Main: Allow specifying which number of files to start with

This patch introduces a new flag `-s` or `--start`, allowing users to
start running the tool with a starting index into the list of hashed
files generated from traversing either the repository or a single
directory.

src/main.rs

@@ -17,6 +17,9 @@ struct Args {
     #[clap(short, long, default_value_t = 0)]
     jobs: u8,
 
+    #[clap(short, long, default_value_t = 0)]
+    start: usize,
+
     /// If the path argument should be used to check all files in a directory it points to instead of a restic repository (use for testing)
     #[clap(long)]
     single_directory: bool
@@ -25,6 +28,7 @@ struct Args {
 fn run() -> Result<()> {
     let args = Args::parse();
     let path = Path::new(&args.path);
+    let start = args.start;
 
     let mut directories_with_hashed_files = Vec::new();
 
@@ -60,6 +64,19 @@ fn run() -> Result<()> {
 
     let pool = ThreadPoolBuilder::new().num_threads(args.jobs as usize).build().unwrap();
 
+    let hashed_files_len = hashed_files.len();
+
+    if start != 0 {
+        match hashed_files_len.checked_sub(start) {
+            None => return Err(Error::new(ErrorKind::Other, format!("Specified start at {} is larger than the total number of files {}", start, hashed_files_len))),
+            Some(remaining) => {
+                println!("Starting at {} files, {} remaining...", start, remaining);
+
+                hashed_files.drain(0..start);
+            }
+        }
+    }
+
     pool.install(|| {
         return hashed_files.par_iter().progress_with(progress_bar).try_for_each(|path| -> Result<()> {
             // Just opening the file and hashing using io::copy is roughly ~2.5x fater compared to sha256::digest_file