reverse engineer 12 days

#include <stdio.h>

/* the original set of strings */

static char *strings =

"@n'+,#'/*{}w+/w#cdnr/+,{}r/*de}+,/*{*+,/w{%+,/w#q#n+,/#{l+,/n{n+,/+#n+,/#\
;#q#n+,/+k#;*+,/'r :'d*'3,}{w+K w'K:'+}e#';dq#'l \
q#'+d'K#!/+k#;q#'r}eKK#}w'r}eKK{nl]'/#;#q#n'){)#}w'){){nl]'/+#n';d}rw' i;# \
){nl]!/n{n#'; r{#w'r nc{nl]'/#{l,+'K {rw' iK{;[{nl]'/w#q#n'wk nw' \
iwk{KK{nl]!/w{%'l##w#' i; :{nl]'/*{q#'ld;r'}{nlwb!/*de}'c \
;;{nl'-{}rw]'/+,}##'*}#nc,',#nw]'/+kd'+e}+;#'rdq#w! nr'/ ') }+}{rl#'{n' ')# \
}'+}##(!!/";

/* the translation mapping */

static char *translate =

"!ek;dc i@bK'(q)-[w]*%n+r3#l,{}:\nuwloca-O;m .vpbks,fxntdCeghiry";

#define FIRST_DAY  1
#define LAST_DAY  12

/* the original "indices" of the various strings */

enum {
   ON_THE = 0,
   FIRST = -1,
   TWELFTH = -12,
   DAY_OF_CHRISTMAS = -13,
   TWELVE_DRUMMERS_DRUMMING = -14,
   ELEVEN_PIPERS_PIPING = -15,
   TWO_TURTLE_DOVES_AND_A = -24,
   PARTRIDGE_IN_A_PEAR_TREE = -25
};

/* skip -n strings (separator is /), where n is a negative value */

char* skip_n_strings(int n,char *s) {
  if (n == 0)
    return s;

  if (*s=='/')
    return skip_n_strings(n+1,s+1);
  else
    return skip_n_strings(n,s+1);
}

/* find the character in the translation buffer matching c and output
   the translation */

void translate_and_put_char(char c, char *trans) {
  if (c == *trans)
    putchar(trans[31]);
  else
    translate_and_put_char(c,trans+1);
}

void output_chars(char *s) {
  if (*s == '/')
    return;
  translate_and_put_char(*s,translate);
  output_chars(s+1);
}

/* skip to the "n^th" string and print it */

void print_string(int n) {
     output_chars(skip_n_strings(n,strings));
}

void inner_loop(int count_day, int current_day) {
  if (count_day == FIRST_DAY) {
    print_string(ON_THE);               /* "On the " */
    print_string(-current_day);         /* twelve days, ranges from -1 to -12 */
    print_string(DAY_OF_CHRISTMAS);     /* "day of Christmas ..." */
  }

  if (count_day < current_day)     /* inner iteration */
    inner_loop(count_day+1,current_day);

  print_string(PARTRIDGE_IN_A_PEAR_TREE+(count_day-1));   /* print the gift */
}

void outer_loop(int count_day, int current_day) {
  inner_loop(count_day,current_day);
  if (count_day == FIRST_DAY && current_day < LAST_DAY)  /* outer iteration */
    outer_loop(1,current_day+1);
}

void main() {
  outer_loop(1,1);
}