use std::{convert::TryInto, io::Write}; use crate::{entry, extension, write::util::CountBytes, State, Version}; /// A way to specify which of the optional extensions to write.#[derive(Debug, Copy, Clone)]pub enum Extensions {    /// Writes all available optional extensions to avoid loosing any information.    All,    /// Only write the given optional extensions, with each extension being marked by a boolean flag.    ///    /// # Note: mandatory extensions    ///    /// Mandatory extensions, like `sdir` or other lower-case ones, may not be configured here as they need to be present    /// or absent depending on the state of the index itself and for it to be valid.    Given {        /// Write the tree-cache extension, if present.        tree_cache: bool,        /// Write the end-of-index-entry extension.        end_of_index_entry: bool,    },    /// Write no optional extension at all for what should be the smallest possible index    None,} impl Default for Extensions {    fn default() -> Self {        Extensions::All    }} impl Extensions {    /// Returns `Some(signature)` if it should be written out.    pub fn should_write(&self, signature: extension::Signature) -> Option<extension::Signature> {        match self {            Extensions::None => None,            Extensions::All => Some(signature),            Extensions::Given {                tree_cache,                end_of_index_entry,            } => match signature {                extension::tree::SIGNATURE => tree_cache,                extension::end_of_index_entry::SIGNATURE => end_of_index_entry,                _ => &false,            }            .then(|| signature),        }    }} /// The options for use when [writing an index][State::write_to()].////// Note that default options write either index V2 or V3 depending on the content of the entries.#[derive(Debug, Default, Clone, Copy)]pub struct Options {    /// Configures which extensions to write    pub extensions: Extensions,} impl State {    /// Serialize this instance to `out` with [`options`][Options].    pub fn write_to(&self, out: impl std::io::Write, Options { extensions }: Options) -> std::io::Result<Version> {        let version = self.detect_required_version();         let mut write = CountBytes::new(out);        let num_entries = self            .entries()            .len()            .try_into()            .expect("definitely not 4billion entries");         let offset_to_entries = header(&mut write, version, num_entries)?;        let offset_to_extensions = entries(&mut write, self, offset_to_entries)?;        let (extension_toc, out) = self.write_extensions(write, offset_to_extensions, extensions)?;         if num_entries > 0            && extensions                .should_write(extension::end_of_index_entry::SIGNATURE)                .is_some()            && !extension_toc.is_empty()        {            extension::end_of_index_entry::write_to(out, self.object_hash, offset_to_extensions, extension_toc)?        }         Ok(version)    }     fn write_extensions<T>(        &self,        mut write: CountBytes<T>,        offset_to_extensions: u32,        extensions: Extensions,    ) -> std::io::Result<(Vec<(extension::Signature, u32)>, T)>    where        T: std::io::Write,    {        type WriteExtFn<'a> = &'a dyn Fn(&mut dyn std::io::Write) -> Option<std::io::Result<extension::Signature>>;        let extensions: &[WriteExtFn<'_>] = &[            &|write| {                extensions                    .should_write(extension::tree::SIGNATURE)                    .and_then(|signature| self.tree().map(|tree| tree.write_to(write).map(|_| signature)))            },            &|write| {                self.is_sparse()                    .then(|| extension::sparse::write_to(write).map(|_| extension::sparse::SIGNATURE))            },        ];         let mut offset_to_previous_ext = offset_to_extensions;        let mut out = Vec::with_capacity(5);        for write_ext in extensions {            if let Some(signature) = write_ext(&mut write).transpose()? {                let offset_past_ext = write.count;                let ext_size = offset_past_ext - offset_to_previous_ext - (extension::MIN_SIZE as u32);                offset_to_previous_ext = offset_past_ext;                out.push((signature, ext_size));            }        }        Ok((out, write.inner))    }} impl State {    fn detect_required_version(&self) -> Version {        self.entries            .iter()            .find_map(|e| e.flags.contains(entry::Flags::EXTENDED).then(|| Version::V3))            .unwrap_or(Version::V2)    }} fn header<T: std::io::Write>(    out: &mut CountBytes<T>,    version: Version,    num_entries: u32,) -> Result<u32, std::io::Error> {    let version = match version {        Version::V2 => 2_u32.to_be_bytes(),        Version::V3 => 3_u32.to_be_bytes(),        Version::V4 => 4_u32.to_be_bytes(),    };     out.write_all(crate::decode::header::SIGNATURE)?;    out.write_all(&version)?;    out.write_all(&num_entries.to_be_bytes())?;     Ok(out.count)} fn entries<T: std::io::Write>(out: &mut CountBytes<T>, state: &State, header_size: u32) -> Result<u32, std::io::Error> {    for entry in state.entries() {        entry.write_to(&mut *out, state)?;        match (out.count - header_size) % 8 {            0 => {}            n => {                let eight_null_bytes = [0u8; 8];                out.write_all(&eight_null_bytes[n as usize..])?;            }        };    }     Ok(out.count)} mod util {    use std::convert::TryFrom;     pub struct CountBytes<T> {        pub count: u32,        pub inner: T,    }     impl<T> CountBytes<T>    where        T: std::io::Write,    {        pub fn new(inner: T) -> Self {            CountBytes { inner, count: 0 }        }    }     impl<T> std::io::Write for CountBytes<T>    where        T: std::io::Write,    {        fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {            let written = self.inner.write(buf)?;            self.count = self                .count                .checked_add(u32::try_from(written).expect("we don't write 4GB buffers"))                .ok_or_else(|| {                    std::io::Error::new(                        std::io::ErrorKind::Other,                        "Cannot write indices larger than 4 gigabytes",                    )                })?;            Ok(written)        }         fn flush(&mut self) -> std::io::Result<()> {            self.inner.flush()        }    }}