use clippy_utils::diagnostics::span_lint_and_then;
use clippy_utils::source::{SpanRangeExt, snippet_indent};
use clippy_utils::tokenize_with_text;
use itertools::Itertools;
use rustc_ast::token::CommentKind;
use rustc_ast::{AssocItemKind, AttrKind, AttrStyle, Attribute, Crate, Item, ItemKind, ModKind, NodeId};
use rustc_errors::{Applicability, Diag, SuggestionStyle};
use rustc_lexer::TokenKind;
use rustc_lint::{EarlyContext, EarlyLintPass, LintContext};
use rustc_session::impl_lint_pass;
use rustc_span::symbol::kw;
use rustc_span::{BytePos, ExpnKind, Ident, InnerSpan, Span, SpanData, Symbol};

declare_clippy_lint! {
    /// ### What it does
    /// Checks for empty lines after outer attributes
    ///
    /// ### Why is this bad?
    /// The attribute may have meant to be an inner attribute (`#![attr]`). If
    /// it was meant to be an outer attribute (`#[attr]`) then the empty line
    /// should be removed
    ///
    /// ### Example
    /// ```no_run
    /// #[allow(dead_code)]
    ///
    /// fn not_quite_good_code() {}
    /// ```
    ///
    /// Use instead:
    /// ```no_run
    /// // Good (as inner attribute)
    /// #![allow(dead_code)]
    ///
    /// fn this_is_fine() {}
    ///
    /// // or
    ///
    /// // Good (as outer attribute)
    /// #[allow(dead_code)]
    /// fn this_is_fine_too() {}
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub EMPTY_LINE_AFTER_OUTER_ATTR,
    suspicious,
    "empty line after outer attribute"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for empty lines after doc comments.
    ///
    /// ### Why is this bad?
    /// The doc comment may have meant to be an inner doc comment, regular
    /// comment or applied to some old code that is now commented out. If it was
    /// intended to be a doc comment, then the empty line should be removed.
    ///
    /// ### Example
    /// ```no_run
    /// /// Some doc comment with a blank line after it.
    ///
    /// fn f() {}
    ///
    /// /// Docs for `old_code`
    /// // fn old_code() {}
    ///
    /// fn new_code() {}
    /// ```
    ///
    /// Use instead:
    /// ```no_run
    /// //! Convert it to an inner doc comment
    ///
    /// // Or a regular comment
    ///
    /// /// Or remove the empty line
    /// fn f() {}
    ///
    /// // /// Docs for `old_code`
    /// // fn old_code() {}
    ///
    /// fn new_code() {}
    /// ```
    #[clippy::version = "1.70.0"]
    pub EMPTY_LINE_AFTER_DOC_COMMENTS,
    suspicious,
    "empty line after doc comments"
}

#[derive(Debug)]
struct ItemInfo {
    kind: &'static str,
    name: Symbol,
    span: Span,
    mod_items: Option<NodeId>,
}

pub struct EmptyLineAfter {
    items: Vec<ItemInfo>,
}

impl_lint_pass!(EmptyLineAfter => [
    EMPTY_LINE_AFTER_OUTER_ATTR,
    EMPTY_LINE_AFTER_DOC_COMMENTS,
]);

impl EmptyLineAfter {
    pub fn new() -> Self {
        Self { items: Vec::new() }
    }
}

#[derive(Debug, PartialEq, Clone, Copy)]
enum StopKind {
    Attr,
    Doc(CommentKind),
}

impl StopKind {
    fn is_doc(self) -> bool {
        matches!(self, StopKind::Doc(_))
    }
}

#[derive(Debug)]
struct Stop {
    span: Span,
    kind: StopKind,
    first: usize,
    last: usize,
}

impl Stop {
    fn convert_to_inner(&self) -> (Span, String) {
        let inner = match self.kind {
            // #![...]
            StopKind::Attr => InnerSpan::new(1, 1),
            // /// or /**
            //   ^      ^
            StopKind::Doc(_) => InnerSpan::new(2, 3),
        };
        (self.span.from_inner(inner), "!".into())
    }

    fn comment_out(&self, cx: &EarlyContext<'_>, suggestions: &mut Vec<(Span, String)>) {
        match self.kind {
            StopKind::Attr => {
                if cx.sess().source_map().is_multiline(self.span) {
                    suggestions.extend([
                        (self.span.shrink_to_lo(), "/* ".into()),
                        (self.span.shrink_to_hi(), " */".into()),
                    ]);
                } else {
                    suggestions.push((self.span.shrink_to_lo(), "// ".into()));
                }
            },
            StopKind::Doc(CommentKind::Line) => suggestions.push((self.span.shrink_to_lo(), "// ".into())),
            StopKind::Doc(CommentKind::Block) => {
                // /** outer */  /*! inner */
                //  ^             ^
                let asterisk = self.span.from_inner(InnerSpan::new(1, 2));
                suggestions.push((asterisk, String::new()));
            },
        }
    }

    fn from_attr(cx: &EarlyContext<'_>, attr: &Attribute) -> Option<Self> {
        let SpanData { lo, hi, .. } = attr.span.data();
        let file = cx.sess().source_map().lookup_source_file(lo);

        Some(Self {
            span: attr.span,
            kind: match attr.kind {
                AttrKind::Normal(_) => StopKind::Attr,
                AttrKind::DocComment(comment_kind, _) => StopKind::Doc(comment_kind),
            },
            first: file.lookup_line(file.relative_position(lo))?,
            last: file.lookup_line(file.relative_position(hi))?,
        })
    }
}

/// Represents a set of attrs/doc comments separated by 1 or more empty lines
///
/// ```ignore
/// /// chunk 1 docs
/// // not an empty line so also part of chunk 1
/// #[chunk_1_attrs] // <-- prev_stop
///
/// /* gap */
///
/// /// chunk 2 docs // <-- next_stop
/// #[chunk_2_attrs]
/// ```
struct Gap<'a> {
    /// The span of individual empty lines including the newline at the end of the line
    empty_lines: Vec<Span>,
    has_comment: bool,
    next_stop: &'a Stop,
    prev_stop: &'a Stop,
    /// The chunk that includes [`prev_stop`](Self::prev_stop)
    prev_chunk: &'a [Stop],
}

impl<'a> Gap<'a> {
    fn new(cx: &EarlyContext<'_>, prev_chunk: &'a [Stop], next_chunk: &'a [Stop]) -> Option<Self> {
        let prev_stop = prev_chunk.last()?;
        let next_stop = next_chunk.first()?;
        let gap_span = prev_stop.span.between(next_stop.span);
        let gap_snippet = gap_span.get_source_text(cx)?;

        let mut has_comment = false;
        let mut empty_lines = Vec::new();

        for (token, source, inner_span) in tokenize_with_text(&gap_snippet) {
            match token {
                TokenKind::BlockComment {
                    doc_style: None,
                    terminated: true,
                }
                | TokenKind::LineComment { doc_style: None } => has_comment = true,
                TokenKind::Whitespace => {
                    let newlines = source.bytes().positions(|b| b == b'\n');
                    empty_lines.extend(
                        newlines
                            .tuple_windows()
                            .map(|(a, b)| InnerSpan::new(inner_span.start + a + 1, inner_span.start + b))
                            .map(|inner_span| gap_span.from_inner(inner_span)),
                    );
                },
                // Ignore cfg_attr'd out attributes as they may contain empty lines, could also be from macro
                // shenanigans
                _ => return None,
            }
        }

        (!empty_lines.is_empty()).then_some(Self {
            empty_lines,
            has_comment,
            next_stop,
            prev_stop,
            prev_chunk,
        })
    }

    fn contiguous_empty_lines(&self) -> impl Iterator<Item = Span> + '_ {
        self.empty_lines
            // The `+ BytePos(1)` means "next line", because each empty line span is "N:1-N:1".
            .chunk_by(|a, b| a.hi() + BytePos(1) == b.lo())
            .map(|chunk| {
                let first = chunk.first().expect("at least one empty line");
                let last = chunk.last().expect("at least one empty line");
                // The BytePos subtraction here is safe, as before an empty line, there must be at least one
                // attribute/comment. The span needs to start at the end of the previous line.
                first.with_lo(first.lo() - BytePos(1)).with_hi(last.hi())
            })
    }
}

impl EmptyLineAfter {
    fn check_gaps(&self, cx: &EarlyContext<'_>, gaps: &[Gap<'_>], id: NodeId) {
        let Some(first_gap) = gaps.first() else {
            return;
        };
        let empty_lines = || gaps.iter().flat_map(|gap| gap.empty_lines.iter().copied());
        let contiguous_empty_lines = || gaps.iter().flat_map(Gap::contiguous_empty_lines);
        let mut has_comment = false;
        let mut has_attr = false;
        for gap in gaps {
            has_comment |= gap.has_comment;
            if !has_attr {
                has_attr = gap.prev_chunk.iter().any(|stop| stop.kind == StopKind::Attr);
            }
        }
        let kind = first_gap.prev_stop.kind;
        let (lint, kind_desc) = match kind {
            StopKind::Attr => (EMPTY_LINE_AFTER_OUTER_ATTR, "outer attribute"),
            StopKind::Doc(_) => (EMPTY_LINE_AFTER_DOC_COMMENTS, "doc comment"),
        };
        let (lines, are, them) = if empty_lines().nth(1).is_some() {
            ("lines", "are", "them")
        } else {
            ("line", "is", "it")
        };
        span_lint_and_then(
            cx,
            lint,
            first_gap.prev_stop.span.to(empty_lines().last().unwrap()),
            format!("empty {lines} after {kind_desc}"),
            |diag| {
                let info = self.items.last().unwrap();
                diag.span_label(
                    info.span,
                    match kind {
                        StopKind::Attr => format!("the attribute applies to this {}", info.kind),
                        StopKind::Doc(_) => format!("the comment documents this {}", info.kind),
                    },
                );

                diag.multipart_suggestion_with_style(
                    format!("if the empty {lines} {are} unintentional, remove {them}"),
                    contiguous_empty_lines()
                        .map(|empty_lines| (empty_lines, String::new()))
                        .collect(),
                    Applicability::MaybeIncorrect,
                    SuggestionStyle::HideCodeAlways,
                );

                if has_comment && kind.is_doc() {
                    // Likely doc comments that applied to some now commented out code
                    //
                    // /// Old docs for Foo
                    // // struct Foo;

                    let mut suggestions = Vec::new();
                    for stop in gaps.iter().flat_map(|gap| gap.prev_chunk) {
                        stop.comment_out(cx, &mut suggestions);
                    }
                    diag.multipart_suggestion_verbose(
                        format!("if the doc comment should not document `{}` comment it out", info.name),
                        suggestions,
                        Applicability::MaybeIncorrect,
                    );
                } else {
                    self.suggest_inner(diag, kind, gaps, id);
                }

                if kind == StopKind::Doc(CommentKind::Line)
                    && gaps
                        .iter()
                        .all(|gap| !gap.has_comment && gap.next_stop.kind == StopKind::Doc(CommentKind::Line))
                {
                    // Commentless empty gaps between line doc comments, possibly intended to be part of the markdown

                    let indent = snippet_indent(cx, first_gap.prev_stop.span).unwrap_or_default();
                    diag.multipart_suggestion_verbose(
                        format!("if the documentation should include the empty {lines} include {them} in the comment"),
                        empty_lines()
                            .map(|empty_line| (empty_line, format!("{indent}///")))
                            .collect(),
                        Applicability::MaybeIncorrect,
                    );
                }
            },
        );
    }

    /// If the node the attributes/docs apply to is the first in the module/crate suggest converting
    /// them to inner attributes/docs
    fn suggest_inner(&self, diag: &mut Diag<'_, ()>, kind: StopKind, gaps: &[Gap<'_>], id: NodeId) {
        if let Some(parent) = self.items.iter().rev().nth(1)
            && (parent.kind == "module" || parent.kind == "crate")
            && parent.mod_items == Some(id)
        {
            let desc = if parent.kind == "module" {
                "parent module"
            } else {
                parent.kind
            };
            diag.multipart_suggestion_verbose(
                match kind {
                    StopKind::Attr => format!("if the attribute should apply to the {desc} use an inner attribute"),
                    StopKind::Doc(_) => format!("if the comment should document the {desc} use an inner doc comment"),
                },
                gaps.iter()
                    .flat_map(|gap| gap.prev_chunk)
                    .map(Stop::convert_to_inner)
                    .collect(),
                Applicability::MaybeIncorrect,
            );
        }
    }

    fn check_item_kind(
        &mut self,
        cx: &EarlyContext<'_>,
        kind: &ItemKind,
        ident: &Ident,
        span: Span,
        attrs: &[Attribute],
        id: NodeId,
    ) {
        self.items.push(ItemInfo {
            kind: kind.descr(),
            name: ident.name,
            span: if span.contains(ident.span) {
                span.with_hi(ident.span.hi())
            } else {
                span.with_hi(span.lo())
            },
            mod_items: match kind {
                ItemKind::Mod(_, ModKind::Loaded(items, _, _, _)) => items
                    .iter()
                    .filter(|i| !matches!(i.span.ctxt().outer_expn_data().kind, ExpnKind::AstPass(_)))
                    .map(|i| i.id)
                    .next(),
                _ => None,
            },
        });

        let mut outer = attrs
            .iter()
            .filter(|attr| attr.style == AttrStyle::Outer && !attr.span.from_expansion())
            .map(|attr| Stop::from_attr(cx, attr))
            .collect::<Option<Vec<_>>>()
            .unwrap_or_default();

        if outer.is_empty() {
            return;
        }

        // Push a fake attribute Stop for the item itself so we check for gaps between the last outer
        // attr/doc comment and the item they apply to
        let span = self.items.last().unwrap().span;
        if !span.from_expansion()
            && let Ok(line) = cx.sess().source_map().lookup_line(span.lo())
        {
            outer.push(Stop {
                span,
                kind: StopKind::Attr,
                first: line.line,
                // last doesn't need to be accurate here, we don't compare it with anything
                last: line.line,
            });
        }

        let mut gaps = Vec::new();
        let mut last = 0;
        for pos in outer
            .array_windows()
            .positions(|[a, b]| b.first.saturating_sub(a.last) > 1)
        {
            // we want to be after the first stop in the window
            let pos = pos + 1;
            if let Some(gap) = Gap::new(cx, &outer[last..pos], &outer[pos..]) {
                last = pos;
                gaps.push(gap);
            }
        }

        self.check_gaps(cx, &gaps, id);
    }
}

impl EarlyLintPass for EmptyLineAfter {
    fn check_crate(&mut self, _: &EarlyContext<'_>, krate: &Crate) {
        self.items.push(ItemInfo {
            kind: "crate",
            name: kw::Crate,
            span: krate.spans.inner_span.with_hi(krate.spans.inner_span.lo()),
            mod_items: krate
                .items
                .iter()
                .filter(|i| !matches!(i.span.ctxt().outer_expn_data().kind, ExpnKind::AstPass(_)))
                .map(|i| i.id)
                .next(),
        });
    }

    fn check_item_post(&mut self, _: &EarlyContext<'_>, _: &Item) {
        self.items.pop();
    }
    fn check_impl_item_post(&mut self, _: &EarlyContext<'_>, _: &Item<AssocItemKind>) {
        self.items.pop();
    }
    fn check_trait_item_post(&mut self, _: &EarlyContext<'_>, _: &Item<AssocItemKind>) {
        self.items.pop();
    }

    fn check_impl_item(&mut self, cx: &EarlyContext<'_>, item: &Item<AssocItemKind>) {
        self.check_item_kind(
            cx,
            &item.kind.clone().into(),
            &item.ident,
            item.span,
            &item.attrs,
            item.id,
        );
    }

    fn check_trait_item(&mut self, cx: &EarlyContext<'_>, item: &Item<AssocItemKind>) {
        self.check_item_kind(
            cx,
            &item.kind.clone().into(),
            &item.ident,
            item.span,
            &item.attrs,
            item.id,
        );
    }

    fn check_item(&mut self, cx: &EarlyContext<'_>, item: &Item) {
        self.check_item_kind(cx, &item.kind, &item.ident, item.span, &item.attrs, item.id);
    }
}