How does the 12 Days of Christmas program work?
In 1988, an impressive and awesome piece of C code, codenamed , won the International Obfuscated C Code Contest.
The program was even smaller than the "compressed" type of its output and represented a completely new approach to text compression standards. According to the judges, the program looks like it was created by randomly hitting the keyboard.
But the program magically prints out the lyrics to the 12 Days of Christmas in just a few sentences of C code!
Here's a step-by-step guide to figuring out how the program works.
1 Author's source code, some of the feeling of chaos characters, a number of main~
#include <> main(t,_,a) char *a; { return!0<t?t<3?main(-79,-13,a+main(-87,1-_,main(-86,0,a+1)+a)): 1,t<_?main(t+1,_,a):3,main(-94,-27+t,a)&&t==2?_<13? main(2,_+1,"%s %d %d\n"):9:16:t<0?t<-72?main(_,t, "@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' ')# \ }'+}##(!!/") :t<-50?_==*a?putchar(31[a]):main(-65,_,a+1):main((*a=='/')+t,_,a+1) :0<t?main(2,2,"%s"):*a=='/'||main(0,main(-61,*a, "!ek;dc i@bK'(q)-[w]*%n+r3#l,{}:\nuwloca-O;m .vpbks,fxntdCeghiry"),a+1); }
2 Compile, run, see the result, well, look at these few messy code, this is the lyrics of The Twelve Days Of Christmas, so amazing ^@^
On the first day of Christmas my true love gave to me a partridge in a pear tree. On the second day of Christmas my true love gave to me two turtle doves and a partridge in a pear tree. On the third day of Christmas my true love gave to me three french hens, two turtle doves and a partridge in a pear tree. On the fourth day of Christmas my true love gave to me four calling birds, three french hens, two turtle doves and a partridge in a pear tree. On the fifth day of Christmas my true love gave to me five gold rings; four calling birds, three french hens, two turtle doves and a partridge in a pear tree. On the sixth day of Christmas my true love gave to me six geese a-laying, five gold rings; four calling birds, three french hens, two turtle doves and a partridge in a pear tree. On the seventh day of Christmas my true love gave to me seven swans -swimming, six geese a-laying, five gold rings; four calling birds, three french hens, two turtle doves and a partridge in a pear tree. On the eighth day of Christmas my true love gave to me eight maids a-milking, seven swans -swimming, six geese a-laying, five gold rings; four calling birds, three french hens, two turtle doves and a partridge in a pear tree. On the ninth day of Christmas my true love gave to me nine ladies dancing, eight maids a-milking, seven swans -swimming, six geese a-laying, five gold rings; four calling birds, three french hens, two turtle doves and a partridge in a pear tree. On the tenth day of Christmas my true love gave to me ten lords a-leaping, nine ladies dancing, eight maids a-milking, seven swans -swimming, six geese a-laying, five gold rings; four calling birds, three french hens, two turtle doves and a partridge in a pear tree. On the eleventh day of Christmas my true love gave to me eleven pipers piping, ten lords a-leaping, nine ladies dancing, eight maids a-milking, seven swans -swimming, six geese a-laying, five gold rings; four calling birds, three french hens, two turtle doves and a partridge in a pear tree. On the twelfth day of Christmas my true love gave to me twelve drummers drumming, eleven pipers piping, ten lords a-leaping, nine ladies dancing, eight maids a-milking, seven swans -swimming, six geese a-laying, five gold rings; four calling birds, three french hens, two turtle doves and a partridge in a pear tree.
"The Twelve Days of Christmas" is a special English Christmas carol published around the 1780s and said to have been written by Catholics in hiding during the persecution of Queen Elizabeth I. It was written to help teach children about the Catholic faith without the attention of government officials, using figurative descriptions as a tool to help children memorize it. It was written to help teach children about the Catholic faith without attracting the attention of government officials, using figurative descriptions as a tool to help children memorize. The song represents the grand gift that is gradually given on each of the twelve days of Christmas. The Twelve Days of Christmas is the joyous holiday that begins on Christmas Day (December 25th). It is also known as the Christmas season (Christmastide and Twelvetide).
4 Review the assembly code
Address Hex dump Command Comments 00401000 /$ 55 PUSH EBP ; a.00401000(guessed Arg1,Arg2,Arg3) 00401001 |. 89E5 MOV EBP,ESP 00401003 |. 81EC 04000000 SUB ESP,4 00401009 |. 90 NOP !0<t 0040100A |. B8 01000000 MOV EAX,1 0040100F |. 8B4D 08 MOV ECX,DWORD PTR SS:[ARG.1] 00401012 |. 39C8 CMP EAX,ECX 00401014 |. B8 00000000 MOV EAX,0 00401019 |. 0F9CC0 SETL AL 0040101C |. 83F8 00 CMP EAX,0 0040101F |. 0F84 51010000 JE 00401176 t<3 00401025 |. 8B45 08 MOV EAX,DWORD PTR SS:[ARG.1] 00401028 |. 83F8 03 CMP EAX,3 0040102B |. B8 00000000 MOV EAX,0 00401030 |. 0F9CC0 SETL AL 00401033 |. 83F8 00 CMP EAX,0 00401036 |. 0F84 5D000000 JE 00401099 1-_ 0040103C |. B8 01000000 MOV EAX,1 00401041 |. 8B4D 0C MOV ECX,DWORD PTR SS:[ARG.2] 00401044 |. 29C8 SUB EAX,ECX main(-86,0,a+1) 00401046 |. 8B4D 10 MOV ECX,DWORD PTR SS:[ARG.3] 00401049 |. 41 INC ECX 0040104A |. 51 PUSH ECX ; /Arg3 0040104B |. B9 00000000 MOV ECX,0 ; | 00401050 |. 51 PUSH ECX ; |Arg2 => 0 00401051 |. B9 AAFFFFFF MOV ECX,-56 ; | 00401056 |. 51 PUSH ECX ; |Arg1 => -56 00401057 |. 8945 FC MOV DWORD PTR SS:[LOCAL.1],EAX ; | 0040105A |. E8 A1FFFFFF CALL 00401000 ; \a.00401000 main(-86,0,a+1) 0040105F |. 83C4 0C ADD ESP,0C main(-87,1-_, main(-86,0,a+1)+a) 00401062 |. 8B4D 10 MOV ECX,DWORD PTR SS:[ARG.3] 00401065 |. 01C1 ADD ECX,EAX 00401067 |. 51 PUSH ECX ; /Arg3 00401068 |. 8B45 FC MOV EAX,DWORD PTR SS:[LOCAL.1] ; | 0040106B |. 50 PUSH EAX ; |Arg2 => [LOCAL.1] 0040106C |. B8 A9FFFFFF MOV EAX,-57 ; | 00401071 |. 50 PUSH EAX ; |Arg1 => -57 00401072 |. E8 89FFFFFF CALL 00401000 ; \a.00401000 main(-87,1-_, main(-86,0,a+1)+a) 00401077 |. 83C4 0C ADD ESP,0C main(-79,-13,a+main(-87,1-_, main(-86,0,a+1)+a)) 0040107A |. 8B4D 10 MOV ECX,DWORD PTR SS:[ARG.3] 0040107D |. 01C1 ADD ECX,EAX 0040107F |. 51 PUSH ECX ; /Arg3 00401080 |. B8 F3FFFFFF MOV EAX,-0D ; | 00401085 |. 50 PUSH EAX ; |Arg2 => -0D 00401086 |. B8 B1FFFFFF MOV EAX,-4F ; | 0040108B |. 50 PUSH EAX ; |Arg1 => -4F 0040108C |. E8 6FFFFFFF CALL 00401000 ; \a.00401000 main(-79,-13,a+main(-87,1-_, main(-86,0,a+1)+a)) 00401091 |. 83C4 0C ADD ESP,0C 00401094 |. E9 0A000000 JMP 004010A3 00401099 |> B8 01000000 MOV EAX,1 0040109E |. E9 00000000 JMP 004010A3 t<_ 004010A3 |> 8B45 08 MOV EAX,DWORD PTR SS:[ARG.1] 004010A6 |. 8B4D 0C MOV ECX,DWORD PTR SS:[ARG.2] 004010A9 |. 39C8 CMP EAX,ECX 004010AB |. B8 00000000 MOV EAX,0 004010B0 |. 0F9CC0 SETL AL 004010B3 |. 83F8 00 CMP EAX,0 004010B6 |. 0F84 1A000000 JE 004010D6 main(t+1,_,a) 004010BC |. 8B45 08 MOV EAX,DWORD PTR SS:[ARG.1] 004010BF |. 40 INC EAX 004010C0 |. 8B4D 10 MOV ECX,DWORD PTR SS:[ARG.3] 004010C3 |. 51 PUSH ECX ; /Arg3 => [ARG.3] 004010C4 |. 8B4D 0C MOV ECX,DWORD PTR SS:[ARG.2] ; | 004010C7 |. 51 PUSH ECX ; |Arg2 => [ARG.2] 004010C8 |. 50 PUSH EAX ; |Arg1 004010C9 |. E8 32FFFFFF CALL 00401000 ; \a.00401000 004010CE |. 83C4 0C ADD ESP,0C 004010D1 |. E9 0A000000 JMP 004010E0 004010D6 |> B8 03000000 MOV EAX,3 004010DB |. E9 00000000 JMP 004010E0 main(-94,-27+t,a) 004010E0 |> 8B45 08 MOV EAX,DWORD PTR SS:[ARG.1] 004010E3 |. 83C0 E5 ADD EAX,-1B 004010E6 |. 8B4D 10 MOV ECX,DWORD PTR SS:[ARG.3] 004010E9 |. 51 PUSH ECX ; /Arg3 => [ARG.3] 004010EA |. 50 PUSH EAX ; |Arg2 004010EB |. B8 A2FFFFFF MOV EAX,-5E ; | 004010F0 |. 50 PUSH EAX ; |Arg1 => -5E 004010F1 |. E8 0AFFFFFF CALL 00401000 ; \a.00401000 main(-94,-27+t,a)&&t==2 004010F6 |. 83C4 0C ADD ESP,0C 004010F9 |. 83F8 00 CMP EAX,0 004010FC |. 0F84 13000000 JE 00401115 t==2 00401102 |. 8B45 08 MOV EAX,DWORD PTR SS:[ARG.1] 00401105 |. 83F8 02 CMP EAX,2 00401108 |. 0F85 07000000 JNE 00401115 0040110E |. B8 01000000 MOV EAX,1 00401113 |. EB 05 JMP SHORT 0040111A 00401115 |> B8 00000000 MOV EAX,0 0040111A |> 83F8 00 CMP EAX,0 0040111D |. 0F84 44000000 JE 00401167 _<13 00401123 |. 8B45 0C MOV EAX,DWORD PTR SS:[ARG.2] 00401126 |. 83F8 0D CMP EAX,0D 00401129 |. B8 00000000 MOV EAX,0 0040112E |. 0F9CC0 SETL AL 00401131 |. 83F8 00 CMP EAX,0 00401134 |. 0F84 1E000000 JE 00401158 main(2,_+1,"%s %d %d\n") 0040113A |. 8B45 0C MOV EAX,DWORD PTR SS:[ARG.2] 0040113D |. 40 INC EAX 0040113E |. B9 00204000 MOV ECX,OFFSET 00402000 ; ASCII "%s %d %d" 00401143 |. 51 PUSH ECX ; /Arg3 => ASCII "%s %d %d" 00401144 |. 50 PUSH EAX ; |Arg2 00401145 |. B8 02000000 MOV EAX,2 ; | 0040114A |. 50 PUSH EAX ; |Arg1 => 2 0040114B |. E8 B0FEFFFF CALL 00401000 ; \a.00401000 main(2,_+1,"%s %d %d\n") 00401150 |. 83C4 0C ADD ESP,0C 00401153 |. E9 0A000000 JMP 00401162 00401158 |> B8 09000000 MOV EAX,9 0040115D |. E9 00000000 JMP 00401162 00401162 |> E9 0A000000 JMP 00401171 00401167 |> B8 10000000 MOV EAX,10 0040116C |. E9 00000000 JMP 00401171 00401171 |> E9 87010000 JMP 004012FD t<0 00401176 |> 8B45 08 MOV EAX,DWORD PTR SS:[ARG.1] 00401179 |. 83F8 00 CMP EAX,0 0040117C |. B8 00000000 MOV EAX,0 00401181 |. 0F9CC0 SETL AL 00401184 |. 83F8 00 CMP EAX,0 00401187 |. 0F84 D4000000 JE 00401261 t<-72 0040118D |. 8B45 08 MOV EAX,DWORD PTR SS:[ARG.1] 00401190 |. 83F8 B8 CMP EAX,-48 00401193 |. B8 00000000 MOV EAX,0 00401198 |. 0F9CC0 SETL AL 0040119B |. 83F8 00 CMP EAX,0 0040119E |. 0F84 1B000000 JE 004011BF main(_,t,strText) 004011A4 |. B8 0A204000 MOV EAX,OFFSET 0040200A ; ASCII "@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"... 004011A9 |. 50 PUSH EAX ; /Arg3 => ASCII "@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". 004011AA |. 8B45 08 MOV EAX,DWORD PTR SS:[ARG.1] ; | 004011AD |. 50 PUSH EAX ; |Arg2 => [ARG.1] 004011AE |. 8B45 0C MOV EAX,DWORD PTR SS:[ARG.2] ; | 004011B1 |. 50 PUSH EAX ; |Arg1 => [ARG.2] 004011B2 |. E8 49FEFFFF CALL 00401000 ; \a.00401000 main(_,t, "@n'+,#'/*{}w+/w#cdnr/+,{}r/*de}+ 004011B7 |. 83C4 0C ADD ESP,0C 004011BA |. E9 9D000000 JMP 0040125C t<-50 004011BF |> 8B45 08 MOV EAX,DWORD PTR SS:[ARG.1] 004011C2 |. 83F8 CE CMP EAX,-32 004011C5 |. B8 00000000 MOV EAX,0 004011CA |. 0F9CC0 SETL AL 004011CD |. 83F8 00 CMP EAX,0 004011D0 |. 0F84 54000000 JE 0040122A _==*a? 004011D6 |. 8B45 10 MOV EAX,DWORD PTR SS:[ARG.3] 004011D9 |. 8B4D 0C MOV ECX,DWORD PTR SS:[ARG.2] 004011DC |. 0FBE10 MOVSX EDX,BYTE PTR DS:[EAX] 004011DF |. 39D1 CMP ECX,EDX 004011E1 |. B8 00000000 MOV EAX,0 004011E6 |. 0F94C0 SETE AL 004011E9 |. 83F8 00 CMP EAX,0 004011EC |. 0F84 17000000 JE 00401209 putchar(31[a]) 004011F2 |. 8B45 10 MOV EAX,DWORD PTR SS:[ARG.3] 004011F5 |. 83C0 1F ADD EAX,1F 004011F8 |. 0FBE08 MOVSX ECX,BYTE PTR DS:[EAX] 004011FB |. 51 PUSH ECX ; /c 004011FC |. E8 AF020000 CALL <JMP.&> ; \ 00401201 |. 83C4 04 ADD ESP,4 00401204 |. E9 1C000000 JMP 00401225 main(-65,_,a+1) 00401209 |> 8B45 10 MOV EAX,DWORD PTR SS:[ARG.3] 0040120C |. 40 INC EAX 0040120D |. 50 PUSH EAX ; /Arg3 0040120E |. 8B45 0C MOV EAX,DWORD PTR SS:[ARG.2] ; | 00401211 |. 50 PUSH EAX ; |Arg2 => [ARG.2] 00401212 |. B8 BFFFFFFF MOV EAX,-41 ; | 00401217 |. 50 PUSH EAX ; |Arg1 => -41 00401218 |. E8 E3FDFFFF CALL 00401000 ; \a.00401000 main(-65,_,a+1) 0040121D |. 83C4 0C ADD ESP,0C 00401220 |. E9 00000000 JMP 00401225 00401225 |> E9 2D000000 JMP 00401257 main((*a=='/')+t,_,a+1) 0040122A |> 8B45 10 MOV EAX,DWORD PTR SS:[ARG.3] 0040122D |. 0FBE08 MOVSX ECX,BYTE PTR DS:[EAX] 00401230 |. 83F9 2F CMP ECX,2F (*a=='/') 00401233 |. B8 00000000 MOV EAX,0 00401238 |. 0F94C0 SETE AL 0040123B |. 8B4D 08 MOV ECX,DWORD PTR SS:[ARG.1] 0040123E |. 01C8 ADD EAX,ECX (*a=='/')+t 00401240 |. 8B4D 10 MOV ECX,DWORD PTR SS:[ARG.3] 00401243 |. 41 INC ECX a+1 00401244 |. 51 PUSH ECX ; /Arg3 00401245 |. 8B4D 0C MOV ECX,DWORD PTR SS:[ARG.2] ; | 00401248 |. 51 PUSH ECX ; |Arg2 => [ARG.2] 00401249 |. 50 PUSH EAX ; |Arg1 0040124A |. E8 B1FDFFFF CALL 00401000 ; \a.00401000 0040124F |. 83C4 0C ADD ESP,0C 00401252 |. E9 00000000 JMP 00401257 00401257 |> E9 00000000 JMP 0040125C 0040125C |> E9 97000000 JMP 004012F8 0<t 00401261 |> B8 00000000 MOV EAX,0 00401266 |. 8B4D 08 MOV ECX,DWORD PTR SS:[ARG.1] 00401269 |. 39C8 CMP EAX,ECX 0040126B |. B8 00000000 MOV EAX,0 00401270 |. 0F9CC0 SETL AL 00401273 |. 83F8 00 CMP EAX,0 00401276 |. 0F84 1F000000 JE 0040129B main(2,2,"%s") 0040127C |. B8 A2214000 MOV EAX,OFFSET 004021A2 ; ASCII "%s" 00401281 |. 50 PUSH EAX ; /Arg3 => ASCII "%s" 00401282 |. B8 02000000 MOV EAX,2 ; | 00401287 |. 50 PUSH EAX ; |Arg2 => 2 00401288 |. B8 02000000 MOV EAX,2 ; | 0040128D |. 50 PUSH EAX ; |Arg1 => 2 0040128E |. E8 6DFDFFFF CALL 00401000 ; \a.00401000 main(2,2,"%s") 00401293 |. 83C4 0C ADD ESP,0C 00401296 |. E9 58000000 JMP 004012F3 *a=='/' 0040129B |> 8B45 10 MOV EAX,DWORD PTR SS:[ARG.3] 0040129E |. 0FBE08 MOVSX ECX,BYTE PTR DS:[EAX] 004012A1 |. 83F9 2F CMP ECX,2F 004012A4 |. 0F84 3F000000 JE 004012E9 main(-61,*a, "!ek;dc i@bK'(q)-[w]*%n+r3#l,{}:\nuwloca-O;m .vpbks,fxntdCeghiry") 004012AA |. 8B45 10 MOV EAX,DWORD PTR SS:[ARG.3] 004012AD |. B9 A5214000 MOV ECX,OFFSET 004021A5 ; ASCII "!ek;dc i@bK'(q)-[w]*%n+r3#l,{}: \nuwloca-O;m .vpbks,fxntdCeghiry" 004012B2 |. 51 PUSH ECX ; /Arg3 => ASCII "!ek;dc i@bK'(q)-[w]*%n+r3#l,{}:uwloca-O;m .vpbks,fxntdCeghiry" 004012B3 |. 0FBE08 MOVSX ECX,BYTE PTR DS:[EAX] ; | 004012B6 |. 51 PUSH ECX ; |Arg2 004012B7 |. B8 C3FFFFFF MOV EAX,-3D ; | 004012BC |. 50 PUSH EAX ; |Arg1 => -3D 004012BD |. E8 3EFDFFFF CALL 00401000 ; \a.00401000 main(-61,*a, "!ek;dc i@bK'(q)-[w]*%n+r3#l,{}:\nuwloca-O;m .vpbks,fxntdCeghiry" 004012C2 |. 83C4 0C ADD ESP,0C main(0,main(-61,*a, "!ek;dc i@bK'(q)-[w]*%n+r3#l,{}:\nuwloca-O;m .vpbks,fxntdCeghiry"),a+1) 004012C5 |. 8B4D 10 MOV ECX,DWORD PTR SS:[ARG.3] 004012C8 |. 41 INC ECX 004012C9 |. 51 PUSH ECX ; /Arg3 004012CA |. 50 PUSH EAX ; |Arg2 004012CB |. B8 00000000 MOV EAX,0 ; | 004012D0 |. 50 PUSH EAX ; |Arg1 => 0 004012D1 |. E8 2AFDFFFF CALL 00401000 ; \a.00401000 main(0,main(-61,*a, "!ek;dc i@bK'(q)-[w]*%n+r3#l,{}:\nu 004012D6 |. 83C4 0C ADD ESP,0C 004012D9 |. 83F8 00 CMP EAX,0 004012DC |. 0F85 07000000 JNE 004012E9 004012E2 |. B8 00000000 MOV EAX,0 004012E7 |. EB 05 JMP SHORT 004012EE 004012E9 |> B8 01000000 MOV EAX,1 004012EE |> \E9 00000000 JMP 004012F3 004012F3 |> E9 00000000 JMP 004012F8 004012F8 |> E9 00000000 JMP 004012FD 004012FD |> C9 LEAVE 004012FE \. C3 RETN
5 Source code breaks
The source code is based on the ? /, / operations for formatting rearrangement . Assembly code is used to assist in determining whether the breaks are executed consistently with the original code.
The two strings are replaced by a macro for ease of understanding. The first is the plaintext and the second string is the key used for encryption.
#include <> #define strText "@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' ')# \ }'+}##(!!/" #define strEnc "!ek;dc i@bK'(q)-[w]*%n+r3#l,{}:\nuwloca-O;m .vpbks,fxntdCeghiry" main(t,_,a) char *a; { return !0<t ? t<3 ? main(-79,-13,a+main(-87,1-_,main(-86,0,a+1)+a)) : 1 , t<_ ? main(t+1,_,a) : 3 , main(-94,-27+t,a) && t==2 ? _<13 ? main(2,_+1,"%s %d %d\n") : 9 : 16 : t<0 ? t<-72 ? main(_,t,strText) : t<-50 ? _==*a ? putchar(31[a]) : main(-65,_,a+1) : main((*a=='/')+t,_,a+1) : 0<t ? main(2,2,"%s") : *a=='/'||main(0,main(-61,*a,strEnc),a+1); }
6 Parsing if-then-else Statements in C
#include <> #define strText "@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' ')# \ }'+}##(!!/" #define strEnc "!ek;dc i@bK'(q)-[w]*%n+r3#l,{}:\nuwloca-O;m .vpbks,fxntdCeghiry" main(t,_,a) char *a; { if ((!0)<t) { if (t<3) { main(-79,-13,a+main(-87,1-_,main(-86,0,a+1)+a)); } else { 1; } if (t<_) { main(t+1,_,a); } else { 3; } if (main(-94,-27+t,a) && t==2) { if (_<13) { return main(2,_+1,"%s %d %d\n"); } else { return 9; } } else { return 16; } } else { if (t<0) { if (t<-72) { return main(_,t,strText ); } else { if (t<-50) { if (_==(*a)) { return putchar(31[a]); } else { return main(-65,_,a+1); } } else { return main((*a=='/')+t,_,a+1); } } } else { if (0<t) { return main(2,2,"%s"); } else { return (*a=='/')||main(0,main(-61,*a,strEnc ),a+1); } } } }
7 Source code analysis
7.1)!0 is the constant 1
7.2)main(-79,-13,a+main(-87,1-_,main(-86,0,a+1)+a));
This statement is nested and can be broken down into
int m1=main(-86,0,a+1); int m2=main(-87,1-_,m1+a); main(-79,-13,a+m2);
7.3) The comma (,) in the A,B statement indicates that the execution of A is completed and continues with B.
7.4) main(-94,-27+t,a) && t==2?A:B This statement can be broken down into
int m3=main(-94,-27+t,a); if(m3&& t==2)A; else B;
7.5)return *a=='/'||main(0,main(-61,*a,strEnc),a+1);This statement can be broken down into
if(*a=='/')
{
return 1;
}else
{
return main(0,main(-61,*a,strEnc),a+1);
}
Because it runs to the current branch t=0, which is actually a recursive function
7.6)putchar(31[a]), note 31[a], the parentheses [] stand for C arrays, because a[31] is equivalent to *(a+31),31[a] is equivalent to *(31+a),so 31[a] is equivalent to a[31].
8 Organize the code
8.1) According to the if-then-else source code, organize the pseudo-code in front of the code according to t
if(t>1) Do2();
else if(t<0) DoN();
else if(t>0) Do1(); //t values that satisfy t<=1&&t>=0&&t>0 can only be 1
else Do0(); //t can only be 0 if none of the above is satisfied
8.2) Organize pseudo-code by t from largest to smallest
if(t>1) Do2();
else if(t==1)Do1();
else if(t==0)Do0();
else DoN();
8.3) Organize pseudo-code organization source code by t from largest to smallest
#include <>
#define strText "@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' ')# \
}'+}##(!!/"
#define strEnc "!ek;dc i@bK'(q)-[w]*%n+r3#l,{}:\nuwloca-O;m .vpbks,fxntdCeghiry"
main(t,_,a)
char *a;
{
if (t>1)
{
if (t<3)
{
main(-79,-13,a+main(-87,1-_,main(-86,0,a+1)+a));
}
else
{
1;
}
if (t<_)
{
main(t+1,_,a);
}
else
{
3;
}
if (main(-94,-27+t,a) && t==2)
{
if (_<13)
{
return main(2,_+1,"%s %d %d\n");
}
else
{
return 9;
}
}
else
{
return 16;
}
}
else if(t==1)
{
return main(2,2,"%s");
}else if(t==0)
{
return (*a=='/')||main(0,main(-61,*a,strEnc),a+1);
}else if(t>=-50)
{
return main((*a=='/')+t,_,a+1);
}else if(t>=-72)
{
if (_==(*a))
{
return putchar(31[a]);
}
else
{
return main(-65,_,a+1);
}
}else
{
return main(_,t,strText );
}
}
9 Output analysis
The source code output statement has only one sentence putchar(31[a]), at this time t should satisfy -50>t>=-72. The source code recursively calls main(-65,_,a+1) until (_==*a), and then prints the decrypted 31[a] characters.
The key used for decryption is as follows
#define strEnc "!ek;dc i@bK'(q)-[w]*%n+r3#l,{}:\nuwloca-O;m .vpbks,fxntdCeghiry"
So serial number 1 character 'e' corresponds to the original character 'u' serial number 32=1+31, and serial number 2 character 'k' has the original character 'w' before encryption, serial number 33=2+31. note that '!' (serial number 0) corresponds to the encryption before the newline '\n' serial number 31.
Decrypting text with this
"@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 declassified original text reads as follows.
"On the /first/second/third/fourth/fifth/sixth/seventh/eigth/ninth/tenth/eleventh/twelfth/ day of Christmas my true love gave to me /twelve drummers drumming, /eleven pipers piping, /ten lords a-leaping, /nine ladies dancing, /eight maids a-milking, /seven swans a-swimming, /six geese a-laying, /five gold rings; /four calling birds, /three french hens, /two turtle doves and /a partridge in a pear tree. "
The character '/' in the code is not encrypted, it is used to separate the sub-strings, than to add first,second.
Rewrite the unencrypted source code, to avoid the character '\' in the line break '\n', use '! in the output character is treated as a newline. in the output character is treated as a newline.
#include <> #define strDeText "On the /first/second/third/fourth/fifth/sixth/seventh/eigth/ninth/tenth/eleventh/twelfth/ day of Christmas my true love gave to me!\ /twelve drummers drumming, /eleven pipers piping, /ten lords a-leaping,!\ /nine ladies dancing, /eight maids a-milking, /seven swans a-swimming,!\ /six geese a-laying, /five gold rings;!\ /four calling birds, /three french hens, /two turtle doves!\ and /a partridge in a pear tree.!!/" main(t,_,a) char *a; { if (t>1) { if (t<3) { main(-79,-13,a+main(-87,1-_,main(-86,0,a+1)+a)); } else { 1; } if (t<_) { main(t+1,_,a); } else { 3; } if (main(-94,-27+t,a) && t==2) { if (_<13) { return main(2,_+1,"%s %d %d\n"); } else { return 9; } } else { return 16; } } else if(t==1) { return main(2,2,"%s"); }else if(t==0) { return (*a=='/')||main(0,main(-61,*a,""),a+1); }else if(t>=-50) { return main((*a=='/')+t,_,a+1); }else if(t>=-72) { if(_=='!')_='\n'; return putchar(_); }else { return main(_,t,strDeText ); } }
To better understand the original code, the '/' separated subcharacters are represented by a single letter.
For example, "first" is replaced by the character 'a', "second" is replaced by 'b', and so on. The simplified code is as follows
#include <>
#define strDeText "On /a/b/c/d/e/f/g/h/i/j/k/l/ day /L,/K,/J,/I,/H,/G,/F,/E,/D,/C,/B,/A.!/"
main(t,_,a)
char *a;
{
if (t>1)
{
if (t<3)
{
main(-79,-13,a+main(-87,1-_,main(-86,0,a+1)+a));
}
else
{
1;
}
if (t<_)
{
main(t+1,_,a);
}
else
{
3;
}
if (main(-94,-27+t,a) && t==2)
{
if (_<13)
{
return main(2,_+1,"%s %d %d\n");
}
else
{
return 9;
}
}
else
{
return 16;
}
}
else if(t==1)
{
return main(2,2,"%s");
}else if(t==0)
{
return (*a=='/')||main(0,main(-61,*a,""),a+1);
}else if(t>=-50)
{
return main((*a=='/')+t,_,a+1);
}else if(t>=-72)
{
if(_=='!')_='\n';
return putchar(_);
}else
{//t<-72
return main(_,t,strDeText );
}
}
The output of running the program is as follows.
On a day A. On b day B,A. On c day C,B,A. On d day D,C,B,A. On e day E,D,C,B,A. On f day F,E,D,C,B,A. On g day G,F,E,D,C,B,A. On h day H,G,F,E,D,C,B,A. On i day I,H,G,F,E,D,C,B,A. On j day J,I,H,G,F,E,D,C,B,A. On k day K,J,I,H,G,F,E,D,C,B,A. On l day L,K,J,I,H,G,F,E,D,C,B,A.
Rewrite t==0 with recursive output string for the normal call to the function, and note that t<-72 when exchanging t and _ and fixed a for strDeText recursive call to main.
#include <>
#define strDeText "On /a/b/c/d/e/f/g/h/i/j/k/l/ day /L,/K,/J,/I,/H,/G,/F,/E,/D,/C,/B,/A.!/"
int funprint(t,_,a)
char *a;
{
while(*a!='/')
{
char c=*a;
if(c=='!')c='\n';
putchar(c);
a++;
}
return 1;
}
main(t,_,a)
char *a;
{
if (t>1)
{
if (t<3)
{
int m1=main(0,-86,strDeText);
int m2=main(1-_,-87,strDeText);
main(-13,-79,strDeText);
}
else
{
1;
}
if (t<_)
{
main(t+1,_,a);
}
else
{
3;
}
if (main(-27+t,-94,strDeText) && t==2)
{
if (_<13)
{
return main(2,_+1,"%s %d %d\n");
}
else
{
return 9;
}
}
else
{
return 16;
}
}
else if(t==1)
{
return main(2,2,"%s");
}else if(t==0)
{
return funprint(t,_,a);
}else if(t>=-50)
{//
return main((*a=='/')+t,_,a+1);
}else if(t>=-72)
{
if(_=='!')_='\n';
return putchar(_);
}else
{
return main(_,t,strDeText );
}
}
The code is a bit clearer, as you can notice int m1=main(0,-86,strDeText) outputs "On ",
int m2=main(1-_,-87,strDeText) outputs 'a' or 'b' or 'c' etc., the
main(-13,-79,strDeText) output ' day ', you can understand to run t>=-50 this branch recursively call main, at this time t represents the number of '/'.
Continuing to rewrite the code, the above three recursions into a function call
1 #include <> 2 3 #define strDeText "On /a/b/c/d/e/f/g/h/i/j/k/l/ day /L,/K,/J,/I,/H,/G,/F,/E,/D,/C,/B,/A.!/" 4 5 int funprint(t,_,a) 6 char *a; 7 { 8 while(*a!='/') 9 { 10 char c=*a; 11 if(c=='!')c='\n'; 12 putchar(c); 13 a++; 14 } 15 return 1; 16 } 17 18 int funOut(t,_,a) 19 char *a; 20 { 21 int i; 22 for(i=t;i<0;i++) 23 { 24 while(*a!='/') 25 { 26 a++; 27 } 28 a++; 29 } 30 return funprint(t,_,a); 31 } 32 33 main(t,_,a) 34 char *a; 35 { 36 if (t>1) 37 { 38 if (t<3) 39 { 40 int m1=funOut(0,-86,strDeText); 41 int m2=funOut(1-_,-87,strDeText); 42 funOut(-13,-79,strDeText); 43 } 44 else 45 { 46 1; 47 } 48 if (t<_) 49 { 50 main(t+1,_,a); 51 } 52 else 53 { 54 3; 55 } 56 57 if (funOut(-27+t,-94,strDeText) && t==2) 58 { 59 if (_<13) 60 { 61 return main(2,_+1,"%s %d %d\n"); 62 } 63 else 64 { 65 return 9; 66 } 67 } 68 else 69 { 70 return 16; 71 } 72 } 73 else if(t==1) 74 { 75 return main(2,2,"%s"); 76 }else if(t==0) 77 { 78 return funprint(t,_,a); 79 }else if(t>=-50) 80 { 81 return main((*a=='/')+t,_,a+1); 82 }else if(t>=-72) 83 { 84 if(_=='!')_='\n'; 85 return putchar(_); 86 }else 87 { 88 return main(_,t,strDeText ); 89 } 90 91 }
The program starts at t=1 and recursively calls t=2,_=2, after printing "On After the first sentence of "a day A." main(2,_+1,"%s %d %d\n") recursively calls main, adding 1 to the value of _ to make it 3.
(of a computer) run
int m1=funOut(0,-86,strDeText); int m2=funOut(1-_,-87,strDeText); funOut(-13,-79,strDeText);
Print "On b day ", since t=2,_=3, recursively call main(t+1,_,a), at which point t=3,_=3, and return to call funOut(-27+t,-94,strDeText) which prints out "B,A.".
Continue calling main(2,_+1,"%s %d %d\n") add 1 to the _ value to become 4,... Until _=13, output "L,K,J,I,H,G,F,E,D,C,B,A."
Once this is understood, the recursion of t>1 is changed to a function call
#include <> #define strDeText "On /a/b/c/d/e/f/g/h/i/j/k/l/ day /L,/K,/J,/I,/H,/G,/F,/E,/D,/C,/B,/A.!/" int funprint(t,_,a) char *a; { while(*a!='/') { char c=*a; if(c=='!')c='\n'; putchar(c); a++; } return 1; } int funOut(t,_,a) char *a; { int i; for(i=t;i<0;i++) { while(*a!='/') { a++; } a++; } return funprint(t,_,a); } main(t,_,a) char *a; { if (t>1) { int i,j; for(;_<13;_++) { int m1=funOut(0,-86,strDeText); int m2=funOut(1-_,-87,strDeText); funOut(-13,-79,strDeText); i=_; while(i>=t) { funOut(-27+i,-94,strDeText); i--; } } } else if(t==1) { return main(2,2,"%s"); }else if(t==0) { return funprint(t,_,a); }else if(t>=-50) { return main((*a=='/')+t,_,a+1); }else if(t>=-72) { if(_=='!')_='\n'; return putchar(_); }else { return main(_,t,strDeText ); } }
Finally remove the useless code, structured rewrite the original code, simple logic prints the result
#include <> //#define strDeText "On /a/b/c/d/e/f/g/h/i/j/k/l/ day /L,/K,/J,/I,/H,/G,/F,/E,/D,/C,/B,/A.!/" #define strDeText "On the /first/second/third/fourth/fifth/sixth/seventh/eigth/ninth/tenth/eleventh/twelfth/ day of Christmas my true love gave to me!\ /twelve drummers drumming, /eleven pipers piping, /ten lords a-leaping,!\ /nine ladies dancing, /eight maids a-milking, /seven swans a-swimming,!\ /six geese a-laying, /five gold rings;!\ /four calling birds, /three french hens, /two turtle doves!\ and /a partridge in a pear tree.!!/" int fun0Print(char *a) { while(*a!='/') { char c=*a; if(c=='!')c='\n'; putchar(c); a++; } return 1; } void funPrint(int k) { char *s=strDeText; int i; for(i=k;i<0;i++) { while(*s!='/') { s++; } s++; } fun0Print(s); } void funDisp() { int _,m; int i; for(_=2;_<=13;_++) { funPrint(0); //Output "On" funPrint(1-_); //Output" a "or" b "or" c "or" d "or .... funPrint(-13); //Output " day " for(m=_;m>=2;m--) { funPrint(-27+m); //Output " L,/K,/J,/I,/H,/G,/F,/E,/D,/C,/B,/A.!" } } } main(int t,int _,char* a) { funDisp(); }