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use std::ops::{Deref, Index, RangeFrom};

/// Input to parsers should either be a `&str` or a slice (i.e. `&[T]`)
///
/// If you have either of those you can convert it into an `Input` by using `.into()` or
/// `Into::from()`
///
/// `Input` implements `Deref<Target = T>`, and so inherits all of `T`s methods.
///
/// `Input` implements `PartialEq<Rhs=T>`, so you can compare an instance of `Input<T>` with a `T`
#[derive(Debug, Clone)]
pub struct Input<T> {
    input: T,
    length: usize,
    position: usize,
}

impl<'i> From<&'i str> for Input<&'i str> {
    fn from(value: &'i str) -> Self {
        Input {
            input: value,
            length: value.len(),
            position: 0,
        }
    }
}

impl<'i, T> From<&'i [T]> for Input<&'i [T]> {
    fn from(value: &'i [T]) -> Self {
        Input {
            input: value,
            length: value.len(),
            position: 0,
        }
    }
}

impl PartialEq<&str> for Input<&str> {
    fn eq(&self, other: &&str) -> bool {
        &self.input[self.position..] == *other
    }
}

impl<T> PartialEq<&[T]> for Input<&[T]>
where
    [T]: Eq,
{
    fn eq(&self, other: &&[T]) -> bool {
        &self.input[self.position..] == *other
    }
}

impl<T> Deref for Input<T>
where
    T: Deref,
    <T as Deref>::Target: Index<RangeFrom<usize>>,
{
    type Target = <<T as Deref>::Target as Index<RangeFrom<usize>>>::Output;

    fn deref(&self) -> &Self::Target {
        // This is where the magic of the `Input` struct comes from. It holds the entire parse
        // input, but to individual parsers, will only appear to hold the part of the input they
        // care about.
        &self.input[self.position..]
    }
}

impl<T> Input<T>
where
    T: Clone,
{
    /// Returns a new instance of `Input` with `count` elements removed from the start
    pub fn pop(&self, count: usize) -> Input<T> {
        let offset = if self.position + count <= self.length {
            count
        } else {
            0
        };

        Input {
            input: self.input.clone(),
            length: self.length,
            position: self.position + offset,
        }
    }
}

impl<T> Input<T> {
    /// Returns the current position
    pub fn get_position(&self) -> usize {
        self.position
    }
}

#[cfg(test)]
mod test {
    use super::*;
    #[test]
    fn test_equality() {
        let str = "A";
        let input = Input::from(str);
        assert_eq!(input, str);
        assert_eq!(input, "A");

        let slice = &[0, 2, 4][..];
        let input = Input::from(slice);
        assert_eq!(input, slice);
        assert_eq!(input, &[0, 2, 4]);
    }

    #[test]
    fn test_deref_magic() {
        let str = "ABC";

        let input = Input::from(str);

        assert_eq!(input, "ABC");

        let input = input.pop(1);
        assert_eq!(input, "BC");
        assert_eq!(&input[0..], "BC");

        let input = input.pop(1);
        assert_eq!(input, "C");
        assert_eq!(&input[0..], "C");

        let input = input.pop(1);
        assert_eq!(input, "");
        assert_eq!(&input[0..], "");

        let input = input.pop(10).pop(1).pop(3);
        assert_eq!(input, "");
        assert_eq!(&input[0..], "");
    }
}