use std::collections::BTreeMap;
use std::io;
use std::path::{Path, PathBuf};
use std::str::FromStr;
use std::sync::LazyLock;

use memchr::memmem::Finder;
use serde::Deserialize;
use thiserror::Error;

use uv_pep440::VersionSpecifiers;
use uv_pep508::PackageName;
use uv_pypi_types::VerbatimParsedUrl;
use uv_settings::{GlobalOptions, ResolverInstallerOptions};
use uv_workspace::pyproject::Sources;

static FINDER: LazyLock<Finder> = LazyLock::new(|| Finder::new(b"# /// script"));

/// A PEP 723 item, either read from a script on disk or provided via `stdin`.
#[derive(Debug)]
pub enum Pep723Item {
    /// A PEP 723 script read from disk.
    Script(Pep723Script),
    /// A PEP 723 script provided via `stdin`.
    Stdin(Pep723Metadata),
    /// A PEP 723 script provided via a remote URL.
    Remote(Pep723Metadata),
}

impl Pep723Item {
    /// Return the [`Pep723Metadata`] associated with the item.
    pub fn metadata(&self) -> &Pep723Metadata {
        match self {
            Self::Script(script) => &script.metadata,
            Self::Stdin(metadata) => metadata,
            Self::Remote(metadata) => metadata,
        }
    }

    /// Consume the item and return the associated [`Pep723Metadata`].
    pub fn into_metadata(self) -> Pep723Metadata {
        match self {
            Self::Script(script) => script.metadata,
            Self::Stdin(metadata) => metadata,
            Self::Remote(metadata) => metadata,
        }
    }
}

/// A PEP 723 script, including its [`Pep723Metadata`].
#[derive(Debug)]
pub struct Pep723Script {
    /// The path to the Python script.
    pub path: PathBuf,
    /// The parsed [`Pep723Metadata`] table from the script.
    pub metadata: Pep723Metadata,
    /// The content of the script before the metadata table.
    pub prelude: String,
    /// The content of the script after the metadata table.
    pub postlude: String,
}

impl Pep723Script {
    /// Read the PEP 723 `script` metadata from a Python file, if it exists.
    ///
    /// See: <https://peps.python.org/pep-0723/>
    pub async fn read(file: impl AsRef<Path>) -> Result<Option<Self>, Pep723Error> {
        let contents = match fs_err::tokio::read(&file).await {
            Ok(contents) => contents,
            Err(err) if err.kind() == io::ErrorKind::NotFound => return Ok(None),
            Err(err) => return Err(err.into()),
        };

        // Extract the `script` tag.
        let ScriptTag {
            prelude,
            metadata,
            postlude,
        } = match ScriptTag::parse(&contents) {
            Ok(Some(tag)) => tag,
            Ok(None) => return Ok(None),
            Err(err) => return Err(err),
        };

        // Parse the metadata.
        let metadata = Pep723Metadata::from_str(&metadata)?;

        Ok(Some(Self {
            path: file.as_ref().to_path_buf(),
            metadata,
            prelude,
            postlude,
        }))
    }

    /// Reads a Python script and generates a default PEP 723 metadata table.
    ///
    /// See: <https://peps.python.org/pep-0723/>
    pub async fn init(
        file: impl AsRef<Path>,
        requires_python: &VersionSpecifiers,
    ) -> Result<Self, Pep723Error> {
        let contents = fs_err::tokio::read(&file).await?;

        // Define the default metadata.
        let default_metadata = indoc::formatdoc! {r#"
            requires-python = "{requires_python}"
            dependencies = []
            "#,
            requires_python = requires_python,
        };
        let metadata = Pep723Metadata::from_str(&default_metadata)?;

        //  Extract the shebang and script content.
        let (shebang, postlude) = extract_shebang(&contents)?;

        Ok(Self {
            path: file.as_ref().to_path_buf(),
            prelude: if shebang.is_empty() {
                String::new()
            } else {
                format!("{shebang}\n")
            },
            metadata,
            postlude,
        })
    }

    /// Create a PEP 723 script at the given path.
    pub async fn create(
        file: impl AsRef<Path>,
        requires_python: &VersionSpecifiers,
        existing_contents: Option<Vec<u8>>,
    ) -> Result<(), Pep723Error> {
        let file = file.as_ref();

        let script_name = file
            .file_name()
            .and_then(|name| name.to_str())
            .ok_or_else(|| Pep723Error::InvalidFilename(file.to_string_lossy().to_string()))?;

        let default_metadata = indoc::formatdoc! {r#"
            requires-python = "{requires_python}"
            dependencies = []
            "#,
        };
        let metadata = serialize_metadata(&default_metadata);

        let script = if let Some(existing_contents) = existing_contents {
            indoc::formatdoc! {r#"
            {metadata}
            {content}
            "#,
            content = String::from_utf8(existing_contents).map_err(|err| Pep723Error::Utf8(err.utf8_error()))?}
        } else {
            indoc::formatdoc! {r#"
            {metadata}

            def main() -> None:
                print("Hello from {name}!")


            if __name__ == "__main__":
                main()
        "#,
                metadata = metadata,
                name = script_name,
            }
        };

        Ok(fs_err::tokio::write(file, script).await?)
    }

    /// Replace the existing metadata in the file with new metadata and write the updated content.
    pub async fn write(&self, metadata: &str) -> Result<(), Pep723Error> {
        let content = format!(
            "{}{}{}",
            self.prelude,
            serialize_metadata(metadata),
            self.postlude
        );

        fs_err::tokio::write(&self.path, content).await?;

        Ok(())
    }
}

/// PEP 723 metadata as parsed from a `script` comment block.
///
/// See: <https://peps.python.org/pep-0723/>
#[derive(Debug, Deserialize)]
#[serde(rename_all = "kebab-case")]
pub struct Pep723Metadata {
    pub dependencies: Option<Vec<uv_pep508::Requirement<VerbatimParsedUrl>>>,
    pub requires_python: Option<VersionSpecifiers>,
    pub tool: Option<Tool>,
    /// The raw unserialized document.
    #[serde(skip)]
    pub raw: String,
}

impl Pep723Metadata {
    /// Parse the PEP 723 metadata from `stdin`.
    pub fn parse(contents: &[u8]) -> Result<Option<Self>, Pep723Error> {
        // Extract the `script` tag.
        let ScriptTag { metadata, .. } = match ScriptTag::parse(contents) {
            Ok(Some(tag)) => tag,
            Ok(None) => return Ok(None),
            Err(err) => return Err(err),
        };

        // Parse the metadata.
        Ok(Some(Self::from_str(&metadata)?))
    }

    /// Read the PEP 723 `script` metadata from a Python file, if it exists.
    ///
    /// See: <https://peps.python.org/pep-0723/>
    pub async fn read(file: impl AsRef<Path>) -> Result<Option<Self>, Pep723Error> {
        let contents = match fs_err::tokio::read(&file).await {
            Ok(contents) => contents,
            Err(err) if err.kind() == io::ErrorKind::NotFound => return Ok(None),
            Err(err) => return Err(err.into()),
        };

        // Extract the `script` tag.
        let ScriptTag { metadata, .. } = match ScriptTag::parse(&contents) {
            Ok(Some(tag)) => tag,
            Ok(None) => return Ok(None),
            Err(err) => return Err(err),
        };

        // Parse the metadata.
        Ok(Some(Self::from_str(&metadata)?))
    }
}

impl FromStr for Pep723Metadata {
    type Err = Pep723Error;

    /// Parse `Pep723Metadata` from a raw TOML string.
    fn from_str(raw: &str) -> Result<Self, Self::Err> {
        let metadata = toml::from_str(raw)?;
        Ok(Self {
            raw: raw.to_string(),
            ..metadata
        })
    }
}

#[derive(Deserialize, Debug)]
#[serde(rename_all = "kebab-case")]
pub struct Tool {
    pub uv: Option<ToolUv>,
}

#[derive(Debug, Deserialize)]
#[serde(deny_unknown_fields)]
pub struct ToolUv {
    #[serde(flatten)]
    pub globals: GlobalOptions,
    #[serde(flatten)]
    pub top_level: ResolverInstallerOptions,
    pub sources: Option<BTreeMap<PackageName, Sources>>,
}

#[derive(Debug, Error)]
pub enum Pep723Error {
    #[error("An opening tag (`# /// script`) was found without a closing tag (`# ///`). Ensure that every line between the opening and closing tags (including empty lines) starts with a leading `#`.")]
    UnclosedBlock,
    #[error(transparent)]
    Io(#[from] io::Error),
    #[error(transparent)]
    Utf8(#[from] std::str::Utf8Error),
    #[error(transparent)]
    Toml(#[from] toml::de::Error),
    #[error("Invalid filename `{0}` supplied")]
    InvalidFilename(String),
}

#[derive(Debug, Clone, Eq, PartialEq)]
pub struct ScriptTag {
    /// The content of the script before the metadata block.
    prelude: String,
    /// The metadata block.
    metadata: String,
    /// The content of the script after the metadata block.
    postlude: String,
}

impl ScriptTag {
    /// Given the contents of a Python file, extract the `script` metadata block with leading
    /// comment hashes removed, any preceding shebang or content (prelude), and the remaining Python
    /// script.
    ///
    /// Given the following input string representing the contents of a Python script:
    ///
    /// ```python
    /// #!/usr/bin/env python3
    /// # /// script
    /// # requires-python = '>=3.11'
    /// # dependencies = [
    /// #   'requests<3',
    /// #   'rich',
    /// # ]
    /// # ///
    ///
    /// import requests
    ///
    /// print("Hello, World!")
    /// ```
    ///
    /// This function would return:
    ///
    /// - Preamble: `#!/usr/bin/env python3\n`
    /// - Metadata: `requires-python = '>=3.11'\ndependencies = [\n  'requests<3',\n  'rich',\n]`
    /// - Postlude: `import requests\n\nprint("Hello, World!")\n`
    ///
    /// See: <https://peps.python.org/pep-0723/>
    pub fn parse(contents: &[u8]) -> Result<Option<Self>, Pep723Error> {
        // Identify the opening pragma.
        let Some(index) = FINDER.find(contents) else {
            return Ok(None);
        };

        // The opening pragma must be the first line, or immediately preceded by a newline.
        if !(index == 0 || matches!(contents[index - 1], b'\r' | b'\n')) {
            return Ok(None);
        }

        // Extract the preceding content.
        let prelude = std::str::from_utf8(&contents[..index])?;

        // Decode as UTF-8.
        let contents = &contents[index..];
        let contents = std::str::from_utf8(contents)?;

        let mut lines = contents.lines();

        // Ensure that the first line is exactly `# /// script`.
        if !lines.next().is_some_and(|line| line == "# /// script") {
            return Ok(None);
        }

        // > Every line between these two lines (# /// TYPE and # ///) MUST be a comment starting
        // > with #. If there are characters after the # then the first character MUST be a space. The
        // > embedded content is formed by taking away the first two characters of each line if the
        // > second character is a space, otherwise just the first character (which means the line
        // > consists of only a single #).
        let mut toml = vec![];

        // Extract the content that follows the metadata block.
        let mut python_script = vec![];

        while let Some(line) = lines.next() {
            // Remove the leading `#`.
            let Some(line) = line.strip_prefix('#') else {
                python_script.push(line);
                python_script.extend(lines);
                break;
            };

            // If the line is empty, continue.
            if line.is_empty() {
                toml.push("");
                continue;
            }

            // Otherwise, the line _must_ start with ` `.
            let Some(line) = line.strip_prefix(' ') else {
                python_script.push(line);
                python_script.extend(lines);
                break;
            };

            toml.push(line);
        }

        // Find the closing `# ///`. The precedence is such that we need to identify the _last_ such
        // line.
        //
        // For example, given:
        // ```python
        // # /// script
        // #
        // # ///
        // #
        // # ///
        // ```
        //
        // The latter `///` is the closing pragma
        let Some(index) = toml.iter().rev().position(|line| *line == "///") else {
            return Err(Pep723Error::UnclosedBlock);
        };
        let index = toml.len() - index;

        // Discard any lines after the closing `# ///`.
        //
        // For example, given:
        // ```python
        // # /// script
        // #
        // # ///
        // #
        // #
        // ```
        //
        // We need to discard the last two lines.
        toml.truncate(index - 1);

        // Join the lines into a single string.
        let prelude = prelude.to_string();
        let metadata = toml.join("\n") + "\n";
        let postlude = python_script.join("\n") + "\n";

        Ok(Some(Self {
            prelude,
            metadata,
            postlude,
        }))
    }
}

/// Extracts the shebang line from the given file contents and returns it along with the remaining
/// content.
fn extract_shebang(contents: &[u8]) -> Result<(String, String), Pep723Error> {
    let contents = std::str::from_utf8(contents)?;

    if contents.starts_with("#!") {
        // Find the first newline.
        let bytes = contents.as_bytes();
        let index = bytes
            .iter()
            .position(|&b| b == b'\r' || b == b'\n')
            .unwrap_or(bytes.len());

        // Support `\r`, `\n`, and `\r\n` line endings.
        let width = match bytes.get(index) {
            Some(b'\r') => {
                if bytes.get(index + 1) == Some(&b'\n') {
                    2
                } else {
                    1
                }
            }
            Some(b'\n') => 1,
            _ => 0,
        };

        // Extract the shebang line.
        let shebang = contents[..index].to_string();
        let script = contents[index + width..].to_string();

        Ok((shebang, script))
    } else {
        Ok((String::new(), contents.to_string()))
    }
}

/// Formats the provided metadata by prefixing each line with `#` and wrapping it with script markers.
fn serialize_metadata(metadata: &str) -> String {
    let mut output = String::with_capacity(metadata.len() + 32);

    output.push_str("# /// script");
    output.push('\n');

    for line in metadata.lines() {
        output.push('#');
        if !line.is_empty() {
            output.push(' ');
            output.push_str(line);
        }
        output.push('\n');
    }

    output.push_str("# ///");
    output.push('\n');

    output
}

#[cfg(test)]
mod tests;