use std::{
    borrow::Cow,
    collections::hash_map::Entry,
    fmt::{Formatter, LowerHex, Write},
    hash::Hash,
};

use anyhow::bail;
use ruff_db::system::{SystemPath, SystemPathBuf};
use rustc_hash::FxHashMap;

use crate::config::MarkdownTestConfig;
use ruff_index::{IndexVec, newtype_index};
use ruff_python_ast::PySourceType;
use ruff_python_trivia::Cursor;
use ruff_source_file::{LineIndex, LineRanges, OneIndexed};
use ruff_text_size::{TextLen, TextRange, TextSize};
use rustc_stable_hash::{FromStableHash, SipHasher128Hash, StableSipHasher128};

/// Parse the Markdown `source` as a test suite with given `title`.
pub(crate) fn parse<'s>(title: &'s str, source: &'s str) -> anyhow::Result<MarkdownTestSuite<'s>> {
    let parser = Parser::new(title, source);
    parser.parse()
}

/// A parsed markdown file containing tests.
///
/// Borrows from the source string and filepath it was created from.
#[derive(Debug)]
pub(crate) struct MarkdownTestSuite<'s> {
    /// Header sections.
    sections: IndexVec<SectionId, Section<'s>>,

    /// Test files embedded within the Markdown file.
    files: IndexVec<EmbeddedFileId, EmbeddedFile<'s>>,
}

impl<'s> MarkdownTestSuite<'s> {
    pub(crate) fn tests(&self) -> MarkdownTestIterator<'_, 's> {
        MarkdownTestIterator {
            suite: self,
            current_file_index: 0,
        }
    }
}

struct Hash128([u64; 2]);

impl FromStableHash for Hash128 {
    type Hash = SipHasher128Hash;

    fn from(SipHasher128Hash(hash): SipHasher128Hash) -> Hash128 {
        Hash128(hash)
    }
}

impl LowerHex for Hash128 {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        let Self(hash) = self;

        // Only write the first half for concision
        write!(f, "{:x}", hash[0])
    }
}

/// A single test inside a [`MarkdownTestSuite`].
///
/// A test is a single header section (or the implicit root section, if there are no Markdown
/// headers in the file), containing one or more embedded Python files as fenced code blocks, and
/// containing no nested header subsections.
#[derive(Debug)]
pub(crate) struct MarkdownTest<'m, 's> {
    suite: &'m MarkdownTestSuite<'s>,
    section: &'m Section<'s>,
    files: &'m [EmbeddedFile<'s>],
}

impl<'m, 's> MarkdownTest<'m, 's> {
    const MAX_TITLE_LENGTH: usize = 20;
    const ELLIPSIS: char = '\u{2026}';

    fn contracted_title(title: &str) -> String {
        if title.len() <= Self::MAX_TITLE_LENGTH {
            return (*title).to_string();
        }

        format!(
            "{}{}",
            title
                .chars()
                .take(Self::MAX_TITLE_LENGTH)
                .collect::<String>(),
            Self::ELLIPSIS
        )
    }

    fn joined_name(&self, contracted: bool) -> String {
        let mut name_fragments = vec![];
        let mut parent_id = self.section.parent_id;

        while let Some(next_id) = parent_id {
            let parent = &self.suite.sections[next_id];
            name_fragments.insert(0, parent.title);
            parent_id = parent.parent_id;
        }

        name_fragments.push(self.section.title);

        let full_name = name_fragments.join(" - ");

        if !contracted {
            return full_name;
        }

        let mut contracted_name = name_fragments
            .iter()
            .map(|fragment| Self::contracted_title(fragment))
            .collect::<Vec<_>>()
            .join(" - ");

        let mut hasher = StableSipHasher128::new();
        full_name.hash(&mut hasher);
        let _ = write!(contracted_name, " ({:x})", hasher.finish::<Hash128>());

        contracted_name
    }

    pub(crate) fn uncontracted_name(&self) -> String {
        self.joined_name(false)
    }

    pub(crate) fn name(&self) -> String {
        self.joined_name(true)
    }

    pub(crate) fn files(&self) -> impl Iterator<Item = &'m EmbeddedFile<'s>> {
        self.files.iter()
    }

    pub(crate) fn configuration(&self) -> &MarkdownTestConfig {
        &self.section.config
    }

    pub(super) fn should_snapshot_diagnostics(&self) -> bool {
        self.section
            .directives
            .has_directive_set(MdtestDirective::SnapshotDiagnostics)
    }

    pub(super) fn should_expect_panic(&self) -> Result<Option<&str>, ()> {
        self.section.directives.get(MdtestDirective::ExpectPanic)
    }

    pub(super) fn should_skip_pulling_types(&self) -> bool {
        self.section
            .directives
            .has_directive_set(MdtestDirective::PullTypesSkip)
    }
}

/// Iterator yielding all [`MarkdownTest`]s in a [`MarkdownTestSuite`].
#[derive(Debug)]
pub(crate) struct MarkdownTestIterator<'m, 's> {
    suite: &'m MarkdownTestSuite<'s>,
    current_file_index: usize,
}

impl<'m, 's> Iterator for MarkdownTestIterator<'m, 's> {
    type Item = MarkdownTest<'m, 's>;

    fn next(&mut self) -> Option<Self::Item> {
        let mut current_file_index = self.current_file_index;
        let mut file = self.suite.files.get(current_file_index.into());
        let section_id = file?.section;
        while file.is_some_and(|file| file.section == section_id) {
            current_file_index += 1;
            file = self.suite.files.get(current_file_index.into());
        }
        let files = &self.suite.files[EmbeddedFileId::from_usize(self.current_file_index)
            ..EmbeddedFileId::from_usize(current_file_index)];
        self.current_file_index = current_file_index;
        Some(MarkdownTest {
            suite: self.suite,
            section: &self.suite.sections[section_id],
            files,
        })
    }
}

#[newtype_index]
struct SectionId;

/// A single header section of a [`MarkdownTestSuite`], or the implicit root "section".
///
/// A header section is the part of a Markdown file beginning with a `#`-prefixed header line, and
/// extending until the next header line at the same or higher outline level (that is, with the
/// same number or fewer `#` characters).
///
/// A header section may either contain one or more embedded Python files (making it a
/// [`MarkdownTest`]), or it may contain nested sections (headers with more `#` characters), but
/// not both.
#[derive(Debug)]
struct Section<'s> {
    title: &'s str,
    level: u8,
    parent_id: Option<SectionId>,
    config: MarkdownTestConfig,
    directives: MdtestDirectives,
}

#[newtype_index]
struct EmbeddedFileId;

/// Holds information about the start and the end of a code block in a Markdown file.
///
/// The start is the offset of the first triple-backtick in the code block, and the end is the
/// offset of the (start of the) closing triple-backtick.
#[derive(Debug, Clone)]
pub(crate) struct BacktickOffsets(TextSize, TextSize);

/// Holds information about the position and length of all code blocks that are part of
/// a single embedded file in a Markdown file. This is used to reconstruct absolute line
/// numbers (in the Markdown file) from relative line numbers (in the embedded file).
///
/// If we have a Markdown section with multiple code blocks like this:
///
///    01   # Test
///    02
///    03   Part 1:
///    04
///    05   ```py
///    06   a = 1    # Relative line number: 1
///    07   b = 2    # Relative line number: 2
///    08   ```
///    09
///    10   Part 2:
///    11
///    12   ```py
///    13   c = 3    # Relative line number: 3
///    14   ```
///
/// We want to reconstruct the absolute line number (left) from relative
/// line numbers. The information we have is the start line and the line
/// count of each code block:
///
///    - Block 1: (start =  5, count = 2)
///    - Block 2: (start = 12, count = 1)
///
/// For example, if we see a relative line number of 3, we see that it is
/// larger than the line count of the first block, so we subtract the line
/// count of the first block, and then add the new relative line number (1)
/// to the absolute start line of the second block (12), resulting in an
/// absolute line number of 13.
pub(crate) struct EmbeddedFileSourceMap {
    start_line_and_line_count: Vec<(usize, usize)>,
}

impl EmbeddedFileSourceMap {
    pub(crate) fn new(
        md_index: &LineIndex,
        dimensions: impl IntoIterator<Item = BacktickOffsets>,
    ) -> EmbeddedFileSourceMap {
        EmbeddedFileSourceMap {
            start_line_and_line_count: dimensions
                .into_iter()
                .map(|d| {
                    let start_line = md_index.line_index(d.0).get();
                    let end_line = md_index.line_index(d.1).get();
                    let code_line_count = (end_line - start_line) - 1;
                    (start_line, code_line_count)
                })
                .collect(),
        }
    }

    /// Returns the absolute line number in the markdown file for a given line number
    /// relative to the concatenated code blocks.
    ///
    /// Returns an `Err` if the relative line number is out of bounds where
    /// the returned value is the absolute line number of the last code block.
    ///
    /// # Panics
    ///  If called when the markdown file has no code blocks.
    pub(crate) fn to_absolute_line_number(
        &self,
        relative_line_number: OneIndexed,
    ) -> std::result::Result<OneIndexed, OneIndexed> {
        let mut relative_line_number = relative_line_number.get();

        for (start_line, line_count) in &self.start_line_and_line_count {
            if relative_line_number > *line_count {
                relative_line_number -= *line_count;
            } else {
                let absolute_line_number = start_line + relative_line_number;
                return Ok(OneIndexed::new(absolute_line_number)
                    .expect("absolute line number must be >= 1"));
            }
        }

        let last_line_number = self
            .start_line_and_line_count
            .last()
            .and_then(|(start_line, line_count)| OneIndexed::new(start_line + line_count));

        Err(last_line_number.expect("markdown file to have at least one code block"))
    }
}

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub(crate) enum EmbeddedFilePath<'s> {
    Autogenerated(PySourceType),
    Explicit(&'s str),
}

impl EmbeddedFilePath<'_> {
    pub(crate) fn as_str(&self) -> &str {
        match self {
            EmbeddedFilePath::Autogenerated(PySourceType::Python) => "mdtest_snippet.py",
            EmbeddedFilePath::Autogenerated(PySourceType::Stub) => "mdtest_snippet.pyi",
            EmbeddedFilePath::Autogenerated(PySourceType::Ipynb) => "mdtest_snippet.ipynb",
            EmbeddedFilePath::Explicit(path) => path,
        }
    }

    fn is_explicit(&self) -> bool {
        matches!(self, EmbeddedFilePath::Explicit(_))
    }

    fn is_allowed_explicit_path(path: &str) -> bool {
        [PySourceType::Python, PySourceType::Stub]
            .iter()
            .all(|source_type| path != EmbeddedFilePath::Autogenerated(*source_type).as_str())
    }
}

/// A single file embedded in a [`Section`] as a fenced code block.
///
/// Currently must be a Python file (`py` language), a type stub (`pyi`) or a [typeshed `VERSIONS`]
/// file.
///
/// TOML configuration blocks are also supported, but are not stored as `EmbeddedFile`s. In the
/// future we plan to support `pth` files as well.
///
/// A Python embedded file makes its containing [`Section`] into a [`MarkdownTest`], and will be
/// type-checked and searched for inline-comment assertions to match against the diagnostics from
/// type checking.
///
/// [typeshed `VERSIONS`]: https://github.com/python/typeshed/blob/c546278aae47de0b2b664973da4edb613400f6ce/stdlib/VERSIONS#L1-L18
#[derive(Debug)]
pub(crate) struct EmbeddedFile<'s> {
    section: SectionId,
    path: EmbeddedFilePath<'s>,
    pub(crate) lang: &'s str,
    pub(crate) code: Cow<'s, str>,
    pub(crate) backtick_offsets: Vec<BacktickOffsets>,
}

impl EmbeddedFile<'_> {
    fn append_code(&mut self, backtick_offsets: BacktickOffsets, new_code: &str) {
        // Treat empty code blocks as non-existent, instead of creating
        // an additional empty line:
        if new_code.is_empty() {
            return;
        }

        self.backtick_offsets.push(backtick_offsets);

        let existing_code = self.code.to_mut();
        existing_code.push('\n');
        existing_code.push_str(new_code);
    }

    pub(crate) fn relative_path(&self) -> &str {
        self.path.as_str()
    }

    /// Returns the full path using unix file-path convention.
    pub(crate) fn full_path(&self, project_root: &SystemPath) -> SystemPathBuf {
        // Don't use `SystemPath::absolute` here because it's platform dependent
        // and we want to use unix file-path convention.
        let relative_path = self.relative_path();
        if relative_path.starts_with('/') {
            SystemPathBuf::from(relative_path)
        } else {
            project_root.join(relative_path)
        }
    }
}

#[derive(Debug)]
struct SectionStack(Vec<SectionId>);

impl SectionStack {
    fn new(root_section_id: SectionId) -> Self {
        Self(vec![root_section_id])
    }

    fn push(&mut self, section_id: SectionId) {
        self.0.push(section_id);
    }

    fn pop(&mut self) -> Option<SectionId> {
        let popped = self.0.pop();
        debug_assert_ne!(popped, None, "Should never pop the implicit root section");
        debug_assert!(
            !self.0.is_empty(),
            "Should never pop the implicit root section"
        );
        popped
    }

    fn top(&mut self) -> SectionId {
        *self
            .0
            .last()
            .expect("Should never pop the implicit root section")
    }
}

/// Parse the source of a Markdown file into a [`MarkdownTestSuite`].
#[derive(Debug)]
struct Parser<'s> {
    /// [`Section`]s of the final [`MarkdownTestSuite`].
    sections: IndexVec<SectionId, Section<'s>>,

    /// [`EmbeddedFile`]s of the final [`MarkdownTestSuite`].
    files: IndexVec<EmbeddedFileId, EmbeddedFile<'s>>,

    /// The unparsed remainder of the Markdown source.
    cursor: Cursor<'s>,

    // Number of consecutive empty lines.
    preceding_blank_lines: usize,

    // Explicitly specified path for the upcoming code block.
    explicit_path: Option<&'s str>,

    source: &'s str,

    /// Stack of ancestor sections.
    stack: SectionStack,

    /// Names of embedded files in current active section.
    current_section_files: FxHashMap<EmbeddedFilePath<'s>, EmbeddedFileId>,

    /// Whether or not the current section has a config block.
    current_section_has_config: bool,

    /// Whether or not any section in the file has external dependencies.
    /// Only one section per file is allowed to have dependencies (for lockfile support).
    file_has_dependencies: bool,
}

impl<'s> Parser<'s> {
    fn new(title: &'s str, source: &'s str) -> Self {
        let mut sections = IndexVec::default();
        let root_section_id = sections.push(Section {
            title,
            level: 0,
            parent_id: None,
            config: MarkdownTestConfig::default(),
            directives: MdtestDirectives::default(),
        });
        Self {
            sections,
            source,
            files: IndexVec::default(),
            cursor: Cursor::new(source),
            preceding_blank_lines: 0,
            explicit_path: None,
            stack: SectionStack::new(root_section_id),
            current_section_files: FxHashMap::default(),
            current_section_has_config: false,
            file_has_dependencies: false,
        }
    }

    fn parse(mut self) -> anyhow::Result<MarkdownTestSuite<'s>> {
        self.parse_impl()?;
        Ok(self.finish())
    }

    fn finish(mut self) -> MarkdownTestSuite<'s> {
        self.sections.shrink_to_fit();
        self.files.shrink_to_fit();

        MarkdownTestSuite {
            sections: self.sections,
            files: self.files,
        }
    }

    fn skip_non_newline_whitespace(&mut self) {
        self.cursor.eat_while(|c| c.is_whitespace() && c != '\n');
    }

    fn skip_to_beginning_of_next_line(&mut self) -> bool {
        if let Some(position) = memchr::memchr(b'\n', self.cursor.as_bytes()) {
            self.cursor.skip_bytes(position + 1);
            true
        } else {
            false
        }
    }

    fn consume_until(&mut self, mut end_predicate: impl FnMut(char) -> bool) -> Option<&'s str> {
        let start = self.cursor.offset().to_usize();

        while !self.cursor.is_eof() {
            if end_predicate(self.cursor.first()) {
                return Some(&self.source[start..self.cursor.offset().to_usize()]);
            }
            self.cursor.bump();
        }

        None
    }

    fn parse_impl(&mut self) -> anyhow::Result<()> {
        const SECTION_CONFIG_SNAPSHOT: &str = "snapshot-diagnostics";
        const SECTION_CONFIG_PULLTYPES: &str = "pull-types:skip";
        const SECTION_CONFIG_EXPECT_PANIC: &str = "expect-panic";
        const HTML_COMMENT_ALLOWLIST: &[&str] = &["blacken-docs:on", "blacken-docs:off"];
        const CODE_BLOCK_END: &[u8] = b"```";
        const HTML_COMMENT_END: &[u8] = b"-->";

        while let Some(first) = self.cursor.bump() {
            match first {
                '<' if self.cursor.eat_char3('!', '-', '-') => {
                    if let Some(position) =
                        memchr::memmem::find(self.cursor.as_bytes(), HTML_COMMENT_END)
                    {
                        let html_comment = self.cursor.as_str()[..position].trim();
                        let (directive, value) = match html_comment.split_once(':') {
                            Some((directive, value)) => {
                                (directive.trim(), Some(value.trim().to_string()))
                            }
                            None => (html_comment, None),
                        };

                        match directive {
                            SECTION_CONFIG_SNAPSHOT => {
                                anyhow::ensure!(
                                    value.is_none(),
                                    "The `{SECTION_CONFIG_SNAPSHOT}` directive does not take a value."
                                );
                                self.process_mdtest_directive(
                                    MdtestDirective::SnapshotDiagnostics,
                                    None,
                                )?;
                            }
                            SECTION_CONFIG_PULLTYPES => {
                                anyhow::ensure!(
                                    value.is_none(),
                                    "The `{SECTION_CONFIG_PULLTYPES}` directive does not take a value."
                                );
                                self.process_mdtest_directive(
                                    MdtestDirective::PullTypesSkip,
                                    None,
                                )?;
                            }
                            SECTION_CONFIG_EXPECT_PANIC => {
                                self.process_mdtest_directive(MdtestDirective::ExpectPanic, value)?;
                            }
                            _ => {
                                if !HTML_COMMENT_ALLOWLIST.contains(&html_comment) {
                                    bail!(
                                        "Unknown HTML comment `{html_comment}` -- possibly a typo? \
                                (Add to `HTML_COMMENT_ALLOWLIST` if this is a false positive)"
                                    );
                                }
                            }
                        }

                        self.cursor.skip_bytes(position + HTML_COMMENT_END.len());
                    } else {
                        bail!("Unterminated HTML comment.");
                    }
                }

                '#' => {
                    self.explicit_path = None;
                    self.preceding_blank_lines = 0;

                    // Determine header level (number of '#' characters)
                    let mut header_level = 1;
                    while self.cursor.eat_char('#') {
                        header_level += 1;
                    }

                    // Parse header title
                    if let Some(title) = self.consume_until(|c| c == '\n') {
                        let title = title.trim();

                        if !title.is_empty() {
                            self.process_header(header_level, title)?;
                        }
                    }
                }
                '`' => {
                    if self.cursor.eat_char2('`', '`') {
                        // We saw the triple-backtick beginning of a code block.

                        let backtick_offset_start = self.cursor.offset() - "```".text_len();

                        if self.preceding_blank_lines < 1 && self.explicit_path.is_none() {
                            bail!(
                                "Code blocks must start on a new line and be preceded by at least one blank line."
                            );
                        }

                        self.skip_non_newline_whitespace();

                        // Parse the code block language specifier
                        let lang = self
                            .consume_until(|c| matches!(c, ' ' | '\n'))
                            .unwrap_or_default();

                        self.skip_non_newline_whitespace();

                        if !self.cursor.eat_char('\n') {
                            bail!(
                                "Trailing code-block metadata is not supported. Only the code block language can be specified."
                            );
                        }

                        if let Some(position) =
                            memchr::memmem::find(self.cursor.as_bytes(), CODE_BLOCK_END)
                        {
                            let mut code = &self.cursor.as_str()[..position];
                            self.cursor.skip_bytes(position + CODE_BLOCK_END.len());

                            if code.ends_with('\n') {
                                code = &code[..code.len() - '\n'.len_utf8()];
                            }

                            let backtick_offset_end = self.cursor.offset() - "```".text_len();

                            self.process_code_block(
                                lang,
                                code,
                                BacktickOffsets(backtick_offset_start, backtick_offset_end),
                            )?;
                        } else {
                            let code_block_start = self.cursor.token_len();
                            let line = self.source.count_lines(TextRange::up_to(code_block_start));
                            bail!("Unterminated code block at line {line}.");
                        }

                        self.explicit_path = None;
                    } else if self.preceding_blank_lines > 0 {
                        // This could be a line that specifies an explicit path for a Markdown code block (`module.py`:)
                        self.explicit_path = None;

                        if let Some(path) = self.consume_until(|c| matches!(c, '`' | '\n')) {
                            if self.cursor.eat_char('`') {
                                self.skip_non_newline_whitespace();
                                if self.cursor.eat_char(':') {
                                    self.explicit_path = Some(path);
                                }
                            }
                        }
                    }

                    self.preceding_blank_lines = 0;
                }
                '\n' => {
                    self.preceding_blank_lines += 1;
                    continue;
                }
                c => {
                    self.preceding_blank_lines = 0;
                    self.explicit_path = None;

                    if c.is_whitespace() {
                        self.skip_non_newline_whitespace();
                        if self.cursor.eat_char('`')
                            && self.cursor.eat_char('`')
                            && self.cursor.eat_char('`')
                        {
                            bail!("Indented code blocks are not supported.");
                        }
                    }
                }
            }

            if !self.skip_to_beginning_of_next_line() {
                break;
            }
        }

        Ok(())
    }

    fn process_header(&mut self, header_level: usize, title: &'s str) -> anyhow::Result<()> {
        self.pop_sections_to_level(header_level);

        let parent = self.stack.top();

        let section = Section {
            title,
            level: header_level.try_into()?,
            parent_id: Some(parent),
            config: self.sections[parent].config.clone(),
            directives: self.sections[parent].directives.clone(),
        };

        if !self.current_section_files.is_empty() {
            bail!(
                "Header '{}' not valid inside a test case; parent '{}' has code files.",
                section.title,
                self.sections[parent].title,
            );
        }

        let section_id = self.sections.push(section);
        self.stack.push(section_id);

        self.current_section_files.clear();
        self.current_section_has_config = false;

        Ok(())
    }

    fn process_code_block(
        &mut self,
        lang: &'s str,
        code: &'s str,
        backtick_offsets: BacktickOffsets,
    ) -> anyhow::Result<()> {
        // We never pop the implicit root section.
        let section = self.stack.top();
        let test_name = self.sections[section].title;

        if lang == "toml" {
            return self.process_config_block(code);
        }

        if lang == "ignore" {
            return Ok(());
        }

        if let Some(explicit_path) = self.explicit_path {
            let expected_extension = if lang == "python" { "py" } else { lang };

            if !expected_extension.is_empty()
                && lang != "text"
                && !SystemPath::new(explicit_path)
                    .extension()
                    .is_none_or(|extension| extension.eq_ignore_ascii_case(expected_extension))
            {
                let backtick_start = self.line_index(backtick_offsets.0);
                bail!(
                    "File extension of test file path `{explicit_path}` in test `{test_name}` does not match language specified `{lang}` of code block at line `{backtick_start}`"
                );
            }
        }

        let path = match self.explicit_path {
            Some(path) => {
                if !EmbeddedFilePath::is_allowed_explicit_path(path) {
                    bail!(
                        "The file name `{path}` in test `{test_name}` must not be used explicitly.",
                    );
                }

                EmbeddedFilePath::Explicit(path)
            }
            None => match lang {
                "py" | "python" => EmbeddedFilePath::Autogenerated(PySourceType::Python),
                "pyi" => EmbeddedFilePath::Autogenerated(PySourceType::Stub),
                "" => {
                    bail!(
                        "Cannot auto-generate file name for code block with empty language specifier in test `{test_name}`"
                    );
                }
                _ => {
                    bail!(
                        "Cannot auto-generate file name for code block with language `{lang}` in test `{test_name}`"
                    );
                }
            },
        };

        let has_merged_snippets = self.current_section_has_merged_snippets();
        let has_explicit_file_paths = self.current_section_has_explicit_file_paths();

        match self.current_section_files.entry(path.clone()) {
            Entry::Vacant(entry) => {
                if has_merged_snippets {
                    bail!(
                        "Merged snippets in test `{test_name}` are not allowed in the presence of other files."
                    );
                }

                let index = self.files.push(EmbeddedFile {
                    path: path.clone(),
                    section,
                    lang,
                    code: Cow::Borrowed(code),
                    backtick_offsets: vec![backtick_offsets],
                });
                entry.insert(index);
            }
            Entry::Occupied(entry) => {
                if path.is_explicit() {
                    bail!(
                        "Test `{test_name}` has duplicate files named `{}`.",
                        path.as_str(),
                    );
                }

                if has_explicit_file_paths {
                    bail!(
                        "Merged snippets in test `{test_name}` are not allowed in the presence of other files."
                    );
                }

                let index = *entry.get();
                self.files[index].append_code(backtick_offsets, code);
            }
        }

        Ok(())
    }

    fn current_section_has_explicit_file_paths(&self) -> bool {
        self.current_section_files
            .iter()
            .any(|(path, _)| path.is_explicit())
    }

    fn current_section_has_merged_snippets(&self) -> bool {
        self.current_section_files
            .values()
            .any(|id| self.files[*id].backtick_offsets.len() > 1)
    }

    fn process_config_block(&mut self, code: &str) -> anyhow::Result<()> {
        if self.current_section_has_config {
            bail!("Multiple TOML configuration blocks in the same section are not allowed.");
        }

        let config = MarkdownTestConfig::from_str(code)?;

        if config.dependencies().is_some() {
            if self.file_has_dependencies {
                bail!(
                    "Multiple sections with `[project]` dependencies in the same file are not allowed. \
                     External dependencies must be specified in a single top-level configuration block."
                );
            }
            self.file_has_dependencies = true;
        }

        let current_section = &mut self.sections[self.stack.top()];
        current_section.config = config;

        self.current_section_has_config = true;

        Ok(())
    }

    fn process_mdtest_directive(
        &mut self,
        directive: MdtestDirective,
        value: Option<String>,
    ) -> anyhow::Result<()> {
        if self.current_section_has_config {
            bail!(
                "Section config to enable {directive} must come before \
                 everything else (including TOML configuration blocks).",
            );
        }
        if !self.current_section_files.is_empty() {
            bail!(
                "Section config to enable {directive} must come before \
                 everything else (including embedded files).",
            );
        }

        let current_section = &mut self.sections[self.stack.top()];
        if current_section.directives.has_directive_set(directive) {
            bail!(
                "Section config to enable {directive} should appear \
                 at most once.",
            );
        }
        current_section.directives.add_directive(directive, value);

        Ok(())
    }

    fn pop_sections_to_level(&mut self, level: usize) {
        while level <= self.sections[self.stack.top()].level.into() {
            self.stack.pop();
            // We would have errored before pushing a child section if there were files, so we know
            // no parent section can have files.
            self.current_section_files.clear();
        }
    }

    fn line_index(&self, char_index: TextSize) -> u32 {
        self.source.count_lines(TextRange::up_to(char_index))
    }
}

#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
enum MdtestDirective {
    /// A directive to enable snapshotting diagnostics.
    SnapshotDiagnostics,
    /// A directive to skip pull types.
    PullTypesSkip,

    ExpectPanic,
}

impl std::fmt::Display for MdtestDirective {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        match self {
            MdtestDirective::SnapshotDiagnostics => f.write_str("snapshotting diagnostics"),
            MdtestDirective::PullTypesSkip => f.write_str("skipping the pull-types visitor"),
            MdtestDirective::ExpectPanic => f.write_str("expect test to panic"),
        }
    }
}

/// The directives applied to a Markdown test section.
#[derive(Default, Debug, Clone, PartialEq, Eq)]
pub(crate) struct MdtestDirectives {
    directives: FxHashMap<MdtestDirective, Option<String>>,
}

impl MdtestDirectives {
    fn has_directive_set(&self, directive: MdtestDirective) -> bool {
        self.directives.contains_key(&directive)
    }

    fn get(&self, directive: MdtestDirective) -> Result<Option<&str>, ()> {
        self.directives
            .get(&directive)
            .map(|s| s.as_deref())
            .ok_or(())
    }

    fn add_directive(&mut self, directive: MdtestDirective, value: Option<String>) {
        self.directives.insert(directive, value);
    }
}

#[cfg(test)]
mod tests {
    use ruff_python_ast::PySourceType;
    use ruff_python_trivia::textwrap::dedent;
    use ruff_source_file::OneIndexed;

    use insta::assert_snapshot;

    use crate::parser::EmbeddedFilePath;

    #[test]
    fn empty() {
        let mf = super::parse("file.md", "").unwrap();

        assert!(mf.tests().next().is_none());
    }

    #[test]
    fn source_map_to_absolute_line_number() {
        let map = super::EmbeddedFileSourceMap {
            start_line_and_line_count: vec![(10, 5), (25, 3)],
        };

        let absolute = map
            .to_absolute_line_number(OneIndexed::new(6).unwrap())
            .unwrap();
        assert_eq!(absolute.get(), 26);
    }

    #[test]
    fn source_map_reports_invalid_relative_line() {
        let map = super::EmbeddedFileSourceMap {
            start_line_and_line_count: vec![(9, 2)],
        };

        let error = map
            .to_absolute_line_number(OneIndexed::new(3).unwrap())
            .unwrap_err();

        assert_eq!(error.get(), 11);
    }

    #[test]
    fn single_file_test() {
        let source = dedent(
            "
            ```py
            x = 1
            ```
            ",
        );
        let mf = super::parse("file.md", &source).unwrap();

        let [test] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected one test");
        };

        assert_snapshot!(test.name(), @"file.md (a8decfe8bd23e259)");

        let [file] = test.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(
            file.path,
            EmbeddedFilePath::Autogenerated(PySourceType::Python)
        );
        assert_eq!(file.lang, "py");
        assert_eq!(file.code, "x = 1");
    }

    #[test]
    fn no_new_line_at_eof() {
        let source = dedent(
            "
            ```py
            x = 1
            ```",
        );
        let mf = super::parse("file.md", &source).unwrap();

        let [test] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected one test");
        };

        assert_snapshot!(test.name(), @"file.md (a8decfe8bd23e259)");

        let [file] = test.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(
            file.path,
            EmbeddedFilePath::Autogenerated(PySourceType::Python)
        );
        assert_eq!(file.lang, "py");
        assert_eq!(file.code, "x = 1");
    }

    #[test]
    fn multiple_tests() {
        let source = dedent(
            "
            # One

            ```py
            x = 1
            ```

            # Two

            ```py
            y = 2
            ```

            # Three

            `mod_a.pyi`:

            ```pyi
            a: int
            ```

            `mod_b.pyi`:

            ```pyi
            b: str
            ```
            ",
        );
        let mf = super::parse("file.md", &source).unwrap();

        let [test1, test2, test3] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected three tests");
        };

        assert_snapshot!(test1.name(), @"file.md - One (9f620a533a21278)");
        assert_snapshot!(test2.name(), @"file.md - Two (1b4d4ef5a2cebbdc)");
        assert_snapshot!(test3.name(), @"file.md - Three (26479e23633dda57)");

        let [file] = test1.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(
            file.path,
            EmbeddedFilePath::Autogenerated(PySourceType::Python)
        );
        assert_eq!(file.lang, "py");
        assert_eq!(file.code, "x = 1");

        let [file] = test2.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(
            file.path,
            EmbeddedFilePath::Autogenerated(PySourceType::Python)
        );
        assert_eq!(file.lang, "py");
        assert_eq!(file.code, "y = 2");

        let [file_1, file_2] = test3.files().collect::<Vec<_>>()[..] else {
            panic!("expected two files");
        };

        assert_eq!(file_1.relative_path(), "mod_a.pyi");
        assert_eq!(file_1.lang, "pyi");
        assert_eq!(file_1.code, "a: int");

        assert_eq!(file_2.relative_path(), "mod_b.pyi");
        assert_eq!(file_2.lang, "pyi");
        assert_eq!(file_2.code, "b: str");
    }

    #[test]
    fn multiple_file_tests() {
        let source = dedent(
            "
            # One

            `main.py`:

            ```py
            from foo import y
            ```

            `foo.py`:

            ```py
            y = 2
            ```

            # Two

            ```py
            y = 2
            ```
            ",
        );
        let mf = super::parse("file.md", &source).unwrap();

        let [test1, test2] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected two tests");
        };

        assert_snapshot!(test1.name(), @"file.md - One (9f620a533a21278)");
        assert_snapshot!(test2.name(), @"file.md - Two (1b4d4ef5a2cebbdc)");

        let [main, foo] = test1.files().collect::<Vec<_>>()[..] else {
            panic!("expected two files");
        };

        assert_eq!(main.relative_path(), "main.py");
        assert_eq!(main.lang, "py");
        assert_eq!(main.code, "from foo import y");

        assert_eq!(foo.relative_path(), "foo.py");
        assert_eq!(foo.lang, "py");
        assert_eq!(foo.code, "y = 2");

        let [file] = test2.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(
            file.path,
            EmbeddedFilePath::Autogenerated(PySourceType::Python)
        );
        assert_eq!(file.lang, "py");
        assert_eq!(file.code, "y = 2");
    }

    #[test]
    fn merged_snippets() {
        let source = dedent(
            "
            # One

            This is the first part of the embedded file:

            ```py
            x = 1
            ```

            And this is the second part:

            ```py
            y = 2
            ```

            And this is the third part:

            ```py
            z = 3
            ```
            ",
        );
        let mf = super::parse("file.md", &source).unwrap();

        let [test] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected one test");
        };

        let [file] = test.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(
            file.path,
            EmbeddedFilePath::Autogenerated(PySourceType::Python)
        );
        assert_eq!(file.lang, "py");
        assert_eq!(file.code, "x = 1\ny = 2\nz = 3");
    }

    #[test]
    fn no_merged_snippets_for_explicit_paths() {
        let source = dedent(
            "
            # One

            `foo.py`:

            ```py
            x = 1
            ```

            `foo.py`:

            ```py
            y = 2
            ```
            ",
        );
        let err = super::parse("file.md", &source).expect_err("Should fail to parse");
        assert_eq!(
            err.to_string(),
            "Test `One` has duplicate files named `foo.py`."
        );
    }

    #[test]
    fn disallow_merged_snippets_in_presence_of_explicit_paths() {
        for source in [
            // Merged snippets first
            "
            # One

            ```py
            x = 1
            ```

            ```py
            y = 2
            ```

            `foo.py`:

            ```py
            print('hello')
            ```
            ",
            // Explicit path first
            "
            # One

            `foo.py`:

            ```py
            print('hello')
            ```

            ```py
            x = 1
            ```

            ```py
            y = 2
            ```
            ",
        ] {
            let err = super::parse("file.md", &dedent(source)).expect_err("Should fail to parse");
            assert_eq!(
                err.to_string(),
                "Merged snippets in test `One` are not allowed in the presence of other files."
            );
        }
    }

    #[test]
    fn disallow_pyi_snippets_in_presence_of_merged_py_snippets() {
        let source = dedent(
            "
            # One

            ```py
            x = 1
            ```

            ```py
            y = 2
            ```

            ```pyi
            x: int
            ```
            ",
        );
        let err = super::parse("file.md", &source).expect_err("Should fail to parse");
        assert_eq!(
            err.to_string(),
            "Merged snippets in test `One` are not allowed in the presence of other files."
        );
    }

    #[test]
    fn custom_file_path() {
        let source = dedent(
            "
            `foo.py`:

            ```py
            x = 1
            ```
            ",
        );
        let mf = super::parse("file.md", &source).unwrap();

        let [test] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected one test");
        };
        let [file] = test.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(file.relative_path(), "foo.py");
        assert_eq!(file.lang, "py");
        assert_eq!(file.code, "x = 1");
    }

    #[test]
    fn multi_line_file() {
        let source = dedent(
            "
            ```py
            x = 1
            y = 2
            ```
            ",
        );
        let mf = super::parse("file.md", &source).unwrap();

        let [test] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected one test");
        };
        let [file] = test.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(file.code, "x = 1\ny = 2");
    }

    #[test]
    fn empty_file() {
        let source = dedent(
            "
            ```py
            ```
            ",
        );

        let mf = super::parse("file.md", &source).unwrap();

        let [test] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected one test");
        };
        let [file] = test.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(file.code, "");
    }

    #[test]
    fn no_lang() {
        let source = dedent(
            "
            # No language specifier

            ```
            x = 10
            ```
            ",
        );

        let err = super::parse("file.md", &source).expect_err("Should fail to parse");
        assert_eq!(
            err.to_string(),
            "Cannot auto-generate file name for code block with empty language specifier in test `No language specifier`"
        );
    }

    #[test]
    fn cannot_generate_name_for_lang() {
        let source = dedent(
            "
            # JSON test?

            ```json
            {}
            ```
            ",
        );
        let err = super::parse("file.md", &source).expect_err("Should fail to parse");
        assert_eq!(
            err.to_string(),
            "Cannot auto-generate file name for code block with language `json` in test `JSON test?`"
        );
    }

    #[test]
    fn mismatching_lang() {
        let source = dedent(
            "
            # Accidental stub

            `a.py`:

            ```pyi
            x = 1
            ```
            ",
        );
        let err = super::parse("file.md", &source).expect_err("Should fail to parse");
        assert_eq!(
            err.to_string(),
            "File extension of test file path `a.py` in test `Accidental stub` does not match language specified `pyi` of code block at line `5`"
        );
    }

    #[test]
    fn files_with_no_extension_can_have_any_lang() {
        let source = dedent(
            "
            `lorem`:

            ```foo
            x = 1
            ```
            ",
        );

        let mf = super::parse("file.md", &source).unwrap();

        let [test] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected one test");
        };
        let [file] = test.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(file.relative_path(), "lorem");
        assert_eq!(file.code, "x = 1");
    }

    #[test]
    fn files_with_lang_text_can_have_any_paths() {
        let source = dedent(
            "
            `lorem.yaml`:

            ```text
            x = 1
            ```
            ",
        );

        let mf = super::parse("file.md", &source).unwrap();

        let [test] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected one test");
        };
        let [file] = test.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(file.relative_path(), "lorem.yaml");
        assert_eq!(file.code, "x = 1");
    }

    #[test]
    fn unterminated_code_block_1() {
        let source = dedent(
            "
            ```
            x = 1
            ",
        );
        let err = super::parse("file.md", &source).expect_err("Should fail to parse");
        assert_eq!(err.to_string(), "Unterminated code block at line 2.");
    }

    #[test]
    fn unterminated_code_block_2() {
        let source = dedent(
            "
            ## A well-fenced block

            ```py
            y = 2
            ```

            ## A not-so-well-fenced block

            ```py
            x = 1
            ",
        );
        let err = super::parse("file.md", &source).expect_err("Should fail to parse");
        assert_eq!(err.to_string(), "Unterminated code block at line 10.");
    }

    #[test]
    fn header_start_at_beginning_of_line() {
        let source = dedent(
            "
            # A test

                # not a header

            ```py
            x = 1
            ```
            ",
        );
        let mf = super::parse("file.md", &source).unwrap();

        let [test] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected one test");
        };

        assert_snapshot!(test.name(), @"file.md - A test (1b4e27e6123dc8e7)");
    }

    #[test]
    fn code_blocks_must_not_be_indented() {
        let source = dedent(
            "
            # A test?

                ```py
                x = 1
                ```
            ",
        );
        let err = super::parse("file.md", &source).expect_err("Should fail to parse");
        assert_eq!(err.to_string(), "Indented code blocks are not supported.");
    }

    #[test]
    fn no_header_inside_test() {
        let source = dedent(
            "
            # One

            ```py
            x = 1
            ```

            ## Two
            ",
        );
        let err = super::parse("file.md", &source).expect_err("Should fail to parse");
        assert_eq!(
            err.to_string(),
            "Header 'Two' not valid inside a test case; parent 'One' has code files."
        );
    }

    #[test]
    fn line_break_in_header_1() {
        let source = dedent(
            "
            #
            Foo

            ```py
            x = 1
            ```
            ",
        );

        let mf = super::parse("file.md", &source).unwrap();

        let [test] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected one test");
        };
        let [file] = test.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(test.section.title, "file.md");
        assert_eq!(
            file.path,
            EmbeddedFilePath::Autogenerated(PySourceType::Python)
        );
        assert_eq!(file.code, "x = 1");
    }

    #[test]
    fn line_break_in_header_2() {
        let source = dedent(
            "
            # Foo

            ##
            Lorem

            ```py
            x = 1
            ```
            ",
        );

        let mf = super::parse("file.md", &source).unwrap();

        let [test] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected one test");
        };
        let [file] = test.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(test.section.title, "Foo");
        assert_eq!(
            file.path,
            EmbeddedFilePath::Autogenerated(PySourceType::Python)
        );
        assert_eq!(file.code, "x = 1");
    }

    #[test]
    fn no_duplicate_name_files_in_test() {
        let source = dedent(
            "
            `foo.py`:

            ```py
            x = 1
            ```

            `foo.py`:

            ```py
            y = 2
            ```
            ",
        );
        let err = super::parse("file.md", &source).expect_err("Should fail to parse");
        assert_eq!(
            err.to_string(),
            "Test `file.md` has duplicate files named `foo.py`."
        );
    }

    #[test]
    fn no_usage_of_autogenerated_name() {
        let source = dedent(
            "
            # Name clash

            `mdtest_snippet.py`:

            ```py
            x = 1
            ```

            ```py
            y = 2
            ```
            ",
        );
        let err = super::parse("file.md", &source).expect_err("Should fail to parse");
        assert_eq!(
            err.to_string(),
            "The file name `mdtest_snippet.py` in test `Name clash` must not be used explicitly."
        );
    }

    #[test]
    fn separate_path() {
        let source = dedent(
            "
            `foo.py`:

            ```py
            x = 1
            ```
            ",
        );

        let mf = super::parse("file.md", &source).unwrap();

        let [test] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected one test");
        };
        let [file] = test.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(file.relative_path(), "foo.py");
        assert_eq!(file.code, "x = 1");
    }

    #[test]
    fn separate_path_whitespace_1() {
        let source = dedent(
            "
            `foo.py` :

            ```py
            x = 1
            ```
            ",
        );

        let mf = super::parse("file.md", &source).unwrap();

        let [test] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected one test");
        };
        let [file] = test.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(file.relative_path(), "foo.py");
        assert_eq!(file.code, "x = 1");
    }

    #[test]
    fn separate_path_whitespace_2() {
        let source = dedent(
            "
            `foo.py`:
            ```py
            x = 1
            ```
            ",
        );

        let mf = super::parse("file.md", &source).unwrap();

        let [test] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected one test");
        };
        let [file] = test.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(file.relative_path(), "foo.py");
        assert_eq!(file.code, "x = 1");
    }

    #[test]
    fn path_with_space() {
        let source = dedent(
            "
            `foo bar.py`:

            ```py
            x = 1
            ```
            ",
        );

        let mf = super::parse("file.md", &source).unwrap();

        let [test] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected one test");
        };
        let [file] = test.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(file.relative_path(), "foo bar.py");
        assert_eq!(file.code, "x = 1");
    }

    #[test]
    fn path_with_line_break() {
        let source = dedent(
            "
            `foo
            .py`:

            ```py
            x = 1
            ```
            ",
        );

        let mf = super::parse("file.md", &source).unwrap();

        let [test] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected one test");
        };
        let [file] = test.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(
            file.path,
            EmbeddedFilePath::Autogenerated(PySourceType::Python)
        );
        assert_eq!(file.code, "x = 1");
    }

    #[test]
    fn path_with_backtick() {
        let source = dedent(
            "
            `foo`bar.py`:

            ```py
            x = 1
            ```
            ",
        );

        let mf = super::parse("file.md", &source).unwrap();

        let [test] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected one test");
        };
        let [file] = test.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(
            file.path,
            EmbeddedFilePath::Autogenerated(PySourceType::Python)
        );
        assert_eq!(file.code, "x = 1");
    }

    #[test]
    fn path_colon_on_next_line() {
        let source = dedent(
            "
            `foo.py`
            :

            ```py
            x = 1
            ```
            ",
        );

        let mf = super::parse("file.md", &source).unwrap();

        let [test] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected one test");
        };
        let [file] = test.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(
            file.path,
            EmbeddedFilePath::Autogenerated(PySourceType::Python)
        );
        assert_eq!(file.code, "x = 1");
    }

    #[test]
    fn random_trailing_backtick_quoted() {
        let source = dedent(
            "
            A long sentence that forces a line break
            `int`:

            ```py
            x = 1
            ```
            ",
        );

        let mf = super::parse("file.md", &source).unwrap();

        let [test] = &mf.tests().collect::<Vec<_>>()[..] else {
            panic!("expected one test");
        };
        let [file] = test.files().collect::<Vec<_>>()[..] else {
            panic!("expected one file");
        };

        assert_eq!(
            file.path,
            EmbeddedFilePath::Autogenerated(PySourceType::Python)
        );
        assert_eq!(file.code, "x = 1");
    }

    #[test]
    fn no_newline_between_prose_and_code() {
        // Regression test for https://github.com/astral-sh/ruff/issues/15923
        let source = dedent(
            "
            Some code:
            No newline between prose and code:
            ```py
            # A syntax error:
            §
            ```
            ",
        );

        super::parse("file.md", &source).expect_err(
            "Code blocks must start on a new line and be preceded by at least one blank line.",
        );
    }

    #[test]
    fn config_no_longer_allowed() {
        let source = dedent(
            "
            ```py foo=bar
            x = 1
            ```
            ",
        );
        let err = super::parse("file.md", &source).expect_err("Should fail to parse");
        assert_eq!(
            err.to_string(),
            "Trailing code-block metadata is not supported. Only the code block language can be specified."
        );
    }

    #[test]
    fn duplicate_section_directive_not_allowed() {
        let source = dedent(
            "
            # Some header

            <!-- snapshot-diagnostics -->
            <!-- snapshot-diagnostics -->

            ```py
            x = 1
            ```
            ",
        );
        let err = super::parse("file.md", &source).expect_err("Should fail to parse");
        assert_eq!(
            err.to_string(),
            "Section config to enable snapshotting diagnostics should appear at most once.",
        );
    }

    #[test]
    fn snapshot_diagnostic_directive_detection_ignores_whitespace() {
        // A bit weird, but the fact that the parser rejects this indicates that
        // we have correctly recognised both of these HTML comments as a directive to have
        // snapshotting enabled for the file, which is what we want (even though they both
        // contain different amounts of whitespace to the "standard" snapshot directive).
        let source = dedent(
            "
            # Some header

            <!--       snapshot-diagnostics   -->
            <!--snapshot-diagnostics-->

            ```py
            x = 1
            ```
            ",
        );
        let err = super::parse("file.md", &source).expect_err("Should fail to parse");
        assert_eq!(
            err.to_string(),
            "Section config to enable snapshotting diagnostics should appear at most once.",
        );
    }

    #[test]
    fn section_directive_must_appear_before_config() {
        let source = dedent(
            "
            # Some header

            ```toml
            [environment]
            typeshed = \"/typeshed\"
            ```

            <!-- snapshot-diagnostics -->

            ```py
            x = 1
            ```
            ",
        );
        let err = super::parse("file.md", &source).expect_err("Should fail to parse");
        assert_eq!(
            err.to_string(),
            "Section config to enable snapshotting diagnostics must \
             come before everything else \
             (including TOML configuration blocks).",
        );
    }

    #[test]
    fn section_directive_must_appear_before_embedded_files() {
        let source = dedent(
            "
            # Some header

            ```py
            x = 1
            ```

            <!-- snapshot-diagnostics -->
            ",
        );
        let err = super::parse("file.md", &source).expect_err("Should fail to parse");
        assert_eq!(
            err.to_string(),
            "Section config to enable snapshotting diagnostics must \
             come before everything else \
             (including embedded files).",
        );
    }

    #[test]
    fn obvious_typos_in_directives_are_detected() {
        let source = dedent(
            "
            # Some header
            <!-- snpshotttt-digggggnosstic -->
            ```py
            x = 1
            ```
            ",
        );
        let err = super::parse("file.md", &source).expect_err("Should fail to parse");
        assert_eq!(
            err.to_string(),
            "Unknown HTML comment `snpshotttt-digggggnosstic` -- possibly a typo? \
            (Add to `HTML_COMMENT_ALLOWLIST` if this is a false positive)",
        );
    }

    #[test]
    fn allowlisted_html_comments_are_permitted() {
        let source = dedent(
            "
            # Some header
            <!-- blacken-docs:off -->

            ```py
            x = 1
            ```

            <!-- blacken-docs:on -->
            ",
        );
        let parse_result = super::parse("file.md", &source);
        assert!(parse_result.is_ok(), "{parse_result:?}");
    }

    #[test]
    fn multiple_sections_with_dependencies_not_allowed() {
        let source = dedent(
            r#"
            # First section

            ```toml
            [project]
            dependencies = ["pydantic==2.12.2"]
            ```

            ```py
            x = 1
            ```

            # Second section

            ```toml
            [project]
            dependencies = ["numpy==2.0.0"]
            ```

            ```py
            y = 2
            ```
            "#,
        );
        let err = super::parse("file.md", &source).expect_err("Should fail to parse");
        assert_eq!(
            err.to_string(),
            "Multiple sections with `[project]` dependencies in the same file are not allowed. \
             External dependencies must be specified in a single top-level configuration block."
        );
    }
}
