use anyhow::Context as _;
use gix::Commit;
use gix::ObjectId;
use gix::Repository;
use gix::bstr::BStr;
use gix::bstr::ByteSlice;
use gix::date::Time;
use gix::object::tree::EntryRef;
use gix::objs::decode::Error as DecodeError;
use gix::revision::walk::Sorting;
use gix::traverse::commit::simple::CommitTimeOrder;
use std::collections::HashSet;

#[cfg(feature = "time-budget")]
use std::time::Duration;
#[cfg(feature = "time-budget")]
use std::time::Instant;

mod cli;

fn main() -> anyhow::Result<()> {
    let arguments = cli::parse();

    #[cfg(feature = "time-budget")]
    let (time_limit, start) = (Duration::from_millis(arguments.time_limit), Instant::now());

    let repository = gix::open(arguments.repository)?;

    let mut current_commit = repository.resolve_revspec(&arguments.from)?;
    let mut current_tree = current_commit
        .tree()
        .context("Failed to get tree for initial commit")?;

    if let Some(subtree) = &arguments.subtree {
        let entry = current_tree
            .lookup_entry_by_path(subtree)?
            .ok_or_else(|| anyhow::anyhow!("Failed to find subtree: {subtree:?}"))?;

        if !entry.mode().is_tree() {
            Err(anyhow::anyhow!("{subtree:?} is not a subtree"))?;
        }

        current_tree = repository
            .find_tree(entry.id())
            .context("Searching for subtree entry's tree")?;
    }

    // Build a set of the entry file names we are interested in.
    let mut interested: HashSet<_> = Default::default();
    for entry in current_tree.iter() {
        interested.insert(entry?.filename().to_owned());
    }

    let name_pad = 2 + interested
        .iter()
        .fold(0, |len, filename| std::cmp::max(len, filename.len()));

    let mut rev_walk = repository
        .rev_walk([current_commit.id])
        .use_commit_graph(true)
        .first_parent_only()
        .sorting(Sorting::ByCommitTime(CommitTimeOrder::NewestFirst));

    if let Some(boundary) = arguments.to {
        let stop_commit = repository.resolve_revspec(&boundary)?;
        rev_walk = rev_walk.with_boundary([stop_commit.id]);
    }

    let output = |filename: &BStr, commit: &Commit| -> anyhow::Result<()> {
        let id = commit.id;
        let ts = format_ts(&commit.time()?, arguments.preserve_tz)?;
        let message = commit.message()?.summary();
        let escaped_filename = format!("{filename:?}");
        println!("{escaped_filename:<name_pad$}\t{id}\t{ts}\t{message}",);
        Ok(())
    };

    for revision in rev_walk.all()? {
        if interested.is_empty() {
            break;
        }

        #[cfg(feature = "time-budget")]
        if start.elapsed() > time_limit {
            break;
        }

        let revision = revision?;
        let Some(&parent_id) = revision.parent_ids.first() else {
            // The revision has no parents. Assume any remaining entries belong
            // to this commit.
            for entry in interested.drain() {
                output(entry.as_bstr(), &current_commit)?;
            }
            break;
        };

        let parent_commit = repository
            .find_commit(parent_id)
            .context("Failed to find parent commit")?;
        let mut parent_tree = parent_commit
            .tree()
            .context("Failed to retrieve parent tree")?;

        if let Some(subtree) = &arguments.subtree {
            let Some(entry) = parent_tree.lookup_entry_by_path(subtree)? else {
                // The subtree does not exist in this revision. Assume any
                // remaining entries of interest belong to this commit.
                for entry in interested.drain() {
                    output(entry.as_bstr(), &current_commit)?;
                }
                break;
            };
            if !entry.mode().is_tree() {
                // The subtree is no longer a subtree in this revision. Assume any
                // remaining entries of interest belong to this commit.
                for entry in interested.drain() {
                    output(entry.as_bstr(), &current_commit)?;
                }
                break;
            }
            parent_tree = repository.find_tree(entry.id())?;
        }

        let mut scanner = EntryScanner::new(current_tree.iter(), parent_tree.iter())?;
        while let Some(change) = scanner.next_change()? {
            if interested.remove(change.filename()) {
                output(change.filename(), &current_commit)?;
            }
        }

        current_commit = parent_commit;
    }

    if !interested.is_empty() {
        for entry in interested.drain() {
            println!("{:?}", entry.as_bstr());
        }
    }

    Ok(())
}

struct EntryScanner<'repo, 'a, I> {
    current_tree: I,
    current_entry: Option<EntryRef<'repo, 'a>>,
    parent_tree: I,
    parent_entry: Option<EntryRef<'repo, 'a>>,
}

impl<'repo, 'a, I> EntryScanner<'repo, 'a, I>
where
    I: Iterator<Item = Result<EntryRef<'repo, 'a>, DecodeError>>,
{
    pub fn new(current_tree: I, parent_tree: I) -> Result<Self, DecodeError> {
        let mut this = Self {
            current_tree,
            parent_tree,
            current_entry: None,
            parent_entry: None,
        };

        this.advance_both()?;
        Ok(this)
    }

    fn advance_current(&mut self) -> Result<Option<EntryRef<'repo, 'a>>, DecodeError> {
        let old = self.current_entry.take();
        self.current_entry = self.current_tree.next().transpose()?;
        Ok(old)
    }

    fn advance_parent(&mut self) -> Result<Option<EntryRef<'repo, 'a>>, DecodeError> {
        let old = self.parent_entry.take();
        self.parent_entry = self.parent_tree.next().transpose()?;
        Ok(old)
    }

    fn advance_both(
        &mut self,
    ) -> Result<(Option<EntryRef<'repo, 'a>>, Option<EntryRef<'repo, 'a>>), DecodeError> {
        Ok((self.advance_current()?, self.advance_parent()?))
    }

    pub fn is_complete(&self) -> bool {
        self.current_entry.is_none() && self.parent_entry.is_none()
    }

    /// Get the next change between the two trees.
    pub fn next_change(&mut self) -> Result<Option<Change<'a>>, DecodeError> {
        while !self.is_complete() {
            match (&self.current_entry, &self.parent_entry) {
                (None, None) => break,
                (Some(current), Some(parent)) if current.filename() == parent.filename() => {
                    if current.id() != parent.id() {
                        return Ok(self
                            .advance_both()?
                            .0
                            .map(|entry| Change::Modify(entry.detach())));
                    }
                    self.advance_both()?;
                    continue;
                }
                (Some(current), Some(parent)) => {
                    if current.filename() < parent.filename() {
                        return Ok(self
                            .advance_current()?
                            .map(|entry| Change::Create(entry.detach())));
                    } else {
                        return Ok(self
                            .advance_parent()?
                            .map(|entry| Change::Delete(entry.detach())));
                    }
                }
                (Some(_), None) => {
                    // Entry was created by the current commit.
                    return Ok(self
                        .advance_current()?
                        .map(|entry| Change::Create(entry.detach())));
                }
                (None, Some(_)) => {
                    // Entry was deleted by the current commit.
                    return Ok(self
                        .advance_parent()?
                        .map(|entry| Change::Delete(entry.detach())));
                }
            }
        }

        Ok(None)
    }
}

#[derive(Debug)]
pub enum Change<'a> {
    Create(gix::objs::tree::EntryRef<'a>),
    Modify(gix::objs::tree::EntryRef<'a>),
    Delete(gix::objs::tree::EntryRef<'a>),
}

impl<'a> Change<'a> {
    pub fn filename(&self) -> &BStr {
        match self {
            Self::Create(entry) => entry.filename,
            Self::Modify(entry) => entry.filename,
            Self::Delete(entry) => entry.filename,
        }
    }
}

fn format_ts(time: &Time, preserve_tz: bool) -> anyhow::Result<String> {
    use time::OffsetDateTime;
    use time::UtcOffset;
    use time::format_description::well_known::Rfc3339;

    let odt = OffsetDateTime::from_unix_timestamp(time.seconds)?
        .to_offset(UtcOffset::from_whole_seconds(time.offset)?);

    let formatted = match preserve_tz {
        true => odt.format(&Rfc3339)?,
        false => odt.to_utc().format(&Rfc3339)?,
    };

    Ok(formatted)
}

trait ResolveRevSpec {
    type Error;
    /// Resolve a commit ID, reference, or symbolic reference to a [`Commit`].
    fn resolve_revspec<'repo>(&'repo self, spec: &str) -> Result<Commit<'repo>, Self::Error>;
}

impl ResolveRevSpec for Repository {
    type Error = anyhow::Error;
    fn resolve_revspec<'repo>(&'repo self, spec: &str) -> Result<Commit<'repo>, Self::Error> {
        use std::str::FromStr;

        let resolved = match ObjectId::from_str(spec) {
            Ok(id) => self
                .find_commit(id)
                .context("Failed to find commit in repository")?,
            Err(_) => {
                let id = self
                    .find_reference(spec)
                    .context("Failed to resolve reference in repository")?
                    .into_fully_peeled_id()?;

                self.find_commit(id)
                    .context("Failed to find commit in repository")?
            }
        };

        Ok(resolved)
    }
}