use std::convert::Infallible;

/// A trait for reacting to an edit script from the "old" version to
/// the "new" version.
pub trait DiffHook: Sized {
    type Error;

    /// Called when lines with indices `old_index` (in the old version) and
    /// `new_index` (in the new version) start an section equal in both
    /// versions, of length `len`.
    fn equal(&mut self, old_index: usize, new_index: usize, len: usize) -> Result<(), Self::Error> {
        let _ = old_index;
        let _ = new_index;
        let _ = len;
        Ok(())
    }

    /// Called when a section of length `old_len`, starting at `old_index`,
    /// needs to be deleted from the old version.
    fn delete(
        &mut self,
        old_index: usize,
        old_len: usize,
        new_index: usize,
    ) -> Result<(), Self::Error> {
        let _ = old_index;
        let _ = old_len;
        let _ = new_index;
        Ok(())
    }

    /// Called when a section of the new version, of length `new_len`
    /// and starting at `new_index`, needs to be inserted at position `old_index'.
    fn insert(
        &mut self,
        old_index: usize,
        new_index: usize,
        new_len: usize,
    ) -> Result<(), Self::Error> {
        let _ = old_index;
        let _ = new_index;
        let _ = new_len;
        Ok(())
    }

    /// Called when a section of the old version, starting at index
    /// `old_index` and of length `old_len`, needs to be replaced with a
    /// section of length `new_len`, starting at `new_index`, of the new
    /// version.
    ///
    /// The default implementations invokes `delete` and `insert`.
    fn replace(
        &mut self,
        old_index: usize,
        old_len: usize,
        new_index: usize,
        new_len: usize,
    ) -> Result<(), Self::Error> {
        self.delete(old_index, old_len, new_index)?;
        self.insert(old_index, new_index, new_len)
    }

    /// Always called at the end of the algorithm.
    fn finish(&mut self) -> Result<(), Self::Error> {
        Ok(())
    }
}

impl<'a, D: DiffHook + 'a> DiffHook for &'a mut D {
    type Error = D::Error;

    fn equal(&mut self, old_index: usize, new_index: usize, len: usize) -> Result<(), Self::Error> {
        (*self).equal(old_index, new_index, len)
    }

    fn delete(
        &mut self,
        old_index: usize,
        old_len: usize,
        new_index: usize,
    ) -> Result<(), Self::Error> {
        (*self).delete(old_index, old_len, new_index)
    }

    fn insert(
        &mut self,
        old_index: usize,
        new_index: usize,
        new_len: usize,
    ) -> Result<(), Self::Error> {
        (*self).insert(old_index, new_index, new_len)
    }

    fn replace(
        &mut self,
        old: usize,
        old_len: usize,
        new: usize,
        new_len: usize,
    ) -> Result<(), Self::Error> {
        (*self).replace(old, old_len, new, new_len)
    }

    fn finish(&mut self) -> Result<(), Self::Error> {
        (*self).finish()
    }
}

/// Utility enum to capture a diff operation.
#[derive(Debug, PartialEq, Eq, Hash, Clone, Copy)]
pub enum DiffOp {
    Equal {
        old_index: usize,
        new_index: usize,
        len: usize,
    },
    Delete {
        old_index: usize,
        old_len: usize,
        new_index: usize,
    },
    Insert {
        old_index: usize,
        new_index: usize,
        new_len: usize,
    },
    Replace {
        old_index: usize,
        old_len: usize,
        new_index: usize,
        new_len: usize,
    },
}

/// Captures diff operations.
#[derive(Default, Clone)]
pub struct CaptureHook(Vec<DiffOp>);

impl CaptureHook {
    /// Creates a new capture hook.
    pub fn new() -> CaptureHook {
        CaptureHook::default()
    }

    /// Converts the capture hook into a vector.
    pub fn into_vec(self) -> Vec<DiffOp> {
        self.0
    }

    /// Accesses the captured operations.
    pub fn ops(&self) -> &[DiffOp] {
        &self.0
    }
}

impl DiffHook for CaptureHook {
    type Error = Infallible;

    fn equal(&mut self, old_index: usize, new_index: usize, len: usize) -> Result<(), Self::Error> {
        self.0.push(DiffOp::Equal {
            old_index,
            new_index,
            len,
        });
        Ok(())
    }

    fn delete(
        &mut self,
        old_index: usize,
        old_len: usize,
        new_index: usize,
    ) -> Result<(), Self::Error> {
        self.0.push(DiffOp::Delete {
            old_index,
            old_len,
            new_index,
        });
        Ok(())
    }

    fn insert(
        &mut self,
        old_index: usize,
        new_index: usize,
        new_len: usize,
    ) -> Result<(), Self::Error> {
        self.0.push(DiffOp::Insert {
            old_index,
            new_index,
            new_len,
        });
        Ok(())
    }

    fn replace(
        &mut self,
        old_index: usize,
        old_len: usize,
        new_index: usize,
        new_len: usize,
    ) -> Result<(), Self::Error> {
        self.0.push(DiffOp::Replace {
            old_index,
            old_len,
            new_index,
            new_len,
        });
        Ok(())
    }
}