feat: don't read non-contributing archives for export

CHANGELOG.md

@@ -7,6 +7,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased](https://git.rustybever.be/Chewing_Bever/alex/src/branch/dev)
 
+### Changed
+
+* Export command no longer reads backups that do not contribute to the final
+  state
+
 ## [0.3.1](https://git.rustybever.be/Chewing_Bever/alex/src/tag/0.3.1)
 
 ### Added

src/backup/delta.rs

@@ -18,11 +18,18 @@ pub struct Delta {
 impl Delta {
     pub fn new() -> Self {
         Self {
-            added: Default::default(),
+            added: State::new(),
-            removed: Default::default(),
+            removed: State::new(),
         }
     }
 
+    /// Returns whether the delta is empty by checking whether both its added and removed state
+    /// return true for their `is_empty`.
+    #[allow(dead_code)]
+    pub fn is_empty(&self) -> bool {
+        self.added.is_empty() && self.removed.is_empty()
+    }
+
     /// Calculate the union of this delta with another delta.
     ///
     /// The union of two deltas is a delta that produces the same state as if you were to apply

src/backup/manager/mod.rs

@@ -226,9 +226,16 @@ where
                         let enc = GzEncoder::new(tar_gz, Compression::default());
                         let mut ar = tar::Builder::new(enc);
 
-                        for (contribution, backup) in
+                        // We only need to consider backups that have a non-empty contribution.
-                            contributions.iter().rev().zip(chain.iter().take(index + 1))
+                        // This allows us to skip reading backups that have been completely
+                        // overwritten by their successors anyways.
+                        for (contribution, backup) in contributions
+                            .iter()
+                            .rev()
+                            .zip(chain.iter().take(index + 1))
+                            .filter(|(contribution, _)| !contribution.is_empty())
                         {
+                            println!("{}", &backup);
                             backup.append(&self.backup_dir, contribution, &mut ar)?;
                         }
 

src/backup/state.rs

@@ -41,6 +41,14 @@ impl State {
             path.starts_with(dir) && files.contains(path.strip_prefix(dir).unwrap())
         })
     }
+
+    /// Returns whether the state is empty.
+    ///
+    /// Note that this does not necessarily mean that the state does not contain any sets, but
+    /// rather that any sets that it does contain are also empty.
+    pub fn is_empty(&self) -> bool {
+        self.0.values().all(|s| s.is_empty())
+    }
 }
 
 impl<T> From<T> for State