implement sort --stable

Made a new function sort_by(lines, compare_fns), which accepts a
list of compare_fns and calls lines.sort_by() with a closure that
calls each compare_fn in turn until one returns something other
than equal.

Default behavior ensures that String::cmp is the last element in the
compare_fns list (referred to as 'last resort' sorting by man sort).
Passing --stable (-s) turns this behaviour off.

Test cases provided for `sort --month` and `sort --month --stable`.

src/sort/sort.rs

@@ -43,6 +43,7 @@ struct Settings {
     mode: SortMode,
     reverse: bool,
     outfile: Option<String>,
+    stable: bool,
     unique: bool,
     check: bool,
 }
@@ -53,6 +54,7 @@ impl Default for Settings {
             mode: SortMode::Default,
             reverse: false,
             outfile: None,
+            stable: false,
             unique: false,
             check: false,
         }
@@ -70,6 +72,7 @@ pub fn uumain(args: Vec<String>) -> i32 {
     opts.optflag("h", "help", "display this help and exit");
     opts.optflag("", "version", "output version information and exit");
     opts.optopt("o", "output", "write output to FILENAME instead of stdout", "FILENAME");
+    opts.optflag("s", "stable", "stabilize sort by disabling last-resort comparison");
     opts.optflag("u", "unique", "output only the first of an equal run");
     opts.optflag("V", "version-sort", "Sort by SemVer version number, eg 1.12.2 > 1.1.2");
     opts.optflag("c", "check", "check for sorted input; do not sort");
@@ -112,6 +115,7 @@ With no FILE, or when FILE is -, read standard input.", NAME, VERSION);
 
     settings.reverse = matches.opt_present("reverse");
     settings.outfile = matches.opt_str("output");
+    settings.stable = matches.opt_present("stable");
     settings.unique = matches.opt_present("unique");
     settings.check = matches.opt_present("check");
 
@@ -146,14 +150,25 @@ fn exec(files: Vec<String>, settings: &Settings) -> i32 {
 
     let original_lines = lines.to_vec();
 
-    match settings.mode {
+    let mut compare_fns = Vec::new();
-        SortMode::Numeric => lines.sort_by(numeric_compare),
+
-        SortMode::HumanNumeric => lines.sort_by(human_numeric_size_compare),
+    compare_fns.push(match settings.mode {
-        SortMode::Month => lines.sort_by(month_compare),
+        SortMode::Numeric => numeric_compare,
-        SortMode::Version => lines.sort_by(version_compare),
+        SortMode::HumanNumeric => human_numeric_size_compare,
-        SortMode::Default => lines.sort()
+        SortMode::Month => month_compare,
+        SortMode::Version => version_compare,
+        SortMode::Default => String::cmp
+    });
+
+    if !settings.stable {
+        match settings.mode {
+            SortMode::Default => {}
+            _ => compare_fns.push(String::cmp)
+        }
     }
 
+    sort_by(&mut lines, compare_fns);
+
     if settings.unique {
         lines.dedup()
     }
@@ -178,6 +193,20 @@ fn exec(files: Vec<String>, settings: &Settings) -> i32 {
 
 }
 
+fn sort_by<F>(lines: &mut Vec<String>, compare_fns: Vec<F>)
+    where F: Fn( &String, &String ) -> Ordering
+{
+    lines.sort_by(|a, b| {
+        for compare_fn in &compare_fns {
+            let cmp = compare_fn(a, b);
+            if cmp != Ordering::Equal {
+                return cmp;
+            }
+        }
+        return Ordering::Equal;
+    })
+}
+
 /// Parse the beginning string into an f64, returning -inf instead of NaN on errors.
 fn permissive_f64_parse(a: &str) -> f64 {
     // Maybe should be split on non-digit, but then 10e100 won't parse properly.

tests/fixtures/sort/month_default.expected

@@ -3,5 +3,8 @@ Jan Lorem ipsum dolor sit amet
 mar laboris nisi ut aliquip ex ea
 May sed do eiusmod tempor incididunt
 JUN nostrud exercitation ullamco
+Jul 1 should remain 2,1,3
+Jul 2 these three lines
+Jul 3 if --stable is provided
 Oct ut labore et dolore magna aliqua
 Dec consectetur adipiscing elit

tests/fixtures/sort/month_default.txt

@@ -5,3 +5,6 @@ Oct ut labore et dolore magna aliqua
 N/A Ut enim ad minim veniam, quis
 JUN nostrud exercitation ullamco
 mar laboris nisi ut aliquip ex ea
+Jul 2 these three lines
+Jul 1 should remain 2,1,3
+Jul 3 if --stable is provided

tests/fixtures/sort/month_stable.expected

@@ -0,0 +1,10 @@
+N/A Ut enim ad minim veniam, quis
+Jan Lorem ipsum dolor sit amet
+mar laboris nisi ut aliquip ex ea
+May sed do eiusmod tempor incididunt
+JUN nostrud exercitation ullamco
+Jul 2 these three lines
+Jul 1 should remain 2,1,3
+Jul 3 if --stable is provided
+Oct ut labore et dolore magna aliqua
+Dec consectetur adipiscing elit

tests/fixtures/sort/month_stable.txt

@@ -0,0 +1,10 @@
+Jan Lorem ipsum dolor sit amet
+Dec consectetur adipiscing elit
+May sed do eiusmod tempor incididunt
+Oct ut labore et dolore magna aliqua
+N/A Ut enim ad minim veniam, quis
+JUN nostrud exercitation ullamco
+mar laboris nisi ut aliquip ex ea
+Jul 2 these three lines
+Jul 1 should remain 2,1,3
+Jul 3 if --stable is provided

tests/test_sort.rs

@@ -37,6 +37,11 @@ fn test_month_default() {
     test_helper("month_default", "-M");
 }
 
+#[test]
+fn test_month_stable() {
+    test_helper("month_stable", "-Ms");
+}
+
 #[test]
 fn test_default_unsorted_ints() {
     test_helper("default_unsorted_ints", "");