[updater (any/c . -> . any/c)]
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[failure-result any/c (lambda ()
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(raise (make-exn:fail:contract ....)))])
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void?]{
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Composes @scheme[hash-ref] and @scheme[hash-set!] to update an
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existing mapping in @scheme[hash], where the optional
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@scheme[failure-result] argument is used as in @scheme[hash-ref] when
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no mapping exists for @scheme[key] already. See the caveat above about
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concurrent updates.
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@see-also-caveats[]}
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@defproc[(hash-update [hash (and/c hash? immutable?)]
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[key any/c]
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[updater (any/c . -> . any/c)]
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[failure-result any/c (lambda ()
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(raise (make-exn:fail:contract ....)))])
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(and/c hash? immutable?)]{
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Composes @scheme[hash-ref] and @scheme[hash-set] to functionally
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update an existing mapping in @scheme[hash], where the optional
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@scheme[failure-result] argument is used as in @scheme[hash-ref] when
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no mapping exists for @scheme[key] already.
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@see-also-mutable-key-caveat[]}
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@defproc[(hash-remove! [hash (and/c hash? (not/c immutable?))]
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[key any/c])
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void?]{
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Removes any existing mapping for @scheme[key] in @scheme[hash].
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@see-also-caveats[]}
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@defproc[(hash-remove [hash (and/c hash? immutable?)]
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[key any/c])
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(and/c hash? immutable?)]{
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Functionally removes any existing mapping for @scheme[key] in
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@scheme[hash], returning the fresh hash table.
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@see-also-mutable-key-caveat[]}
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@defproc[(hash-map [hash hash?]
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[proc (any/c any/c . -> . any/c)])
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(listof any/c)]{
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Applies the procedure @scheme[proc] to each element in
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@scheme[hash] in an unspecified order, accumulating the results
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into a list. The procedure @scheme[proc] is called each time with a
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key and its value.
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If a hash table is extended with new keys (either through
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@scheme[proc] or by another thread) while a @scheme[hash-map] or
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@scheme[hash-for-each] traversal is in process, arbitrary key--value
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pairs can be dropped or duplicated in the traversal. Key mappings can
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be deleted or remapped (by any thread) with no adverse affects; the
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change does not affect a traversal if the key has been seen already,
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otherwise the traversal skips a deleted key or uses the remapped key's
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new value.
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@see-also-concurrency-caveat[]}
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@defproc[(hash-keys [hash hash?])
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(listof any/c)]{
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Returns a list of the keys of @scheme[hash] in an unspecified order.
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See @scheme[hash-map] for information about modifying @scheme[hash]
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during @scheme[hash-keys]. @see-also-concurrency-caveat[]}
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@defproc[(hash-values [hash hash?])
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(listof any/c)]{
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Returns a list of the values of @scheme[hash] in an unspecified order.
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See @scheme[hash-map] for information about modifying @scheme[hash]
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during @scheme[hash-values]. @see-also-concurrency-caveat[]}
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@defproc[(hash->list [hash hash?])
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(listof (cons/c any/c any/c))]{
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Returns a list of the key--value pairs of @scheme[hash] in an unspecified order.
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See @scheme[hash-map] for information about modifying @scheme[hash]
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during @scheme[hash->list]. @see-also-concurrency-caveat[]}
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@defproc[(hash-for-each [hash hash?]
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[proc (any/c any/c . -> . any)])
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void?]{
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Applies @scheme[proc] to each element in @scheme[hash] (for the
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side-effects of @scheme[proc]) in an unspecified order. The procedure
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@scheme[proc] is called each time with a key and its value.
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See @scheme[hash-map] for information about modifying @scheme[hash]
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within @scheme[proc]. @see-also-concurrency-caveat[]}
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@defproc[(hash-count [hash hash?])
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exact-nonnegative-integer?]{
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Returns the number of keys mapped by @scheme[hash]. If @scheme[hash]
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is not created with @scheme['weak], then the result is computed in
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constant time and atomically. If @scheme[hash] is created with
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@scheme['weak], see the @concurrency-caveat[] above.}
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@defproc[(hash-iterate-first [hash hash?])
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(or/c #f exact-nonnegative-integer?)]{
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Returns @scheme[#f] if @scheme[hash] contains no elements, otherwise
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it returns an integer that is an index to the first element in the hash
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table; ``first'' refers to an unspecified ordering of the table
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elements, and the index values are not necessarily consecutive
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integers. For a mutable @scheme[hash], this index is guaranteed to
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refer to the first item only as long as no items are added to or
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removed from @scheme[hash].}
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@defproc[(hash-iterate-next [hash hash?]
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[pos exact-nonnegative-integer?])
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(or/c #f exact-nonnegative-integer?)]{
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Returns either an integer that is an index to the element in
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@scheme[hash] after the element indexed by @scheme[pos] (which is not
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necessarily one more than @scheme[pos]) or @scheme[#f] if @scheme[pos]
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refers to the last element in @scheme[hash]. If @scheme[pos] is not a
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valid index, then the @exnraise[exn:fail:contract]. For a mutable
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@scheme[hash], the result index is guaranteed to refer to its item
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only as long as no items are added to or removed from @scheme[hash].}
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@defproc[(hash-iterate-key [hash hash?]
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[pos exact-nonnegative-integer?])
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any]{
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Returns the key for the element in @scheme[hash] at index
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@scheme[pos]. If @scheme[pos] is not a valid index for
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@scheme[hash], the @exnraise[exn:fail:contract].}
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@defproc[(hash-iterate-value [hash hash?]
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[pos exact-nonnegative-integer?])
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any]{
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Returns the value for the element in @scheme[hash] at index
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@scheme[pos]. If @scheme[pos] is not a valid index for
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@scheme[hash], the @exnraise[exn:fail:contract].}
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@defproc[(hash-copy [hash hash?])
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(and/c hash? (not/c immutable?))]{
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Returns a mutable hash table with the same mappings, same
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key-comparison mode, and same key-holding strength as @scheme[hash].}
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@defproc[(eq-hash-code [v any/c]) fixnum?]{
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Returns a @tech{fixnum}; for any two calls with @scheme[eq?] values,
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the returned number is the same.
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@margin-note{Equal @tech{fixnums} are always @racket[eq?].}}
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@defproc[(eqv-hash-code [v any/c]) fixnum?]{
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Returns a @tech{fixnum}; for any two calls with @scheme[eqv?] values,
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the returned number is the same.}
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@defproc[(equal-hash-code [v any/c]) fixnum?]{
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Returns a @tech{fixnum}; for any two calls with @scheme[equal?] values,
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the returned number is the same. A hash code is computed even when
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@scheme[v] contains a cycle through pairs, vectors, boxes, and/or
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inspectable structure fields. See also @scheme[prop:equal+hash].}
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@defproc[(equal-secondary-hash-code [v any/c]) fixnum?]{
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Like @scheme[equal-hash-code], but computes a secondary value suitable
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for use in double hashing.}
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