Generate the shortest De Bruijn

Question

A De Bruijn sequence is interesting: It is the shortest, cyclic sequence that contains all possible sequences of a given alphabet of a given length. For example, if we were considering the alphabet A,B,C and a length of 3, a possible output is:

AAABBBCCCABCACCBBAACBCBABAC

You will notice that every possible 3-character sequence using the letters A, B, and C are in there.

Your challenge is to generate a De Bruijn sequence in as few characters as possible. Your function should take two parameters, an integer representing the length of the sequences, and a list containing the alphabet. Your output should be the sequence in list form.

You may assume that every item in the alphabet is distinct.

An example generator can be found here

Standard loopholes apply

Answer 1

Pyth, 31 bytes

This is the direct conversion of the algorithm used in my CJam answer. Tips for golfing welcome!

Mu?G}H+GG+G>Hefq<HT>G-lGTUH^GHk

This code defines a function g which takes two arguments, the string of list of characters and the number.

Example usage:

Mu?G}H+GG+G>Hefq<HT>G-lGTUH^GHkg"ABC"3

Output:

AAABAACABBABCACBACCBBBCBCCC

Code expansion:

M                 # def g(G,H):
 u                #   return reduce(lambda G, H:
  ?G              #     (G if
    }H            #       (H in
      +GG         #          add(G,G)) else
    +G            #       add(G,
      >H          #         slice_end(H,
        e         #           last_element(
         f        #             Pfilter(lambda T:
          q       #               equal(
           <HT    #                 slice_start(H,T),
           >G     #                 slice_end(G,
             -lGT #                   minus(Plen(G),T))),
          UH      #               urange(H)))))),
  ^GH             #     cartesian_product(G,H),
  k               #     "")

Try it here

Answer 2

CJam, 52 49 48 bytes

This is surprisingly long. This can be golfed a lot, taking in tips from the Pyth translation.

q~a*{m*:s}*{:H\:G_+\#)GGHH,,{_H<G,@-G>=},W=>+?}*

The input goes like

3 "ABC"

i.e. - String of list of characters and the length.

and output is the De Bruijn string

AAABAACABBABCACBACCBBBCBCCC

Try it online here

Answer 3

CJam, 52 49 bytes

Here is a different approach in CJam:

l~:N;:L,(:Ma{_N*N<0{;)_!}g(+_0a=!}g]{,N\%!},:~Lf=

Takes input like this:

"ABC" 3

and produces a Lyndon work like

CCCBCCACBBCBACABCAABBBABAAA

Try it here.

This makes use of the relation with Lyndon words. It generates all Lyndon words of length n in lexicographic order (as outlined in that Wikipedia article), then drops those whose length doesn't divide n. This already yields the De Bruijn sequence, but since I'm generating the Lyndon words as strings of digits, I also need to replace those with the corresponding letters at the end.

For golfing reasons, I consider the later letters in the alphabet to have lower lexicographic order.

Answer 4

Jelly, 15 bytes

ṁL*¥Œ!;`wⱮẠɗƇṗḢ

Try it online!

Pretty slow, uses a brute force approach with around an \$O(n \times m \times (n \times m)!)\$ complexity, where \$n\$ is the integer input and \$m\$ is the length of the string. Times out if both \$n\$ and \$m\$ are greater than 3 on TIO

How it works

The length of the De Bruijn sequence will always be \$m^n\$ and each symbol in the provided alphabet will occur the same number of times, \$m^{n-1}\$. Therefore, we generate the string with that many symbols, then filter its permutations to find valid De Bruijn sequences.

ṁL*¥Œ!ẋ2wⱮẠʋƇṗḢ - Main link. Takes an alphabet A on the left and n on the right
   ¥            - Last 2 links as a dyad f(A, n):
 L              -   Length of A
  *             -   Raised to the power n
ṁ               - Mold A to that length
    Œ!          - All permutations
             ṗ  - Powerset; Get all length n combinations of A. Call that C
           ʋƇ   - Filter the permutations P on the following dyad g(P, C):
      ẋ2        -   Repeat P twice
         Ɱ      -   For each element E in C:
        w       -     Is it a sublist of P?
          Ạ     -   Is this true for all elements of C?
              Ḣ - Take the first one

Answer 5

JavaScript (ES6) 143

Using Lyndon words, like Martin's aswer, just 3 times long...

F=(a,n)=>{
  for(w=[-a[l='length']],r='';w[0];)
  {
    n%w[l]||w.map(x=>r+=a[~x]);
    for(;w.push(...w)<=n;);
    for(w[l]=n;!~(z=w.pop()););
    w.push(z+1)
  }
  return r
}

Test In FireFox/FireBug console

console.log(F("ABC",3),F("10",4))

Output

CCCBCCACBBCBACABCAABBBABAAA 0000100110101111

Answer 6

Python 2, 114 bytes

I'm not really sure how to golf it more, due to my approach.

def f(a,n):
 s=a[-1]*n
 while 1:
    for c in a:
     if((s+c)[len(s+c)-n:]in s)<1:s+=c;break
    else:break
 print s[:1-n]

Try it online

Ungolfed:

This code is a trivial modification from my solution to more recent challenge.

def f(a,n):
    s=a[-1]*n
    while 1:
        for c in a:
            p=s+c
            if p[len(p)-n:]in s:
                continue
            else:
                s=p
                break
        else:
            break
    print s[:1-n]

The only reason [:1-n] is needed is because the sequence includes the wrap-around.

Answer 7

Powershell, 164 96 bytes

-68 bytes with -match O($n*2^n) instead recursive generator O(n*log(n))

param($s,$n)for(;$z=$s|% t*y|?{"$($s[-1])"*($n-1)+$x-notmatch-join"$x$_"[-$n..-1]}){$x+=$z[0]}$x

Ungolfed & test script:

$f = {

param($s,$n)                    # $s is a alphabet, $n is a subsequence length
for(;                           # repeat until...
    $z=$s|% t*y|?{              # at least a character from the alphabet returns $true for expression:
        "$($s[-1])"*($n-1)+$x-notmatch  # the old sequence that follows two characters (the last letter from the alphabet) not contains
        -join"$x$_"[-$n..-1]    # n last characters from the new sequence
}){
    $x+=$z[0]                   # replace old sequence with new sequence
}
$x                              # return the sequence

}

@(
    ,("ABC",  2, "AABACBBCC")
    ,("ABC",  3, "AAABAACABBABCACBACCBBBCBCCC")
    ,("ABC",  4, "AAAABAAACAABBAABCAACBAACCABABACABBBABBCABCBABCCACACBBACBCACCBACCCBBBBCBBCCBCBCCCC")
    ,("ABC",  5, "AAAAABAAAACAAABBAAABCAAACBAAACCAABABAABACAABBBAABBCAABCBAABCCAACABAACACAACBBAACBCAACCBAACCCABABBABABCABACBABACCABBACABBBBABBBCABBCBABBCCABCACABCBBABCBCABCCBABCCCACACBACACCACBBBACBBCACBCBACBCCACCBBACCBCACCCBACCCCBBBBBCBBBCCBBCBCBBCCCBCBCCBCCCCC")
    ,("ABC",  6, "AAAAAABAAAAACAAAABBAAAABCAAAACBAAAACCAAABABAAABACAAABBBAAABBCAAABCBAAABCCAAACABAAACACAAACBBAAACBCAAACCBAAACCCAABAABAACAABABBAABABCAABACBAABACCAABBABAABBACAABBBBAABBBCAABBCBAABBCCAABCABAABCACAABCBBAABCBCAABCCBAABCCCAACAACABBAACABCAACACBAACACCAACBABAACBACAACBBBAACBBCAACBCBAACBCCAACCABAACCACAACCBBAACCBCAACCCBAACCCCABABABACABABBBABABBCABABCBABABCCABACACABACBBABACBCABACCBABACCCABBABBABCABBACBABBACCABBBACABBBBBABBBBCABBBCBABBBCCABBCACABBCBBABBCBCABBCCBABBCCCABCABCACBABCACCABCBACABCBBBABCBBCABCBCBABCBCCABCCACABCCBBABCCBCABCCCBABCCCCACACACBBACACBCACACCBACACCCACBACBACCACBBBBACBBBCACBBCBACBBCCACBCBBACBCBCACBCCBACBCCCACCACCBBBACCBBCACCBCBACCBCCACCCBBACCCBCACCCCBACCCCCBBBBBBCBBBBCCBBBCBCBBBCCCBBCBBCBCCBBCCBCBBCCCCBCBCBCCCBCCBCCCCCC")
    ,("01",   3, "00010111")
    ,("01",   4, "0000100110101111")
    ,("abcd", 2, "aabacadbbcbdccdd")
    ,("0123456789", 3, "0001002003004005006007008009011012013014015016017018019021022023024025026027028029031032033034035036037038039041042043044045046047048049051052053054055056057058059061062063064065066067068069071072073074075076077078079081082083084085086087088089091092093094095096097098099111211311411511611711811912212312412512612712812913213313413513613713813914214314414514614714814915215315415515615715815916216316416516616716816917217317417517617717817918218318418518618718818919219319419519619719819922232242252262272282292332342352362372382392432442452462472482492532542552562572582592632642652662672682692732742752762772782792832842852862872882892932942952962972982993334335336337338339344345346347348349354355356357358359364365366367368369374375376377378379384385386387388389394395396397398399444544644744844945545645745845946546646746846947547647747847948548648748848949549649749849955565575585595665675685695765775785795865875885895965975985996667668669677678679687688689697698699777877978878979879988898999")
    ,("9876543210", 3, "9998997996995994993992991990988987986985984983982981980978977976975974973972971970968967966965964963962961960958957956955954953952951950948947946945944943942941940938937936935934933932931930928927926925924923922921920918917916915914913912911910908907906905904903902901900888788688588488388288188087787687587487387287187086786686586486386286186085785685585485385285185084784684584484384284184083783683583483383283183082782682582482382282182081781681581481381281181080780680580480380280180077767757747737727717707667657647637627617607567557547537527517507467457447437427417407367357347337327317307267257247237227217207167157147137127117107067057047037027017006665664663662661660655654653652651650645644643642641640635634633632631630625624623622621620615614613612611610605604603602601600555455355255155054454354254154053453353253153052452352252152051451351251151050450350250150044434424414404334324314304234224214204134124114104034024014003332331330322321320312311310302301300222122021121020120011101000")
) |% {
    $s,$n,$e = $_
    $r = &$f $s $n
    "$($r-eq$e): $r"
}

Output:

True: AABACBBCC
True: AAABAACABBABCACBACCBBBCBCCC
True: AAAABAAACAABBAABCAACBAACCABABACABBBABBCABCBABCCACACBBACBCACCBACCCBBBBCBBCCBCBCCCC
True: AAAAABAAAACAAABBAAABCAAACBAAACCAABABAABACAABBBAABBCAABCBAABCCAACABAACACAACBBAACBCAACCBAACCCABABBABABCABACBABACCABBACABBBBABBBCABBCBABBCCABCACABCBBABCBCABCCBABCCCACACBACACCACBBBACBBCACBCBACBCCACCBBACCBCACCCBACCCCBBBBBCBBBCCBBCBCBBCCCBCBCCBCCCCC
True: AAAAAABAAAAACAAAABBAAAABCAAAACBAAAACCAAABABAAABACAAABBBAAABBCAAABCBAAABCCAAACABAAACACAAACBBAAACBCAAACCBAAACCCAABAABAACAABABBAABABCAABACBAABACCAABBABAABBACAABBBBAABBBCAABBCBAABBCCAABCABAABCACAABCBBAABCBCAABCCBAABCCCAACAACABBAACABCAACACBAACACCAACBABAACBACAACBBBAACBBCAACBCBAACBCCAACCABAACCACAACCBBAACCBCAACCCBAACCCCABABABACABABBBABABBCABABCBABABCCABACACABACBBABACBCABACCBABACCCABBABBABCABBACBABBACCABBBACABBBBBABBBBCABBBCBABBBCCABBCACABBCBBABBCBCABBCCBABBCCCABCABCACBABCACCABCBACABCBBBABCBBCABCBCBABCBCCABCCACABCCBBABCCBCABCCCBABCCCCACACACBBACACBCACACCBACACCCACBACBACCACBBBBACBBBCACBBCBACBBCCACBCBBACBCBCACBCCBACBCCCACCACCBBBACCBBCACCBCBACCBCCACCCBBACCCBCACCCCBACCCCCBBBBBBCBBBBCCBBBCBCBBBCCCBBCBBCBCCBBCCBCBBCCCCBCBCBCCCBCCBCCCCCC
True: 00010111
True: 0000100110101111
True: aabacadbbcbdccdd
True: 0001002003004005006007008009011012013014015016017018019021022023024025026027028029031032033034035036037038039041042043044045046047048049051052053054055056057058059061062063064065066067068069071072073074075076077078079081082083084085086087088089091092093094095096097098099111211311411511611711811912212312412512612712812913213313413513613713813914214314414514614714814915215315415515615715815916216316416516616716816917217317417517617717817918218318418518618718818919219319419519619719819922232242252262272282292332342352362372382392432442452462472482492532542552562572582592632642652662672682692732742752762772782792832842852862872882892932942952962972982993334335336337338339344345346347348349354355356357358359364365366367368369374375376377378379384385386387388389394395396397398399444544644744844945545645745845946546646746846947547647747847948548648748848949549649749849955565575585595665675685695765775785795865875885895965975985996667668669677678679687688689697698699777877978878979879988898999
True: 9998997996995994993992991990988987986985984983982981980978977976975974973972971970968967966965964963962961960958957956955954953952951950948947946945944943942941940938937936935934933932931930928927926925924923922921920918917916915914913912911910908907906905904903902901900888788688588488388288188087787687587487387287187086786686586486386286186085785685585485385285185084784684584484384284184083783683583483383283183082782682582482382282182081781681581481381281181080780680580480380280180077767757747737727717707667657647637627617607567557547537527517507467457447437427417407367357347337327317307267257247237227217207167157147137127117107067057047037027017006665664663662661660655654653652651650645644643642641640635634633632631630625624623622621620615614613612611610605604603602601600555455355255155054454354254154053453353253153052452352252152051451351251151050450350250150044434424414404334324314304234224214204134124114104034024014003332331330322321320312311310302301300222122021121020120011101000

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See also: One Ring to rule them all. One String to contain them all

Answer 8

Python 3, 72 bytes

f=lambda a,n:(a[:n]in a[1:])*a[n:]or max((f(c+a,n)for c in{*a}),key=len)

Try it online!