//! Assert a number is within epsilon of another number.
//!
//! Pseudocode:<br>
//! | a - b | ≤ ε * min(a, b)
//!
//! # Example
//!
//! ```rust
//! # #[macro_use] extern crate assertables;
//! # fn main() {
//! let a: i8 = 10;
//! let b: i8 = 20;
//! let epsilon: i8 = 1;
//! assert_in_epsilon!(a, b, epsilon);
//! # }
//! ```
//!
//! The macros `assert_in_delta` and `assert_in_epsilon` can test
//! approximations:
//!
//! * For an approximation, the absolute error (i.e. delta) is the magnitude of
//!   the difference between the exact value and the approximation.
//!
//! * For an approximation, the relative error (i.e. epsilon) is the absolute
//!   error divided by the magnitude of the exact value. This can be used to
//!   compare approximations of numbers of wildly differing size.
//!
//! * For example, approximating the number 1,000 with an absolute error of 3
//!   is, in most applications, much worse than approximating the number
//!   1,000,000 with an absolute error of 3; in the first case the relative
//!   error is 0.003 and in the second it is only 0.000003.
//!
//! * Thanks to Ruby minitest for the example and documentation.
//!
//! # Module macros
//!
//! * [`assert_in_epsilon`](macro@crate::assert_in_epsilon)
//! * [`assert_in_epsilon_as_result`](macro@crate::assert_in_epsilon_as_result)
//! * [`debug_assert_in_epsilon`](macro@crate::debug_assert_in_epsilon)

/// Assert a number is within epsilon of another number.
///
/// Pseudocode:<br>
/// | a - b | ≤ ε * min(a, b)
///
/// * If true, return Result `Ok(())`.
///
/// * When false, return [`Err`] with a message and the values of the
///   expressions with their debug representations.
///
/// This macro provides the same statements as [`assert_`](macro.assert_.html),
/// except this macro returns a Result, rather than doing a panic.
///
/// This macro is useful for runtime checks, such as checking parameters,
/// or sanitizing inputs, or handling different results in different ways.
///
/// # Module macros
///
/// * [`assert_in_epsilon`](macro@crate::assert_in_epsilon)
/// * [`assert_in_epsilon_as_result`](macro@crate::assert_in_epsilon_as_result)
/// * [`debug_assert_in_epsilon`](macro@crate::debug_assert_in_epsilon)
///
#[macro_export]
macro_rules! assert_in_epsilon_as_result {
    ($a:expr, $b:expr, $epsilon:expr $(,)?) => ({
        match (&$a, &$b, &$epsilon) {
            (a, b, epsilon) => {
                if a == b {
                    Ok(())
                } else {
                    let diff = if (a > b) { a - b } else { b - a };
                    let min = if (a < b) { a } else { b };
                    let rhs = *epsilon * min;
                    if diff <= rhs {
                        Ok(())
                    } else {
                        Err(format!(
                            concat!(
                                "assertion failed: `assert_in_epsilon!(a, b, epsilon)`\n",
                                "https://docs.rs/assertables/8.7.0/assertables/macro.assert_in_epsilon.html\n",
                                "                         a label: `{}`,\n",
                                "                         a debug: `{:?}`,\n",
                                "                         b label: `{}`,\n",
                                "                         b debug: `{:?}`,\n",
                                "                   epsilon label: `{}`,\n",
                                "                   epsilon debug: `{:?}`,\n",
                                "                       | a - b |: `{:?}`,\n",
                                "             epsilon * min(a, b): `{:?}`,\n",
                                " | a - b | ≤ epsilon * min(a, b): {}",
                            ),
                            stringify!($a),
                            a,
                            stringify!($b),
                            b,
                            stringify!($epsilon),
                            epsilon,
                            diff,
                            rhs,
                            false
                        ))
                    }
                }
            }
        }
    });
}

#[cfg(test)]
mod tests {

    #[test]
    fn test_assert_in_epsilon_as_result_x_success() {
        let a: i8 = 10;
        let b: i8 = 20;
        let epsilon: i8 = 1;
        let result = assert_in_epsilon_as_result!(a, b, epsilon);
        assert_eq!(result, Ok(()));
    }

    #[test]
    fn test_assert_in_epsilon_as_result_x_failure() {
        let a: i8 = 10;
        let b: i8 = 30;
        let e: i8 = 1;
        let result = assert_in_epsilon_as_result!(a, b, e);
        assert!(result.is_err());
        assert_eq!(
            result.unwrap_err(),
            concat!(
                "assertion failed: `assert_in_epsilon!(a, b, epsilon)`\n",
                "https://docs.rs/assertables/8.7.0/assertables/macro.assert_in_epsilon.html\n",
                "                         a label: `a`,\n",
                "                         a debug: `10`,\n",
                "                         b label: `b`,\n",
                "                         b debug: `30`,\n",
                "                   epsilon label: `e`,\n",
                "                   epsilon debug: `1`,\n",
                "                       | a - b |: `20`,\n",
                "             epsilon * min(a, b): `10`,\n",
                " | a - b | ≤ epsilon * min(a, b): false"
            )
        );
    }
}

/// Assert a number is within epsilon of another number.
///
/// Pseudocode:<br>
/// | a - b | ≤ ε * min(a, b)
///
/// * If true, return `()`.
///
/// * Otherwise, call [`panic!`] with a message and the values of the
///   expressions with their debug representations.
///
/// # Examples
///
/// ```rust
/// # #[macro_use] extern crate assertables;
/// # use std::panic;
/// # fn main() {
/// let a: i8 = 10;
/// let b: i8 = 20;
/// let epsilon: i8 = 1;
/// assert_in_epsilon!(a, b, epsilon);
///
/// # let result = panic::catch_unwind(|| {
/// let a: i8 = 10;
/// let b: i8 = 30;
/// let e: i8 = 1;
/// assert_in_epsilon!(a, b, e);
/// # });
/// // assertion failed: `assert_in_epsilon!(a, b, epsilon)`
/// // https://docs.rs/assertables/8.7.0/assertables/macro.assert_in_epsilon.html
/// //                          a label: `a`,
/// //                          a debug: `10`,
/// //                          b label: `b`,
/// //                          b debug: `30`,
/// //                    epsilon label: `e`,
/// //                    epsilon debug: `1`,
/// //                        | a - b |: `20`,
/// //              epsilon * min(a, b): `10`,\n",
/// //  | a - b | ≤ epsilon * min(a, b): false"
/// # let actual = result.unwrap_err().downcast::<String>().unwrap().to_string();
/// # let expect = concat!(
/// #     "assertion failed: `assert_in_epsilon!(a, b, epsilon)`\n",
/// #     "https://docs.rs/assertables/8.7.0/assertables/macro.assert_in_epsilon.html\n",
/// #     "                         a label: `a`,\n",
/// #     "                         a debug: `10`,\n",
/// #     "                         b label: `b`,\n",
/// #     "                         b debug: `30`,\n",
/// #     "                   epsilon label: `e`,\n",
/// #     "                   epsilon debug: `1`,\n",
/// #     "                       | a - b |: `20`,\n",
/// #     "             epsilon * min(a, b): `10`,\n",
/// #     " | a - b | ≤ epsilon * min(a, b): false"
/// # );
/// # assert_eq!(actual, expect);
/// # }
/// ```
///
/// The macros `assert_in_delta` and `assert_in_epsilon` can test
/// approximations:
///
/// * For an approximation, the absolute error (i.e. delta) is the magnitude of
///   the difference between the exact value and the approximation. For this,
///  use the macro
///
/// * For an approximation, the relative error (i.e. epsilon) is the absolute
///   error divided by the magnitude of the exact value. This can be used to
///   compare approximations of numbers of wildly differing size.
///
/// * For example, approximating the number 1,000 with an absolute error of 3
///   is, in most applications, much worse than approximating the number
///   1,000,000 with an absolute error of 3; in the first case the relative
///   error is 0.003 and in the second it is only 0.000003.
///
/// * Thanks to Ruby minitest for the example and documentation.
///
/// # Module macros
///
/// * [`assert_in_epsilon`](macro@crate::assert_in_epsilon)
/// * [`assert_in_epsilon_as_result`](macro@crate::assert_in_epsilon_as_result)
/// * [`debug_assert_in_epsilon`](macro@crate::debug_assert_in_epsilon)
///
#[macro_export]
macro_rules! assert_in_epsilon {
    ($a:expr, $b:expr, $epsilon:expr $(,)?) => ({
        match assert_in_epsilon_as_result!($a, $b, $epsilon) {
            Ok(()) => (),
            Err(err) => panic!("{}", err),
        }
    });
    ($a:expr, $b:expr, $epsilon:expr, $($message:tt)+) => ({
        match assert_in_epsilon_as_result!($a, $b, $epsilon) {
            Ok(()) => (),
            Err(_err) => panic!("{}", $($message)+),
        }
    });
}

/// Assert a number is within epsilon of another number.
///
/// Pseudocode:<br>
/// | a - b | ≤ ε * min(a, b)
///
/// This macro provides the same statements as [`assert_in_epsilon`](macro.assert_in_epsilon.html),
/// except this macro's statements are only enabled in non-optimized
/// builds by default. An optimized build will not execute this macro's
/// statements unless `-C debug-assertions` is passed to the compiler.
///
/// This macro is useful for checks that are too expensive to be present
/// in a release build but may be helpful during development.
///
/// The result of expanding this macro is always type checked.
///
/// An unchecked assertion allows a program in an inconsistent state to
/// keep running, which might have unexpected consequences but does not
/// introduce unsafety as long as this only happens in safe code. The
/// performance cost of assertions, however, is not measurable in general.
/// Replacing `assert*!` with `debug_assert*!` is thus only encouraged
/// after thorough profiling, and more importantly, only in safe code!
///
/// This macro is intended to work in a similar way to
/// [`std::debug_assert`](https://doc.rust-lang.org/std/macro.debug_assert.html).
///
/// # Module macros
///
/// * [`assert_in_epsilon`](macro@crate::assert_in_epsilon)
/// * [`assert_in_epsilon`](macro@crate::assert_in_epsilon)
/// * [`debug_assert_in_epsilon`](macro@crate::debug_assert_in_epsilon)
///
#[macro_export]
macro_rules! debug_assert_in_epsilon {
    ($($arg:tt)*) => {
        if $crate::cfg!(debug_assertions) {
            $crate::assert_in_epsilon!($($arg)*);
        }
    };
}