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// Copyright (c) 2017, Nick Stevens <nick@bitcurry.com>
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/license/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! Definition of `Predicate`s for comparisons over `Ord` and `Eq` types.

use Predicate;

#[derive(Clone, Copy, Debug)]
enum EqOps {
    Equal,
    NotEqual,
}

/// Predicate that returns `true` if `variable` matches the pre-defined `Eq`
/// value, otherwise returns `false`.
///
/// This is created by the `predicate::{eq, ne}` functions.
#[derive(Debug)]
pub struct EqPredicate<T> {
    constant: T,
    op: EqOps,
}

impl<T> Predicate for EqPredicate<T>
where
    T: PartialEq,
{
    type Item = T;

    fn eval(&self, variable: &Self::Item) -> bool {
        match self.op {
            EqOps::Equal => variable.eq(&self.constant),
            EqOps::NotEqual => variable.ne(&self.constant),
        }
    }
}

/// Creates a new predicate that will return `true` when the given `variable` is
/// equal to a pre-defined value.
///
/// # Examples
///
/// ```
/// use predicates::predicate::*;
///
/// let predicate_fn = eq(5);
/// assert_eq!(true, predicate_fn.eval(&5));
/// assert_eq!(false, predicate_fn.eval(&10));
/// ```
pub fn eq<T>(constant: T) -> EqPredicate<T>
where
    T: PartialEq,
{
    EqPredicate {
        constant: constant,
        op: EqOps::Equal,
    }
}

/// Creates a new predicate that will return `true` when the given `variable` is
/// _not_ equal to a pre-defined value.
///
/// # Examples
///
/// ```
/// use predicates::predicate::*;
///
/// let predicate_fn = ne(5);
/// assert_eq!(false, predicate_fn.eval(&5));
/// assert_eq!(true, predicate_fn.eval(&10));
/// ```
pub fn ne<T>(constant: T) -> EqPredicate<T>
where
    T: PartialEq,
{
    EqPredicate {
        constant: constant,
        op: EqOps::NotEqual,
    }
}

#[derive(Clone, Copy, Debug)]
enum OrdOps {
    LessThan,
    LessThanOrEqual,
    GreaterThanOrEqual,
    GreaterThan,
}

/// Predicate that returns `true` if `variable` matches the pre-defined `Ord`
/// value, otherwise returns `false`.
///
/// This is created by the `predicate::{gt, ge, lt, le}` functions.
#[derive(Debug)]
pub struct OrdPredicate<T> {
    constant: T,
    op: OrdOps,
}

impl<T> Predicate for OrdPredicate<T>
where
    T: PartialOrd,
{
    type Item = T;

    fn eval(&self, variable: &Self::Item) -> bool {
        match self.op {
            OrdOps::LessThan => variable.lt(&self.constant),
            OrdOps::LessThanOrEqual => variable.le(&self.constant),
            OrdOps::GreaterThanOrEqual => variable.ge(&self.constant),
            OrdOps::GreaterThan => variable.gt(&self.constant),
        }
    }
}

/// Creates a new predicate that will return `true` when the given `variable` is
/// less than a pre-defined value.
///
/// # Examples
///
/// ```
/// use predicates::predicate::*;
///
/// let predicate_fn = lt(5);
/// assert_eq!(true, predicate_fn.eval(&4));
/// assert_eq!(false, predicate_fn.eval(&6));
/// ```
pub fn lt<T>(constant: T) -> OrdPredicate<T>
where
    T: PartialOrd,
{
    OrdPredicate {
        constant: constant,
        op: OrdOps::LessThan,
    }
}

/// Creates a new predicate that will return `true` when the given `variable` is
/// less than or equal to a pre-defined value.
///
/// # Examples
///
/// ```
/// use predicates::predicate::*;
///
/// let predicate_fn = le(5);
/// assert_eq!(true, predicate_fn.eval(&4));
/// assert_eq!(true, predicate_fn.eval(&5));
/// assert_eq!(false, predicate_fn.eval(&6));
/// ```
pub fn le<T>(constant: T) -> OrdPredicate<T>
where
    T: PartialOrd,
{
    OrdPredicate {
        constant: constant,
        op: OrdOps::LessThanOrEqual,
    }
}

/// Creates a new predicate that will return `true` when the given `variable` is
/// greater than or equal to a pre-defined value.
///
/// # Examples
///
/// ```
/// use predicates::predicate::*;
///
/// let predicate = ge(5);
/// assert_eq!(false, predicate.eval(&4));
/// assert_eq!(true, predicate.eval(&5));
/// assert_eq!(true, predicate.eval(&6));
/// ```
pub fn ge<T>(constant: T) -> OrdPredicate<T>
where
    T: PartialOrd,
{
    OrdPredicate {
        constant: constant,
        op: OrdOps::GreaterThanOrEqual,
    }
}

/// Creates a new predicate that will return `true` when the given `variable` is
/// greater than a pre-defined value.
///
/// # Examples
///
/// ```
/// use predicates::predicate::*;
///
/// let predicate_fn = gt(5);
/// assert_eq!(false, predicate_fn.eval(&4));
/// assert_eq!(false, predicate_fn.eval(&5));
/// assert_eq!(true, predicate_fn.eval(&6));
/// ```
pub fn gt<T>(constant: T) -> OrdPredicate<T>
where
    T: PartialOrd,
{
    OrdPredicate {
        constant: constant,
        op: OrdOps::GreaterThan,
    }
}