patch: add support for configuring suppress_blank_empty (#38)

Add support for configuring `--suppress-blank-empty` as is possible with
GNU diff and git-diff. This allows suppressing the trailing space that
would otherwise exist when formatting a patch with an empty context
line.

Author: bmwill

src/diff/tests.rs

@@ -642,6 +642,81 @@ void Chunk_copy(Chunk *src, size_t src_start, Chunk *dst, size_t dst_start, size
     assert_patch!(original, a, expected_diffy);
 }
 
+#[test]
+fn suppress_blank_empty() {
+    let original = "\
+1
+2
+3
+
+4
+";
+
+    let modified = "\
+1
+2
+3
+
+5
+";
+
+    // Note that there is a space " " on the line after 3
+    let expected = "\
+--- original
++++ modified
+@@ -2,4 +2,4 @@
+ 2
+ 3
+ 
+-4
++5
+";
+
+    let f = PatchFormatter::new().suppress_blank_empty(false);
+    let patch = create_patch(original, modified);
+    let bpatch = create_patch_bytes(original.as_bytes(), modified.as_bytes());
+    let patch_str = format!("{}", f.fmt_patch(&patch));
+    let mut patch_bytes = Vec::new();
+    f.write_patch_into(&bpatch, &mut patch_bytes).unwrap();
+
+    assert_eq!(patch_str, expected);
+    assert_eq!(patch_bytes, patch_str.as_bytes());
+    assert_eq!(patch_bytes, expected.as_bytes());
+    assert_eq!(apply(original, &patch).unwrap(), modified);
+    assert_eq!(
+        crate::apply_bytes(original.as_bytes(), &bpatch).unwrap(),
+        modified.as_bytes()
+    );
+
+    // Note that there is no space " " on the line after 3
+    let expected_suppressed = "\
+--- original
++++ modified
+@@ -2,4 +2,4 @@
+ 2
+ 3
+
+-4
++5
+";
+
+    let f = PatchFormatter::new().suppress_blank_empty(true);
+    let patch = create_patch(original, modified);
+    let bpatch = create_patch_bytes(original.as_bytes(), modified.as_bytes());
+    let patch_str = format!("{}", f.fmt_patch(&patch));
+    let mut patch_bytes = Vec::new();
+    f.write_patch_into(&bpatch, &mut patch_bytes).unwrap();
+
+    assert_eq!(patch_str, expected_suppressed);
+    assert_eq!(patch_bytes, patch_str.as_bytes());
+    assert_eq!(patch_bytes, expected_suppressed.as_bytes());
+    assert_eq!(apply(original, &patch).unwrap(), modified);
+    assert_eq!(
+        crate::apply_bytes(original.as_bytes(), &bpatch).unwrap(),
+        modified.as_bytes()
+    );
+}
+
 // In the event that a patch has an invalid hunk range we want to ensure that when apply is
 // attempting to search for a matching position to apply a hunk that the search algorithm runs in
 // time bounded by the length of the original image being patched. Before clamping the search space

src/patch/format.rs

@@ -10,6 +10,7 @@ use std::{
 pub struct PatchFormatter {
     with_color: bool,
     with_missing_newline_message: bool,
+    suppress_blank_empty: bool,
 
     context: Style,
     delete: Style,
@@ -26,6 +27,10 @@ impl PatchFormatter {
             with_color: false,
             with_missing_newline_message: true,
 
+            // TODO the default in git-diff and GNU diff is to have this set to false, on the next
+            // semver breaking release we should contemplate switching this to be false by default
+            suppress_blank_empty: true,
+
             context: Style::new(),
             delete: Color::Red.normal(),
             insert: Color::Green.normal(),
@@ -54,6 +59,20 @@ impl PatchFormatter {
         self
     }
 
+    /// Sets whether to suppress printing of a space before empty lines.
+    ///
+    /// Defaults to `true`.
+    ///
+    /// For more information you can refer to the [Omitting trailing blanks] manual page of GNU
+    /// diff or the [diff.suppressBlankEmpty] config for `git-diff`.
+    ///
+    /// [Omitting trailing blanks]: https://www.gnu.org/software/diffutils/manual/html_node/Trailing-Blanks.html
+    /// [diff.suppressBlankEmpty]: https://git-scm.com/docs/git-diff#Documentation/git-diff.txt-codediffsuppressBlankEmptycode
+    pub fn suppress_blank_empty(mut self, enable: bool) -> Self {
+        self.suppress_blank_empty = enable;
+        self
+    }
+
     /// Returns a `Display` impl which can be used to print a Patch
     pub fn fmt_patch<'a>(&'a self, patch: &'a Patch<'a, str>) -> impl Display + 'a {
         PatchDisplay { f: self, patch }
@@ -240,7 +259,7 @@ impl<T: AsRef<[u8]> + ?Sized> LineDisplay<'_, T> {
             write!(w, "{}", style.prefix())?;
         }
 
-        if sign == ' ' && line == b"\n" {
+        if self.f.suppress_blank_empty && sign == ' ' && line == b"\n" {
             w.write_all(line)?;
         } else {
             write!(w, "{}", sign)?;
@@ -274,7 +293,7 @@ impl Display for LineDisplay<'_, str> {
             write!(f, "{}", style.prefix())?;
         }
 
-        if sign == ' ' && *line == "\n" {
+        if self.f.suppress_blank_empty && sign == ' ' && *line == "\n" {
             write!(f, "{}", line)?;
         } else {
             write!(f, "{}{}", sign, line)?;