Czech ACM Student Chapter
Czech Technical University in Prague
Charles University in Prague
Technical University of Ostrava
acm
ˇ a
Slovak University of Technology
Pavol Jozef Saf´rik University in Koˇice
s
cz
ˇ
University of Zilina
Masaryk University
Matej Bel University in Bansk´ Bystrica
a
University of West Bohemia
CTU Open Contest 2013
False Sense of Security
fs.c, fs.cpp, Fs.java
Freddy discovered a new procedure to grow much bigger cauliflowers. He wants to share this
finding with his fellow gardener Tommy but he does not want anyone to steal the procedure. So
the two gardeners agreed upon using a simple encryption technique proposed by M. E. Ohaver.
The encryption is based on the Morse code, which represents characters as variable-length
sequences of dots and dashes. The following table shows the Morse code sequences for all
letters:
A
H
O
V
.-
....
---
...-
B
I
P
W
-...
..
.--.
.--
C
J
Q
X
-.-.
.---
--.-
-..-
D
K
R
Y
-..
-.-
.-.
-.--
E
L
S
Z
.
.-..
...
--..
F
M
T
..-.
--
-
G
N
U
--.
-.
..-
Note that four possible dot-dash combinations are unassigned. For the purposes of this problem
we will assign them as follows (note these are not the assignments for actual Morse code):
underscore (" ")
period (".")
..--
---.
comma (",")
question mark ("?")
.-.-
----
In practice, characters in a message are delimited by short pauses, typically displayed as spaces.
Thus, the message ACM GREATER NY REGION is encoded as:
.- -.-. -- ..-- --. .-. . .- - . .-. ..-- -. -.-- ..-- .-. . --. .. --- -.
The Ohaver's encryption scheme is based on mutilating Morse code, namely by removing the
pauses between letters. Since the pauses are necessary (because Morse is a variable-length
encoding that is not prefix-free), a string is added that identifies the number of dots and dashes
in each character. For example, consider the message ".--.-.--". Without knowing where the
pauses should be, this could be "ACM", "ANK", or several other possibilities. If we add length
information, such as ".--.-.-- 242", then the code is unambiguous.
Ohaver's scheme has three steps, the same for encryption and decryption:
1. Convert the text to Morse code without pauses but with a string of numbers to indicate
code lengths.
2. Reverse the string of numbers.
3. Convert the dots and dashes back into the text using the reversed string of numbers as
code lengths.
As an example, consider the encrypted message "AKADTOF IBOETATUK IJN". Converting to Morse
code with a length string yields:
.--.-.--..----..-...--..-...---.-.--..--.-..--...----. 232313442431121334242
By reversing the numbers and decoding, we get the original message "ACM GREATER NY REGION".
The security of this encoding scheme is not too high but Freddy believes it is sufficient for
his purposes. Will you help Freddy to implement this encoding algorithm and to protect his
sensitive information?
Input Specification
The input will consist of several messages encoded with Ohaver's algorithm, each of them on one
line. Each message will use only the twenty-six capital letters, underscores, commas, periods,
and question marks. Messages will not exceed 1000 characters in length.
Output Specification
For each message in the input, output the decoded message on one line.
Sample Input
FENDSVTSLHW.EDATS,EULAY
TRDNWPLOEF
NTTTGAZEJUIIGDUZEHKUE
QEWOISE.EIVCAEFNRXTBELYTGD.
?EJHUT.TSMYGW?EJHOT
DSU.XFNCJEVE.OE_UJDXNO_YHU?VIDWDHPDJIKXZT?E
ADAWEKHZN,OTEATWRZMZN_IDWCZGTEPION
Output for Sample Input
FALSE_SENSE_OF_SECURITY
CTU_PRAGUE
TWO_THOUSAND_THIRTEEN
QUOTH_THE_RAVEN,_NEVERMORE.
TO_BE_OR_NOT_TO_BE?
THE_QUICK_BROWN_FOX_JUMPS_OVER_THE_LAZY_DOG
ADAPTED_FROM_ACM_GREATER_NY_REGION
