//! Phase 4: Re-export chain resolution, propagate references through barrel files.

mod propagate;
#[cfg(test)]
mod tests;

use std::path::PathBuf;

use rustc_hash::{FxHashMap, FxHashSet};

use fallow_types::discover::FileId;

use super::ModuleGraph;

use propagate::{StarReExportPropagation, propagate_named_re_export, propagate_star_re_export};

/// A re-export cycle or self-loop detected during Phase 4 chain resolution.
///
/// The graph-layer mirror of `fallow_types::results::ReExportCycle`. Kept in
/// the graph crate so the types crate does not need a dependency arrow back
/// into graph for the conversion; `fallow_core::analyze::re_export_cycles`
/// performs the `GraphReExportCycle` to `ReExportCycle` mapping by reading
/// `is_self_loop` and routing to the matching `ReExportCycleKind` variant.
#[derive(Debug, Clone)]
pub struct GraphReExportCycle {
    /// Member files participating in the cycle, sorted lexicographically by
    /// the `Path::display()` form (matches the existing diagnostic-output
    /// sort). For a self-loop, exactly one entry.
    pub files: Vec<PathBuf>,
    /// Parallel array to `files`: the FileId for each member. Kept alongside
    /// the paths so the core-layer detector can call
    /// `suppressions.is_file_suppressed(id, IssueKind::ReExportCycle)`
    /// without an extra path-to-FileId lookup.
    pub file_ids: Vec<FileId>,
    /// `true` for single-file self-re-exports (`export * from './'`), `false`
    /// for multi-node strongly connected components.
    pub is_self_loop: bool,
}

/// A single re-export edge collected from the module graph.
///
/// Replaces an earlier ad-hoc 5-tuple so the propagation loop is more
/// readable and the new `is_type_only` field carried into
/// [`propagate_star_re_export`] does not get lost in tuple-index plumbing.
struct ReExportTuple {
    barrel: FileId,
    source: FileId,
    imported_name: String,
    exported_name: String,
    /// `true` when the triggering re-export edge is `export type * from ...`
    /// or `export type { foo } from ...`. Threaded into star propagation so
    /// any synthetic stub created on the source module reflects the chain's
    /// type-only-ness instead of defaulting to `false`.
    is_type_only: bool,
}

impl ModuleGraph {
    /// Resolve re-export chains: when module A re-exports from B,
    /// any reference to A's re-exported symbol should also count as a reference
    /// to B's original export (and transitively through the chain).
    ///
    /// Returns the list of re-export cycles and self-loops detected during
    /// the upfront Tarjan SCC pass. The caller stores this on the
    /// `ModuleGraph` so the `re-export-cycle` finding type can surface them
    /// to users instead of relying on `RUST_LOG=warn` (see issue #515).
    pub(super) fn resolve_re_export_chains(&mut self) -> Vec<GraphReExportCycle> {
        let re_export_info: Vec<ReExportTuple> = self
            .modules
            .iter()
            .flat_map(|m| {
                m.re_exports.iter().map(move |re| ReExportTuple {
                    barrel: m.file_id,
                    source: re.source_file,
                    imported_name: re.imported_name.clone(),
                    exported_name: re.exported_name.clone(),
                    is_type_only: re.is_type_only,
                })
            })
            .collect();

        if re_export_info.is_empty() {
            return Vec::new();
        }

        let cycles = find_re_export_cycles(&self.modules, &re_export_info);

        let mut entry_star_targets: FxHashSet<FileId> = self
            .modules
            .iter()
            .filter(|m| m.is_entry_point())
            .flat_map(|m| {
                m.re_exports
                    .iter()
                    .filter(|re| re.exported_name == "*")
                    .map(|re| re.source_file)
            })
            .collect();
        let mut entry_star_stack: Vec<FileId> = entry_star_targets.iter().copied().collect();
        while let Some(file_id) = entry_star_stack.pop() {
            let idx = file_id.0 as usize;
            if idx >= self.modules.len() {
                continue;
            }

            for re in self.modules[idx]
                .re_exports
                .iter()
                .filter(|re| re.exported_name == "*")
            {
                if entry_star_targets.insert(re.source_file) {
                    entry_star_stack.push(re.source_file);
                }
            }
        }

        let mut edges_by_target: FxHashMap<FileId, Vec<usize>> = FxHashMap::default();
        for (idx, edge) in self.edges.iter().enumerate() {
            edges_by_target.entry(edge.target).or_default().push(idx);
        }

        let safety_cap = re_export_info.len().saturating_add(1);
        let mut changed = true;
        let mut iteration: usize = 0;
        let mut existing_refs: FxHashSet<FileId> = FxHashSet::default();
        let mut synthetic_stubs: FxHashSet<(FileId, String)> = FxHashSet::default();

        while changed && iteration < safety_cap {
            changed = false;
            iteration += 1;

            for entry in &re_export_info {
                let barrel_idx = entry.barrel.0 as usize;
                let source_idx = entry.source.0 as usize;

                if barrel_idx >= self.modules.len() || source_idx >= self.modules.len() {
                    continue;
                }

                if entry.exported_name == "*" {
                    changed |= propagate_star_re_export(StarReExportPropagation {
                        modules: &mut self.modules,
                        edges: &self.edges,
                        edges_by_target: &edges_by_target,
                        barrel_id: entry.barrel,
                        barrel_idx,
                        source_id: entry.source,
                        source_idx,
                        entry_star_targets: &entry_star_targets,
                        triggering_is_type_only: entry.is_type_only,
                        synthetic_stubs: &mut synthetic_stubs,
                    });
                } else {
                    changed |= propagate_named_re_export(
                        &mut self.modules,
                        entry.barrel,
                        barrel_idx,
                        source_idx,
                        &entry.imported_name,
                        &entry.exported_name,
                        &mut existing_refs,
                    );
                }
            }
        }

        if iteration >= safety_cap && changed {
            tracing::error!(
                iterations = iteration,
                safety_cap,
                re_export_edges = re_export_info.len(),
                "Re-export chain fixpoint exceeded safety cap; \
                 propagation may be non-monotonic. Please file a bug at \
                 https://github.com/fallow-rs/fallow/issues with the repro."
            );
        }

        cycles
    }
}

/// Find SCCs of size >= 2 in the re-export subgraph and self-re-export
/// edges, emit one `tracing::warn!` per cycle, AND return structured cycle
/// data for the user-visible `re-export-cycle` finding type.
///
/// The `tracing::warn!` emissions remain unchanged from #442 (RUST_LOG=warn
/// operators still see them). The returned `Vec<GraphReExportCycle>` is the
/// structured surface that `fallow_core::analyze::re_export_cycles` consumes
/// and wraps in typed `ReExportCycleFinding`s for end-user output. See
/// issue #515.
fn find_re_export_cycles(
    modules: &[super::types::ModuleNode],
    re_export_info: &[ReExportTuple],
) -> Vec<GraphReExportCycle> {
    let mut cycles: Vec<GraphReExportCycle> = Vec::new();

    let mut node_index: FxHashMap<FileId, usize> = FxHashMap::default();
    let mut nodes: Vec<FileId> = Vec::new();
    for entry in re_export_info {
        for &id in &[entry.barrel, entry.source] {
            node_index.entry(id).or_insert_with(|| {
                let idx = nodes.len();
                nodes.push(id);
                idx
            });
        }
    }

    let n = nodes.len();
    if n == 0 {
        return cycles;
    }

    let mut adj: Vec<Vec<usize>> = vec![Vec::new(); n];
    let mut seen_edge: FxHashSet<(usize, usize)> = FxHashSet::default();
    let mut seen_self_loop: FxHashSet<FileId> = FxHashSet::default();
    for entry in re_export_info {
        let from = node_index[&entry.barrel];
        let to = node_index[&entry.source];
        if from == to {
            if seen_self_loop.insert(entry.barrel) {
                let i = entry.barrel.0 as usize;
                let (path_buf, path_display) = if i < modules.len() {
                    let p = modules[i].path.clone();
                    let d = p.display().to_string();
                    (p, d)
                } else {
                    (
                        PathBuf::from(format!("<file id {i}>")),
                        format!("<file id {i}>"),
                    )
                };
                tracing::warn!(
                    file = path_display.as_str(),
                    "Re-export self-loop detected: this file re-exports from \
                     itself. Chain propagation is structurally a no-op for \
                     these edges. Inspect the barrel for an accidental \
                     `export * from './<this-file>'` after a rename or move."
                );
                cycles.push(GraphReExportCycle {
                    files: vec![path_buf],
                    file_ids: vec![entry.barrel],
                    is_self_loop: true,
                });
            }
            continue;
        }
        if seen_edge.insert((from, to)) {
            adj[from].push(to);
        }
    }

    let sccs = tarjan_scc(n, &adj);

    for scc in &sccs {
        if scc.len() < 2 {
            continue;
        }
        let mut triples: Vec<(PathBuf, String, FileId)> = scc
            .iter()
            .map(|&idx| {
                let file_id = nodes[idx];
                let i = file_id.0 as usize;
                if i < modules.len() {
                    let p = modules[i].path.clone();
                    let d = p.display().to_string();
                    (p, d, file_id)
                } else {
                    let placeholder = format!("<file id {i}>");
                    (PathBuf::from(&placeholder), placeholder, file_id)
                }
            })
            .collect();
        triples.sort_by(|a, b| a.1.cmp(&b.1));
        let members = triples
            .iter()
            .map(|(_, d, _)| d.as_str())
            .collect::<Vec<_>>()
            .join(" <-> ");
        tracing::warn!(
            cycle_size = scc.len(),
            members = members.as_str(),
            "Re-export cycle detected: chain propagation may be incomplete \
             for symbols on this barrel loop. Break the cycle to restore \
             full reachability analysis."
        );
        let (files, file_ids) = triples.into_iter().fold(
            (Vec::new(), Vec::new()),
            |(mut paths, mut ids), (p, _, id)| {
                paths.push(p);
                ids.push(id);
                (paths, ids)
            },
        );
        cycles.push(GraphReExportCycle {
            files,
            file_ids,
            is_self_loop: false,
        });
    }

    cycles
}

/// Iterative Tarjan's strongly connected components, returns SCCs that
/// contain at least one node. The graph is given as adjacency-by-index;
/// the caller maps node indices back to FileIds.
fn tarjan_scc(n: usize, adj: &[Vec<usize>]) -> Vec<Vec<usize>> {
    use fixedbitset::FixedBitSet;

    let mut index_counter: u32 = 0;
    let mut indices: Vec<u32> = vec![u32::MAX; n];
    let mut lowlinks: Vec<u32> = vec![0; n];
    let mut on_stack = FixedBitSet::with_capacity(n);
    let mut stack: Vec<usize> = Vec::new();
    let mut sccs: Vec<Vec<usize>> = Vec::new();

    struct Frame {
        node: usize,
        next_succ: usize,
    }

    for start in 0..n {
        if indices[start] != u32::MAX {
            continue;
        }
        let mut dfs: Vec<Frame> = vec![Frame {
            node: start,
            next_succ: 0,
        }];
        indices[start] = index_counter;
        lowlinks[start] = index_counter;
        index_counter = index_counter.saturating_add(1);
        stack.push(start);
        on_stack.insert(start);

        while let Some(frame) = dfs.last_mut() {
            let v = frame.node;
            if frame.next_succ < adj[v].len() {
                let w = adj[v][frame.next_succ];
                frame.next_succ = frame.next_succ.saturating_add(1);
                if indices[w] == u32::MAX {
                    indices[w] = index_counter;
                    lowlinks[w] = index_counter;
                    index_counter = index_counter.saturating_add(1);
                    stack.push(w);
                    on_stack.insert(w);
                    dfs.push(Frame {
                        node: w,
                        next_succ: 0,
                    });
                } else if on_stack.contains(w) {
                    lowlinks[v] = lowlinks[v].min(indices[w]);
                }
            } else {
                if lowlinks[v] == indices[v] {
                    let mut scc = Vec::new();
                    while let Some(w) = stack.pop() {
                        on_stack.remove(w);
                        scc.push(w);
                        if w == v {
                            break;
                        }
                    }
                    sccs.push(scc);
                }
                dfs.pop();
                if let Some(parent) = dfs.last_mut() {
                    let pv = parent.node;
                    lowlinks[pv] = lowlinks[pv].min(lowlinks[v]);
                }
            }
        }
    }

    sccs
}
