Cody Boone Ferguson
41abeda97a
In the line in README.md and index.html files that said:
### NOTICE to those who wish for a greater challenge
I added a ':' at the end of the line.
There were some entries that had a link fix in the index.html file but I
only looked up through 1995/leo as those still have to have a 'look and
feel' check and it would just slow things down.
146 lines
3.7 KiB
Markdown
146 lines
3.7 KiB
Markdown
## To build:
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``` <!---sh-->
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make
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```
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## To use:
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``` <!---sh-->
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./hou 'expression'
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```
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## Try:
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``` <!---sh-->
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./try.sh
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```
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As the script asks you, what does the last one mean? Can you get it to report a
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different value? Hint: it's in this file, kind of.
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## Judges' remarks:
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This self-documenting scientific calculator compiles clean with no
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warnings under the most strict pedantic mode of gcc and clang.
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The first example, along with the correct result appears in the
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source code. What does it mean?
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The calculator obeys the precedence rules and allows parenthesis.
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``` <!---sh-->
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./hou '1+2*3'
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./hou '(1+2)*3'
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```
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What exactly does the text formatting do?
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What does this evaluate to in C?
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``` <!---c-->
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+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
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```
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### Self documenting source
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Comments would almost be redundant. :-}
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### The ASCII art is valid C code
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... and not just fancy formatted comment removed by the C Pre-processor.
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### The only multi-byte strings are format strings
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The code does not contain some simple string-encoded lookup table for calculator
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operations.
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### Example input and output in the source
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The example input and output in the C code is significant. Can you find out
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why?
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### The button layout
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The button layout in the C code is significant ... not just a nice looking
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layout.
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### Input parsing
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The parser is functionally equivalent to a normal arithmetic parser for all
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valid inputs. For another fun challenge: try to determine how it parses the
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command line argument.
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## Author's remarks:
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### Obfuscations
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This program is designed to be self-documenting. It doesn't just provide an
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example command line. It also provides the expected output and illustrates all
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supported functionality using embedded ASCII art.
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- A few line numbers are significant and careless beautification will break the
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program.
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- The entire ASCII art is valid code. The preprocessor won't remove any part of it.
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- The only strings longer than 1 byte are `printf(3)`/`sscanf(3)` format strings, even
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after preprocessing. No string-encoded lookup table is used.
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- The example input and its expected output are both algorithmically
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significant.
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- The button layout is also significant.
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- The actual syntax being parsed is not that obvious, despite being functionally
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equivalent for all correct inputs.
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### Portability
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- This program requires `argv` to point to writable memory. It doesn't require
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the actual `argv` strings to be writable, though.
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- This program requires ASCII.
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- This program doesn't work with `%e` style numbers like `1e-4`.
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- Some seemingly redundant code works to eliminate warnings and evade a gcc bug.
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### NOTICE to those who wish for a greater challenge:
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**If you want a greater challenge, don't read any further**:
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just try to understand the program via the source.
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If you get stuck, come back and read below for additional hints and information.
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### How this entry works:
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The parser parses function names, subtraction, and division as single-letter
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unary operators. A bitmask encoded hash table is used to check no-ops/digits
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and another hash function maps the ASCII to a function ordinal, which is then
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executed by the button panel.
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Both hash functions were found using a separate search program. Another program
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was used to find a way to format one hash function as example input such that
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it produces something resembling the bitmask hash table as part of its output.
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<!--
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Copyright © 1984-2024 by Landon Curt Noll. All Rights Reserved.
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You are free to share and adapt this file under the terms of this license:
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Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)
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For more information, see:
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https://creativecommons.org/licenses/by-sa/4.0/
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-->
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