Modern perfect hashing for strings

http://0x80.pl/notesen/2023-04-30-lookup-in-strings.html

by jandrewrogers • 3 years ago

73 14 3 years ago

drpixie2• 3 years ago

Nice shortcut if you know the input is correct, but seems like it might be prone to failure with carefully constructed input strings. If I can find an input token that produces the same hash as a normal keyword, a program using this might misbehave in "interesting" fashions.

anonymoushn4• 3 years ago

I think all of these techniques check whether the input string is correct. For example see here https://github.com/WojciechMula/toys/blob/master/lookup-in-s...

drpixie2• 3 years ago

If I was parsing the input, then parse-time is the time to assign a token-id (I'd use an enum). Then you have a unique identifier and wouldn't need to use the fancy hash function. It's a nice function, takes good advantage of the instruction set, but seems brittle or redundant.

anonymoushn4• 3 years ago

This looks cool, we'll try it out as an alternative to Zig's std.meta.stringToEnum

pstoll• 3 years ago

Neat. But I suggest moving the summary to the top.

There is a reason summaries / tldr are at the top. They help your reader understand / digest all the details that follow.

benwills3• 3 years ago

If the author sees this, there's a comment on your HTTP Verb parsing post that says:

"The major drawback of gperf is that it generates function "exists", while we need a "lookup"."

gperf allows for returning actual values. I use it in a whole lot of ways (enough that I've written code that imports data to then generate enums, execute gperf and import generated code, etc), including for generating lookups for HTTP Verbs.

Let me know if you want to know how that's done and I can give more details.

anonymoushn4• 3 years ago

Sure how do you do that?

benwills3• 3 years ago

Here's a sample gperf config file I just recently used/auto-generated (with the data labels modified):

  struct SomeData_Gperf_st {
    const char* strKey;
    SomeData_et enumVal;
  };
  %compare-lengths
  %compare-strncmp
  %enum
  %ignore-case
  %includes
  %readonly-tables
  %struct-type
  %define slot-name            strKey
  %define lookup-function-name SomeData_Find
  %%
  val 1, SOME_DATA__VAL_1
  val 2, SOME_DATA__VAL_2

The important option here is the %struct-type option. Using that, it will find and parse the struct definition (above in the configuration), and use that to create the objects in the lookup table.

The lookup table is defined after the two percent signs; %%. You simply use comma separated values. And then you just have to define the const char* to use for the lookup string, as defined by `%define slot-name`

So, gperf will then generate a function, SomeData_Find(), which returns a pointer to a SomeData_Gperf_st object. Or NULL if it's not found.

There are a number of other options available, but what you're seeing above are the options I use most often.

And, finally, I execute a command like this to run it with that config:

  gperf SomeData.gperf.confg --output-file=SomeData_Find.c --multiple-iterations=100

benwills3• 3 years ago

Also, if you get stuck with it or want to configure it differently (eg: using a struct definition or values defined elsewhere), feel free to reach out to my username at gmail.

Just make sure to use a dot between my first and last name.

rurban• 3 years ago

It's better when you pad the const keys for 64bit access with zero's. Otherwise you need to prey the compiler does it for you.

Modern perfect hashing for strings

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