feature: use textwrap crate for wrapping help texts

The textwrap crate uses a simpler linear-time algorithm for wrapping
the text. The current algorithm in wrap_help uses several O(n) calls
to String::insert and String::remove, which makes it potentially
quadratic in complexity.

... benchcmp

Notice how the wrapping_newline_chars test was updated -- the old
algorithm had a subtle bug where it would break lines too early. That
is, it wrapped the text like

    ARGS:
        <mode>    x, max, maximum   20 characters, contains
                  symbols.
                  l, long           Copy-friendly,
                  14 characters, contains symbols.
                  m, med, medium    Copy-friendly, 8
                  characters, contains symbols.";

when it should really have wrapped it like

    ARGS:
        <mode>    x, max, maximum   20 characters, contains
                  symbols.
                  l, long           Copy-friendly, 14
                  characters, contains symbols.
                  m, med, medium    Copy-friendly, 8
                  characters, contains symbols.";

Notice how the word "14" was incorrectly moved to the next line. There
is clearly room for the word on the line with the "l, long" option
since there is room for "contains" just above it.

The algorithm in textwrap handles this case correctly.

Cargo.toml

@@ -20,6 +20,7 @@ bitflags              = "0.8.0"
 vec_map               = "0.8"
 unicode-width         = "0.1.4"
 unicode-segmentation  = "1.0.1"
+textwrap              = "0.6.0"
 strsim    = { version = "0.6.0",  optional = true }
 ansi_term = { version = "0.9.0",  optional = true }
 term_size = { version = "0.3.0",  optional = true }

src/app/help.rs

@@ -17,7 +17,7 @@ use app::usage;
 use unicode_width::UnicodeWidthStr;
 #[cfg(feature = "wrap_help")]
 use term_size;
-use unicode_segmentation::UnicodeSegmentation;
+use textwrap;
 use vec_map::VecMap;
 
 #[cfg(not(feature = "wrap_help"))]
@@ -955,45 +955,13 @@ impl<'a> Help<'a> {
 }
 
 fn wrap_help(help: &mut String, longest_w: usize, avail_chars: usize) {
-    debugln!("Help::wrap_help: longest_w={}, avail_chars={}",
-             longest_w,
-             avail_chars);
-    debug!("Help::wrap_help: Enough space to wrap...");
+    // Keep previous behavior of not wrapping at all if one of the
+    // words would overflow the line.
     if longest_w < avail_chars {
-        sdebugln!("Yes");
-        let mut prev_space = 0;
-        let mut j = 0;
-        for (idx, g) in (&*help.clone()).grapheme_indices(true) {
-            debugln!("Help::wrap_help:iter: idx={}, g={}", idx, g);
-            if g == "\n" {
-                debugln!("Help::wrap_help:iter: Newline found...");
-                debugln!("Help::wrap_help:iter: Still space...{:?}",
-                         str_width(&help[j..idx]) < avail_chars);
-                if str_width(&help[j..idx]) < avail_chars {
-                    j = idx;
-                    continue;
-                }
-            } else if g != " " {
-                if idx != help.len() - 1 || str_width(&help[j..idx]) < avail_chars {
-                    continue;
-                }
-                debugln!("Help::wrap_help:iter: Reached the end of the line and we're over...");
-            } else if str_width(&help[j..idx]) <= avail_chars {
-                debugln!("Help::wrap_help:iter: Space found with room...");
-                prev_space = idx;
-                continue;
-            }
-            debugln!("Help::wrap_help:iter: Adding Newline...");
-            j = prev_space;
-            debugln!("Help::wrap_help:iter: prev_space={}, j={}", prev_space, j);
-            debugln!("Help::wrap_help:iter: Removing...{}", j);
-            debugln!("Help::wrap_help:iter: Char at {}: {:?}", j, &help[j..j + 1]);
-            help.remove(j);
-            help.insert(j, '\n');
-            prev_space = idx;
-        }
-    } else {
-        sdebugln!("No");
+        *help = help.lines()
+            .map(|line| textwrap::fill(line, avail_chars))
+            .collect::<Vec<String>>()
+            .join("\n");
     }
 }
 

src/lib.rs

@@ -541,6 +541,7 @@ extern crate bitflags;
 extern crate vec_map;
 #[cfg(feature = "wrap_help")]
 extern crate term_size;
+extern crate textwrap;
 extern crate unicode_segmentation;
 #[cfg(feature = "color")]
 extern crate atty;

tests/help.rs

@@ -253,8 +253,8 @@ FLAGS:
 ARGS:
     <mode>    x, max, maximum   20 characters, contains
               symbols.
-              l, long           Copy-friendly,
-              14 characters, contains symbols.
+              l, long           Copy-friendly, 14
+              characters, contains symbols.
               m, med, medium    Copy-friendly, 8
               characters, contains symbols.";