On this last question, most (maybe all) Clojure immutable collections store
their hash value after it has been calculated once, so it need not be
calculated again.
Even the first time a collection's hash is calculated, it is a function of
the hash of its elements (set elements, or map keys & values), and if any
of those are collections, and their hash has been earlier calculated and
stored, then the stored value is used.
Andy
On Sat, Jun 27, 2015 at 7:47 AM, Jacob Goodson <submissionfight...@gmx.com>
wrote:
> One more thing, is it cheap to hash immutable maps vs mutable ones?
>
>
> On Saturday, June 27, 2015 at 10:44:44 AM UTC-4, Jacob Goodson wrote:
>>
>> Thanks.
>>
>> What about the perf of something like this...
>>
>> ({{:x 1 :y 2} :point} {:x 1 :y 2})
>>
>> vs
>>
>> ({1 2} 1)
>>
>> and what if the map was mutable as a key?
>>
>> On Saturday, June 27, 2015 at 10:08:19 AM UTC-4, Andy Fingerhut wrote:
>>>
>>> (let [foo {:x 1}]
>>> (= foo foo))
>>>
>>> is fast, because they are identical, and = is fast for things that are
>>> identical.
>>>
>>> In general, two maps that are = are often not identical.
>>>
>>> If two maps have different numbers of elements, = should quickly return
>>> false because of the way equiv() is implemented for maps, here:
>>> https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/APersistentMap.java#L73-L94
>>>
>>> There is a check whether the maps are the same size, and if not, return
>>> false without checking all key/value pairs.
>>>
>>> If they have equal number of key/value pairs, then it will iterate
>>> through all key/value pairs of one, checking whether the key of one is
>>> present in the other, and if so, whether their associated values are
>>> equiv() or not.
>>>
>>> Those checks can be fast if the keys are quickly comparable, and the
>>> values are quickly comparable.
>>>
>>> If you take one persistent map m1 and remove a few keys and/or add a few
>>> keys to produce m2, then all of the unchanged keys and values they have in
>>> common will be identical, and very quickly comparable. The slowest part of
>>> comparing them would be comparing their non-identical parts.
>>>
>>> Andy
>>>
>>> On Sat, Jun 27, 2015 at 5:01 AM, Jacob Goodson <submissio...@gmx.com>
>>> wrote:
>>>
>>>> I was wondering something...
>>>>
>>>> Would (= {:x 1} {:x 1}) be just as fast as (= 1 1) since maps are
>>>> immutable? The idea is that since clojure data structures are just values
>>>> can they be compared much faster than there mutable counter parts? Thanks.
>>>>
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>>>
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