Author:

• Name: Qiming HOU
  Location: CN - People’s Republic of China (China)

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 argv to point to writable memory. It doesn’t require the actual argv strings to be writable, though.
• This program requires ASCII.
• This program doesn’t work with %e style numbers like 1e-4.
• Some seemingly redundant code works to eliminate warnings and evade a gcc bug.

NOTICE to those who wish for a greater challenge:

If you want a greater challenge, don’t read any further: just try to understand the program via the source.

If you get stuck, come back and read below for additional hints and information.

How this entry works:

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.

Primary files

• hou.c - entry source code
• Makefile - entry Makefile
• hou.orig.c - original source code
• try.sh - script to try entry

Secondary files

• 2011_hou.tar.bz2 - download entry tarball
• README.md - markdown source for this web page
• .entry.json - entry summary and manifest in JSON
• .gitignore - list of files that should not be committed under git
• .path - directory path from top level directory
• index.html - this web page

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