introduce backend modules

src/backend/mod.rs

@@ -0,0 +1,14 @@
+mod pacman;
+
+use std::collections::HashSet;
+
+use crate::Package;
+
+pub use pacman::Pacman;
+
+pub trait Backend {
+    /// Get all packages that are installed in the system.
+    fn get_all_installed_packages() -> HashSet<Package>;
+    /// Get all packages that were installed in the system explicitly.
+    fn get_explicitly_installed_packages() -> HashSet<Package>;
+}

src/backend/pacman.rs

@@ -0,0 +1,42 @@
+use std::collections::HashSet;
+
+use alpm::Alpm;
+use alpm::PackageReason::Explicit;
+
+use super::Backend;
+use crate::Package;
+
+pub struct Pacman;
+
+impl Backend for Pacman {
+    fn get_all_installed_packages() -> HashSet<Package> {
+        convert_to_pacdef_packages(get_all_installed_packages_from_alpm())
+    }
+
+    fn get_explicitly_installed_packages() -> HashSet<Package> {
+        convert_to_pacdef_packages(get_explicitly_installed_packages_from_alpm())
+    }
+}
+
+fn get_all_installed_packages_from_alpm() -> HashSet<String> {
+    let db = Alpm::new("/", "/var/lib/pacman").unwrap();
+    db.localdb()
+        .pkgs()
+        .iter()
+        .map(|p| p.name().to_string())
+        .collect()
+}
+
+fn get_explicitly_installed_packages_from_alpm() -> HashSet<String> {
+    let db = Alpm::new("/", "/var/lib/pacman").unwrap();
+    db.localdb()
+        .pkgs()
+        .iter()
+        .filter(|p| p.reason() == Explicit)
+        .map(|p| p.name().to_string())
+        .collect()
+}
+
+fn convert_to_pacdef_packages(packages: HashSet<String>) -> HashSet<Package> {
+    packages.into_iter().map(Package::from).collect()
+}