stmt_function_def.rs

use crate::comments::format::empty_lines_before_trailing_comments;
use ruff_formatter::write;
use ruff_python_ast::{Parameters, StmtFunctionDef};
use ruff_python_trivia::{SimpleTokenKind, SimpleTokenizer};
use ruff_text_size::Ranged;

use crate::comments::SourceComment;
use crate::expression::maybe_parenthesize_expression;
use crate::expression::parentheses::{Parentheses, Parenthesize};
use crate::prelude::*;
use crate::statement::clause::{clause_body, clause_header, ClauseHeader};
use crate::statement::stmt_class_def::FormatDecorators;
use crate::statement::suite::SuiteKind;

#[derive(Default)]
pub struct FormatStmtFunctionDef;

impl FormatNodeRule<StmtFunctionDef> for FormatStmtFunctionDef {
    fn fmt_fields(&self, item: &StmtFunctionDef, f: &mut PyFormatter) -> FormatResult<()> {
        let StmtFunctionDef {
            range: _,
            is_async,
            decorator_list,
            name,
            type_params,
            parameters,
            returns,
            body,
        } = item;

        let comments = f.context().comments().clone();

        let dangling_comments = comments.dangling(item);
        let trailing_definition_comments_start =
            dangling_comments.partition_point(|comment| comment.line_position().is_own_line());

        let (leading_definition_comments, trailing_definition_comments) =
            dangling_comments.split_at(trailing_definition_comments_start);

        write!(
            f,
            [
                FormatDecorators {
                    decorators: decorator_list,
                    leading_definition_comments,
                },
                clause_header(
                    ClauseHeader::Function(item),
                    trailing_definition_comments,
                    &format_with(|f| {
                        if *is_async {
                            write!(f, [token("async"), space()])?;
                        }

                        write!(f, [token("def"), space(), name.format()])?;

                        if let Some(type_params) = type_params.as_ref() {
                            write!(f, [type_params.format()])?;
                        }

                        let format_inner = format_with(|f: &mut PyFormatter| {
                            write!(f, [parameters.format()])?;

                            if let Some(return_annotation) = returns.as_ref() {
                                write!(f, [space(), token("->"), space()])?;

                                if return_annotation.is_tuple_expr() {
                                    let parentheses =
                                        if comments.has_leading(return_annotation.as_ref()) {
                                            Parentheses::Always
                                        } else {
                                            Parentheses::Never
                                        };
                                    write!(
                                        f,
                                        [return_annotation.format().with_options(parentheses)]
                                    )?;
                                } else if comments.has_trailing(return_annotation.as_ref()) {
                                    // Intentionally parenthesize any return annotations with trailing comments.
                                    // This avoids an instability in cases like:
                                    // ```python
                                    // def double(
                                    //     a: int
                                    // ) -> (
                                    //     int  # Hello
                                    // ):
                                    //     pass
                                    // ```
                                    // If we allow this to break, it will be formatted as follows:
                                    // ```python
                                    // def double(
                                    //     a: int
                                    // ) -> int:  # Hello
                                    //     pass
                                    // ```
                                    // On subsequent formats, the `# Hello` will be interpreted as a dangling
                                    // comment on a function, yielding:
                                    // ```python
                                    // def double(a: int) -> int:  # Hello
                                    //     pass
                                    // ```
                                    // Ideally, we'd reach that final formatting in a single pass, but doing so
                                    // requires that the parent be aware of how the child is formatted, which
                                    // is challenging. As a compromise, we break those expressions to avoid an
                                    // instability.
                                    write!(
                                        f,
                                        [return_annotation
                                            .format()
                                            .with_options(Parentheses::Always)]
                                    )?;
                                } else {
                                    write!(
                                        f,
                                        [maybe_parenthesize_expression(
                                            return_annotation,
                                            item,
                                            if empty_parameters(parameters, f.context().source()) {
                                                // If the parameters are empty, add parentheses if the return annotation
                                                // breaks at all.
                                                Parenthesize::IfBreaksOrIfRequired
                                            } else {
                                                // Otherwise, use our normal rules for parentheses, which allows us to break
                                                // like:
                                                // ```python
                                                // def f(
                                                //     x,
                                                // ) -> Tuple[
                                                //     int,
                                                //     int,
                                                // ]:
                                                //     ...
                                                // ```
                                                Parenthesize::IfBreaks
                                            },
                                        )]
                                    )?;
                                }
                            }
                            Ok(())
                        });

                        group(&format_inner).fmt(f)
                    }),
                ),
                clause_body(body, trailing_definition_comments).with_kind(SuiteKind::Function),
            ]
        )?;

        // If the function contains trailing comments, insert newlines before them.
        // For example, given:
        // ```python
        // def func():
        //     ...
        // # comment
        // ```
        //
        // At the top-level in a non-stub file, reformat as:
        // ```python
        // def func():
        //     ...
        //
        //
        // # comment
        // ```
        empty_lines_before_trailing_comments(f, comments.trailing(item)).fmt(f)
    }

    fn fmt_dangling_comments(
        &self,
        _dangling_comments: &[SourceComment],
        _f: &mut PyFormatter,
    ) -> FormatResult<()> {
        // Handled in `fmt_fields`
        Ok(())
    }
}

/// Returns `true` if [`Parameters`] is empty (no parameters, no comments, etc.).
fn empty_parameters(parameters: &Parameters, source: &str) -> bool {
    let mut tokenizer = SimpleTokenizer::new(source, parameters.range())
        .filter(|token| !matches!(token.kind, SimpleTokenKind::Whitespace));

    let Some(lpar) = tokenizer.next() else {
        return false;
    };
    if !matches!(lpar.kind, SimpleTokenKind::LParen) {
        return false;
    }

    let Some(rpar) = tokenizer.next() else {
        return false;
    };
    if !matches!(rpar.kind, SimpleTokenKind::RParen) {
        return false;
    }

    true
}