// Copyright (c) 2025-2026 Zensical and contributors

// SPDX-License-Identifier: MIT
// All contributions are certified under the DCO

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// of this software and associated documentation files (the "Software"), to
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// The above copyright notice and this permission notice shall be included in
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// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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// IN THE SOFTWARE.

// ----------------------------------------------------------------------------

//! Iterator over paths between two nodes.

use crate::graph::topology::Adjacency;
use crate::graph::Graph;

// ----------------------------------------------------------------------------
// Structs
// ----------------------------------------------------------------------------

/// Iterator over paths between two nodes.
pub struct Paths<'a> {
    /// Outgoing edges.
    outgoing: &'a Adjacency,
    /// Target node.
    target: usize,
    /// Stack for depth-first search.
    stack: Vec<(usize, usize)>,
    /// Current path.
    path: Vec<usize>,
}

// ----------------------------------------------------------------------------
// Implementations
// ----------------------------------------------------------------------------

impl<T> Graph<T> {
    /// Creates an iterator over the paths between the given nodes.
    ///
    /// # Panics
    ///
    /// Panics if the node does not exist, as this indicates that there's a bug
    /// in the code that creates or uses the iterator. While the [`Builder`][]
    /// is designed to be fallible to ensure the structure is valid, methods
    /// that operate on [`Graph`] panic on violated invariants.
    ///
    /// [`Builder`]: crate::graph::Builder
    ///
    /// # Examples
    ///
    /// ```
    /// # use std::error::Error;
    /// # fn main() -> Result<(), Box<dyn Error>> {
    /// use zrx_graph::Graph;
    ///
    /// // Create graph builder and add nodes
    /// let mut builder = Graph::builder();
    /// let a = builder.add_node("a");
    /// let b = builder.add_node("b");
    /// let c = builder.add_node("c");
    ///
    /// // Create edges between nodes
    /// builder.add_edge(a, b)?;
    /// builder.add_edge(b, c)?;
    ///
    /// // Create graph from builder
    /// let graph = builder.build();
    ///
    /// // Create iterator over paths
    /// for path in graph.paths(a, c) {
    ///     println!("{path:?}");
    /// }
    /// # Ok(())
    /// # }
    /// ```
    #[inline]
    #[must_use]
    pub fn paths(&self, source: usize, target: usize) -> Paths<'_> {
        Paths {
            outgoing: self.topology.outgoing(),
            target,
            stack: Vec::from([(source, 0)]),
            path: Vec::from([source]),
        }
    }
}

// ----------------------------------------------------------------------------
// Trait implementations
// ----------------------------------------------------------------------------

impl Iterator for Paths<'_> {
    type Item = Vec<usize>;

    /// Returns the next path.
    fn next(&mut self) -> Option<Self::Item> {
        // Perform a depth-first search to find all paths from the source to
        // the target, and emit them in the order of discovery
        while let Some((node, depth)) = self.stack.pop() {
            // Backtrack by truncating the current path to the depth of the
            // current node, and then add the current node to the path
            self.path.truncate(depth);
            self.path.push(node);

            // In case we've reached the target, emit the current path. Note
            // that we need to clone it, since we can't return a reference
            if node == self.target {
                return Some(self.path.clone());
            }

            // Add descendants to stack in reverse order for consistent depth-
            // first ordering. Additionally, perform a debug assertion to ensure
            // that we don't revisit nodes within the current path, which would
            // lead to infinite loops, but should never happen in a DAG.
            for &descendant in self.outgoing[node].iter().rev() {
                debug_assert!(!self.path.contains(&descendant));
                self.stack.push((descendant, depth + 1));
            }
        }

        // No more paths to return
        None
    }
}

// ----------------------------------------------------------------------------
// Tests
// ----------------------------------------------------------------------------

#[cfg(test)]
mod tests {

    mod paths {
        use crate::graph;

        #[test]
        fn handles_graph() {
            let graph = graph! {
                "a" => "b", "a" => "c",
                "b" => "d", "b" => "e",
                "c" => "f",
                "d" => "g",
                "e" => "g", "e" => "h",
                "f" => "h",
                "g" => "i",
                "h" => "i",
            };
            assert_eq!(
                graph.paths(0, 8).collect::<Vec<_>>(),
                vec![
                    vec![0, 1, 3, 6, 8],
                    vec![0, 1, 4, 6, 8],
                    vec![0, 1, 4, 7, 8],
                    vec![0, 2, 5, 7, 8],
                ]
            );
        }

        #[test]
        fn handles_graph_and_self_path() {
            let graph = graph! {
                "a" => "b", "a" => "c",
                "b" => "d", "b" => "e",
                "c" => "f",
                "d" => "g",
                "e" => "g", "e" => "h",
                "f" => "h",
                "g" => "i",
                "h" => "i",
            };
            assert_eq!(
                graph.paths(0, 0).collect::<Vec<_>>(), // fmt
                vec![vec![0]]
            );
        }

        #[test]
        fn handles_graph_and_non_existing_path() {
            let graph = graph! {
                "a" => "b", "a" => "c",
                "b" => "d", "b" => "e",
                "c" => "f",
                "d" => "g",
                "e" => "g", "e" => "h",
                "f" => "h",
                "g" => "i",
                "h" => "i",
            };
            assert_eq!(
                graph.paths(8, 0).collect::<Vec<_>>(),
                vec![] as Vec<Vec<usize>>
            );
        }

        #[test]
        fn handles_multi_graph() {
            let graph = graph! {
                "a" => "b", "a" => "c", "a" => "c",
                "b" => "d", "b" => "e",
                "c" => "f",
                "d" => "g",
                "e" => "g", "e" => "h",
                "f" => "h",
                "g" => "i",
                "h" => "i",
            };
            assert_eq!(
                graph.paths(0, 8).collect::<Vec<_>>(),
                vec![
                    vec![0, 1, 3, 6, 8],
                    vec![0, 1, 4, 6, 8],
                    vec![0, 1, 4, 7, 8],
                    vec![0, 2, 5, 7, 8],
                    vec![0, 2, 5, 7, 8],
                ]
            );
        }

        #[test]
        fn handles_multi_graph_and_self_path() {
            let graph = graph! {
                "a" => "b", "a" => "c", "a" => "c",
                "b" => "d", "b" => "e",
                "c" => "f",
                "d" => "g",
                "e" => "g", "e" => "h",
                "f" => "h",
                "g" => "i",
                "h" => "i",
            };
            assert_eq!(
                graph.paths(0, 0).collect::<Vec<_>>(), // fmt
                vec![vec![0]]
            );
        }

        #[test]
        fn handles_multi_graph_and_non_existing_path() {
            let graph = graph! {
                "a" => "b", "a" => "c", "a" => "c",
                "b" => "d", "b" => "e",
                "c" => "f",
                "d" => "g",
                "e" => "g", "e" => "h",
                "f" => "h",
                "g" => "i",
                "h" => "i",
            };
            assert_eq!(
                graph.paths(8, 0).collect::<Vec<_>>(),
                vec![] as Vec<Vec<usize>>
            );
        }
    }
}
