Fixes and updates in 1998/dorssel
Format fixes in README.md. Rename dorssel.hint -> dorssel.md and update formatting. Updated Makefile DATA variable for rename of dorssel.hint.
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@@ -111,7 +111,7 @@ ENTRY= dorssel
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PROG= ${ENTRY}
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#
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OBJ= ${PROG}.o
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DATA= dorssel.hint
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DATA= dorssel.md
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TARGET= ${PROG}
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#
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ALT_OBJ=
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+41
-30
@@ -1,29 +1,35 @@
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# Obsolescent Feature
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Frans van Dorsselaer
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Bakker Industrial Automation
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Kaiserstraat 35
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NL-2311 GP Leiden
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The Netherlands
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Frans van Dorsselaer
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Bakker Industrial Automation
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Kaiserstraat 35
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NL-2311 GP Leiden
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The Netherlands
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## To build:
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make all
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```sh
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make all
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```
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## To run:
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./dorssel
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# input some text
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```sh
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./dorssel
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# input some text
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echo text | ./dorssel
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echo text | ./dorssel
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```
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## Try:
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echo Simple as 123 | ./dorssel
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echo 'Binary as simple as 1, 10 11.' | ./dorssel
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echo '.... --- .-- -.. --- . ... .. - -.. --- - .... .- -' | ./dorssel
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echo 'HOW DOES IT DO THAT' | ./dorssel
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echo 'HOW DOES IT DO THAT' | ./dorssel | ./dorssel
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```sh
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echo Simple as 123 | ./dorssel
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echo 'Binary as simple as 1, 10 11.' | ./dorssel
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echo '.... --- .-- -.. --- . ... .. - -.. --- - .... .- -' | ./dorssel
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echo 'HOW DOES IT DO THAT' | ./dorssel
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echo 'HOW DOES IT DO THAT' | ./dorssel | ./dorssel
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```
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## Judges' remarks:
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@@ -52,15 +58,14 @@ You might also want to read the [dorssel.hint](dorssel.hint) file.
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## Author's remarks:
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Usage
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=====
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### Usage
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This program converts ASCII to Morse code, and vice versa.
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The program reads in lines from standard input and writes its result to
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standard output. The direction of conversion is determined on a per
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line basis. Each line must consist of a maximum of 952 characters
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followed by a new line character, they must not contain any NUL
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followed by a new line character and they must not contain any NUL
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characters. Otherwise, some characters may be lost in the conversion.
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- Encoding to Morse code: The program knows the Morse codes for all Latin
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@@ -69,40 +74,46 @@ are replaced by an ASCII space ' '. Morse code per converted character is
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written as a sequence of '.' and '-' characters, each sequence followed by a
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space.
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- Decoding from Morse code: Morse code should be typed as sequences of '.' and
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'-' characters, separated by a space. Extra spaces will be echoed back (to
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separate words). Unknown sequences will be converted to a '?' character.
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- Decoding from Morse code: Morse code should be typed as sequences of `.` and
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`-` characters, separated by a space. Extra spaces will be echoed back (to
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separate words). Unknown sequences will be converted to a `?` character.
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Notes
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=====
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### Notes
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The program assumes both source and execution character sets to be
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ASCII. As far as I can tell, the program is conforming and does not
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invoke undefined behavior.
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Hints
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=====
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### Hints
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Try
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Try:
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./dorssel < dorssel.c | ./dorssel
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```sh
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./dorssel < dorssel.c | ./dorssel
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```
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to see what variables, numbers, and functions are used. (Does a
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better job than most C beautifiers).
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- Filter your text twice (as in the previous sample) to obtain a version of your
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text that can be reversibly morsified, i.e.
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text that can be reversibly Morsified, i.e.
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./dorssel | ./dorssel | ./dorssel | ./dorssel
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```sh
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./dorssel < dorssel.c | ./dorssel | ./dorssel | ./dorssel
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```
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is the same as
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./dorssel | ./dorssel
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```sh
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./dorssel < dorssel.c | ./dorssel
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```
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- The file "dorssel.hint" contains all the information needed to understand this
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program. For an obfuscated spoiler, try
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./dorssel < dorssel.hint
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```sh
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./dorssel < dorssel.hint
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```
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## Copyright and CC BY-SA 4.0 License:
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@@ -1,14 +1,18 @@
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# Information about this entry
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Morse codes for letters and digits use sequences of 1 to 5 (inclusive)
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Morse symbols (denoted as '.' or '-'), e.g.
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```
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E .
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Z --..
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3 ...--
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```
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(BTW: digits always have 5 symbols, letters 1 to 4. I didn't use this fact)
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(BTW: digits always have 5 symbols, letters 1 to 4. I didn't use this fact.)
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Of course we can encode these symbols with bits in a byte. I chose '-'
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to be a bit set to 0, '.' to be a bit set to 1. Characters in C are at
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Of course we can encode these symbols with bits in a byte. I chose `-`
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to be a bit set to 0, `.` to be a bit set to 1. Characters in C are at
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least 8 bits, but they are either signed or unsigned. To be portable,
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I've used only values from 0 to 127 (inclusive) (both in the encoding
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of Morse codes, as well as in subexpressions in the program). Hence we
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@@ -18,42 +22,52 @@ the source code excessively large ;-)
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The least significant bit contains the first symbol of a Morse code, so
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we have
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```
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E > 0......1
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Z > 0...1100
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3 > 0..00111
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```
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The dots have still to be determined. But we also need to encode the
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length of the sequence. Since there are only 4 different kind of lengths,
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two bits would suffice, e.g.
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```
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E > 000....1
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Z > 010.1100
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3 > 01100111
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```
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That means bits 6 and 5 set to 00 is length 1, set to 01 is length 2, etc.
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But this would mean that we would have to extract the length into a
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counter before shifting out each symbol, such as
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```c
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length = (encoding & 96) >> 5;
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do {
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putchar(encoding & 1 ? '-' : '.');
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encoding >>= 1;
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} while (length--);
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```
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We can also use the remaining bits to *include* the counter
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```
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length 1: 00000.1x E > 00000.11
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length 2: 0000.1xx
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length 3: 000.1xxx
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length 4: 00.1xxxx Z > 00.11100
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length 5: 0.1xxxxx 3 > 0.100111
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```
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With 1 bit still to be chosen freely (let's assume it is 0). Now we can do
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```c
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do {
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putchar(encoding & 1 ? '-' : '.');
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encoding >>= 1;
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while (encoding > 1);
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```
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As you see, the encoded Morse sequence can be the counter itself. But
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this is not very pleasant, as some Morse sequences will end up having
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@@ -61,66 +75,75 @@ an ASCII value < 32, i.e. they will be control codes. This would make
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the initializing (encoding) string awkward. Hence I came up with the
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following
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```
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length 1: 0010010x E > 00100101 (= '%')
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length 2: 001010xx
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length 3: 00110xxx
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length 4: 0100xxxx Z > 01001100 (= 'L')
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length 5: 011xxxxx 3 > 01100111 (= 'g')
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```
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Notice these encodings in the initialization string. Only the coding for
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'5' needs an escape (5 = ..... > 01111111 = '\177'). The way the length
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`5` needs an escape (`5 = ..... > 01111111 = '\177'`). The way the length
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is encoded still allows the encoding to be its own counter:
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```c
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do {
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putchar(encoding & 1 ? '-' : '.');
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encoding = (encoding + 32) >> 1;
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} while (encoding > 35);
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```
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The last shift will always result in 00100010, hence the test for >35.
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Note that the calculation for the next value of `encoding' is done at int
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precision, so that the actual value of the char remains <= 127.
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The last shift will always result in `00100010`, hence the test for `>35`.
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Note that the calculation for the next value of `encoding` is done at int
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precision, so that the actual value of the char remains `<= 127`.
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The character '-' (ASCII 45) and '.' (ASCII 46) are conveniently in
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The character `'-'` (ASCII 45) and `'.'` (ASCII 46) are conveniently in
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sequence, so the putchar can simply be
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```c
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putchar(45 + encoding % 2);
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```
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I used the modulo operator this time, because '+' takes precedence over '&'.
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I used the modulo operator this time, because `+` takes precedence over `&`.
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The assignment can be incorporated into the while control test, and a
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do-while loop with a body of a single expression statement can be written
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as a while loop with a comma operator in the test:
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```c
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while(putchar(45 + encoding % 2), encoding = encoding + 32 >> 1);
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```
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Voila.
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Now, the decoding of Morse codes is just the reverse. We shift in 0's
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and 1's as we scan the line. But: we have to scan from back to front
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Now, the decoding of Morse codes is just the reverse. We shift in `0`s
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and `1`s as we scan the line. But: we have to scan from back to front
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(the left most Morse symbol is in the least significant bit, and thus
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has to shifted in last). The test for the scanning while loop must
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stop when there are no more '.' or '-' on the left. Hence we need a
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stop when there are no more `.` or `-` on the left. Hence we need a
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terminating character in front of the line. The input line is read one
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past the encoding string (into the same character array) the terminating
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character for the reverse scan is therefore provided by the implicit '\0'
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character for the reverse scan is therefore provided by the implicit `'\0'`
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of the array initializer ! The test also stops when at most 5 symbols
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have been shifted in.
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The encoded character is then looked up in the encoding string with
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memchr (why memchr is explained later). Either a '?' (when there are
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`memchr()` (why `memchr()` is explained later). Either a `?` (when there are
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more than 5 consecutive Morse symbols, or if the sequence is unknown)
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or the decoded character is printed. Or a ' ' if there was an extra
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or the decoded character is printed. Or a `' '` if there was an extra
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space in the source line. Note the use of "or" in these last sentences,
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as they explain the use of the conditional operator.
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The strspn calls need either " .-" or ".-" as argument. Of course they
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The `strspn` calls need either `" .-"` or `".-"` as argument. Of course they
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are overlapped. I put these strings between the encodings for digits
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and letters. This is OK, since neither of these characters, or the '\0',
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is a valid encoding ! We must, however, use memchr instead of strchr in
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the decoding lookup because of this '\0'. Since it is far more obscure
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and letters. This is OK, since neither of these characters, or the `'\0'`,
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is a valid encoding ! We must, however, use `memchr()` instead of `strchr()` in
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the decoding lookup because of this `'\0'`. Since it is far more obscure
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this way, I hope you'll forgive me.
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The outline of the program is roughly as follows
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```
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while (more lines) {
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if (line is in morse) {
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while (more characters) {
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@@ -137,23 +160,25 @@ while (more lines) {
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putchar(' ');
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}
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}
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```
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Because of the *two* while(more characters) loops are the same, I've
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exchanged the if(line is in morse) and the two while loops (so there
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is *single* while loop). However, we must now store the if(line is in
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morse) test for efficiency (it would be stupid to test the entire line
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for every character). This is done in l[0] ! (the first encoding is
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thus in l[1]). The memchr decoding lookup is not disturbed, because the
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test is always 0 for Morse lines, and '\0' is not a possible encoding.
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l[0] is also used as the temporary shift character while encoding a
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line of text. This does not disturb the if (line is in morse) test,
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as the temporary l[0] will remain != 0.
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Because of the *two* `while(more characters)` loops are the same, I've
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exchanged the `if(line is in morse)` and the two while loops (so there
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is *single* while loop). However, we must now store the `if(line is in
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morse)` test for efficiency (it would be stupid to test the entire line
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for every character). This is done in `l[0]` ! (the first encoding is
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thus in `l[1]`). The `memchr()` decoding lookup is not disturbed, because the
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test is always `0` for Morse lines, and `'\0'` is not a possible encoding.
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`l[0]` is also used as the temporary shift character while encoding a
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line of text. This does not disturb the `if (line is in morse)` test,
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as the temporary `l[0]` will remain `!= 0`.
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I've put the if's in the above layout into a single switch. Hope you don't
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I've put the `if`s in the above layout into a single switch. Hope you don't
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mind ;-)
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Summary of the use of l[999]:
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```
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l[0] result of "is this line in morse" test
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l[0] temporary shift during encoding
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l[0] - l[33] Morse codings (letters and digits, plus garbage)
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@@ -161,13 +186,14 @@ l[11] - l[14] strspn argument
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l[12] - l[14] strspn argument
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l[34] terminator for backward decoding
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l[35] - l[998] input line
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```
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*****
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---
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Selected notes:
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- We don't need to declare isalnum() (or include ctype.h) as its implicit
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- We don't need to declare `isalnum()` (or include ctype.h) as its implicit
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declaration is correct (int argument, int result). This is not so for
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strlen(), strspn(), and memchr() as they use size_t.
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`strlen()`, `strspn()`, and `memchr()` as they use `size_t`.
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- During development, my gcc 2.7.2.3 had an internal compiler error (signal
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6) on code that was correct !
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- lclint 2.4b thinks that "the observer is modified" a couple of times,
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@@ -176,7 +202,7 @@ Selected notes:
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- lclint 2.4b is positive the second while loop is infinite, whereas it is
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not.
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*****
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---
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.. - .... .. -. -.- - .... .. ... ... .--. --- .. .-.. . .-. .. ... -- --- .-. . --- -... ..-. ..- ... -.-. .- - . -.. - .... .- -. - .... . .--. .-. --- --. .-. .- --
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Reference in new issue
Block a user