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digitalmars.D.learn - hash design problem: both string and int[] keys

reply Charles Hixson <charleshixsn earthlink.net> writes:
I'm trying to figure out how to construct an associative array whose 
keys will be a combination of strings and immutable int[]'s, but every 
approach I've looked at has run into problems.  It should be relatively 
easy, as strings are really just an immutable list of ints, but I 
haven't been able to find how strings are hashed.  (It's looking as if 
the details are handled in C...which makes it difficult.)  I could 
define my own hash code, but I don't find it at all clear what would be 
appropriate given that I don't know the size.  The data is rather 
sparse, so a hash table seems appropriate.  (My only other alternative 
is a database, and that imposes the heavy slowdown of I/O ... even 
though it does automate persistence it doesn't seem like a good tradeoff.)

So far my best idea is to build two tables, and then look at the key 
when a retrieval is attempted to figure out which table it's in.  That 
would probably work, but it feels like a kludge.
Nov 20 2012
parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 11/20/2012 11:39 AM, Charles Hixson wrote:

 I'm trying to figure out how to construct an associative array whose
 keys will be a combination of strings and immutable int[]'s, but every
 approach I've looked at has run into problems.

Can you show some code?
 It should be relatively
 easy, as strings are really just an immutable list of ints,

A list of dchars is more accurate of course, but yes, dchar can be casted to int.
 but I
 haven't been able to find how strings are hashed. (It's looking as if
 the details are handled in C...which makes it difficult.)

I bet the algorithm considers all of the characters of the strings. The following has been mentioned in a recent forum thread: http://eternallyconfuzzled.com/tuts/algorithms/jsw_tut_hashing.aspx
 I could define
 my own hash code, but I don't find it at all clear what would be
 appropriate given that I don't know the size.

I wonder how much slow iterating over string would be, compared to directly accessing the elements of a dstring. (After all, strings must be decoded to produce dchars.) In any case, you don't need the length of the string; a loop would do: while (!s.empty) { // ... use s.front and s.popFront() ... } Alternatively: foreach (c; stride(s, 1)) { // ... use c ... } UFCS looks better: :) foreach (c; s.stride(1)) { // ... use c ... }
 The data is rather sparse,
 so a hash table seems appropriate. (My only other alternative is a
 database, and that imposes the heavy slowdown of I/O ... even though it
 does automate persistence it doesn't seem like a good tradeoff.)

 So far my best idea is to build two tables, and then look at the key
 when a retrieval is attempted to figure out which table it's in. That
 would probably work, but it feels like a kludge.

How do you differentiate between the key type being int[] vs. string? Ali
Nov 20 2012
next sibling parent reply Charles Hixson <charleshixsn earthlink.net> writes:
Ali Çehreli wrote:
 On 11/20/2012 11:39 AM, Charles Hixson wrote:

  > I'm trying to figure out how to construct an associative array whose
  > keys will be a combination of strings and immutable int[]'s, but every
  > approach I've looked at has run into problems.

 Can you show some code?

limitations, and that's all I could use to write code right now.
  > It should be relatively
  > easy, as strings are really just an immutable list of ints,

 A list of dchars is more accurate of course, but yes, dchar can be
 casted to int.

ints, which are smaller.
  > but I
  > haven't been able to find how strings are hashed. (It's looking as if
  > the details are handled in C...which makes it difficult.)

 I bet the algorithm considers all of the characters of the strings. The
 following has been mentioned in a recent forum thread:

 http://eternallyconfuzzled.com/tuts/algorithms/jsw_tut_hashing.aspx

lot of room for alternatives. For ASCII there are several plausible approaches, since all one needs to do is minimize collisions. But there are too many plausible unicode chars for most of them to be reasonable. I'd probably go for multiply by a medium size prime, add to the accumulator, mod the accumulator by a large prime, stepping through the loop. And I'd initialize the accumulator by the length of the current string. (Note that this would apply equally to strings or int[]'s.) So if that's what you mean by "considers all the characters", then, yes.
  > I could define
  > my own hash code, but I don't find it at all clear what would be
  > appropriate given that I don't know the size.

 I wonder how much slow iterating over string would be, compared to
 directly accessing the elements of a dstring. (After all, strings must
 be decoded to produce dchars.)

 In any case, you don't need the length of the string; a loop would do:

different strings, and it's a convenient built-in function.
 while (!s.empty) {
 // ... use s.front and s.popFront() ...
 }

 Alternatively:

 foreach (c; stride(s, 1)) {
 // ... use c ...
 }

 UFCS looks better: :)

 foreach (c; s.stride(1)) {
 // ... use c ...
 }

foreach (char c; lst) { ... } A hash code doesn't need to reflect the chars of the string, merely to separate different cases.
  > The data is rather sparse,
  > so a hash table seems appropriate. (My only other alternative is a
  > database, and that imposes the heavy slowdown of I/O ... even though it
  > does automate persistence it doesn't seem like a good tradeoff.)
  >
  > So far my best idea is to build two tables, and then look at the key
  > when a retrieval is attempted to figure out which table it's in. That
  > would probably work, but it feels like a kludge.

 How do you differentiate between the key type being int[] vs. string?

 Ali

different parameter types. But as I said, that feels like a kludge. I'm just contemplating it because I want rehash to work properly. So I'd have two indexes: Items[string] stringKeys; Items[int] listKeys; This lets the compiler handle resizing the arrays properly, at the cost of doubly hashing the listKeys variable. This isn't too bad, as I expect all the lists to be 5 items or shorter, but it feels like a kludge.
Nov 20 2012
parent reply Charles Hixson <charleshixsn earthlink.net> writes:
Charles Hixson wrote:
 Ali Çehreli wrote:
 On 11/20/2012 11:39 AM, Charles Hixson wrote:

 I'm trying to figure out how to construct an associative array whose
 keys will be a combination of strings and immutable int[]'s, but every
 approach I've looked at has run into problems.

Can you show some code?

limitations, and that's all I could use to write code right now.
 It should be relatively
 easy, as strings are really just an immutable list of ints,

A list of dchars is more accurate of course, but yes, dchar can be casted to int.

which are smaller.
 but I
 haven't been able to find how strings are hashed. (It's looking as if
 the details are handled in C...which makes it difficult.)

I bet the algorithm considers all of the characters of the strings. The following has been mentioned in a recent forum thread: http://eternallyconfuzzled.com/tuts/algorithms/jsw_tut_hashing.aspx

lot of room for alternatives. For ASCII there are several plausible approaches, since all one needs to do is minimize collisions. But there are too many plausible unicode chars for most of them to be reasonable. I'd probably go for multiply by a medium size prime, add to the accumulator, mod the accumulator by a large prime, stepping through the loop. And I'd initialize the accumulator by the length of the current string. (Note that this would apply equally to strings or int[]'s.) So if that's what you mean by "considers all the characters", then, yes.
 I could define
 my own hash code, but I don't find it at all clear what would be
 appropriate given that I don't know the size.


 I wonder how much slow iterating over string would be, compared to
 directly accessing the elements of a dstring. (After all, strings must
 be decoded to produce dchars.)

 In any case, you don't need the length of the string; a loop would do:

strings, and it's a convenient built-in function.
 while (!s.empty) {
 // ... use s.front and s.popFront() ...
 }

 Alternatively:

 foreach (c; stride(s, 1)) {
 // ... use c ...
 }

 UFCS looks better: :)

 foreach (c; s.stride(1)) {
 // ... use c ...
 }

foreach (char c; lst) { ... } A hash code doesn't need to reflect the chars of the string, merely to separate different cases.
 The data is rather sparse,
 so a hash table seems appropriate. (My only other alternative is a
 database, and that imposes the heavy slowdown of I/O ... even though it
 does automate persistence it doesn't seem like a good tradeoff.)

 So far my best idea is to build two tables, and then look at the key
 when a retrieval is attempted to figure out which table it's in. That
 would probably work, but it feels like a kludge.

How do you differentiate between the key type being int[] vs. string? Ali

different parameter types. But as I said, that feels like a kludge. I'm just contemplating it because I want rehash to work properly. So I'd have two indexes: Items[string] stringKeys; Items[int] listKeys; This lets the compiler handle resizing the arrays properly, at the cost of doubly hashing the listKeys variable. This isn't too bad, as I expect all the lists to be 5 items or shorter, but it feels like a kludge.

ints of the list, because if I did then any collision would cause the old value to be replaces...or would it if they didn't compare to equal? Which different items wouldn't, but the *keys* would compare to equal, so they probably would replace, which isn't at all what I desire. But if I just try to combine the ints, I can't guarantee unique keys...certainly not for any list longer than 2 items, as int.max * int would require a long, but adding a third term is a "can't do". BigInt defines opCmp, but doesn't seem to define toHash. So that seems to mean I need to convert the list to a string. UGH!! On the one hand, it allows me to get away with only using one hashtable, but on the other I've got this ugly key conversion...but maybe I won't need to convert both ways. But it's still ugly. Even the two table approach was nicer.
Nov 20 2012
parent Charles Hixson <charleshixsn earthlink.net> writes:
Charles Hixson wrote:
 Charles Hixson wrote:
 Ali Çehreli wrote:
 On 11/20/2012 11:39 AM, Charles Hixson wrote:

 I'm trying to figure out how to construct an associative array whose
 keys will be a combination of strings and immutable int[]'s, but every
 approach I've looked at has run into problems.

Can you show some code?

limitations, and that's all I could use to write code right now.
 It should be relatively
 easy, as strings are really just an immutable list of ints,

A list of dchars is more accurate of course, but yes, dchar can be casted to int.

which are smaller.
 but I
 haven't been able to find how strings are hashed. (It's looking as if
 the details are handled in C...which makes it difficult.)

I bet the algorithm considers all of the characters of the strings. The following has been mentioned in a recent forum thread: http://eternallyconfuzzled.com/tuts/algorithms/jsw_tut_hashing.aspx

lot of room for alternatives. For ASCII there are several plausible approaches, since all one needs to do is minimize collisions. But there are too many plausible unicode chars for most of them to be reasonable. I'd probably go for multiply by a medium size prime, add to the accumulator, mod the accumulator by a large prime, stepping through the loop. And I'd initialize the accumulator by the length of the current string. (Note that this would apply equally to strings or int[]'s.) So if that's what you mean by "considers all the characters", then, yes.
 I could define
 my own hash code, but I don't find it at all clear what would be
 appropriate given that I don't know the size.


 I wonder how much slow iterating over string would be, compared to
 directly accessing the elements of a dstring. (After all, strings must
 be decoded to produce dchars.)

 In any case, you don't need the length of the string; a loop would do:

strings, and it's a convenient built-in function.
 while (!s.empty) {
 // ... use s.front and s.popFront() ...
 }

 Alternatively:

 foreach (c; stride(s, 1)) {
 // ... use c ...
 }

 UFCS looks better: :)

 foreach (c; s.stride(1)) {
 // ... use c ...
 }

foreach (char c; lst) { ... } A hash code doesn't need to reflect the chars of the string, merely to separate different cases.
 The data is rather sparse,
 so a hash table seems appropriate. (My only other alternative is a
 database, and that imposes the heavy slowdown of I/O ... even

 does automate persistence it doesn't seem like a good tradeoff.)

 So far my best idea is to build two tables, and then look at the key
 when a retrieval is attempted to figure out which table it's in. That
 would probably work, but it feels like a kludge.

How do you differentiate between the key type being int[] vs. string? Ali

different parameter types. But as I said, that feels like a kludge. I'm just contemplating it because I want rehash to work properly. So I'd have two indexes: Items[string] stringKeys; Items[int] listKeys; This lets the compiler handle resizing the arrays properly, at the cost of doubly hashing the listKeys variable. This isn't too bad, as I expect all the lists to be 5 items or shorter, but it feels like a kludge.

ints of the list, because if I did then any collision would cause the old value to be replaces...or would it if they didn't compare to equal? Which different items wouldn't, but the *keys* would compare to equal, so they probably would replace, which isn't at all what I desire. But if I just try to combine the ints, I can't guarantee unique keys...certainly not for any list longer than 2 items, as int.max * int would require a long, but adding a third term is a "can't do". BigInt defines opCmp, but doesn't seem to define toHash. So that seems to mean I need to convert the list to a string. UGH!! On the one hand, it allows me to get away with only using one hashtable, but on the other I've got this ugly key conversion...but maybe I won't need to convert both ways. But it's still ugly. Even the two table approach was nicer.

This is the approach I've decided to use, i.e., generate string keys from the list of ints. /** Convert a list of non-negative ints into an invalid utf-8 * string. * \todo The generated string both begins and ends with 0xff ... * these aren't needed as a hash code, and could be trimmed. Think * about removing them. */ string lst2sbytes (int[] lst) { char[] retc; char[] num; retc ~= 0xff; foreach (int i; lst) { enforce (i >= 0, "only non-negative ints are allowed in an sbytes"); int val = i; num.length = 0; while (val > 0) { num ~= val % 128; val = val / 128; } // N.B.: the reverse is needed if you wish to decode the list // later, but is excess baggage when generating a hash code. // reverse (num); retc ~= num ~ 0xff; } return retc.idup; } A bit of a kludge, but it seems to work, and should be fairly fast and reasonably compact. And it allows the components of the key to be any reasonable number. (Well, reasonable for my purposes. I don't want to handle anything but positive ints. Even allowing 0 isn't really reasonable, it just doesn't cost anything.)
Nov 22 2012
prev sibling parent Jonathan M Davis <jmdavisProg gmx.com> writes:
On Tuesday, November 20, 2012 12:42:50 Charles Hixson wrote:
  > It should be relatively
  > easy, as strings are really just an immutable list of ints,
 
 A list of dchars is more accurate of course, but yes, dchar can be
 casted to int.

Well, longs anyway. Which is a minor problem, as I'd prefer to use ints, which are smaller.

Um, no. int is 32 bits. dchar is 32 bits. long is 64 bits. So, both int and dchar will implicitly convert to long, but long will not implicitly to them. int and dchar however _do_ implicitly convert both ways. But if you're using string, then you're dealing with char (8 bits), which means that it's byte or ubyte (also 8 bits) that it implicitly converts to, not int. - Jonathan M Davis
Nov 20 2012