Erlang binary longest common substring

[Marc Worrell]

Maybe you would like to take UTF8 strings into account.
As copying is sometimes better, and actually quite fast, you can also build a new binary.


common_prefix(A, B) ->
    common_prefix(A, B, <<>>).

common_prefix(<<C/utf8, A/binary>>, <<C/utf8, B/binary>>, Acc) ->
    common_prefix(A, B, <<Acc/binary, C/utf8>>);
common_prefix(_A, _B, Acc) ->
    Acc.


Or use the offset method and return a sub-binary:


common_prefix(A, B) ->
    common_prefix(A, B, 0).

common_prefix(A, B, Offs) ->
    case {A, B} of
        {<<_:Offs/binary, C/utf8, _/binary>>, <<_:Offs/binary, C/utf8, _/binary>>} ->
            common_prefix(A, B, Offs + size(<<C/utf8>>));
        {<<Prefix:Offs/binary, _/binary>>, _} ->
            Prefix
    end.


Cheers, Marc



> On 30 Mar 2020, at 13:23, Valentin Micic <v@REDACTED> wrote:
> 
> 
>> On 30 Mar 2020, at 12:18, I Gusti Ngurah Oka Prinarjaya <okaprinarjaya@REDACTED <mailto:okaprinarjaya@REDACTED>> wrote:
>> 
>> Hi,
>> 
>> I need longest common substring erlang implementation. Please help me
>> 
>> I've tried https://rosettacode.org/wiki/Longest_common_subsequence#Erlang <https://rosettacode.org/wiki/Longest_common_subsequence#Erlang> but very very slow T_T
>> 
>> and i've tried binary:longest_common_prefix([<<"i love you 2020-03-28">>,<<"i love you    2020-03-01">>]) not effective / not valid
>> 
>> Thank you 
>> 
> 
> It may be faster to do this with lists, but this should do the trick:
> 
> lcs( <<A/binary>>, <<B/binary>>, Off )
> ->
>     case {A, B} of
>        {<<_:Off/binary-unit:8, C:8, _/binary>>, <<_:Off/binary-unit:8, C:8, _/binary>>} -> lcs( A, B, Off+1 );
>        {<<Common:Off/binary-unit:8, _/binary>>,  _}			 	        -> Common
>     end
> .
> 
> Fro example, a call to  lcs( <<“Hello World”>>, <<"Hellao World”>>, 0 )will return  <<“Hell”>>.
> 
> NOTE (and just in case): you’d need to put this function in your own module.
> 
> V/
> 
> 

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