Add longest-shortest-path command

brood/src/commands/longest_shortest_path.rs

@@ -0,0 +1,170 @@
+use std::collections::BinaryHeap;
+use std::fs::File;
+use std::io::{self, BufReader};
+use std::path::Path;
+
+use crate::data::{AdjacencyList, LinkInfo, PageInfo};
+use crate::util;
+
+struct DijkstraPageInfo {
+    cost: u32,
+    prev_page_idx: u32,
+    redirect: bool,
+}
+
+impl DijkstraPageInfo {
+    fn from_page_info(info: PageInfo) -> Self {
+        Self {
+            cost: u32::MAX,
+            prev_page_idx: u32::MAX,
+            redirect: info.redirect,
+        }
+    }
+}
+
+struct DijkstraLinkInfo {
+    cost: u32,
+}
+
+impl DijkstraLinkInfo {
+    fn from_link_info(info: LinkInfo) -> Self {
+        Self {
+            cost: 1,
+            // cost: 1000 + info.start,
+            // cost: 10000 + info.start,
+            // cost: 1000 + info.start / 10,
+        }
+    }
+}
+
+#[derive(Clone, Copy, PartialEq, Eq)]
+struct Entry {
+    cost: u32,
+    page_idx: u32,
+}
+
+impl Entry {
+    pub fn new(cost: u32, page_idx: u32) -> Self {
+        Self { cost, page_idx }
+    }
+}
+
+// Manual implementation so the queue is a min-heap instead of a max-heap.
+impl Ord for Entry {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        other
+            .cost
+            .cmp(&self.cost)
+            .then_with(|| self.page_idx.cmp(&other.page_idx))
+    }
+}
+
+impl PartialOrd for Entry {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+/// Closely matches the dijkstra example in [std::collections::binary_heap].
+fn full_dijkstra(
+    data: AdjacencyList<PageInfo, LinkInfo>,
+    from_idx: u32,
+) -> AdjacencyList<DijkstraPageInfo, DijkstraLinkInfo> {
+    println!("> Prepare state");
+    let mut data = data
+        .change_page_data(DijkstraPageInfo::from_page_info)
+        .change_link_data(DijkstraLinkInfo::from_link_info);
+    let mut queue = BinaryHeap::new();
+    data.page_mut(from_idx).data.cost = 0;
+    queue.push(Entry::new(0, from_idx));
+
+    println!("> Run dijkstra");
+    while let Some(Entry { cost, page_idx }) = queue.pop() {
+        let page = data.page(page_idx);
+        if cost > page.data.cost {
+            // This queue entry is outdated
+            continue;
+        }
+
+        let redirect = page.data.redirect;
+        for link_idx in data.link_range(page_idx) {
+            let link = data.link(link_idx);
+
+            let next = Entry {
+                cost: cost + if redirect { 0 } else { link.data.cost },
+                page_idx: link.to,
+            };
+
+            let mut target_page = data.page_mut(link.to);
+            if next.cost < target_page.data.cost {
+                target_page.data.cost = next.cost;
+                target_page.data.prev_page_idx = page_idx;
+                queue.push(next);
+            }
+        }
+    }
+
+    data
+}
+
+fn find_longest_shortest_path(
+    data: AdjacencyList<DijkstraPageInfo, DijkstraLinkInfo>,
+    from_idx: u32,
+) -> Option<Vec<u32>> {
+    let to_idx = data
+        .pages
+        .iter()
+        .enumerate()
+        .filter(|(_, p)| p.data.cost != u32::MAX)
+        .max_by_key(|(_, p)| p.data.cost)?
+        .0 as u32;
+
+    let mut steps = vec![];
+    let mut at_idx = to_idx;
+    loop {
+        steps.push(at_idx);
+        at_idx = data.page(at_idx).data.prev_page_idx;
+        if at_idx == u32::MAX {
+            break;
+        };
+    }
+    steps.reverse();
+    if steps.first() == Some(&from_idx) {
+        Some(steps)
+    } else {
+        None
+    }
+}
+
+pub fn run(datafile: &Path, from: &str) -> io::Result<()> {
+    println!(">> Import");
+    let mut databuf = BufReader::new(File::open(datafile)?);
+    let data = AdjacencyList::read(&mut databuf)?;
+    let pages = data.pages.clone();
+
+    println!(">> Locate from and to");
+    let from_idx = util::resolve_redirects(&data, util::find_index_of_title(&pages, from));
+    println!("From: {:?}", data.page(from_idx).data.title);
+
+    println!(">> Find all shortest paths");
+    let data = full_dijkstra(data, from_idx);
+
+    println!(">> Find longest shortest path");
+    let path = find_longest_shortest_path(data, from_idx);
+
+    if let Some(path) = path {
+        println!("Path found:");
+        for page_idx in path {
+            let page = &pages[page_idx as usize];
+            if page.data.redirect {
+                println!(" v {:?}", page.data.title);
+            } else {
+                println!(" - {:?}", page.data.title);
+            }
+        }
+    } else {
+        println!("No path found");
+    }
+
+    Ok(())
+}