//! "Diff"ing iterators for caching elements to sequential collections without requiring the new

//! elements' iterator to be `Clone`.

//!

//! - [`Diff`] (produced by the [`diff_with`] function)

//! describes the difference between two non-`Clone` iterators `I` and `J` after breaking ASAP from

//! a lock-step comparison.

use std::fmt;

use crate::free::put_back;

use crate::structs::PutBack;

/// A type returned by the [`diff_with`] function.

///

/// `Diff` represents the way in which the elements yielded by the iterator `I` differ to some

/// iterator `J`.

pub enum Diff<I, J>

where

    I: Iterator,

    J: Iterator,

{

    /// The index of the first non-matching element along with both iterator's remaining elements

    /// starting with the first mis-match.

    FirstMismatch(usize, PutBack<I>, PutBack<J>),

    /// The total number of elements that were in `J` along with the remaining elements of `I`.

    Shorter(usize, PutBack<I>),

    /// The total number of elements that were in `I` along with the remaining elements of `J`.

    Longer(usize, PutBack<J>),

}

impl<I, J> fmt::Debug for Diff<I, J>

where

    I: Iterator,

    J: Iterator,

    PutBack<I>: fmt::Debug,

    PutBack<J>: fmt::Debug,

{

    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {

        match self {

            Self::FirstMismatch(idx, i, j) => f

                .debug_tuple("FirstMismatch")

                .field(idx)

                .field(i)

                .field(j)

                .finish(),

            Self::Shorter(idx, i) => f.debug_tuple("Shorter").field(idx).field(i).finish(),

            Self::Longer(idx, j) => f.debug_tuple("Longer").field(idx).field(j).finish(),

        }

    }

}

impl<I, J> Clone for Diff<I, J>

where

    I: Iterator,

    J: Iterator,

    PutBack<I>: Clone,

    PutBack<J>: Clone,

{

    fn clone(&self) -> Self {

        match self {

            Self::FirstMismatch(idx, i, j) => Self::FirstMismatch(*idx, i.clone(), j.clone()),

            Self::Shorter(idx, i) => Self::Shorter(*idx, i.clone()),

            Self::Longer(idx, j) => Self::Longer(*idx, j.clone()),

        }

    }

}

/// Compares every element yielded by both `i` and `j` with the given function in lock-step and

/// returns a [`Diff`] which describes how `j` differs from `i`.

///

/// If the number of elements yielded by `j` is less than the number of elements yielded by `i`,

/// the number of `j` elements yielded will be returned along with `i`'s remaining elements as

/// `Diff::Shorter`.

///

/// If the two elements of a step differ, the index of those elements along with the remaining

/// elements of both `i` and `j` are returned as `Diff::FirstMismatch`.

///

/// If `i` becomes exhausted before `j` becomes exhausted, the number of elements in `i` along with

/// the remaining `j` elements will be returned as `Diff::Longer`.

pub fn diff_with<I, J, F>(i: I, j: J, is_equal: F) -> Option<Diff<I::IntoIter, J::IntoIter>>

where

    I: IntoIterator,

    J: IntoIterator,

    F: Fn(&I::Item, &J::Item) -> bool,

{

    let mut i = i.into_iter();

    let mut j = j.into_iter();

    let mut idx = 0;

    while let Some(i_elem) = i.next() {

        match j.next() {

            None => return Some(Diff::Shorter(idx, put_back(i).with_value(i_elem))),

            Some(j_elem) => {

                if !is_equal(&i_elem, &j_elem) {

                    let remaining_i = put_back(i).with_value(i_elem);

                    let remaining_j = put_back(j).with_value(j_elem);

                    return Some(Diff::FirstMismatch(idx, remaining_i, remaining_j));

                }

            }

        }

        idx += 1;

    }

    j.next()

        .map(|j_elem| Diff::Longer(idx, put_back(j).with_value(j_elem)))

}