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+//! Atward implements TVL's redirection service, living at
+//! atward.tvl.fyi
+//!
+//! This service is designed to be added as a search engine to web
+//! browsers and attempts to send users to useful locations based on
+//! their search query (falling back to another search engine).
+use regex::Regex;
+use rouille::input::cookies;
+use rouille::{Request, Response};
+
+#[cfg(test)]
+mod tests;
+
+/// A query handler supported by atward. It consists of a pattern on
+/// which to match and trigger the query, and a function to execute
+/// that returns the target URL.
+struct Handler {
+    /// Regular expression on which to match the query string.
+    pattern: Regex,
+
+    /// Function to construct the target URL. If the pattern matches,
+    /// this is invoked with the captured matches and the entire URI.
+    ///
+    /// Returning `None` causes atward to fall through to the next
+    /// query (and eventually to the default search engine).
+    target: for<'s> fn(&Query, regex::Captures<'s>) -> Option<String>,
+}
+
+/// An Atward query supplied by a user.
+#[derive(Debug, PartialEq)]
+struct Query {
+    /// Query string itself.
+    query: String,
+
+    /// Should Sourcegraph be used instead of cgit?
+    cs: bool,
+}
+
+/// Helper function for setting a parameter based on a query
+/// parameter.
+fn query_setting(req: &Request, config: &mut bool, param: &str) {
+    match req.get_param(param) {
+        Some(s) if s == "true" => *config = true,
+        Some(s) if s == "false" => *config = false,
+        _ => {}
+    }
+}
+
+impl Query {
+    fn from_request(req: &Request) -> Option<Query> {
+        // First extract the actual search query ...
+        let mut query = match req.get_param("q") {
+            Some(query) => Query { query, cs: false },
+            None => return None,
+        };
+
+        // ... then apply settings to it. Settings in query parameters
+        // take precedence over cookies.
+        for cookie in cookies(req) {
+            match cookie {
+                ("cs", "true") => {
+                    query.cs = true;
+                }
+                _ => {}
+            }
+        }
+
+        query_setting(req, &mut query.cs, "cs");
+
+        Some(query)
+    }
+}
+
+#[cfg(test)]
+impl From<&str> for Query {
+    fn from(query: &str) -> Query {
+        Query {
+            query: query.to_string(),
+            cs: false,
+        }
+    }
+}
+
+/// Create a URL to a file (and, optionally, specific line) in cgit.
+fn cgit_url(path: &str) -> String {
+    if path.ends_with(".md") {
+        format!("https://code.tvl.fyi/about/{}", path)
+    } else {
+        format!("https://code.tvl.fyi/tree/{}", path)
+    }
+}
+
+/// Create a URL to a path in Sourcegraph.
+fn sourcegraph_path_url(path: &str) -> String {
+    format!("https://cs.tvl.fyi/depot/-/tree/{}", path)
+}
+/// Definition of all supported query handlers in atward.
+fn handlers() -> Vec<Handler> {
+    vec![
+        // Bug IDs (e.g. b/123)
+        Handler {
+            pattern: Regex::new("^b/(?P<bug>\\d+)$").unwrap(),
+            target: |_, captures| Some(format!("https://b.tvl.fyi/{}", &captures["bug"])),
+        },
+        // Changelists (e.g. cl/42)
+        Handler {
+            pattern: Regex::new("^cl/(?P<cl>\\d+)$").unwrap(),
+            target: |_, captures| Some(format!("https://cl.tvl.fyi/{}", &captures["cl"])),
+        },
+        // Non-parameterised short hostnames should redirect to $host.tvl.fyi
+        Handler {
+            pattern: Regex::new("^(?P<host>b|cl|cs|code|at|todo)$").unwrap(),
+            target: |_, captures| Some(format!("https://{}.tvl.fyi/", &captures["host"])),
+        },
+        // Depot revisions (e.g. r/3002)
+        Handler {
+            pattern: Regex::new("^r/(?P<rev>\\d+)$").unwrap(),
+            target: |_, captures| {
+                Some(format!(
+                    "https://code.tvl.fyi/commit/?id=refs/r/{}",
+                    &captures["rev"]
+                ))
+            },
+        },
+        // Depot paths (e.g. //web/atward or //ops/nixos/whitby/default.nix)
+        // TODO(tazjin): Add support for specifying lines in a query parameter
+        Handler {
+            pattern: Regex::new("^//(?P<path>[a-zA-Z].*)?$").unwrap(),
+            target: |query, captures| {
+                // Pass an empty string if the path is missing, to
+                // redirect to the depot root.
+                let path = captures.name("path").map(|m| m.as_str()).unwrap_or("");
+
+                if query.cs {
+                    Some(sourcegraph_path_url(path))
+                } else {
+                    Some(cgit_url(path))
+                }
+            },
+        },
+    ]
+}
+
+/// Attempt to match against all known query types, and return the
+/// destination URL if one is found.
+fn dispatch(handlers: &[Handler], query: &Query) -> Option<String> {
+    for handler in handlers {
+        if let Some(captures) = handler.pattern.captures(&query.query) {
+            if let Some(destination) = (handler.target)(query, captures) {
+                return Some(destination);
+            }
+        }
+    }
+
+    None
+}
+
+/// Return the opensearch.xml file which is required for adding atward
+/// as a search engine in Firefox.
+fn opensearch() -> Response {
+    Response::text(include_str!("opensearch.xml"))
+        .with_unique_header("Content-Type", "application/opensearchdescription+xml")
+}
+
+/// Render the atward index page which gives users some information
+/// about how to use the service.
+fn index() -> Response {
+    Response::html(include_str!(env!("ATWARD_INDEX_HTML")))
+}
+
+/// Render the fallback page which informs users that their query is
+/// unsupported.
+fn fallback() -> Response {
+    Response::text("error for emphasis that i am angery and the query whimchst i angery atward")
+        .with_status_code(404)
+}
+
+fn main() {
+    let queries = handlers();
+    let address = std::env::var("ATWARD_LISTEN_ADDRESS")
+        .expect("ATWARD_LISTEN_ADDRESS environment variable must be set");
+
+    rouille::start_server(&address, move |request| {
+        rouille::log(&request, std::io::stderr(), || {
+            if request.url() == "/opensearch.xml" {
+                return opensearch();
+            }
+
+            let query = match Query::from_request(&request) {
+                Some(q) => q,
+                None => return index(),
+            };
+
+            match dispatch(&queries, &query) {
+                None => fallback(),
+                Some(destination) => Response::redirect_303(destination),
+            }
+        })
+    });
+}