Desafio

Em cada um dos 12 dias de Natal, reproduza o verso apropriado de "12 dias de Natal", de acordo com a data, caso contrário, faça o refrão de "Gostaria que pudesse ser Natal todos os dias".

Novas linhas à esquerda e / ou à direita ( \n) e espaços à direita são permitidos. Você não pode ler de fontes externas.

Basicamente, o que você precisa fazer é ler a data do sistema e emitir o versículo correspondente aos "12 dias de Natal".

12 Dias de Natal

Os Doze Dias de Natal são a época cristã festiva, começando no dia de Natal (25 de dezembro) e terminando no dia 5 de janeiro.

Exemplos

Se a data for 25 de dezembro (imprima o menor / primeiro verso):

On the first day of Christmas 
my true love sent to me:
a Partridge in a Pear Tree

Se a data for 31 de dezembro (imprima o sétimo verso):

On the seventh day of Christmas
my true love sent to me:
7 Swans a Swimming
6 Geese a Laying
5 Golden Rings
4 Calling Birds
3 French Hens
2 Turtle Doves
and a Partridge in a Pear Tree

Se a data for 5 de janeiro (imprima o versículo mais longo / último / 12):

On the twelth day of Christmas
my true love sent to me:
12 Drummers Drumming
11 Pipers Piping
10 Lords a Leaping
9 Ladies Dancing
8 Maids a Milking
7 Swans a Swimming
6 Geese a Laying
5 Golden Rings
4 Calling Birds
3 French Hens
2 Turtle Doves
and a Partridge in a Pear Tree

Se a data não estiver entre 25 de dezembro e 5 de janeiro, inclusive:

Well I wish it could be Christmas every day
When the kids start singing and the band begins to play
Oh I wish it could be Christmas every day
So let the bells ring out for Christmas 

Feliz Natal!

CJam, 557 550 530 528 522 519 bytes

et3<(;" Christmas":P;:(31b7+372%_C<{"On the ""first second third"S/"four fif six seven eigh nin ten eleven twelf"S/"th"f++@:X=" day of"P"
my true love sent to me:
""and a Partridge in a Pear Tree."a[D,2>Sm*"Turtle Dove
French Hen
Calling Bird
Golden Ring
Geese a Lay
Swans a Swimm
Maids a Milk
Ladies Danc
Lords a Leap
Pipers Pip
Drummers Drumm"N/'s4*"ing"a7*+]z+X)<W%N*X!{4>}*}{;"Well"" I wish it could be"P" every day
"++_"When the kids start singing and the band begins to play
Oh"\"So let the bells ring out for"P}?

Teste aqui. Para experimentar datas diferentes, substitua et3<(;por algo como [12 25].

Provavelmente ainda há espaço para melhorias.

et                    "Read the current datetime as an array [year month day hour ...].":
  3<                  "Discard hours, minutes etc.";
    (;                "Discard year.";
      " Christmas":P; "Store the string in P and discard.";

:(                    "Decrement month and day.";
  31b                 "Interpret as base-31 digits. So on [12 25] we get 365 (what a coincidence).
                       On [12 31] we get 371. On [1 1] we get 0. On [1 5] we get 4.";
     7+               "Add 7.";
       372%           "Take modulo 372. Now the 12 days result in 0 to 11, others are greater.";
           _C<        "Duplicate and check if it's less than 12.";

{...}{...}?           "Run the first block for the 12 days, the second block otherwise.";

"On the 12 days:";
"On the "             "Push the string.";
"first ... third"S/   "Push first to third. Split by spaces.";
"four ... telf"S/     "Push other ordinal numbers, split by spaces.";
"th"f+                "Add th to each.";
+                     "Append to get first to twelfth.";
@:X                   "Pull day number to the top. Store in X.";
=                     "Select ordinal number.";
" day of"P            "Push ` day of Christmas`.";
"\nmy true...\n"      "Push that string, too".;
"and a Part..."a      "Push day 1 string and wrap in array.";
[                     "Remember beginning of array.";
D,2>                  "Push range [2 3 ... 12].";
Sm*                   "Append a space to each.";
"Turtle..."N/         "Push all the gifts. Split into lines.";
's4*                  "Push `ssss`.";
"ing"a7*              "Push an array with 7 copies of `ing`.";
+                     "Add the two.";
]                     "Close array, containing numbers, gifts and endings.";
z                     "Zip/transpose the array.";
+                     "Add day-1 line.";
X)                    "Push day and decrement.";
<                     "Slice off unwanted days.";
W%                    "Reverse array.";
N*                    "Join lines with line breaks.";
X!                    "Logical not of day, is 1 on day 1 and 0 on all other days.";
{4>}*                 "On day 1, remove first 4 characters (`and `).";

"On all days:";
;                     "Discard day from stack.";
"Well"                "Push the string.";
" I..."P"...day\n"    "Push the rest of the line in 3 strings.";
++_                   "Join those three into one, and duplicate.";
"When...\nOh"         "Push next line and `Oh`.";
\                     "Swap with duplicate line.";
"So ... for"P         "Push final line.";

Python 2, 681 676 673 670 667 bytes

import time
N=time.localtime();D,M=N.tm_mday,N.tm_mon
Q=((D<6>M<2)*(31+D)+(M>11)*D)-24
print["Well%sWhen the kids start singing and the band begins to play\nOh%sSo let the bells ring out for~"%((" I wish it could be~ every day\n",)*2),"On the %s day of~\nmy true love sent to me:\n"%("twel first second third four fif six seven eigh nin ten eleven".split()[Q%12]+"th"*(Q>3))+"\n".join("%d %s"%(i,"Turtle Doves$French Hens$Calling Birds$Golden Rings$Geese a Lay$Swans a Swimm$Maids a Milk$Ladies Danc$Lords a Leap$Pipers Pip$Drummers Drumm".split("$")[i-2]+"ing"*(i>5))for i in range(Q,1,-1))+"\nand "*(Q>1)+"a Partridge in a Pear Tree"][Q>0].replace("~"," Christmas")

Eu tentei competir contra CJam ... e falhei como esperado. Talvez se eu pudesse descobrir como zlib, mas eu não sei ...

(Obrigado a @FryAmTheEggman por sugestões via chat)

É grande! Isso significa! É feio! Está...

Java: 894.887.883.882 bytes

Golfe:

class C{public static void main(String[]a){java.util.Calendar d=java.util.Calendar.getInstance();String p="a Partridge in a Pear Tree\n",t="Well Z\nWhen the kids start singing and the band begins to play\nOh Z\nSo let the bells ring out for X".replaceAll("Z","I wish it could be X every day"),r="";String[]s=(" day of X\nMy true love gave to me:\nçfirstçsecondçthirdçfourthçfifthçsixthçseventhçeighthçninthçtenthçeleventhçtwelfthç Turtle Doves\nç French Hens\nç Calling Birds\nç Golden Rings\nç Geese a LayY\nç Swans a SwimmY\nç Maids a MilkY\nç Ladies DancY\nç Lords a LeapY\nç Pipers PipY\nç Drummers DrummY\n").replaceAll("Y","ing").split("ç");int n=d.get(d.DAY_OF_YEAR),o=(n>358)?(n-358):(n<6)?(n+7):-1,j;if(o==-1)r=t;else{r+=("On the "+s[o]+s[0]);for(j=o+11;j-->12;r+=(j-11+s[j]));r+=(o!=1)?"and "+p:p;}System.out.print(r.replaceAll("X","Christmas"));}}

Ungolfed:

class C {

    public static void main(String[] a) {
        java.util.Calendar d = java.util.Calendar.getInstance();
        String p = "a Partridge in a Pear Tree\n", t = "Well Z\nWhen the kids start singing and the band begins to play\nOh Z\nSo let the bells ring out for X".replaceAll("Z", "I wish it could be X every day"), r = "";
        String[] s = (" day of X\nMy true love gave to me:\nçfirstçsecondçthirdçfourthçfifthçsixthçseventhçeighthçninthçtenthçeleventhçtwelfthç Turtle Doves\nç French Hens\nç Calling Birds\nç Golden Rings\nç Geese a LayY\nç Swans a SwimmY\nç Maids a MilkY\nç Ladies DancY\nç Lords a LeapY\nç Pipers PipY\nç Drummers DrummY\n").replaceAll("Y", "ing").split("ç");
        int n = d.get(d.DAY_OF_YEAR), o = (n > 358) ? (n - 358) : (n < 6) ? (n + 7) : -1, j;
        if (o == -1) {
            r = t;
        } else {
            r += ("On the " + s[o] + s[0]);
            for (j = o + 11; j-- > 12; r += (j - 11 + s[j]));
            r += (o != 1) ? "and " + p : p;
        }
        System.out.print(r.replaceAll("X", "Christmas"));
    }
}

Depois de mexer bastante e ganhar 14 bytes, acho que vou parar de brincar.

E sim, eu sei que, em anos bissextos, minha ideia pode ter um pouco de problema, mas ... era minha maneira de cortar alguns bytes do código. Ah bem.

Erlang, 748 bytes

Uma versão limpa de 7 bits sem gzip-9 é de 748 bytes:

f()->[maps:get(C,#{$N=>"ing",$x=>10,$q=>"th",$z=>" Christmas"},C)||C<-s(t(calendar:local_time()))].
t({{_,1,D},_})->D+7;t({{_,12,D},_})->D-24;t(_)->1.
s(X)when X<1;X>12->L=" I wish it could bez every dayx","Well"++L++"When qe kids start sNN and qe band begins to playxOh"++L++"So let qe bells rN out forz";
s(I)->"On qe "++element(I,{"first","second","qird","fourq","fifq","sixq","sevenq","eighq","ninq","tenq","elevenq","twelq"})++" day ofzxmy true love sent to me:x"++r(I).
r(0)->"";r(I)->element(I,{"a Partridge in a Pear Tree","2 Turtle Dovesxand ","3 French Hensx","4 CallN Birdsx","5 Golden RNsx","6 Geese a LayNx","7 Swans a SwimmNx","8 Maids a MilkNx","9 Ladies DancNx","10 Lords a LeapNx","11 Pipers PipNx","12 Drummers DrummNx"})++r(I-1).

Uma solução gzip-9 + base64 tem 750 bytes:

f()->s(t(calendar:local_time())).
t({{_,1,D},_})->D+8;t({{_,12,D},_})->D-23;t(_)->1.
s(I)->try element(I,binary_to_term(base64:decode("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")))catch _:_->s(1)end.

Uma função Erlang codificada em UTF-8 tem 421 caracteres (935 bytes).

f()->s(t(calendar:local_time())).
t({{_,1,D},_})->D+8;t({{_,12,D},_})->D-23;t(_)->1.
s(I)->try element(I,binary_to_term(<<<<(C-$㐀):14>>||C<-"哔㐀㐤䑸檷䵅焸伱㠱怶悑券摝汈佊玏㵊塐專㔒染孡㵯徵峆䫽㳳欄捐拵㟔㸽䒻焄䋓垐啂庐嚯栏奕挮浫柍濦斝憻䮽埅牃栺整䲸䔢仇椕斡㯜愹䙣㔭㘅溄暫玡乘㖐㶵廁㴠撞㰋䃟䨄㻗刚淥朞壽㖊娞泓拙母倅戒奴㸩娠㼷䶰䞦填显彻己㵲克㪐傊䈿䠛㭾悐戉压咀㲅夘䖧彤欸䡛㭭䨳吂焤擎䉲㷪㮛䡈䢙䕝欕䏛宠拎䫓巣㝐凩淄應东歹墝䧉㾘敞䃡䲜䡬㹒媽㧕勆屑憈䊱濚䇷扃滱䕝犍沿䥝犙㤐䟜㔇檗坷庈㬫焰厺憮潴䤱䍱䐉㹆㲎杒妷撯宥帶㧢夈㿤䁧䛨湽㮣庚㶠珥㯋䌛傜圵怓爜䗉姳䐙㯎槆匶煯液恄姅堭刨䬹䢃㓑喒㱿柖慾漻幜㾠丼撊挭圢㳏䎘㟰䋛冸婽灜䖠旃囦攤嵼柤含怭儿䧭㶾朿曨佰橍徰䎦䥶㟿届䇰㩌猌浀㐀㐀㐀">>))catch _:_->s(1)end.

Javascript (665)

__=new Date("12/31");M=_.getMonth();D=_.getDate();d=M?(M==11)*(D-24):D+7;if(!d||d>12)s="Well K\nWhen the kids start singing and the bands begin to play\nOh K\nSo let the bells ring out for Christmas".replace(/K/g,"I wish it could be Christmas every day");else{for(s="On the "+"0first0second0third0fourth0fifth0sixth0seventh0eight0ninth0tenth0elevent0twelth".split(0)[d]+" day of Chirstmas\nmy true love sent to me:";--d;)s+="\n"+(d+1)+" "+"0Turtle Doves0French Hens0CallZ Birds0Golden RZs0Geese a LayZ0Swans a SwimmZ0Maids a MilkZ0Ladies DancZ0Lords a LeapZ0Pipers PipZ0Drummers DrummZ".split(0)[d].replace("Z","ing");s+="\n"+(D!=25?"and ":"")+"a Partridge in a Pear Tree"}alert(s)

Você pode testá-lo adicionando parênteses à primeira new Datealteração, ou seja:

_=new Date;M=_...

para

_=new Date("1/5");M=_...

Aqui é embelezado:

_ = new Date("12/31");
M = _.getMonth();
D = _.getDate();
d = M ? (M == 11) * (D - 24) : D + 7;
if (!d || d > 12) s = "Well K\nWhen the kids start singing and the bands begin to play\nOh K\nSo let the bells ring out for Christmas".replace(/K/g, "I wish it could be Christmas every day");
else {
    for (s = "On the " + "0first0second0third0fourth0fifth0sixth0seventh0eight0ninth0tenth0elevent0twelth".split(0)[d] + " day of Chirstmas\nmy true love sent to me:"; --d;) s += "\n" + (d + 1) + " " + "0Turtle Doves0French Hens0CallZ Birds0Golden RZs0Geese a LayZ0Swans a SwimmZ0Maids a MilkZ0Ladies DancZ0Lords a LeapZ0Pipers PipZ0Drummers DrummZ".split(0)[d].replace("Z", "ing");
    s += "\n" + (D != 25 ? "and " : "") + "a Partridge in a Pear Tree"
}
alert(s)

C # no Linqpad - 866 bytes

Bem, pelo menos ele venceu o Java;) Os anos bissextos são tratados dentro do código.

Não formatado:

void Main(){var w=DateTime.Now;int y=w.Year%4,d=w.DayOfYear,c=0,p=0;string t="Well Z\nWhen the kids start singing and the band begins to play\nOh Z\nSo let the bells ring out for X".Replace("Z","I wish it could be X every day").Replace("X","Christmas"),o="On the ",x=" day of Christmas\nmy true love sent to me:\n";string[]z={"first","second","third","forth","fifth","sixth","seventh","eighth","ninth","tenth","eleventh","twelth"},b={"a Partridge in a Pear Tree\n","and ","2 Turtle Doves\n","3 French Hens\n","4 Calling Birds\n","5 Golden Rings\n","6 Geese a Laying\n","7 Swans a Swimming\n","8 Maids a Milking\n","9 Ladies Dancing\n","10 Lords a Leaping\n","11 Pipers Piping\n","12 Drummers Drumming\n"};if(y==0){p=1;}if(d>358){c=d-358-p;}else if(d<6){c=d+7;}else{t.Dump();return;}if(c==1){o+=z[0]+x+b[0];}else{o+=z[c-1]+x;for(int i=c;i>=0;i--){o+=b[i];}}o.Dump();}

Formatado:

void Main()
{
var w=DateTime.Now;
int y=w.Year%4,d=w.DayOfYear,c=0,p=0;
string t="Well Z\nWhen the kids start singing and the band begins to play\nOh Z\nSo let the bells ring out for X".Replace("Z","I wish it could be X every day").Replace("X","Christmas"),
o="On the ",x=" day of Christmas\nmy true love sent to me:\n";

string[]z={"first","second","third","forth","fifth","sixth","seventh","eighth","ninth","tenth","eleventh","twelth"},

b={"a Partridge in a Pear Tree\n","and ","2 Turtle Doves\n","3 French Hens\n","4 Calling Birds\n","5 Golden Rings\n","6 Geese a Laying\n",
"7 Swans a Swimming\n","8 Maids a Milking\n","9 Ladies Dancing\n","10 Lords a Leaping\n","11 Pipers Piping\n","12 Drummers Drumming\n"};

if(y==0)
{
    p=1;
}

if(d>358)
{
    c=d-358-p;
}
else if(d<6)
{
    c=d+7;
}
else
{
    t.Dump();
    return;
}

if(c==1)
{
    o+=z[0]+x+b[0];
}
else
{
    o+=z[c-1]+x;
    for(int i=c;i>=0;i--)
    {
        o+=b[i];
    }
}
o.Dump();
}