add imara-diff::UnifiedDiffBuilder as basis.

It should be the basis for providing a more general way to obtain such diffs.

gix-diff/src/blob/mod.rs

@@ -11,6 +11,10 @@ pub mod pipeline;
 ///
 pub mod platform;
 
+///
+pub mod unified_diff;
+pub use unified_diff::_impl::UnifiedDiff;
+
 /// Information about the diff performed to detect similarity.
 #[derive(Debug, Default, Clone, Copy, PartialEq, PartialOrd)]
 pub struct DiffLineStats {

gix-diff/src/blob/unified_diff.rs

@@ -0,0 +1,174 @@
+//! Originally based on https://github.com/pascalkuthe/imara-diff/pull/14.
+//!
+
+/// Defines the size of the context printed before and after each change.
+///
+/// Similar to the `-U` option in git diff or gnu-diff. If the context overlaps
+/// with previous or next change, the context gets reduced accordingly.
+#[derive(Debug, Copy, Clone, Hash, PartialEq, Eq, Ord, PartialOrd)]
+pub struct ContextSize {
+    /// Defines the size of the context printed before and after each change.
+    symmetrical: u32,
+}
+
+impl Default for ContextSize {
+    fn default() -> Self {
+        ContextSize::symmetrical(3)
+    }
+}
+
+/// Instantiation
+impl ContextSize {
+    /// Create a symmetrical context with `n` lines before and after a changed hunk.
+    pub fn symmetrical(n: u32) -> Self {
+        ContextSize { symmetrical: n }
+    }
+}
+
+pub(super) mod _impl {
+    use imara_diff::{intern, Sink};
+    use std::fmt::{Display, Write};
+    use std::hash::Hash;
+    use std::ops::Range;
+
+    use super::ContextSize;
+    use intern::{InternedInput, Interner, Token};
+
+    /// A [`Sink`] that creates a textual diff
+    /// in the format typically output by git or gnu-diff if the `-u` option is used
+    pub struct UnifiedDiff<'a, W, T>
+    where
+        W: Write,
+        T: Hash + Eq + Display,
+    {
+        before: &'a [Token],
+        after: &'a [Token],
+        interner: &'a Interner<T>,
+
+        pos: u32,
+        before_hunk_start: u32,
+        after_hunk_start: u32,
+        before_hunk_len: u32,
+        after_hunk_len: u32,
+        /// Symmetrical context before and after the changed hunk.
+        ctx_size: u32,
+
+        buffer: String,
+        dst: W,
+    }
+
+    impl<'a, T> UnifiedDiff<'a, String, T>
+    where
+        T: Hash + Eq + Display,
+    {
+        /// Create a new `UnifiedDiffBuilder` for the given `input`,
+        /// displaying `context_size` lines of context around each change,
+        /// that will return a [`String`].
+        pub fn new(input: &'a InternedInput<T>, context_size: ContextSize) -> Self {
+            Self {
+                before_hunk_start: 0,
+                after_hunk_start: 0,
+                before_hunk_len: 0,
+                after_hunk_len: 0,
+                buffer: String::with_capacity(8),
+                dst: String::new(),
+                interner: &input.interner,
+                before: &input.before,
+                after: &input.after,
+                pos: 0,
+                ctx_size: context_size.symmetrical,
+            }
+        }
+    }
+
+    impl<'a, W, T> UnifiedDiff<'a, W, T>
+    where
+        W: Write,
+        T: Hash + Eq + Display,
+    {
+        /// Create a new `UnifiedDiffBuilder` for the given `input`,
+        /// displaying `context_size` lines of context around each change,
+        /// that will writes it output to the provided implementation of [`Write`].
+        pub fn with_writer(input: &'a InternedInput<T>, writer: W, context_size: Option<u32>) -> Self {
+            Self {
+                before_hunk_start: 0,
+                after_hunk_start: 0,
+                before_hunk_len: 0,
+                after_hunk_len: 0,
+                buffer: String::with_capacity(8),
+                dst: writer,
+                interner: &input.interner,
+                before: &input.before,
+                after: &input.after,
+                pos: 0,
+                ctx_size: context_size.unwrap_or(3),
+            }
+        }
+
+        fn print_tokens(&mut self, tokens: &[Token], prefix: char) {
+            for &token in tokens {
+                writeln!(&mut self.buffer, "{prefix}{}", self.interner[token]).unwrap();
+            }
+        }
+
+        fn flush(&mut self) {
+            if self.before_hunk_len == 0 && self.after_hunk_len == 0 {
+                return;
+            }
+
+            let end = (self.pos + self.ctx_size).min(self.before.len() as u32);
+            self.update_pos(end, end);
+
+            writeln!(
+                &mut self.dst,
+                "@@ -{},{} +{},{} @@",
+                self.before_hunk_start + 1,
+                self.before_hunk_len,
+                self.after_hunk_start + 1,
+                self.after_hunk_len,
+            )
+            .unwrap();
+            write!(&mut self.dst, "{}", &self.buffer).unwrap();
+            self.buffer.clear();
+            self.before_hunk_len = 0;
+            self.after_hunk_len = 0
+        }
+
+        fn update_pos(&mut self, print_to: u32, move_to: u32) {
+            self.print_tokens(&self.before[self.pos as usize..print_to as usize], ' ');
+            let len = print_to - self.pos;
+            self.pos = move_to;
+            self.before_hunk_len += len;
+            self.after_hunk_len += len;
+        }
+    }
+
+    impl<W, T> Sink for UnifiedDiff<'_, W, T>
+    where
+        W: Write,
+        T: Hash + Eq + Display,
+    {
+        type Out = W;
+
+        fn process_change(&mut self, before: Range<u32>, after: Range<u32>) {
+            if ((self.pos == 0) && (before.start - self.pos > self.ctx_size))
+                || (before.start - self.pos > 2 * self.ctx_size)
+            {
+                self.flush();
+                self.pos = before.start - self.ctx_size;
+                self.before_hunk_start = self.pos;
+                self.after_hunk_start = after.start - self.ctx_size;
+            }
+            self.update_pos(before.start, before.end);
+            self.before_hunk_len += before.end - before.start;
+            self.after_hunk_len += after.end - after.start;
+            self.print_tokens(&self.before[before.start as usize..before.end as usize], '-');
+            self.print_tokens(&self.after[after.start as usize..after.end as usize], '+');
+        }
+
+        fn finish(mut self) -> Self::Out {
+            self.flush();
+            self.dst
+        }
+    }
+}

gix-diff/tests/diff/blob/mod.rs

@@ -1,2 +1,3 @@
 pub(crate) mod pipeline;
 mod platform;
+mod unified_diff;