//! Handle syntactic aspects of merging UseTrees.
use std::cmp::Ordering;

use itertools::{EitherOrBoth, Itertools};
use syntax::{
    ast::{self, make, AstNode, AttrsOwner, PathSegmentKind, VisibilityOwner},
    ted,
};

/// What type of merges are allowed.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum MergeBehavior {
    /// Merge imports from the same crate into a single use statement.
    Crate,
    /// Merge imports from the same module into a single use statement.
    Module,
}

impl MergeBehavior {
    #[inline]
    fn is_tree_allowed(&self, tree: &ast::UseTree) -> bool {
        match self {
            MergeBehavior::Crate => true,
            // only simple single segment paths are allowed
            MergeBehavior::Module => {
                tree.use_tree_list().is_none() && tree.path().map(path_len) <= Some(1)
            }
        }
    }
}

pub fn try_merge_imports(
    lhs: &ast::Use,
    rhs: &ast::Use,
    merge_behavior: MergeBehavior,
) -> Option<ast::Use> {
    // don't merge imports with different visibilities
    if !eq_visibility(lhs.visibility(), rhs.visibility()) {
        return None;
    }
    if !eq_attrs(lhs.attrs(), rhs.attrs()) {
        return None;
    }

    let lhs = lhs.clone_subtree().clone_for_update();
    let lhs_tree = lhs.use_tree()?;
    let rhs_tree = rhs.use_tree()?;
    let merged = try_merge_trees(&lhs_tree, &rhs_tree, merge_behavior)?;
    ted::replace(lhs_tree.syntax(), merged.syntax());
    Some(lhs)
}

pub fn try_merge_trees(
    lhs: &ast::UseTree,
    rhs: &ast::UseTree,
    merge: MergeBehavior,
) -> Option<ast::UseTree> {
    let lhs_path = lhs.path()?;
    let rhs_path = rhs.path()?;

    let (lhs_prefix, rhs_prefix) = common_prefix(&lhs_path, &rhs_path)?;
    let (lhs, rhs) = if lhs.is_simple_path()
        && rhs.is_simple_path()
        && lhs_path == lhs_prefix
        && rhs_path == rhs_prefix
    {
        (lhs.clone(), rhs.clone())
    } else {
        (lhs.split_prefix(&lhs_prefix), rhs.split_prefix(&rhs_prefix))
    };
    recursive_merge(&lhs, &rhs, merge).map(|it| it.clone_for_update())
}

/// Recursively "zips" together lhs and rhs.
fn recursive_merge(
    lhs: &ast::UseTree,
    rhs: &ast::UseTree,
    merge: MergeBehavior,
) -> Option<ast::UseTree> {
    let mut use_trees = lhs
        .use_tree_list()
        .into_iter()
        .flat_map(|list| list.use_trees())
        // We use Option here to early return from this function(this is not the
        // same as a `filter` op).
        .map(|tree| match merge.is_tree_allowed(&tree) {
            true => Some(tree),
            false => None,
        })
        .collect::<Option<Vec<_>>>()?;
    use_trees.sort_unstable_by(|a, b| path_cmp_for_sort(a.path(), b.path()));
    for rhs_t in rhs.use_tree_list().into_iter().flat_map(|list| list.use_trees()) {
        if !merge.is_tree_allowed(&rhs_t) {
            return None;
        }
        let rhs_path = rhs_t.path();
        match use_trees.binary_search_by(|lhs_t| {
            let (lhs_t, rhs_t) = match lhs_t
                .path()
                .zip(rhs_path.clone())
                .and_then(|(lhs, rhs)| common_prefix(&lhs, &rhs))
            {
                Some((lhs_p, rhs_p)) => (lhs_t.split_prefix(&lhs_p), rhs_t.split_prefix(&rhs_p)),
                None => (lhs_t.clone(), rhs_t.clone()),
            };

            path_cmp_bin_search(lhs_t.path(), rhs_t.path())
        }) {
            Ok(idx) => {
                let lhs_t = &mut use_trees[idx];
                let lhs_path = lhs_t.path()?;
                let rhs_path = rhs_path?;
                let (lhs_prefix, rhs_prefix) = common_prefix(&lhs_path, &rhs_path)?;
                if lhs_prefix == lhs_path && rhs_prefix == rhs_path {
                    let tree_is_self = |tree: ast::UseTree| {
                        tree.path().as_ref().map(path_is_self).unwrap_or(false)
                    };
                    // Check if only one of the two trees has a tree list, and
                    // whether that then contains `self` or not. If this is the
                    // case we can skip this iteration since the path without
                    // the list is already included in the other one via `self`.
                    let tree_contains_self = |tree: &ast::UseTree| {
                        tree.use_tree_list()
                            .map(|tree_list| tree_list.use_trees().any(tree_is_self))
                            .unwrap_or(false)
                    };
                    match (tree_contains_self(&lhs_t), tree_contains_self(&rhs_t)) {
                        (true, false) => continue,
                        (false, true) => {
                            *lhs_t = rhs_t;
                            continue;
                        }
                        _ => (),
                    }

                    // Glob imports aren't part of the use-tree lists so we need
                    // to special handle them here as well this special handling
                    // is only required for when we merge a module import into a
                    // glob import of said module see the `merge_self_glob` or
                    // `merge_mod_into_glob` tests.
                    if lhs_t.star_token().is_some() || rhs_t.star_token().is_some() {
                        *lhs_t = make::use_tree(
                            make::path_unqualified(make::path_segment_self()),
                            None,
                            None,
                            false,
                        );
                        use_trees.insert(idx, make::use_tree_glob());
                        continue;
                    }

                    if lhs_t.use_tree_list().is_none() && rhs_t.use_tree_list().is_none() {
                        continue;
                    }
                }
                let lhs = lhs_t.split_prefix(&lhs_prefix);
                let rhs = rhs_t.split_prefix(&rhs_prefix);
                match recursive_merge(&lhs, &rhs, merge) {
                    Some(use_tree) => use_trees[idx] = use_tree,
                    None => return None,
                }
            }
            Err(_)
                if merge == MergeBehavior::Module
                    && use_trees.len() > 0
                    && rhs_t.use_tree_list().is_some() =>
            {
                return None
            }
            Err(idx) => {
                use_trees.insert(idx, rhs_t);
            }
        }
    }

    let lhs = lhs.clone_subtree().clone_for_update();
    if let Some(old) = lhs.use_tree_list() {
        ted::replace(old.syntax(), make::use_tree_list(use_trees).syntax().clone_for_update());
    }
    ast::UseTree::cast(lhs.syntax().clone_subtree())
}

/// Traverses both paths until they differ, returning the common prefix of both.
fn common_prefix(lhs: &ast::Path, rhs: &ast::Path) -> Option<(ast::Path, ast::Path)> {
    let mut res = None;
    let mut lhs_curr = lhs.first_qualifier_or_self();
    let mut rhs_curr = rhs.first_qualifier_or_self();
    loop {
        match (lhs_curr.segment(), rhs_curr.segment()) {
            (Some(lhs), Some(rhs)) if lhs.syntax().text() == rhs.syntax().text() => (),
            _ => break res,
        }
        res = Some((lhs_curr.clone(), rhs_curr.clone()));

        match lhs_curr.parent_path().zip(rhs_curr.parent_path()) {
            Some((lhs, rhs)) => {
                lhs_curr = lhs;
                rhs_curr = rhs;
            }
            _ => break res,
        }
    }
}

/// Orders paths in the following way:
/// the sole self token comes first, after that come uppercase identifiers, then lowercase identifiers
// FIXME: rustfmt sorts lowercase idents before uppercase, in general we want to have the same ordering rustfmt has
// which is `self` and `super` first, then identifier imports with lowercase ones first, then glob imports and at last list imports.
// Example foo::{self, foo, baz, Baz, Qux, *, {Bar}}
fn path_cmp_for_sort(a: Option<ast::Path>, b: Option<ast::Path>) -> Ordering {
    match (a, b) {
        (None, None) => Ordering::Equal,
        (None, Some(_)) => Ordering::Less,
        (Some(_), None) => Ordering::Greater,
        (Some(ref a), Some(ref b)) => match (path_is_self(a), path_is_self(b)) {
            (true, true) => Ordering::Equal,
            (true, false) => Ordering::Less,
            (false, true) => Ordering::Greater,
            (false, false) => path_cmp_short(a, b),
        },
    }
}

/// Path comparison func for binary searching for merging.
fn path_cmp_bin_search(lhs: Option<ast::Path>, rhs: Option<ast::Path>) -> Ordering {
    match (
        lhs.as_ref().and_then(ast::Path::first_segment),
        rhs.as_ref().and_then(ast::Path::first_segment),
    ) {
        (None, None) => Ordering::Equal,
        (None, Some(_)) => Ordering::Less,
        (Some(_), None) => Ordering::Greater,
        (Some(ref a), Some(ref b)) => path_segment_cmp(a, b),
    }
}

/// Short circuiting comparison, if both paths are equal until one of them ends they are considered
/// equal
fn path_cmp_short(a: &ast::Path, b: &ast::Path) -> Ordering {
    let a = a.segments();
    let b = b.segments();
    // cmp_by would be useful for us here but that is currently unstable
    // cmp doesn't work due the lifetimes on text's return type
    a.zip(b)
        .find_map(|(a, b)| match path_segment_cmp(&a, &b) {
            Ordering::Equal => None,
            ord => Some(ord),
        })
        .unwrap_or(Ordering::Equal)
}

/// Compares two paths, if one ends earlier than the other the has_tl parameters decide which is
/// greater as a a path that has a tree list should be greater, while one that just ends without
/// a tree list should be considered less.
pub(super) fn use_tree_path_cmp(
    a: &ast::Path,
    a_has_tl: bool,
    b: &ast::Path,
    b_has_tl: bool,
) -> Ordering {
    let a_segments = a.segments();
    let b_segments = b.segments();
    // cmp_by would be useful for us here but that is currently unstable
    // cmp doesn't work due the lifetimes on text's return type
    a_segments
        .zip_longest(b_segments)
        .find_map(|zipped| match zipped {
            EitherOrBoth::Both(ref a, ref b) => match path_segment_cmp(a, b) {
                Ordering::Equal => None,
                ord => Some(ord),
            },
            EitherOrBoth::Left(_) if !b_has_tl => Some(Ordering::Greater),
            EitherOrBoth::Left(_) => Some(Ordering::Less),
            EitherOrBoth::Right(_) if !a_has_tl => Some(Ordering::Less),
            EitherOrBoth::Right(_) => Some(Ordering::Greater),
        })
        .unwrap_or(Ordering::Equal)
}

fn path_segment_cmp(a: &ast::PathSegment, b: &ast::PathSegment) -> Ordering {
    let a = a.kind().and_then(|kind| match kind {
        PathSegmentKind::Name(name_ref) => Some(name_ref),
        _ => None,
    });
    let b = b.kind().and_then(|kind| match kind {
        PathSegmentKind::Name(name_ref) => Some(name_ref),
        _ => None,
    });
    a.as_ref().map(ast::NameRef::text).cmp(&b.as_ref().map(ast::NameRef::text))
}

fn eq_visibility(vis0: Option<ast::Visibility>, vis1: Option<ast::Visibility>) -> bool {
    match (vis0, vis1) {
        (None, None) => true,
        // FIXME: Don't use the string representation to check for equality
        // spaces inside of the node would break this comparison
        (Some(vis0), Some(vis1)) => vis0.to_string() == vis1.to_string(),
        _ => false,
    }
}

fn eq_attrs(
    attrs0: impl Iterator<Item = ast::Attr>,
    attrs1: impl Iterator<Item = ast::Attr>,
) -> bool {
    let attrs0 = attrs0.map(|attr| attr.to_string());
    let attrs1 = attrs1.map(|attr| attr.to_string());
    attrs0.eq(attrs1)
}

fn path_is_self(path: &ast::Path) -> bool {
    path.segment().and_then(|seg| seg.self_token()).is_some() && path.qualifier().is_none()
}

fn path_len(path: ast::Path) -> usize {
    path.segments().count()
}