Basic diff output

Cargo.toml

@@ -34,6 +34,7 @@ memmap2 = "0.9.0"
 tempfile = "3.8.0"
 thiserror = "1.0.50"
 clap.workspace = true
+similar = { version = "2.3.0", features = ["inline", "bytes"] }
 
 [dev-dependencies]
 assert_cmd = "2.0.12"

src/cli.rs

@@ -20,6 +20,11 @@ pub struct Options {
     /// format are likely to change in the future).
     pub preview: bool,
 
+    #[arg(short, long)]
+    /// Show changes as a unified diff. You may want to use a rich
+    /// color pager to colorize it.
+    pub diff: bool,
+
     #[arg(
         short = 'F',
         long = "fixed-strings",

src/diff.rs

@@ -0,0 +1,28 @@
+use similar::TextDiff;
+use std::path::Path;
+
+use crate::error::{Error, Result};
+
+pub(crate) fn create_udiff<UO: AsRef<[u8]>, UN: AsRef<[u8]>>(
+    old: UO,
+    new: UN,
+    context: usize,
+    path: Option<&Path>,
+) -> Result<String> {
+    let diff = TextDiff::from_lines(old.as_ref(), new.as_ref());
+    let path = if let Some(path) = path {
+        if let Some(spath) = path.to_str() {
+            spath
+        } else {
+            return Err(Error::InvalidPath(path.into()));
+        }
+    } else {
+        ""
+    };
+
+    let mut udiff = diff.unified_diff();
+    udiff.header(path, path);
+    udiff.context_radius(context);
+
+    Ok(udiff.to_string())
+}

src/main.rs

@@ -2,6 +2,7 @@ mod cli;
 mod error;
 mod input;
 
+pub(crate) mod diff;
 pub(crate) mod replacer;
 
 use clap::Parser;
@@ -69,7 +70,9 @@ fn try_main() -> Result<()> {
             .collect()
     };
 
-    if options.preview || sources.first() == Some(&Source::Stdin) {
+    if options.preview
+        || sources.first() == Some(&Source::Stdin) && !options.diff
+    {
         let mut handle = stdout().lock();
 
         for (source, replaced) in sources.iter().zip(replaced) {
@@ -78,6 +81,21 @@ fn try_main() -> Result<()> {
             }
             handle.write_all(&replaced)?;
         }
+    } else if options.diff {
+        use crate::diff::create_udiff;
+
+        let mut handle = stdout().lock();
+
+        for ((source, mmap), replaced) in
+            sources.iter().zip(&mmaps).zip(replaced)
+        {
+            let path = match source {
+                Source::Stdin => None,
+                Source::File(path) => Some(path.as_path()),
+            };
+            // TODO: custom context radius
+            write!(handle, "{}", create_udiff(mmap, replaced, 3, path)?)?;
+        }
     } else {
         // Windows requires closing mmap before writing:
         // > The requested operation cannot be performed on a file with a user-mapped section open