use std::{cmp::Ordering, io}; pub(crate) const LARGE_OFFSET_THRESHOLD: u64 = 0x7fff_ffff;pub(crate) const HIGH_BIT: u32 = 0x8000_0000; use git_features::{    hash,    progress::{self, Progress},}; use crate::index::{util::Count, V2_SIGNATURE}; pub(crate) fn write_to(    out: impl io::Write,    entries_sorted_by_oid: Vec<crate::cache::delta::Item<crate::index::write::TreeEntry>>,    pack_hash: &git_hash::ObjectId,    kind: crate::index::Version,    mut progress: impl Progress,) -> io::Result<git_hash::ObjectId> {    use io::Write;    assert_eq!(kind, crate::index::Version::V2, "Can only write V2 packs right now");    assert!(        entries_sorted_by_oid.len() <= u32::MAX as usize,        "a pack cannot have more than u32::MAX objects"    );     // Write header    let mut out = Count::new(std::io::BufWriter::with_capacity(        8 * 4096,        hash::Write::new(out, kind.hash()),    ));    out.write_all(V2_SIGNATURE)?;    out.write_all(&(kind as u32).to_be_bytes())?;     progress.init(Some(4), progress::steps());    let start = std::time::Instant::now();    let _info = progress.add_child_with_id("writing fan-out table", git_features::progress::UNKNOWN);    let fan_out = fanout(entries_sorted_by_oid.iter().map(|e| e.data.id.first_byte()));     for value in fan_out.iter() {        out.write_all(&value.to_be_bytes())?;    }     progress.inc();    let _info = progress.add_child_with_id("writing ids", git_features::progress::UNKNOWN);    for entry in &entries_sorted_by_oid {        out.write_all(entry.data.id.as_slice())?;    }     progress.inc();    let _info = progress.add_child_with_id("writing crc32", git_features::progress::UNKNOWN);    for entry in &entries_sorted_by_oid {        out.write_all(&entry.data.crc32.to_be_bytes())?;    }     progress.inc();    let _info = progress.add_child_with_id("writing offsets", git_features::progress::UNKNOWN);    {        let mut offsets64 = Vec::<u64>::new();        for entry in &entries_sorted_by_oid {            let offset: u32 = if entry.offset > LARGE_OFFSET_THRESHOLD {                assert!(                    offsets64.len() < LARGE_OFFSET_THRESHOLD as usize,                    "Encoding breakdown - way too many 64bit offsets"                );                offsets64.push(entry.offset);                ((offsets64.len() - 1) as u32) | HIGH_BIT            } else {                entry.offset as u32            };            out.write_all(&offset.to_be_bytes())?;        }        for value in offsets64 {            out.write_all(&value.to_be_bytes())?;        }    }     out.write_all(pack_hash.as_slice())?;     let bytes_written_without_trailer = out.bytes;    let mut out = out.inner.into_inner()?;    let index_hash: git_hash::ObjectId = out.hash.digest().into();    out.inner.write_all(index_hash.as_slice())?;    out.inner.flush()?;     progress.inc();    progress.show_throughput_with(        start,        (bytes_written_without_trailer + 20) as usize,        progress::bytes().expect("unit always set"),        progress::MessageLevel::Success,    );     Ok(index_hash)} pub(crate) fn fanout(iter: impl ExactSizeIterator<Item = u8>) -> [u32; 256] {    let mut fan_out = [0u32; 256];    let entries_len = iter.len() as u32;    let mut iter = iter.enumerate();    let mut idx_and_entry = iter.next();    let mut upper_bound = 0;     for (offset_be, byte) in fan_out.iter_mut().zip(0u8..=255) {        *offset_be = match idx_and_entry.as_ref() {            Some((_idx, first_byte)) => match first_byte.cmp(&byte) {                Ordering::Less => unreachable!("ids should be ordered, and we make sure to keep ahead with them"),                Ordering::Greater => upper_bound,                Ordering::Equal => {                    if byte == 255 {                        entries_len                    } else {                        idx_and_entry = iter.find(|(_, first_byte)| *first_byte != byte);                        upper_bound = idx_and_entry                            .as_ref()                            .map(|(idx, _)| *idx as u32)                            .unwrap_or(entries_len);                        upper_bound                    }                }            },            None => entries_len,        };    }     fan_out}