split: use iterator to produce filenames

Replace the `FilenameFactory` with `FilenameIterator` and calls to
`FilenameFactory::make()` with calls to `FilenameIterator::next()`. We
did not need the fully generality of being able to produce the
filename for an arbitrary chunk index. Instead we need only iterate
over filenames one after another. This allows for a less
mathematically dense algorithm that is easier to understand and
maintain. Furthermore, it can be connected to some familiar concepts
from the representation of numbers as a sequence of digits.

This does not change the behavior of the `split` program, just the
implementation of how filenames are produced.

Co-authored-by: Terts Diepraam <terts.diepraam@gmail.com>

src/uu/split/src/split.rs

@@ -8,9 +8,10 @@
 // spell-checker:ignore (ToDO) PREFIXaa
 
 mod filenames;
+mod number;
 mod platform;
 
-use crate::filenames::FilenameFactory;
+use crate::filenames::FilenameIterator;
 use clap::{crate_version, App, AppSettings, Arg, ArgMatches};
 use std::convert::TryFrom;
 use std::env;
@@ -384,7 +385,7 @@ where
     let chunk_size = (num_bytes / (num_chunks as u64)) as usize;
 
     // This object is responsible for creating the filename for each chunk.
-    let filename_factory = FilenameFactory::new(
+    let mut filename_iterator = FilenameIterator::new(
         &settings.prefix,
         &settings.additional_suffix,
         settings.suffix_length,
@@ -394,9 +395,9 @@ where
     // Create one writer for each chunk. This will create each
     // of the underlying files (if not in `--filter` mode).
     let mut writers = vec![];
-    for i in 0..num_chunks {
-        let filename = filename_factory
-            .make(i)
+    for _ in 0..num_chunks {
+        let filename = filename_iterator
+            .next()
             .ok_or_else(|| USimpleError::new(1, "output file suffixes exhausted"))?;
         let writer = platform::instantiate_current_writer(&settings.filter, filename.as_str());
         writers.push(writer);
@@ -462,17 +463,16 @@ fn split(settings: Settings) -> UResult<()> {
     };
 
     // This object is responsible for creating the filename for each chunk.
-    let filename_factory = FilenameFactory::new(
+    let mut filename_iterator = FilenameIterator::new(
         &settings.prefix,
         &settings.additional_suffix,
         settings.suffix_length,
         settings.numeric_suffix,
     );
-    let mut fileno = 0;
     loop {
         // Get a new part file set up, and construct `writer` for it.
-        let filename = filename_factory
-            .make(fileno)
+        let filename = filename_iterator
+            .next()
             .ok_or_else(|| USimpleError::new(1, "output file suffixes exhausted"))?;
         let mut writer = platform::instantiate_current_writer(&settings.filter, filename.as_str());
 
@@ -509,8 +509,6 @@ fn split(settings: Settings) -> UResult<()> {
         if settings.verbose {
             println!("creating file {}", filename.quote());
         }
-
-        fileno += 1;
     }
     Ok(())
 }