main: new argument -m to give a minimal saving

Some compressions yield only a few hundred lines saving which isn't worth
the database update and the time needed to validate the compression. With
the new commandline argument `-m` the output would be suppressed, if not
at least the given number of database records would be saved.

src/main.rs

@@ -119,6 +119,13 @@ fn main() {
                 .help("The maximum state group to process up to")
                 .takes_value(true)
                 .required(false),
+        ).arg(
+            Arg::with_name("min_saved_rows")
+                .short("m")
+                .value_name("COUNT")
+                .help("Suppress output if fewer than COUNT rows would be saved")
+                .takes_value(true)
+                .required(false),
         ).arg(
             Arg::with_name("output_file")
                 .short("o")
@@ -165,6 +172,10 @@ fn main() {
         .value_of("max_state_group")
         .map(|s| s.parse().expect("max_state_group must be an integer"));
 
+    let min_saved_rows = matches
+        .value_of("min_saved_rows")
+        .map(|v| v.parse().expect("COUNT must be an integer"));
+
     let transactions = matches.is_present("transactions");
 
     let level_sizes = value_t_or_exit!(matches, "level_sizes", LevelSizes);
@@ -217,6 +228,17 @@ fn main() {
         compressor.stats.state_groups_changed
     );
 
+    if let Some(min) = min_saved_rows {
+        let saving = (original_summed_size - compressed_summed_size) as i32;
+        if saving < min {
+            println!(
+                "Only {} rows would be saved by this compression. Skipping output.",
+                saving
+            );
+            return;
+        }
+    }
+
     // If we are given an output file, we output the changes as SQL. If the
     // `transactions` argument is set we wrap each change to a state group in a
     // transaction.