Author: Yotam Gingold <yotam (strudel) yotamgingold.com>
License: Public Domain.  (I, Yotam Gingold, the author, release this code into the public domain.)
Homepage: http://yotamgingold.com/code/
GitHub: https://github.com/yig/miniparser

MiniParser:
    A simple C++ expression parser with no external dependencies whatsoever
    that can
        - generate a parse tree from an expression
            - parentheses
            - constants
            - symbolic variables
            - binary operators:
                - add: +
                - subtract: -
                - multiply: *
                - divide: /
                - modulus: %
                - exponent: ^, **
            - unary operators:
                - negate: -
            - functions:
                - sin()
                - cos()
                - log() (base 10)
                - ln() (base e)
                - exp()
                - sqrt()
            - constants (override any variable with the same name):
                - PI = 3.14...
    NOTE: Order-of-operations is entirely left-associative.  There is no
          operator precedence.  Use parentheses everywhere!
    TODO: Operator precedence can be added easily with several lines of
          find/replace in the function "Node* Parse( const std::string& str );"
          See http://en.wikipedia.org/wiki/Operator-precedence_parser#Alternatives_to_Dijkstra.27s_Algorithm

example:
    An example command line application that demonstrates a class
    with a predefined expression evaluated via operator()
    as well as an expression with an arbitrary number of variables.
    Can be called with two values (x,y) or with an expression followed
    by an arbitrary number of values assigned to the letters of the
    alphabet (a, b, c, ...).


Version History:
    - 2006-04-08:
        initial release
    - 2006-12-09:
        added math functions (sin, cos, log, ln, exp, sqrt) and constants (PI)
        improved error handling (removed an infinite loop)
    - 2007-04-23:
        made the grammar left associative to match natural arithmetic and C
    - 2007-09-20:
        added the modulus operator %.  attempted to add order-of-operations,
        but was foiled by unary minus (e.g. -a).