lib: Implement 'only' that creates a dependency chain from stdin

When building closed loops of packages you sometime don't want the
entire chain to be resolved to just figure out the order of 5 packages.
This implement `--only` which resolves the chain and ensures only the
packages from stdin are included in the output chain.

This is useful for rebuild lists or when building tight loops of
packages.

    $ arch-rebuild-order -o cmake meson jsoncpp
    meson jsoncpp cmake
    $ arch-rebuild-order -o jsoncpp meson cmake
    meson jsoncpp cmake

Signed-off-by: Morten Linderud <morten@linderud.pw>

src/args.rs

@@ -22,4 +22,8 @@ pub struct Args {
     /// Write a dotfile into the given file
     #[structopt(short, long)]
     pub dotfile: Option<String>,
+
+    /// Only use the pkgnames provided as input
+    #[structopt(short, long)]
+    pub only: bool,
 }

src/lib.rs

@@ -79,6 +79,7 @@ pub fn run(
     dbpath: Option<String>,
     repos: Vec<String>,
     dotfile: Option<String>,
+    only: bool,
 ) -> Result<String> {
     let pacman = match dbpath {
         Some(path) => alpm::Alpm::new(ROOT_DIR, &path),
@@ -165,15 +166,14 @@ pub fn run(
     // shows it as last package
     rebuild_order_packages.reverse();
 
-    let mut output = String::new();
-    for elem in rebuild_order_packages.iter() {
-        output.push_str(elem);
-        output.push(' ');
-    }
+    // We only retain the packages we want to when using `--only`
+    // This logic is hard to parse because retain is an inverse filter,
+    // thus we use the negated form of: only && !pkgnames.contains(&pkg.to_string()
+    rebuild_order_packages.retain(|pkg| !only || pkgnames.contains(&pkg.to_string()));
 
     if let Some(filename) = dotfile {
         write_dotfile(filename, graph)?;
     }
 
-    Ok(output)
+    Ok(rebuild_order_packages.join(" "))
 }

src/main.rs

@@ -4,7 +4,13 @@ use arch_rebuild_order::args::Args;
 
 fn main() {
     let args = Args::from_args();
-    match arch_rebuild_order::run(args.pkgnames, args.dbpath, args.repos, args.dotfile) {
+    match arch_rebuild_order::run(
+        args.pkgnames,
+        args.dbpath,
+        args.repos,
+        args.dotfile,
+        args.only,
+    ) {
         Ok(output) => {
             println!("{}", output);
             std::process::exit(0);