Landon Curt Noll
4ee4c731f7
While not strictly necessary, replacing the two ASCII tab characters in the markdown copyright comment makes it easier to detect problematic TAB indentation within markdown files. After this commit, only 90 markdown files have leading ASCII TAB characters. And again, while not strictly necessary, we can consider running expand on those 90 markdown files to see what effect that might have on the HTML files they produce. Replace leading ASCII TAB characters with multiples of 8 ASCII SPACE characters (the default action of the `expand(1)` tool) might have an negative effect on some HTML formatting where **hack-a-round** formatting was attempted to fix broken indentation caused by problematic ASCII TAB characters. Think of this as a baseline for evaluating leading TAB characters within markdown files. TODO: Consider either using `expand(1)` to replace leading ASCII TAB characters with multiples of 8 ASCII SPACE characters AND/OR modifying the appropriate `bin` tool to convert leading ASCII TAB characters into multiples of 8 ASCII SPACE characters.
To build:
make
To use:
./hou 'expression'
Try:
./try.sh
As the script asks you, what does the last one mean? Can you get it to report a different value? Hint: it's in this file, kind of.
Judges' remarks:
This self-documenting scientific calculator compiles clean with no warnings under the most strict pedantic mode of gcc and clang.
The first example, along with the correct result appears in the source code. What does it mean?
The calculator obeys the precedence rules and allows parenthesis.
./hou '1+2*3'
./hou '(1+2)*3'
What exactly does the text formatting do?
What does this evaluate to in C?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
Self documenting source
Comments would almost be redundant. :-}
The ASCII art is valid C code
... and not just fancy formatted comment removed by the C Pre-processor.
The only multi-byte strings are format strings
The code does not contain some simple string-encoded lookup table for calculator operations.
Example input and output in the source
The example input and output in the C code is significant. Can you find out why?
The button layout
The button layout in the C code is significant ... not just a nice looking layout.
Input parsing
The parser is functionally equivalent to a normal arithmetic parser for all valid inputs. For another fun challenge: try to determine how it parses the command line argument.
Author's remarks:
Obfuscations
This program is designed to be self-documenting. It doesn't just provide an example command line. It also provides the expected output and illustrates all supported functionality using embedded ASCII art.
- A few line numbers are significant and careless beautification will break the program.
- The entire ASCII art is valid code. The preprocessor won't remove any part of it.
- The only strings longer than 1 byte are
printf(3)/sscanf(3)format strings, even after preprocessing. No string-encoded lookup table is used. - The example input and its expected output are both algorithmically significant.
- The button layout is also significant.
- The actual syntax being parsed is not that obvious, despite being functionally equivalent for all correct inputs.
Portability
- This program requires
argvto point to writable memory. It doesn't require the actualargvstrings to be writable, though. - This program requires ASCII.
- This program doesn't work with
%estyle numbers like1e-4. - Some seemingly redundant code works to eliminate warnings and evade a gcc bug.
Spoilers
The parser parses function names, subtraction, and division as single-letter unary operators. A bitmask encoded hash table is used to check no-ops/digits and another hash function maps the ASCII to a function ordinal, which is then executed by the button panel.
Both hash functions were found using a separate search program. Another program was used to find a way to format one hash function as example input such that it produces something resembling the bitmask hash table as part of its output.