split: added --chunks option

apart from specifying how many rows per chunk with `--size`, you can now also use the mutually exclusive `--chunks` option to specify the number of chunks and `split` will determine the chunk size.

src/cmd/split.rs

@@ -1,6 +1,8 @@
 static USAGE: &str = r#"
 Splits the given CSV data into chunks.
 
+Uses multithreading to go faster if the given CSV data has an index.
+
 The files are written to the directory given with the name '{start}.csv',
 where {start} is the index of the first record of the chunk (starting at 0).
 
@@ -11,12 +13,16 @@ Examples:
 
     qsv split . -s 100 input.csv
 
-    cat in.csv | qsv split outdir -s 1000 
+    cat in.csv | qsv split outdir -s 1000
+
+    qsv split outdir --chunks 10 input.csv
+
+    qsv split outdir -c 10 -j 4 input.csv
 
 For more examples, see https://github.com/jqnatividad/qsv/blob/master/tests/test_split.rs.
 
 Usage:
-    qsv split [options] <outdir> [<input>]
+    qsv split [options] (--size <arg> | --chunks <arg>) <outdir> [<input>]
     qsv split --help
 
 split arguments:
@@ -28,6 +34,14 @@ split arguments:
 split options:
     -s, --size <arg>       The number of records to write into each chunk.
                            [default: 500]
+    -c, --chunks <arg>     The number of chunks to split the data into.
+                           This option is mutually exclusive with --size.
+                           The number of rows in each chunk is determined by
+                           the number of records in the CSV data and the number
+                           of desired chunks. If the number of records is not evenly
+                           divisible by the number of chunks, the last chunk will
+                           have fewer records.
+
     -j, --jobs <arg>       The number of splitting jobs to run in parallel.
                            This only works when the given CSV data has
                            an index already created. Note that a file handle
@@ -69,6 +83,7 @@ struct Args {
     arg_input:       Option<String>,
     arg_outdir:      String,
     flag_size:       usize,
+    flag_chunks:     Option<usize>,
     flag_jobs:       Option<usize>,
     flag_filename:   FilenameTemplate,
     flag_pad:        usize,
@@ -101,11 +116,22 @@ impl Args {
         let mut rdr = rconfig.reader()?;
         let headers = rdr.byte_headers()?.clone();
 
+        let chunk_size = if self.flag_chunks.is_some() {
+            let count = util::count_rows(&rconfig)?;
+            let chunk = self.flag_chunks.unwrap();
+            if chunk == 0 {
+                return fail_incorrectusage_clierror!("--chunk must be greater than 0.");
+            }
+            (count as f64 / chunk as f64).ceil() as usize
+        } else {
+            self.flag_size
+        };
+
         let mut wtr = self.new_writer(&headers, 0, self.flag_pad)?;
         let mut i = 0;
         let mut row = csv::ByteRecord::new();
         while rdr.read_byte_record(&mut row)? {
-            if i > 0 && i % self.flag_size == 0 {
+            if i > 0 && i % chunk_size == 0 {
                 wtr.flush()?;
                 wtr = self.new_writer(&headers, i, self.flag_pad)?;
             }
@@ -117,13 +143,21 @@ impl Args {
     }
 
     fn parallel_split(&self, idx: &Indexed<fs::File, fs::File>) -> CliResult<()> {
-        let nchunks = util::num_of_chunks(idx.count() as usize, self.flag_size);
+        let args = self.clone();
+        let chunk_size;
+        let nchunks = if let Some(flag_chunks) = args.flag_chunks {
+            chunk_size = idx.count() as usize / flag_chunks;
+            flag_chunks
+        } else {
+            chunk_size = args.flag_size;
+            util::num_of_chunks(idx.count() as usize, self.flag_size)
+        };
         if nchunks == 1 {
             // there's only one chunk, we can just do a sequential split
             // which has less overhead and better error handling
             return self.sequential_split();
         }
-        let args = self.clone();
+
         util::njobs(args.flag_jobs);
 
         // safety: we cannot use ? here because we're in a closure
@@ -138,13 +172,13 @@ impl Args {
 
             let mut wtr = args
                 // safety: the only way this can fail is if we cannot create a file
-                .new_writer(headers, i * args.flag_size, args.flag_pad)
+                .new_writer(headers, i * chunk_size, args.flag_pad)
                 .unwrap();
 
             // safety: we know that there is more than one chunk, so we can safely
             // seek to the start of the chunk
-            idx.seek((i * args.flag_size) as u64).unwrap();
-            for row in idx.byte_records().take(args.flag_size) {
+            idx.seek((i * chunk_size) as u64).unwrap();
+            for row in idx.byte_records().take(chunk_size) {
                 let row = row.unwrap();
                 wtr.write_byte_record(&row).unwrap();
             }