//! Trees: sorted directory listings. A flattened tree ([`Files`]) maps
//! `a/b/c.txt` paths to file entries.

use std::collections::{BTreeMap, HashMap};

use crate::object::{Kind, Object};
use crate::{Id, Result, Source, corrupt};

#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
#[repr(u8)]
pub enum EntryKind {
    File = 1,
    Exec = 2,
    Link = 3,
    Tree = 4,
    /// A git submodule: its content is the commit it points at (40 hex digits).
    Gitlink = 5,
}

impl EntryKind {
    pub fn from_u8(b: u8) -> Option<EntryKind> {
        match b {
            1 => Some(EntryKind::File),
            2 => Some(EntryKind::Exec),
            3 => Some(EntryKind::Link),
            4 => Some(EntryKind::Tree),
            5 => Some(EntryKind::Gitlink),
            _ => None,
        }
    }

    pub fn name(self) -> &'static str {
        match self {
            EntryKind::File => "file",
            EntryKind::Exec => "exec",
            EntryKind::Link => "link",
            EntryKind::Tree => "tree",
            EntryKind::Gitlink => "submodule",
        }
    }
}

/// A file in a flattened tree. `id` names a blob or, for big files, a chunk list.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct Entry {
    pub kind: EntryKind,
    pub id: Id,
}

pub type Files = BTreeMap<String, Entry>;

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct TreeEntry {
    pub name: String,
    pub kind: EntryKind,
    pub id: Id,
}

pub fn encode(entries: &[TreeEntry]) -> Vec<u8> {
    let mut sorted: Vec<&TreeEntry> = entries.iter().collect();
    sorted.sort_by(|a, b| a.name.as_bytes().cmp(b.name.as_bytes()));
    let mut out = Vec::with_capacity(entries.len() * 48);
    for e in sorted {
        out.push(e.kind as u8);
        out.extend_from_slice(&(e.name.len() as u16).to_le_bytes());
        out.extend_from_slice(e.name.as_bytes());
        out.extend_from_slice(&e.id.0);
    }
    out
}

pub fn decode(data: &[u8]) -> Result<Vec<TreeEntry>> {
    let mut out = Vec::new();
    let mut at = 0;
    while at < data.len() {
        let kind = EntryKind::from_u8(data[at]).ok_or_else(|| corrupt("bad tree entry kind"))?;
        let len = u16::from_le_bytes(data.get(at + 1..at + 3).ok_or_else(|| corrupt("short tree"))?.try_into().unwrap()) as usize;
        let name_end = at + 3 + len;
        let name = std::str::from_utf8(data.get(at + 3..name_end).ok_or_else(|| corrupt("short tree"))?)
            .map_err(|_| corrupt("tree name isn't UTF-8"))?
            .to_string();
        if name.is_empty() || name.contains('/') || name == "." || name == ".." {
            return Err(corrupt("bad name in tree"));
        }
        let id = Id::from_slice(data.get(name_end..name_end + 32).ok_or_else(|| corrupt("short tree"))?).unwrap();
        out.push(TreeEntry { name, kind, id });
        at = name_end + 32;
    }
    Ok(out)
}

pub fn empty() -> Object {
    Object::new(Kind::Tree, Vec::new())
}

pub fn empty_id() -> Id {
    empty().id()
}

pub fn read(src: &dyn Source, id: &Id) -> Result<Vec<TreeEntry>> {
    let object = src.must(id)?;
    if object.kind != Kind::Tree {
        return Err(corrupt(format!("{} isn't a tree", id.short())));
    }
    decode(&object.data)
}

/// The entry at `path` under `root` (None: not there), reading only the
/// folders on the path. The root itself is a folder.
pub fn lookup(src: &dyn Source, root: &Id, path: &str) -> Result<Option<Entry>> {
    let mut at = Entry { kind: EntryKind::Tree, id: *root };
    for part in path.split('/').filter(|p| !p.is_empty()) {
        if at.kind != EntryKind::Tree {
            return Ok(None);
        }
        match read(src, &at.id)?.into_iter().find(|e| e.name == part) {
            Some(e) => at = Entry { kind: e.kind, id: e.id },
            None => return Ok(None),
        }
    }
    Ok(Some(at))
}

/// Every file under `tree`.
pub fn flatten(src: &dyn Source, tree: &Id) -> Result<Files> {
    let mut out = Files::new();
    walk(src, tree, "", &mut out)?;
    Ok(out)
}

fn walk(src: &dyn Source, tree: &Id, prefix: &str, out: &mut Files) -> Result<()> {
    for e in read(src, tree)? {
        let path = if prefix.is_empty() { e.name.clone() } else { format!("{prefix}/{}", e.name) };
        if e.kind == EntryKind::Tree {
            walk(src, &e.id, &path, out)?;
        } else {
            out.insert(path, Entry { kind: e.kind, id: e.id });
        }
    }
    Ok(())
}

/// Every object ID a tree reaches (the trees themselves, file blobs and
/// chunk lists, not the chunks inside chunk lists).
pub fn reachable(src: &dyn Source, tree: &Id, out: &mut Vec<Id>) -> Result<()> {
    out.push(*tree);
    for e in read(src, tree)? {
        if e.kind == EntryKind::Tree {
            reachable(src, &e.id, out)?;
        } else {
            out.push(e.id);
        }
    }
    Ok(())
}

/// Trees reached from `new` but not from `old`, walking only subtrees that
/// differ, plus the file IDs those subtrees hold that `old` doesn't have at
/// the same path.
pub fn new_since(src: &dyn Source, old: Option<&Id>, new: &Id, out: &mut Vec<Id>) -> Result<()> {
    if Some(new) == old {
        return Ok(());
    }
    out.push(*new);
    let old_entries: HashMap<String, TreeEntry> = match old {
        Some(o) => read(src, o)?.into_iter().map(|e| (e.name.clone(), e)).collect(),
        None => HashMap::new(),
    };
    for e in read(src, new)? {
        let before = old_entries.get(&e.name);
        if before.is_some_and(|b| b.id == e.id) {
            continue;
        }
        if e.kind == EntryKind::Tree {
            let old_sub = before.filter(|b| b.kind == EntryKind::Tree).map(|b| b.id);
            new_since(src, old_sub.as_ref(), &e.id, out)?;
        } else {
            out.push(e.id);
        }
    }
    Ok(())
}

/// Builds tree objects for `files`. Returns the root ID and every tree
/// object (the host stores the ones it doesn't have).
pub fn build(files: &Files) -> (Id, Vec<Object>) {
    let mut dirs: HashMap<&str, Vec<TreeEntry>> = HashMap::new();
    let mut subdirs: HashMap<&str, Vec<&str>> = HashMap::new();
    dirs.entry("").or_default();
    for (path, entry) in files {
        let (dir, name) = split(path);
        dirs.entry(dir).or_default().push(TreeEntry { name: name.to_string(), kind: entry.kind, id: entry.id });
        let mut d = dir;
        while !d.is_empty() {
            let (parent, _) = split(d);
            let known = subdirs.entry(parent).or_default();
            if known.contains(&d) {
                break;
            }
            known.push(d);
            dirs.entry(d).or_default();
            d = parent;
        }
    }
    let mut order: Vec<&str> = dirs.keys().copied().collect();
    order.sort_by_key(|d| std::cmp::Reverse(if d.is_empty() { 0 } else { d.matches('/').count() + 1 }));
    let mut written: HashMap<&str, Id> = HashMap::with_capacity(order.len());
    let mut objects = Vec::with_capacity(order.len());
    for dir in order {
        let mut entries = dirs.remove(dir).unwrap_or_default();
        for child in subdirs.get(dir).map(Vec::as_slice).unwrap_or(&[]) {
            entries.push(TreeEntry { name: split(child).1.to_string(), kind: EntryKind::Tree, id: written[child] });
        }
        let object = Object::new(Kind::Tree, encode(&entries));
        let id = object.id();
        objects.push(object);
        written.insert(dir, id);
    }
    (written[""], objects)
}

/// "a/b/c" → ("a/b", "c"); "c" → ("", "c").
pub fn split(path: &str) -> (&str, &str) {
    match path.rfind('/') {
        Some(i) => (&path[..i], &path[i + 1..]),
        None => ("", path),
    }
}

/// Whether `path` is `prefix` or inside it.
pub fn under(path: &str, prefix: &str) -> bool {
    prefix.is_empty() || path == prefix || (path.starts_with(prefix) && path.as_bytes().get(prefix.len()) == Some(&b'/'))
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Change {
    Added(Entry),
    Removed(Entry),
    Modified(Entry, Entry),
}

/// What changed from `a` to `b`, by path.
pub fn diff(a: &Files, b: &Files) -> BTreeMap<String, Change> {
    let mut out = BTreeMap::new();
    for (path, old) in a {
        match b.get(path) {
            None => {
                out.insert(path.clone(), Change::Removed(*old));
            }
            Some(new) if new != old => {
                out.insert(path.clone(), Change::Modified(*old, *new));
            }
            _ => {}
        }
    }
    for (path, new) in b {
        if !a.contains_key(path) {
            out.insert(path.clone(), Change::Added(*new));
        }
    }
    out
}

/// A file to write into a tree: its path, kind and content.
pub struct Put {
    pub path: String,
    pub kind: EntryKind,
    pub content: Vec<u8>,
}

/// Whether `path` can name a file: relative, every part a [`safe_name`].
pub fn valid_path(path: &str) -> bool {
    !path.is_empty() && path.split('/').all(safe_name)
}

/// Whether one part of a path is safe on every system. Not empty, `.` or
/// `..`; no backslash (a separator on Windows) or NUL; and not Onset's
/// (`.onset`, or `.vcs` from before it had its name) or git's own folder in
/// any spelling some disk treats as the same (`.GIT`,
/// `.git.`, `git~1`, or with characters macOS ignores), since a project
/// holding `.git/hooks/...` would run code in the folders it's written to.
pub fn safe_name(name: &str) -> bool {
    if name.is_empty() || name == "." || name == ".." || name.len() > u16::MAX as usize || name.contains(['\\', '\0']) {
        return false;
    }
    let plain: String = name
        .chars()
        .filter(|c| !matches!(*c as u32, 0x200C..=0x200F | 0x202A..=0x202E | 0x206A..=0x206F | 0xFEFF))
        .collect::<String>()
        .trim_end_matches(['.', ' '])
        .to_ascii_lowercase();
    !matches!(plain.as_str(), ".git" | ".onset" | ".vcs" | "git~1" | "onset~1" | "vcs~1")
}

/// The first path `new` adds or changes over `old` that isn't a
/// [`valid_path`], if any. Paths already there aren't looked at again.
pub fn unsafe_path(src: &dyn Source, old: &Id, new: &Id) -> Result<Option<String>> {
    Ok(diff_trees(src, old, new)?.into_iter().find(|(p, c)| !matches!(c, Change::Removed(_)) && !valid_path(p)).map(|(p, _)| p))
}

/// `tree` with files written and deleted. Writing a path replaces whatever
/// was there (a folder of that name included); deleting a folder deletes
/// what's in it. Returns the new root and every object it needs that's new
/// (file contents, big files' chunks and trees).
pub fn edit(src: &dyn Source, tree: &Id, put: &[Put], delete: &[String]) -> Result<std::result::Result<(Id, Vec<Object>), String>> {
    if let Some(bad) = put.iter().map(|p| p.path.as_str()).chain(delete.iter().map(String::as_str)).find(|p| !valid_path(p)) {
        return Ok(Err(bad.to_string()));
    }
    let mut objects = Vec::new();
    let mut ops: Vec<EditOp> = delete.iter().map(|d| EditOp::Delete(d)).collect();
    for p in put {
        let parts = crate::chunk::file_objects(&p.content);
        ops.push(EditOp::Put(&p.path, Entry { kind: p.kind, id: parts[0].id() }));
        objects.extend(parts);
    }
    // Only the folders on the edited paths are read and written again.
    let root = match edit_dir(src, Some(tree), &ops, &mut objects)? {
        Some(root) => root,
        None => {
            let empty = empty();
            let id = empty.id();
            objects.push(empty);
            id
        }
    };
    Ok(Ok((root, objects)))
}

/// An edit, with its path relative to the folder it's applied in. Deletes
/// come first, then writes in order.
#[derive(Clone, Copy)]
enum EditOp<'a> {
    Put(&'a str, Entry),
    Delete(&'a str),
}

/// `tree` (None: a new folder) with `ops` applied; None when nothing's left
/// in it. A file and a folder can't share a path, so writing one replaces
/// the other.
fn edit_dir(src: &dyn Source, tree: Option<&Id>, ops: &[EditOp], objects: &mut Vec<Object>) -> Result<Option<Id>> {
    enum Child<'a> {
        Gone,
        File(Entry),
        Dir(Option<Id>, Vec<EditOp<'a>>),
    }
    let mut entries: BTreeMap<String, Entry> = match tree {
        Some(t) => read(src, t)?.into_iter().map(|e| (e.name, Entry { kind: e.kind, id: e.id })).collect(),
        None => BTreeMap::new(),
    };
    let mut touched: BTreeMap<&str, Child> = BTreeMap::new();
    for op in ops {
        let path = match op {
            EditOp::Put(p, _) | EditOp::Delete(p) => *p,
        };
        let (name, rest) = match path.split_once('/') {
            Some((name, rest)) => (name, Some(rest)),
            None => (path, None),
        };
        let child = touched.entry(name).or_insert_with(|| match entries.get(name) {
            Some(e) if e.kind == EntryKind::Tree => Child::Dir(Some(e.id), Vec::new()),
            Some(e) => Child::File(*e),
            None => Child::Gone,
        });
        match (op, rest) {
            (EditOp::Delete(_), None) => *child = Child::Gone,
            (EditOp::Delete(_), Some(rest)) => {
                if let Child::Dir(_, inner) = child {
                    inner.push(EditOp::Delete(rest));
                }
            }
            (EditOp::Put(_, e), None) => *child = Child::File(*e),
            (EditOp::Put(_, e), Some(rest)) => {
                if !matches!(child, Child::Dir(..)) {
                    *child = Child::Dir(None, Vec::new());
                }
                if let Child::Dir(_, inner) = child {
                    inner.push(EditOp::Put(rest, *e));
                }
            }
        }
    }
    let mut changed = false;
    for (name, child) in touched {
        let next = match child {
            Child::Gone => None,
            Child::File(e) => Some(e),
            Child::Dir(base, inner) if inner.is_empty() => base.map(|id| Entry { kind: EntryKind::Tree, id }),
            Child::Dir(base, inner) => edit_dir(src, base.as_ref(), &inner, objects)?.map(|id| Entry { kind: EntryKind::Tree, id }),
        };
        if entries.get(name) != next.as_ref() {
            changed = true;
            match next {
                Some(e) => entries.insert(name.to_string(), e),
                None => entries.remove(name),
            };
        }
    }
    if !changed && tree.is_some() {
        return Ok(tree.copied());
    }
    if entries.is_empty() {
        return Ok(None);
    }
    let list: Vec<TreeEntry> = entries.into_iter().map(|(name, e)| TreeEntry { name, kind: e.kind, id: e.id }).collect();
    let object = Object::new(Kind::Tree, encode(&list));
    let id = object.id();
    objects.push(object);
    Ok(Some(id))
}

/// What changed from tree `a` to tree `b`, by path, reading only the
/// folders that differ (the same answer as `diff` on both flattened).
pub fn diff_trees(src: &dyn Source, a: &Id, b: &Id) -> Result<BTreeMap<String, Change>> {
    let mut out = BTreeMap::new();
    diff_into(src, Some(a), Some(b), "", &mut out)?;
    Ok(out)
}

fn diff_into(src: &dyn Source, a: Option<&Id>, b: Option<&Id>, prefix: &str, out: &mut BTreeMap<String, Change>) -> Result<()> {
    if a == b {
        return Ok(());
    }
    let entries = |t: Option<&Id>| -> Result<BTreeMap<String, TreeEntry>> {
        Ok(match t {
            Some(t) => read(src, t)?.into_iter().map(|e| (e.name.clone(), e)).collect(),
            None => BTreeMap::new(),
        })
    };
    let (ea, eb) = (entries(a)?, entries(b)?);
    let path = |name: &str| if prefix.is_empty() { name.to_string() } else { format!("{prefix}/{name}") };
    let entry = |e: &TreeEntry| Entry { kind: e.kind, id: e.id };
    for (name, x) in &ea {
        let p = path(name);
        match eb.get(name) {
            Some(y) if x.kind == y.kind && x.id == y.id => {}
            Some(y) => match (x.kind == EntryKind::Tree, y.kind == EntryKind::Tree) {
                (true, true) => diff_into(src, Some(&x.id), Some(&y.id), &p, out)?,
                (true, false) => {
                    diff_into(src, Some(&x.id), None, &p, out)?;
                    out.insert(p, Change::Added(entry(y)));
                }
                (false, true) => {
                    out.insert(p.clone(), Change::Removed(entry(x)));
                    diff_into(src, None, Some(&y.id), &p, out)?;
                }
                (false, false) => {
                    out.insert(p, Change::Modified(entry(x), entry(y)));
                }
            },
            None if x.kind == EntryKind::Tree => diff_into(src, Some(&x.id), None, &p, out)?,
            None => {
                out.insert(p, Change::Removed(entry(x)));
            }
        }
    }
    for (name, y) in &eb {
        if ea.contains_key(name) {
            continue;
        }
        let p = path(name);
        if y.kind == EntryKind::Tree {
            diff_into(src, None, Some(&y.id), &p, out)?;
        } else {
            out.insert(p, Change::Added(entry(y)));
        }
    }
    Ok(())
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::MemSource;

    #[test]
    fn diff_trees_matches_flat_diff() {
        let mut src = MemSource::default();
        let mut put = |list: &[(&str, &str, EntryKind)]| -> Id {
            let files: Files = list.iter().map(|(p, c, k)| (p.to_string(), Entry { kind: *k, id: src.put(Object::blob(c.as_bytes().to_vec())) })).collect();
            let (root, trees) = build(&files);
            for t in trees {
                src.put(t);
            }
            root
        };
        let f = EntryKind::File;
        let a = put(&[("a.txt", "1", f), ("dir/x", "x", f), ("dir/sub/y", "y", f), ("same/z", "z", f), ("gone/q", "q", f), ("file_then_dir", "f", f), ("run", "r", f)]);
        let b = put(&[("a.txt", "2", f), ("dir/x", "x", f), ("dir/sub/y2", "y", f), ("same/z", "z", f), ("new/n", "n", f), ("file_then_dir/inner", "i", f), ("run", "r", EntryKind::Exec)]);
        let flat = diff(&flatten(&src, &a).unwrap(), &flatten(&src, &b).unwrap());
        assert_eq!(diff_trees(&src, &a, &b).unwrap(), flat);
        assert!(diff_trees(&src, &a, &a).unwrap().is_empty());
    }

    #[test]
    fn edit_writes_and_deletes() {
        let mut src = MemSource::default();
        let files: Files = [("a.txt", "1"), ("dir/x", "x"), ("dir/y", "y"), ("f", "f")]
            .iter()
            .map(|(p, c)| (p.to_string(), Entry { kind: EntryKind::File, id: src.put(Object::blob(c.as_bytes().to_vec())) }))
            .collect();
        let (root, trees) = build(&files);
        for t in trees {
            src.put(t);
        }
        let put = vec![
            Put { path: "a.txt".into(), kind: EntryKind::File, content: b"2".to_vec() },
            Put { path: "new/deep/n.sh".into(), kind: EntryKind::Exec, content: b"#!/bin/sh".to_vec() },
            Put { path: "f/inside".into(), kind: EntryKind::File, content: b"i".to_vec() },
        ];
        let (out, objects) = edit(&src, &root, &put, &["dir".to_string()]).unwrap().unwrap();
        for o in objects {
            src.put(o);
        }
        let got = flatten(&src, &out).unwrap();
        assert_eq!(got.keys().cloned().collect::<Vec<_>>(), vec!["a.txt", "f/inside", "new/deep/n.sh"]);
        assert_eq!(got["new/deep/n.sh"].kind, EntryKind::Exec);
        assert_eq!(edit(&src, &root, &[], &["../x".to_string()]).unwrap().unwrap_err(), "../x");
    }

    #[test]
    fn unsafe_names() {
        for bad in [".git", ".GIT", ".git.", ".git ", "Git~1", ".onset", ".Onset.", "onset~1", ".vcs", ".VCS..", "vcs~1", ".g\u{200c}it", "a\\b", "..", ".", "", "x\0y"] {
            assert!(!safe_name(bad), "{bad:?}");
        }
        for good in [".github", ".gitignore", "git", "onset.toml", "vcs.toml", "aux.c", "a:b", ".onsetignore", "...x"] {
            assert!(safe_name(good), "{good:?}");
        }
        assert!(!valid_path("src/.git/hooks/post-checkout"));
        assert!(!valid_path("a/.onset/id"));
        assert!(!valid_path("a/.vcs/id"));
        assert!(valid_path("src/.github/workflows/ci.yml"));

        let mut src = MemSource::default();
        let ok = files(&mut src, &[("a.txt", "a"), ("old\\name", "legacy")]);
        let (old, trees) = build(&ok);
        for t in trees {
            src.put(t);
        }
        let mut worse = ok.clone();
        worse.insert("sub/.git/hooks/post-checkout".into(), Entry { kind: EntryKind::Exec, id: src.put(Object::blob(b"#!/bin/sh\n".to_vec())) });
        let (new, trees) = build(&worse);
        for t in trees {
            src.put(t);
        }
        assert_eq!(unsafe_path(&src, &old, &new).unwrap().as_deref(), Some("sub/.git/hooks/post-checkout"));
        // A path already there isn't held against a change that leaves it alone.
        assert_eq!(unsafe_path(&src, &old, &old).unwrap(), None);
    }

    /// The same root as editing every file and building the tree again.
    #[test]
    fn edit_matches_a_full_rebuild() {
        let mut src = MemSource::default();
        let base = files(&mut src, &[("a.txt", "a"), ("src/b.rs", "b"), ("src/x/c.rs", "c"), ("src/x/d.rs", "d"), ("docs/x.md", "x"), ("f", "f"), ("lone/only", "o")]);
        let (root, trees) = build(&base);
        for t in trees {
            src.put(t);
        }
        let p = |path: &str, content: &str| Put { path: path.into(), kind: EntryKind::File, content: content.as_bytes().to_vec() };
        let cases: Vec<(Vec<Put>, Vec<&str>)> = vec![
            (vec![p("src/x/c.rs", "c2")], vec![]),
            (vec![p("new/deep/file", "n")], vec![]),
            (vec![], vec!["src/x/c.rs", "src/x/d.rs"]),
            (vec![], vec!["lone/only"]),
            (vec![], vec!["src"]),
            (vec![], vec!["nope/missing", "f/inside"]),
            (vec![p("f/inside", "i")], vec![]),
            (vec![p("src", "now a file")], vec![]),
            (vec![p("a.txt", "1"), p("a.txt/b", "2"), p("a.txt", "3")], vec![]),
            (vec![p("docs/x.md", "x")], vec![]),
            (vec![p("docs/y.md", "y")], vec!["docs"]),
            (vec![], vec!["a.txt", "src", "docs", "f", "lone"]),
        ];
        for (put, delete) in cases {
            let delete: Vec<String> = delete.into_iter().map(String::from).collect();
            let mut want = base.clone();
            for d in &delete {
                want.retain(|q, _| !under(q, d));
            }
            for x in &put {
                want.retain(|q, _| !under(q, &x.path) && !under(&x.path, q));
                want.insert(x.path.clone(), Entry { kind: x.kind, id: Object::blob(x.content.clone()).id() });
            }
            let (want_root, _) = build(&want);
            let (got, objects) = edit(&src, &root, &put, &delete).unwrap().unwrap();
            assert_eq!(got, want_root, "put {:?} delete {delete:?}", put.iter().map(|x| &x.path).collect::<Vec<_>>());
            let mut after = src.clone();
            for o in objects {
                after.put(o);
            }
            assert_eq!(flatten(&after, &got).unwrap(), want);
        }
    }

    fn files(src: &mut MemSource, list: &[(&str, &str)]) -> Files {
        list.iter()
            .map(|(p, c)| (p.to_string(), Entry { kind: EntryKind::File, id: src.put(Object::blob(c.as_bytes().to_vec())) }))
            .collect()
    }

    #[test]
    fn build_and_flatten() {
        let mut src = MemSource::default();
        let f = files(&mut src, &[("a.txt", "a"), ("src/b.rs", "b"), ("src/x/c.rs", "c"), ("src.rs", "d")]);
        let (root, trees) = build(&f);
        for t in trees {
            src.put(t);
        }
        assert_eq!(flatten(&src, &root).unwrap(), f);
        let (empty, _) = build(&Files::new());
        assert_eq!(empty, empty_id());
    }

    #[test]
    fn new_objects_only() {
        let mut src = MemSource::default();
        let a = files(&mut src, &[("a.txt", "a"), ("src/b.rs", "b"), ("docs/x.md", "x")]);
        let (ra, ta) = build(&a);
        for t in ta {
            src.put(t);
        }
        let mut b = a.clone();
        b.insert("src/b.rs".into(), Entry { kind: EntryKind::File, id: src.put(Object::blob(b"b2".to_vec())) });
        let (rb, tb) = build(&b);
        for t in tb {
            src.put(t);
        }
        let mut out = Vec::new();
        new_since(&src, Some(&ra), &rb, &mut out).unwrap();
        // The root, src/ and the new blob; docs/ and a.txt are shared.
        assert_eq!(out.len(), 3);
    }

    #[test]
    fn under_paths() {
        assert!(under("src/a.rs", "src"));
        assert!(under("src", "src"));
        assert!(!under("srcx/a.rs", "src"));
        assert!(under("anything", ""));
    }
}