refactor(cheddar): Split out a library with rendering logic

Splits `main.rs` into `lib.rs` and `bin/cheddar.rs`, which enables
reuse of cheddar's rendering logic in other Rust applications.

Change-Id: Ifd1a44a8d1620c595550a0a497a25b0563e917ca
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3060
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tools/cheddar/src/bin/cheddar.rs

@@ -0,0 +1,135 @@
+//! This file defines the binary for cheddar, which can be interacted
+//! with in two different ways:
+//!
+//! 1. As a CLI tool that acts as a cgit filter.
+//! 2. As a long-running HTTP server that handles rendering requests
+//!    (matching the SourceGraph protocol).
+use clap::{App, Arg};
+use rouille::Response;
+use rouille::{router, try_or_400};
+use serde::Deserialize;
+use serde_json::json;
+use std::collections::HashMap;
+use std::io;
+
+use cheddar::{THEMES, format_code, format_markdown};
+
+// Server endpoint for rendering the syntax of source code. This
+// replaces the 'syntect_server' component of Sourcegraph.
+fn code_endpoint(request: &rouille::Request) -> rouille::Response {
+    #[derive(Deserialize)]
+    struct SourcegraphQuery {
+        filepath: String,
+        theme: String,
+        code: String,
+    }
+
+    let query: SourcegraphQuery = try_or_400!(rouille::input::json_input(request));
+    let mut buf: Vec<u8> = Vec::new();
+
+    // We don't use syntect with the sourcegraph themes bundled
+    // currently, so let's fall back to something that is kind of
+    // similar (tm).
+    let theme = &THEMES.themes[match query.theme.as_str() {
+        "Sourcegraph (light)" => "Solarized (light)",
+        _ => "Solarized (dark)",
+    }];
+
+    format_code(theme, &mut query.code.as_bytes(), &mut buf, &query.filepath);
+
+    Response::json(&json!({
+        "is_plaintext": false,
+        "data": String::from_utf8_lossy(&buf)
+    }))
+}
+
+// Server endpoint for rendering a Markdown file.
+fn markdown_endpoint(request: &rouille::Request) -> rouille::Response {
+    let mut texts: HashMap<String, String> = try_or_400!(rouille::input::json_input(request));
+
+    for text in texts.values_mut() {
+        let mut buf: Vec<u8> = Vec::new();
+        format_markdown(&mut text.as_bytes(), &mut buf);
+        *text = String::from_utf8_lossy(&buf).to_string();
+    }
+
+    Response::json(&texts)
+}
+
+fn highlighting_server(listen: &str) {
+    println!("Starting syntax highlighting server on '{}'", listen);
+
+    rouille::start_server(listen, move |request| {
+        router!(request,
+                // Markdown rendering route
+                (POST) (/markdown) => {
+                    markdown_endpoint(request)
+                },
+
+                // Code rendering route
+                (POST) (/) => {
+                    code_endpoint(request)
+                },
+
+                _ => {
+                    rouille::Response::empty_404()
+                },
+        )
+    });
+}
+
+fn main() {
+    // Parse the command-line flags passed to cheddar to determine
+    // whether it is running in about-filter mode (`--about-filter`)
+    // and what file extension has been supplied.
+    let matches = App::new("cheddar")
+        .about("TVL's syntax highlighter")
+        .arg(
+            Arg::with_name("about-filter")
+                .help("Run as a cgit about-filter (renders Markdown)")
+                .long("about-filter")
+                .takes_value(false),
+        )
+        .arg(
+            Arg::with_name("sourcegraph-server")
+                .help("Run as a Sourcegraph compatible web-server")
+                .long("sourcegraph-server")
+                .takes_value(false),
+        )
+        .arg(
+            Arg::with_name("listen")
+                .help("Address to listen on")
+                .long("listen")
+                .takes_value(true),
+        )
+        .arg(Arg::with_name("filename").help("File to render").index(1))
+        .get_matches();
+
+    if matches.is_present("sourcegraph-server") {
+        highlighting_server(
+            matches
+                .value_of("listen")
+                .expect("Listening address is required for server mode"),
+        );
+        return;
+    }
+
+    let filename = matches.value_of("filename").expect("filename is required");
+
+    let stdin = io::stdin();
+    let mut in_handle = stdin.lock();
+
+    let stdout = io::stdout();
+    let mut out_handle = stdout.lock();
+
+    if matches.is_present("about-filter") && filename.ends_with(".md") {
+        format_markdown(&mut in_handle, &mut out_handle);
+    } else {
+        format_code(
+            &THEMES.themes["InspiredGitHub"],
+            &mut in_handle,
+            &mut out_handle,
+            filename,
+        );
+    }
+}