ssp/README.md

# Static split parser

A header only "csv" parser which is fast and versatile with modern C++ api. Requires compiler with C++17 support. 

Conversion for floating point values invoked using [fast-float](https://github.com/fastfloat/fast_float) .   
Function traits taken from [qt-creator](https://code.woboq.org/qt5/qt-creator/src/libs/utils/functiontraits.h.html) .   

# Example 
Lets say we have a csv file containing students in a given format (NAME,AGE,GRADE) and we want to parse and print all the valid values:

```
$ cat students.csv
James Bailey,65,2.5
Brian S. Wolfe,40,1.9
Nathan Fielder,37,Really good grades
Bill (Heath) Gates,65,3.3
```
```cpp
#include <iostream>
#include <ss/parser.hpp>

int main() {
    ss::parser p{"students.csv", ","};
    if (!p.valid()) {
        std::cout << p.error_msg() << std::endl;
        exit(EXIT_FAILURE);
    }

    while (!p.eof()) {
        auto [name, age, grade] = p.get_next<std::string, int, double>();

        if (p.valid()) {
            std::cout << name << ' ' << age << ' ' << grade << std::endl;
        }
    }

    return 0;
}
```
And if we compile and execute the program we get the following output:
```
$ ./a.out
James Bailey 65 2.5
Brian S. Wolfe 40 1.9
Bill (Heath) Gates 65 3.3
```
# Features
 * Works on any type
 * Easy to use
 * No exceptions
 * Works with quotes, escapes and spacings
 * Columns and rows can be ignored
 * Works with any type of delimiter
 * Can return whole objects composed of converted values
 * Descriptive error handling can be enabled
 * Restrictions can be added for each column
 * Works with `std::optional` and `std::variant`
 * Works with **CRLF** and **LF**
 * Conversions can be chained if invalid
 * Fast

# Installation
TODO

# Usage

## Conversions
The above example will be used to show some of the features of the library. As seen above, the **get_next** method returns a tuple of objects specified inside the template type list.

If a conversion could not be applied, the method would return a tuple of default constructed objects, and the **valid** method would return **false**, for example if the third (grade) column in our csv could not be converted to a double the conversion would fail. 

If **get_next** is called with a **tuple** as template parameter it would behave identically to passing the same tuple parameters to **get_next**:
```cpp
using student = std::tuple<std::string, int, double>;

// returns std::tuple<std::string, int, double>
auto [name, age, grade] = p.get_next<student>();
```
*Note, it does not always return a student tuple since the returned tuples parameters may be altered as explained below (no void, no restrictions, ...)*

Whole objects can be returned using the **get_object** function which takes the tuple, created in a similar way as **get_next** does it, and creates an object out of it:
```cpp
struct student {
    std::string name;
    int age;
    double grade;
};
```
```cpp
// returns student
auto student = p.get_object<student, std::string, int, double>();
```
This works with any object if the constructor could be invoked using the template arguments given to **get_object**:
```cpp
// returns std::vector<std::string> containing 3 elements
auto vec = p.get_object<std::vector<std::string>, std::string, std::string, 
                        std::string>();
```
And finally, using something I personally like to do, a struct (class) with a **tied** method which returns a tuple of references to to the members of the struct.
```cpp
struct student {
    std::string name;
    int age;
    double grade;

    auto tied() { return std::tie(name, age, grade); }
};
```
The method can be used to compare the object, serialize it, deserialize it, etc. Now **get_next** can accept such a struct and deduce the types to which to convert the csv.
```cpp
// returns student
auto s = p.get_next<student>();
```
*Note, the order in which the members of the tied method are returned must match the order of the elements in the csv*

### Special types 

Passing **void** makes the parser ignore a column. In the given example **void** could be given as the second template parameter to ignore the second (age) column in the csv, a tuple of only 2 parameters would be retuned:
```cpp
// returns std::tuple<std::string, double>
auto [name, grade] = p.get_next<std::string, void, double>();
```
Works with different types of conversions too:
```cpp
using student = std::tuple<std::string, void, double>;

// returns std::tuple<std::string, double>
auto [name, grade] = p.get_next<student>();
```
To ignore a whole row, **ignore_next** could be used, returns **false** if **eof**: 
```cpp
bool parser::ignore_next();
```
**std::optional** could be passed if we wanted the conversion to proceed in the case of a failure returning **std::nullopt** for the specified column: 

```cpp
// returns std::tuple<std::string, int, std::optional<double>>
auto [name, age, grade] = p.get_next<std::string, int, std::optional<double>();
if(grade) {
    // do something with grade
}
```
Similar to **std::optional**, **std::variant** could be used to try other conversions if the previous failed _(Note, conversion to std::string will always pass)_:
```cpp
// returns std::tuple<std::string, int, std::variant<double, char>>
auto [name, age, grade] = 
    p.get_next<std::string, int, std::variant<double, char>();
if(std::holds_alternative<double>(grade)) {
    // grade set as double
} else if(std::holds_alternative<char>(grade)) {
    // grade set as char
}
```
### Restrictions

Custom **restrictions** can be used to narrow down the conversions of unwanted values. **ss::ir** (in range) and **ss::ne** (none empty) are one of those:
```cpp
// ss::ne makes sure that the name is not empty
// ss::ir makes sure that the grade will be in range [0, 10]
// returns std::tuple<std::string, int, double>
auto [name, age, grade] = 
    p.get_next<ss::ne<std::string>, int, ss::ir<double, 0, 10>>();
```
If the restrictions are not met, the conversion will fail. Other predefined restrictions are **ss::ax** (all except), **ss::nx** (none except) and **ss::oor** (out of range), **ss::lt** (less than), ...(see *restrictions.hpp*):
```cpp
// all ints exept 10 and 20
ss::ax<int, 10, 20>
// only 10 and 20
ss::nx<int, 10, 20>
// all values except the range [0, 10]
ss::oor<int, 0, 10>
```
To define a restriction, a class/struct needs to be made which has a **ss_valid** method which returns a **bool** and accepts one object. The type of the conversion will be the same as the type of the passed object within **ss_valid** and not the restriction itself. Optionally, an **error** method can be made to describe the invalid conversion.
```cpp
template <typename T>
struct even {
    bool ss_valid(const T& value) const {
        return value % 2 == 0;
    }

    // optional
    const char* error() const {
        return "number not even";
    }
};
```
```cpp
// only even numbers will pass
// returns std::tuple<std::string, int>
auto [name, age] = p.get_next<std::string, even<int>, void>();
```
## Custom conversions

Custom types can be used when converting values. A specialization of the **ss::extract** function needs to be made and you are good to go. A custom conversion for an enum would look like this: 
```cpp
enum class shape { circle, square, rectangle, triangle };

template <>
inline bool ss::extract(const char* begin, const char* end, shape& dst) {
    const static std::unordered_map<std::string, shape>
        shapes{{"circle", shape::circle},
               {"square", shape::square},
               {"rectangle", shape::rectangle},
               {"triangle", shape::triangle}};

    if (auto it = shapes.find(std::string(begin, end)); it != shapes.end()) {
        dst = it->second;
        return true;
    }
    return false;
}
```
The shape enum will be used in an example below. The **inline** is there just to prevent multiple definition errors. The function returns **true** if the conversion was a success, and **false** otherwise. The function uses **const char*** begin and end for performance reasons. 

## Quoting
Not yet documented.

## Escaping
Not yet documented.

## Spacing
Not yet documented.

## Error handling

Detailed error messages can be accessed via the **error_msg** method, and to enable them the error mode has to be changed to **error_mode::error_string** using the **set_error_mode** method:
```cpp
void parser::set_error_mode(ss::error_mode);
const std::string& parser::error_msg();
bool parser::valid();
bool parser::eof();
```
Error messages can always be disabled by setting the error mode to **error_mode::error_bool**. An error can be detected using the **valid** method which would return **false** if the file could not be opened, or if the conversion could not be made (invalid types, invalid number of columns, ...). The **eof** method can be used to detect if the end of the file was reached.

## Substitute conversions

The parser can also be used to effectively parse files whose rows are not always in the same format (not a classical csv but still csv-like). A more complicated example would be the best way to demonstrate such a scenario.

Supposing we have a file containing different shapes in given formats: 
 * circle RADIUS
 * square SIDE
 * rectangle SIDE_A SIDE_B
 * triangle SIDE_A SIDE_B SIDE_C

```
rectangle 2 3
circle 10
triangle 3 4 5
...
```

The delimiter is " ", and the number of columns varies depending on which shape it is. We are required to read the file and to store information (shape and area) of the shapes into a data structure in the same order as they are in the file. 
```cpp
ss::parser p{"shapes.txt", " "};
if (!p.valid()) {
    std::cout << p.error_msg() << std::endl;
    exit(EXIT_FAILURE);
}

std::vector<std::pair<shape, double>> shapes;

while (!p.eof()) {
    // non negative double
    using udbl = ss::gte<double, 0>;

    auto [circle_or_square, rectangle, triangle] =
        p.try_next<ss::nx<shape, shape::circle, shape::square>, udbl>()
            .or_else<ss::nx<shape, shape::rectangle>, udbl, udbl>()
            .or_else<ss::nx<shape, shape::triangle>, udbl, udbl, udbl>()
            .values();

    if (circle_or_square) {
        auto& [s, x] = circle_or_square.value();
        double area = (s == shape::circle) ? x * x * M_PI : x * x;
        shapes.emplace_back(s, area);
    }

    if (rectangle) {
        auto& [s, a, b] = rectangle.value();
        shapes.emplace_back(s, a * b);
    }

    if (triangle) {
        auto& [s, a, b, c] = triangle.value();
        double sh = (a + b + c) / 2;
        if (sh >= a && sh >= b && sh >= c) {
            double area = sqrt(sh * (sh - a) * (sh - b) * (sh - c));
            shapes.emplace_back(s, area);
        }
    }
}

/* do something with the stored shapes */
/* ... */
```
It is quite hard to make an error this way since most things will be checked at compile time.

The **try_next** method works in a similar way as **get_next** but returns a **composit** which holds a **tuple** with an **optional** to the **tuple** returned by **get_next**. This **composite** has an **or_else** method (looks a bit like tl::expected) which is able to try additional conversions if the previous failed. **or_else** also returns a **composite**, but in its tuple is the **optional** to the **tuple** of the previous conversions and an **optional** to the **tuple** of the new conversion. (sounds more complicated than it is.

To fetch the **tuple** from the **composite** the **values** method is used. The value of the above used conversion would look something like this:
```cpp
std::tuple<
    std::optional<std::tuple<shape, double>>,
    std::optional<std::tuple<shape, double, double>>,
    std::optional<std::tuple<shape, double, double, double>>
    >
```
Similar to the way that **get_next** has a **get_object** alternative, **try_next** has a **try_object** alternative, and **or_else** has a **or_object** alternative. Also all rules applied to **get_next** also work with **try_next** , **or_else**, and all the other **composite** conversions.

Each of those **composite** conversions can accept a lambda (or anything callable) as an argument and invoke it in case of a valid conversion. That lambda itself need not have any arguments, but if it does, it must either accept the whole **tuple**/object as one argument or all the elements of the tuple separately. If the lambda returns something that can be interpreted as **false** the conversion will fail, and the next conversion will try to apply. Rewriting the whole while loop using lambdas would look like this:
```cpp
// non negative double
using udbl = ss::gte<double, 0>;

p.try_next<ss::nx<shape, shape::circle, shape::square>, udbl>(
     [&](const auto& data) {
         const auto& [s, x] = data;
         double area = (s == shape::circle) ? x * x * M_PI : x * x;
         shapes.emplace_back(s, area);
     })
    .or_else<ss::nx<shape, shape::rectangle>, udbl, udbl>(
        [&](const shape s, const double a, const double b) {
            shapes.emplace_back(s, a * b);
        })
    .or_else<ss::nx<shape, shape::triangle>, udbl, udbl, udbl>(
        [&](auto&& s, auto& a, const double& b, double& c) {
            double sh = (a + b + c) / 2;
            if (sh >= a && sh >= b && sh >= c) {
                double area = sqrt(sh * (sh - a) * (sh - b) * (sh - c));
                shapes.emplace_back(s, area);
            }
        });
```
It is a bit less readable, but it removes the need to check which conversion was invoked. The **composite** also has an **on_error** method which accepts a lambda which will be invoked if no previous conversions were successful. The lambda can take no arguments or just one argument, an **std::string**, in which the error message is stored if **error_mode** is set to **error_mode::error_string**: 
```cpp
p.try_next<int>()
    .on_error([](const std::string& e) { /* int conversion failed */ })
    .or_object<x, double>()
    .on_error([] { /* int and x (all) conversions failed */ });
```
*See unit tests for more examples.*

# Rest of the library

First of all, *type_traits.hpp* and *function_traits.hpp* contain many handy traits used in the parser. Most of them are operating on tuples of elements and can be utilized in projects. 

## The converter

**ss::parser** is used to manipulate on files. It has a builtin file reader, but the conversions themselves are done using the **ss::converter**.

To convert a string the **convert** method can be used. It accepts a c-string as input and a delimiter, as **std::string**, and retruns a **tuple** of objects in the same way **get_next** does it for the parser. A whole object can be returned too using the **convert_object** method, again in an identical way **get_object** doest it for the parser.  
```cpp
ss::converter c;

auto [x, y, z] = c.convert<int, double, char>("10::2.2::3", "::");
if (c.valid()) {
    // do something with x y z
}

auto s = c.convert_object<student, std::string, int, double>("name,20,10", ",");
if (c.valid()) {
    // do something with s
}
```
All special types and restrictions work on the converter too. Error handling is
also identical to error handling of the parser.

The converter has also the ability to just split the line, tho it does not change it (kinda statically), hence the name of the library. It returns an **std::vector** of pairs of pointers, begin and end, each pair representing a split segment (column) of the whole string. The vector can then be used in a overloaded **convert** method. This allows the reuse of the same line without splitting it on every conversion. 
```cpp
ss::converter c;
auto split_line = c.split("circle 10", " ");
auto [s, r] = c.convert<shape, int>(split_line);
```
Using the converter is also an easy and fast way to convert single values.
```cpp
ss::converter c;
std::string s;
std::cin >> s;
int num = c.convert<int>(s.c_str());
```