At first, I wanted to use digit swapping, like @coreyja
, but it was harder than I initially thought (e.g. 351 needs two swaps, because the next larger number is 513, not 531.)

To validate the swapping method, I wrote a blunt version that converts a number into an array of digits, lists all the permutations, sorts them and finds the next occurence. For that I searched for a permutation function. It uses generators and list comprehensions, and I haven’t taken the time to understand it yet, but I’ll definitely try that in future challenges.

So my current solution is the blunt one (and I removed the unit tests for the intermediate functions, that are useful for development but not interesting for the solution).

-module(next).-include_lib("eunit/include/eunit.hrl").% convert an integer to an array of digits
digits(N)whenN>=0->digits(N,[]).digits(N,Digits)whenN<10->[N|Digits];digits(N,Digits)->digits(Ndiv10,[Nrem10|Digits]).% convert an array of digits to an integer
number(Digits)->number(Digits,0).number([D|Rest],N)->number(Rest,N*10+D);number([],N)->N.% list all permutations of an array, taken from
% http://erlang.org/doc/programming_examples/list_comprehensions.html#permutations
perms([])->[[]];perms(L)->[[H|T]||H<-L,T<-perms(L--[H])].% return the first element occuring after N in a list
find_next(N,[N,M|_])->M;find_next(N,[_|Rest])->find_next(N,Rest);find_next(_,[])->none.next(N)->Digits=digits(N),Next=find_next(Digits,lists:sort(perms(digits(N)))),caseNextofnone->none;_->number(Next)end.next_test_()->[?_assertEqual(none,next(5)),?_assertEqual(51,next(15)),?_assertEqual(536,next(365)),?_assertEqual(21,next(12)),?_assertEqual(531,next(513)),?_assertEqual(513,next(351)),?_assertEqual(none,next(531)),?_assertEqual(2091,next(2019))].

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Erlang.At first, I wanted to use digit swapping, like @coreyja , but it was harder than I initially thought (e.g.

`351`

needs two swaps, because the next larger number is`513`

, not`531`

.)To validate the swapping method, I wrote a blunt version that converts a number into an array of digits, lists all the permutations, sorts them and finds the next occurence. For that I searched for a permutation function. It uses generators and list comprehensions, and I haven’t taken the time to understand it yet, but I’ll definitely try that in future challenges.

So my current solution is the blunt one (and I removed the unit tests for the intermediate functions, that are useful for development but not interesting for the solution).