This library was developed during my honors project at the University of Victoria under the supervision of [Bill Bird](https://github.com/billbird). The original development occurred in this [Metaprogramming Optimization](https://github.com/mkitzan/metaprogramming-optimization) repository, but was moved into a new, dedicated, home repository. The project was inspired and influenced by [Hana Dusíková](https://github.com/hanickadot)'s great [Compile Time Regular Expression](https://github.com/hanickadot/compile-time-regular-expressions) library (CTRE).

g++ -std=c++2a -O3 -Isingle-header/ -o example example.cpp && ./example

The expression tree created while parsing is a template recursive tree which shares similar properties to the template linked-list (discussed [earlier](https://github.com/mkitzan/constexpr-sql#class-template-sqlrow)). A notable benefit to this data structure is that because the tree is composed of types rather than data values, the tree can be used to express computation models (expression trees) rather than just a tree based container. Fundamentally, the parsing is accomplished by calling a template recursive `static constexpr` parsing function member parameterized on the token position which the parser's "cursor" is standing on ([`Pos`](https://github.com/mkitzan/metaprogramming-optimization/blob/master/include/cexpr/prefix.hpp#L39) template parameter). At each "cursor" position, the function uses the token to decide the how to proceed. If the token indicates the start of an operation, the parser recurses the immediate left subexpression ("cursor" + 1). On return, the left subexpression will report the depth in the token stream it recursed at which point the right subexpression will pick up at this position. This control flow is expressed in [this line](https://github.com/mkitzan/metaprogramming-optimization/blob/master/include/cexpr/prefix.hpp#L47) of [`cexpr::prefix::parse`](https://github.com/mkitzan/metaprogramming-optimization/blob/master/include/cexpr/prefix.hpp#L40). Once both left and right subexpressions are parsed, the new node's type is formed ([`decltype`](https://en.cppreference.com/w/cpp/language/decltype) left and right subexpressions) which is then propagated to the caller. Otherwise, if the token indicates a terminal, then an appropriate terminal node is constructed. It is necessary that the "cursor" position is unique across template instantiations, otherwise template memoization will lead to "infinite" recursion.

The few ancillary class templates used to support this parsing and the math node struct templates can be found in the [`templ` namespace](https://github.com/mkitzan/metaprogramming-optimization/tree/master/include/templ) of the old repository. There is also a [driver program](https://github.com/mkitzan/metaprogramming-optimization/blob/master/resources/parser/equation.cpp) using the parsers in the old repository. In the Constexpr SQL parser, all of the entities in the `templ` namespace were replaced for more template metaprogramming idiomatic structures.