Added deadlines to all algorithms (#18)

This adds a deadline to all algorithms which lets one bail in a fixed amount of
time to avoid pathological cases.

CHANGELOG.md

@@ -6,6 +6,8 @@ All notable changes to similar are documented here.
 
 * Performance improvements for the LCS algorithm.
 * Small performance improvments by adding an early opt-out for and inline highlighting.
+* Added deadlines to all diffing algorithms to bail early.
+* Deprecated slice diffing methods in the individual algorithm modules.
 
 ## 1.2.2
 

src/algorithms/capture.rs

@@ -94,7 +94,7 @@ impl DiffHook for Capture {
 
 #[test]
 fn test_capture_hook_grouping() {
-    use crate::algorithms::{myers, Replace};
+    use crate::algorithms::{diff_slices, Algorithm, Replace};
 
     let rng = (1..100).collect::<Vec<_>>();
     let mut rng_new = rng.clone();
@@ -104,7 +104,7 @@ fn test_capture_hook_grouping() {
     rng_new[34] = 1000;
 
     let mut d = Replace::new(Capture::new());
-    myers::diff_slices(&mut d, &rng, &rng_new).unwrap();
+    diff_slices(Algorithm::Myers, &mut d, &rng, &rng_new).unwrap();
 
     let ops = d.into_inner().into_grouped_ops(3);
     let tags = ops

src/algorithms/lcs.rs

@@ -4,12 +4,18 @@
 //! * space `O(MN)`
 use std::collections::BTreeMap;
 use std::ops::{Index, Range};
+use std::time::Instant;
 
 use crate::algorithms::DiffHook;
 
 /// Hunt–McIlroy / Hunt–Szymanski LCS diff algorithm.
 ///
 /// Diff `old`, between indices `old_range` and `new` between indices `new_range`.
+///
+/// This diff is done with an optional deadline that defines the maximal
+/// execution time permitted before it bails and falls back to an very bad
+/// approximation.  Deadlines with LCS do not make a lot of sense and should
+/// not be used.
 pub fn diff<Old, New, D>(
     d: &mut D,
     old: &Old,
@@ -17,6 +23,29 @@ pub fn diff<Old, New, D>(
     new: &New,
     new_range: Range<usize>,
 ) -> Result<(), D::Error>
+where
+    Old: Index<usize> + ?Sized,
+    New: Index<usize> + ?Sized,
+    D: DiffHook,
+    New::Output: PartialEq<Old::Output>,
+{
+    diff_deadline(d, old, old_range, new, new_range, None)
+}
+
+/// Hunt–McIlroy / Hunt–Szymanski LCS diff algorithm.
+///
+/// Diff `old`, between indices `old_range` and `new` between indices `new_range`.
+///
+/// This diff is done with an optional deadline that defines the maximal
+/// execution time permitted before it bails and falls back to an approximation.
+pub fn diff_deadline<Old, New, D>(
+    d: &mut D,
+    old: &Old,
+    old_range: Range<usize>,
+    new: &New,
+    new_range: Range<usize>,
+    deadline: Option<Instant>,
+) -> Result<(), D::Error>
 where
     Old: Index<usize> + ?Sized,
     New: Index<usize> + ?Sized,
@@ -44,11 +73,12 @@ where
         .take_while(|x| new[x.1] == old[x.0])
         .count();
 
-    let table = make_table(
+    let maybe_table = make_table(
         old,
         prefix_len..(old_range.len() - suffix_len),
         new,
         prefix_len..(new_range.len() - suffix_len),
+        deadline,
     );
     let mut old_idx = 0;
     let mut new_idx = 0;
@@ -59,23 +89,30 @@ where
         d.equal(old_range.start, new_range.start, prefix_len)?;
     }
 
-    while new_idx < new_len && old_idx < old_len {
+    if let Some(table) = maybe_table {
+        while new_idx < new_len && old_idx < old_len {
+            let old_orig_idx = old_range.start + prefix_len + old_idx;
+            let new_orig_idx = new_range.start + prefix_len + new_idx;
+
+            if new[new_orig_idx] == old[old_orig_idx] {
+                d.equal(old_orig_idx, new_orig_idx, 1)?;
+                old_idx += 1;
+                new_idx += 1;
+            } else if table.get(&(new_idx, old_idx + 1)).map_or(0, |&x| x)
+                >= table.get(&(new_idx + 1, old_idx)).map_or(0, |&x| x)
+            {
+                d.delete(old_orig_idx, 1, new_orig_idx)?;
+                old_idx += 1;
+            } else {
+                d.insert(old_orig_idx, new_orig_idx, 1)?;
+                new_idx += 1;
+            }
+        }
+    } else {
         let old_orig_idx = old_range.start + prefix_len + old_idx;
         let new_orig_idx = new_range.start + prefix_len + new_idx;
-
+        d.delete(old_orig_idx, old_len, new_orig_idx)?;
-        if new[new_orig_idx] == old[old_orig_idx] {
+        d.insert(old_orig_idx, new_orig_idx, new_len)?;
-            d.equal(old_orig_idx, new_orig_idx, 1)?;
-            old_idx += 1;
-            new_idx += 1;
-        } else if table.get(&(new_idx, old_idx + 1)).map_or(0, |&x| x)
-            >= table.get(&(new_idx + 1, old_idx)).map_or(0, |&x| x)
-        {
-            d.delete(old_orig_idx, 1, new_orig_idx)?;
-            old_idx += 1;
-        } else {
-            d.insert(old_orig_idx, new_orig_idx, 1)?;
-            new_idx += 1;
-        }
     }
 
     if old_idx < old_len {
@@ -107,6 +144,10 @@ where
 }
 
 /// Shortcut for diffing slices.
+#[deprecated(
+    since = "1.4.0",
+    note = "slice utility function is now only available via similar::algorithms::diff_slices"
+)]
 pub fn diff_slices<D, T>(d: &mut D, old: &[T], new: &[T]) -> Result<(), D::Error>
 where
     D: DiffHook,
@@ -120,7 +161,8 @@ fn make_table<Old, New>(
     old_range: Range<usize>,
     new: &New,
     new_range: Range<usize>,
-) -> BTreeMap<(usize, usize), u32>
+    deadline: Option<Instant>,
+) -> Option<BTreeMap<(usize, usize), u32>>
 where
     Old: Index<usize> + ?Sized,
     New: Index<usize> + ?Sized,
@@ -131,6 +173,13 @@ where
     let mut table = BTreeMap::new();
 
     for i in (0..new_len).rev() {
+        // are we running for too long?  give up on the table
+        if let Some(deadline) = deadline {
+            if Instant::now() > deadline {
+                return None;
+            }
+        }
+
         for j in (0..old_len).rev() {
             let val = if new[i] == old[j] {
                 table.get(&(i + 1, j + 1)).map_or(0, |&x| x) + 1
@@ -146,12 +195,12 @@ where
         }
     }
 
-    table
+    Some(table)
 }
 
 #[test]
 fn test_table() {
-    let table = make_table(&vec![2, 3], 0..2, &vec![0, 1, 2], 0..3);
+    let table = make_table(&vec![2, 3], 0..2, &vec![0, 1, 2], 0..3, None).unwrap();
     let expected = {
         let mut m = BTreeMap::new();
         m.insert((1, 0), 1);
@@ -168,7 +217,7 @@ fn test_diff() {
     let b: &[usize] = &[0, 1, 2, 9, 4];
 
     let mut d = crate::algorithms::Replace::new(crate::algorithms::Capture::new());
-    diff_slices(&mut d, a, b).unwrap();
+    diff(&mut d, a, 0..a.len(), b, 0..b.len()).unwrap();
     insta::assert_debug_snapshot!(d.into_inner().ops());
 }
 
@@ -178,7 +227,7 @@ fn test_contiguous() {
     let b: &[usize] = &[0, 1, 2, 8, 9, 4, 4, 7];
 
     let mut d = crate::algorithms::Replace::new(crate::algorithms::Capture::new());
-    diff_slices(&mut d, a, b).unwrap();
+    diff(&mut d, a, 0..a.len(), b, 0..b.len()).unwrap();
     insta::assert_debug_snapshot!(d.into_inner().ops());
 }
 
@@ -188,6 +237,6 @@ fn test_pat() {
     let b: &[usize] = &[0, 1, 4, 5, 8, 9];
 
     let mut d = crate::algorithms::Capture::new();
-    diff_slices(&mut d, a, b).unwrap();
+    diff(&mut d, a, 0..a.len(), b, 0..b.len()).unwrap();
     insta::assert_debug_snapshot!(d.ops());
 }

src/algorithms/mod.rs

@@ -39,6 +39,7 @@ mod replace;
 
 use std::hash::Hash;
 use std::ops::{Index, Range};
+use std::time::Instant;
 
 pub use capture::Capture;
 pub use hook::{DiffHook, NoFinishHook};
@@ -62,6 +63,34 @@ pub fn diff<Old, New, D>(
     new: &New,
     new_range: Range<usize>,
 ) -> Result<(), D::Error>
+where
+    Old: Index<usize> + ?Sized,
+    New: Index<usize> + ?Sized,
+    D: DiffHook,
+    Old::Output: Hash + Eq + Ord,
+    New::Output: PartialEq<Old::Output> + Hash + Eq + Ord,
+{
+    diff_deadline(alg, d, old, old_range, new, new_range, None)
+}
+
+/// Creates a diff between old and new with the given algorithm with deadline.
+///
+/// Diffs `old`, between indices `old_range` and `new` between indices `new_range`.
+///
+/// This diff is done with an optional deadline that defines the maximal
+/// execution time permitted before it bails and falls back to an approximation.
+/// Note that not all algorithms behave well if they reach the deadline (LCS
+/// for instance produces a very simplistic diff when the deadline is reached
+/// in all cases).
+pub fn diff_deadline<Old, New, D>(
+    alg: Algorithm,
+    d: &mut D,
+    old: &Old,
+    old_range: Range<usize>,
+    new: &New,
+    new_range: Range<usize>,
+    deadline: Option<Instant>,
+) -> Result<(), D::Error>
 where
     Old: Index<usize> + ?Sized,
     New: Index<usize> + ?Sized,
@@ -70,9 +99,9 @@ where
     New::Output: PartialEq<Old::Output> + Hash + Eq + Ord,
 {
     match alg {
-        Algorithm::Myers => myers::diff(d, old, old_range, new, new_range),
+        Algorithm::Myers => myers::diff_deadline(d, old, old_range, new, new_range, deadline),
-        Algorithm::Patience => patience::diff(d, old, old_range, new, new_range),
+        Algorithm::Patience => patience::diff_deadline(d, old, old_range, new, new_range, deadline),
-        Algorithm::Lcs => lcs::diff(d, old, old_range, new, new_range),
+        Algorithm::Lcs => lcs::diff_deadline(d, old, old_range, new, new_range, deadline),
     }
 }
 
@@ -84,3 +113,18 @@ where
 {
     diff(alg, d, old, 0..old.len(), new, 0..new.len())
 }
+
+/// Shortcut for diffing slices with a specific algorithm.
+pub fn diff_slices_deadline<D, T>(
+    alg: Algorithm,
+    d: &mut D,
+    old: &[T],
+    new: &[T],
+    deadline: Option<Instant>,
+) -> Result<(), D::Error>
+where
+    D: DiffHook,
+    T: Eq + Hash + Ord,
+{
+    diff_deadline(alg, d, old, 0..old.len(), new, 0..new.len(), deadline)
+}

src/algorithms/myers.rs

@@ -8,8 +8,19 @@
 //!
 //! The implementation of this algorithm is based on the implementation by
 //! Brandon Williams.
+//!
+//! # Heuristics
+//!
+//! At present this implementation of Myers' does not implement any more advanced
+//! heuristics that would solve some pathological cases.  For instane passing two
+//! large and completely distinct sequences to the algorithm will make it spin
+//! without making reasonable progress.  Currently the only protection in the
+//! library against this is to pass a deadline to the diffing algorithm.
+//!
+//! For potential improvements here see [similar#15](https://github.com/mitsuhiko/similar/issues/15).
 
 use std::ops::{Index, IndexMut, Range};
+use std::time::Instant;
 
 use crate::algorithms::DiffHook;
 
@@ -23,6 +34,29 @@ pub fn diff<Old, New, D>(
     new: &New,
     new_range: Range<usize>,
 ) -> Result<(), D::Error>
+where
+    Old: Index<usize> + ?Sized,
+    New: Index<usize> + ?Sized,
+    D: DiffHook,
+    New::Output: PartialEq<Old::Output>,
+{
+    diff_deadline(d, old, old_range, new, new_range, None)
+}
+
+/// Myers' diff algorithm with deadline.
+///
+/// Diff `old`, between indices `old_range` and `new` between indices `new_range`.
+///
+/// This diff is done with an optional deadline that defines the maximal
+/// execution time permitted before it bails and falls back to an approximation.
+pub fn diff_deadline<Old, New, D>(
+    d: &mut D,
+    old: &Old,
+    old_range: Range<usize>,
+    new: &New,
+    new_range: Range<usize>,
+    deadline: Option<Instant>,
+) -> Result<(), D::Error>
 where
     Old: Index<usize> + ?Sized,
     New: Index<usize> + ?Sized,
@@ -32,11 +66,17 @@ where
     let max_d = max_d(old_range.len(), new_range.len());
     let mut vf = V::new(max_d);
     let mut vb = V::new(max_d);
-    conquer(d, old, old_range, new, new_range, &mut vf, &mut vb)?;
+    conquer(
+        d, old, old_range, new, new_range, &mut vf, &mut vb, deadline,
+    )?;
     d.finish()
 }
 
 /// Shortcut for diffing slices.
+#[deprecated(
+    since = "1.4.0",
+    note = "slice utility function is now only available via similar::algorithms::diff_slices"
+)]
 pub fn diff_slices<D, T>(d: &mut D, old: &[T], new: &[T]) -> Result<(), D::Error>
 where
     D: DiffHook,
@@ -172,7 +212,8 @@ fn find_middle_snake<Old, New>(
     new_range: Range<usize>,
     vf: &mut V,
     vb: &mut V,
-) -> Snake
+    deadline: Option<Instant>,
+) -> Option<Snake>
 where
     Old: Index<usize> + ?Sized,
     New: Index<usize> + ?Sized,
@@ -197,6 +238,13 @@ where
     assert!(vb.len() >= d_max);
 
     for d in 0..d_max as isize {
+        // are we running for too long?
+        if let Some(deadline) = deadline {
+            if Instant::now() > deadline {
+                break;
+            }
+        }
+
         // Forward path
         for k in (-d..=d).rev().step_by(2) {
             let mut x = if k == -d || (k != d && vf[k - 1] < vf[k + 1]) {
@@ -230,10 +278,10 @@ where
                 // TODO optimize this so we don't have to compare against n
                 if vf[k] + vb[-(k - delta)] >= n {
                     // Return the snake
-                    return Snake {
+                    return Some(Snake {
                         x_start: x0 + old_range.start,
                         y_start: y0 + new_range.start,
-                    };
+                    });
                 }
             }
         }
@@ -266,10 +314,10 @@ where
                 // TODO optimize this so we don't have to compare against n
                 if vb[k] + vf[-(k - delta)] >= n {
                     // Return the snake
-                    return Snake {
+                    return Some(Snake {
                         x_start: n - x + old_range.start,
                         y_start: m - y + new_range.start,
-                    };
+                    });
                 }
             }
         }
@@ -277,9 +325,11 @@ where
         // TODO: Maybe there's an opportunity to optimize and bail early?
     }
 
-    unreachable!("unable to find a middle snake");
+    // deadline reached
+    None
 }
 
+#[allow(clippy::too_many_arguments)]
 fn conquer<Old, New, D>(
     d: &mut D,
     old: &Old,
@@ -288,6 +338,7 @@ fn conquer<Old, New, D>(
     mut new_range: Range<usize>,
     vf: &mut V,
     vb: &mut V,
+    deadline: Option<Instant>,
 ) -> Result<(), D::Error>
 where
     Old: Index<usize> + ?Sized,
@@ -326,12 +377,30 @@ where
             new_range.start,
             new_range.end - new_range.start,
         )?;
-    } else {
+    } else if let Some(snake) = find_middle_snake(
-        let snake = find_middle_snake(old, old_range.clone(), new, new_range.clone(), vf, vb);
+        old,
+        old_range.clone(),
+        new,
+        new_range.clone(),
+        vf,
+        vb,
+        deadline,
+    ) {
         let (old_a, old_b) = split_at(old_range, snake.x_start);
         let (new_a, new_b) = split_at(new_range, snake.y_start);
-        conquer(d, old, old_a, new, new_a, vf, vb)?;
+        conquer(d, old, old_a, new, new_a, vf, vb, deadline)?;
-        conquer(d, old, old_b, new, new_b, vf, vb)?;
+        conquer(d, old, old_b, new, new_b, vf, vb, deadline)?;
+    } else {
+        d.delete(
+            old_range.start,
+            old_range.end - old_range.start,
+            new_range.start,
+        )?;
+        d.insert(
+            old_range.start,
+            new_range.start,
+            new_range.end - new_range.start,
+        )?;
     }
 
     if common_suffix_len > 0 {
@@ -348,7 +417,7 @@ fn test_find_middle_snake() {
     let max_d = max_d(a.len(), b.len());
     let mut vf = V::new(max_d);
     let mut vb = V::new(max_d);
-    let snake = find_middle_snake(a, 0..a.len(), b, 0..b.len(), &mut vf, &mut vb);
+    let snake = find_middle_snake(a, 0..a.len(), b, 0..b.len(), &mut vf, &mut vb, None).unwrap();
     assert_eq!(snake.x_start, 4);
     assert_eq!(snake.y_start, 1);
 }
@@ -359,7 +428,7 @@ fn test_diff() {
     let b: &[usize] = &[0, 1, 2, 9, 4];
 
     let mut d = crate::algorithms::Replace::new(crate::algorithms::Capture::new());
-    diff_slices(&mut d, a, b).unwrap();
+    diff(&mut d, a, 0..a.len(), b, 0..b.len()).unwrap();
     insta::assert_debug_snapshot!(d.into_inner().ops());
 }
 
@@ -369,7 +438,7 @@ fn test_contiguous() {
     let b: &[usize] = &[0, 1, 2, 8, 9, 4, 4, 7];
 
     let mut d = crate::algorithms::Replace::new(crate::algorithms::Capture::new());
-    diff_slices(&mut d, a, b).unwrap();
+    diff(&mut d, a, 0..a.len(), b, 0..b.len()).unwrap();
     insta::assert_debug_snapshot!(d.into_inner().ops());
 }
 
@@ -379,6 +448,45 @@ fn test_pat() {
     let b: &[usize] = &[0, 1, 4, 5, 8, 9];
 
     let mut d = crate::algorithms::Capture::new();
-    diff_slices(&mut d, a, b).unwrap();
+    diff(&mut d, a, 0..a.len(), b, 0..b.len()).unwrap();
     insta::assert_debug_snapshot!(d.ops());
 }
+
+#[test]
+fn test_deadline_reached() {
+    use std::ops::Index;
+    use std::time::Duration;
+
+    let a = (0..100).collect::<Vec<_>>();
+    let mut b = (0..100).collect::<Vec<_>>();
+    b[10] = 99;
+    b[50] = 99;
+    b[25] = 99;
+
+    struct SlowIndex<'a>(&'a [usize]);
+
+    impl<'a> Index<usize> for SlowIndex<'a> {
+        type Output = usize;
+
+        fn index(&self, index: usize) -> &Self::Output {
+            std::thread::sleep(Duration::from_millis(1));
+            &self.0[index]
+        }
+    }
+
+    let slow_a = SlowIndex(&a);
+    let slow_b = SlowIndex(&b);
+
+    // don't give it enough time to do anything interesting
+    let mut d = crate::algorithms::Replace::new(crate::algorithms::Capture::new());
+    diff_deadline(
+        &mut d,
+        &slow_a,
+        0..a.len(),
+        &slow_b,
+        0..b.len(),
+        Some(Instant::now() + Duration::from_millis(50)),
+    )
+    .unwrap();
+    insta::assert_debug_snapshot!(d.into_inner().ops());
+}

src/algorithms/patience.rs

@@ -12,6 +12,7 @@ use std::collections::hash_map::Entry;
 use std::collections::HashMap;
 use std::hash::Hash;
 use std::ops::{Index, Range};
+use std::time::Instant;
 
 use crate::algorithms::{myers, DiffHook, NoFinishHook, Replace};
 
@@ -25,6 +26,30 @@ pub fn diff<Old, New, D>(
     new: &New,
     new_range: Range<usize>,
 ) -> Result<(), D::Error>
+where
+    Old: Index<usize> + ?Sized,
+    New: Index<usize> + ?Sized,
+    Old::Output: Hash + Eq,
+    New::Output: PartialEq<Old::Output> + Hash + Eq,
+    D: DiffHook,
+{
+    diff_deadline(d, old, old_range, new, new_range, None)
+}
+
+/// Patience diff algorithm with deadline.
+///
+/// Diff `old`, between indices `old_range` and `new` between indices `new_range`.
+///
+/// This diff is done with an optional deadline that defines the maximal
+/// execution time permitted before it bails and falls back to an approximation.
+pub fn diff_deadline<Old, New, D>(
+    d: &mut D,
+    old: &Old,
+    old_range: Range<usize>,
+    new: &New,
+    new_range: Range<usize>,
+    deadline: Option<Instant>,
+) -> Result<(), D::Error>
 where
     Old: Index<usize> + ?Sized,
     New: Index<usize> + ?Sized,
@@ -45,18 +70,24 @@ where
         new_current: new_range.start,
         new_end: new_range.end,
         new_indexes: &new_indexes,
+        deadline,
     });
-    myers::diff(
+    myers::diff_deadline(
         &mut d,
         &old_indexes,
         0..old_indexes.len(),
         &new_indexes,
         0..new_indexes.len(),
+        deadline,
     )?;
     Ok(())
 }
 
 /// Shortcut for diffing slices.
+#[deprecated(
+    since = "1.4.0",
+    note = "slice utility function is now only available via similar::algorithms::diff_slices"
+)]
 pub fn diff_slices<D, T>(d: &mut D, old: &[T], new: &[T]) -> Result<(), D::Error>
 where
     D: DiffHook,
@@ -128,6 +159,7 @@ struct Patience<'old, 'new, 'd, Old: ?Sized, New: ?Sized, D> {
     new_current: usize,
     new_end: usize,
     new_indexes: &'new [Indexable<'new, New>],
+    deadline: Option<Instant>,
 }
 
 impl<'old, 'new, 'd, Old, New, D> DiffHook for Patience<'old, 'new, 'd, Old, New, D>
@@ -153,12 +185,13 @@ where
                 self.d.equal(a0, b0, self.old_current - a0)?;
             }
             let mut no_finish_d = NoFinishHook::new(&mut self.d);
-            myers::diff(
+            myers::diff_deadline(
                 &mut no_finish_d,
                 self.old,
                 self.old_current..self.old_indexes[old].index,
                 self.new,
                 self.new_current..self.new_indexes[new].index,
+                self.deadline,
             )?;
             self.old_current = self.old_indexes[old].index;
             self.new_current = self.new_indexes[new].index;
@@ -167,12 +200,13 @@ where
     }
 
     fn finish(&mut self) -> Result<(), D::Error> {
-        myers::diff(
+        myers::diff_deadline(
             self.d,
             self.old,
             self.old_current..self.old_end,
             self.new,
             self.new_current..self.new_end,
+            self.deadline,
         )
     }
 }
@@ -183,7 +217,7 @@ fn test_patience() {
     let b: &[usize] = &[10, 1, 2, 2, 8, 9, 4, 4, 7, 47, 18];
 
     let mut d = Replace::new(crate::algorithms::Capture::new());
-    diff_slices(&mut d, a, b).unwrap();
+    diff(&mut d, a, 0..a.len(), b, 0..b.len()).unwrap();
 
     insta::assert_debug_snapshot!(d.into_inner().ops());
 }
@@ -195,7 +229,7 @@ fn test_patience_out_of_bounds_bug() {
     let b: &[usize] = &[1, 2, 3];
 
     let mut d = Replace::new(crate::algorithms::Capture::new());
-    diff_slices(&mut d, a, b).unwrap();
+    diff(&mut d, a, 0..a.len(), b, 0..b.len()).unwrap();
 
     insta::assert_debug_snapshot!(d.into_inner().ops());
 }

src/algorithms/replace.rs

@@ -134,7 +134,7 @@ impl<D: DiffHook> DiffHook for Replace<D> {
 
 #[test]
 fn test_mayers_replace() {
-    use crate::algorithms::myers;
+    use crate::algorithms::{diff_slices, Algorithm};
     let a: &[&str] = &[
         ">>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>\n",
         "a\n",
@@ -159,7 +159,7 @@ fn test_mayers_replace() {
     ];
 
     let mut d = Replace::new(crate::algorithms::Capture::new());
-    myers::diff_slices(&mut d, a, b).unwrap();
+    diff_slices(Algorithm::Myers, &mut d, a, b).unwrap();
 
     insta::assert_debug_snapshot!(&d.into_inner().ops(), @r###"
     [
@@ -196,11 +196,13 @@ fn test_mayers_replace() {
 
 #[test]
 fn test_replace() {
+    use crate::algorithms::{diff_slices, Algorithm};
+
     let a: &[usize] = &[0, 1, 2, 3, 4];
     let b: &[usize] = &[0, 1, 2, 7, 8, 9];
 
     let mut d = Replace::new(crate::algorithms::Capture::new());
-    crate::algorithms::myers::diff_slices(&mut d, a, b).unwrap();
+    diff_slices(Algorithm::Myers, &mut d, a, b).unwrap();
     insta::assert_debug_snapshot!(d.into_inner().ops(), @r###"
     [
         Equal {

src/algorithms/snapshots/similar__algorithms__myers__deadline_reached.snap

@@ -0,0 +1,22 @@
+---
+source: src/algorithms/myers.rs
+expression: d.into_inner().ops()
+---
+[
+    Equal {
+        old_index: 0,
+        new_index: 0,
+        len: 10,
+    },
+    Replace {
+        old_index: 10,
+        old_len: 41,
+        new_index: 10,
+        new_len: 41,
+    },
+    Equal {
+        old_index: 51,
+        new_index: 51,
+        len: 49,
+    },
+]

src/common.rs

@@ -1,13 +1,15 @@
 use std::hash::Hash;
 use std::ops::{Index, Range};
+use std::time::Instant;
 
-use crate::algorithms::{diff, diff_slices, Capture, Replace};
+use crate::algorithms::{diff_deadline, diff_slices_deadline, Capture, Replace};
 use crate::{Algorithm, DiffOp};
 
 /// Creates a diff between old and new with the given algorithm capturing the ops.
 ///
-/// This is like [`diff`] but instead of using an arbitrary hook this will
+/// This is like [`diff`](crate::algorithms::diff) but instead of using an
-/// always use [`Replace`] + [`Capture`] and return the captured [`DiffOp`]s.
+/// arbitrary hook this will always use [`Replace`] + [`Capture`] and return the
+/// captured [`DiffOp`]s.
 pub fn capture_diff<Old, New>(
     alg: Algorithm,
     old: &Old,
@@ -15,6 +17,26 @@ pub fn capture_diff<Old, New>(
     new: &New,
     new_range: Range<usize>,
 ) -> Vec<DiffOp>
+where
+    Old: Index<usize> + ?Sized,
+    New: Index<usize> + ?Sized,
+    Old::Output: Hash + Eq + Ord,
+    New::Output: PartialEq<Old::Output> + Hash + Eq + Ord,
+{
+    capture_diff_deadline(alg, old, old_range, new, new_range, None)
+}
+
+/// Creates a diff between old and new with the given algorithm capturing the ops.
+///
+/// Works like [`capture_diff`] but with an optional deadline.
+pub fn capture_diff_deadline<Old, New>(
+    alg: Algorithm,
+    old: &Old,
+    old_range: Range<usize>,
+    new: &New,
+    new_range: Range<usize>,
+    deadline: Option<Instant>,
+) -> Vec<DiffOp>
 where
     Old: Index<usize> + ?Sized,
     New: Index<usize> + ?Sized,
@@ -22,17 +44,32 @@ where
     New::Output: PartialEq<Old::Output> + Hash + Eq + Ord,
 {
     let mut d = Replace::new(Capture::new());
-    diff(alg, &mut d, old, old_range, new, new_range).unwrap();
+    diff_deadline(alg, &mut d, old, old_range, new, new_range, deadline).unwrap();
     d.into_inner().into_ops()
 }
 
 /// Creates a diff between old and new with the given algorithm capturing the ops.
 pub fn capture_diff_slices<T>(alg: Algorithm, old: &[T], new: &[T]) -> Vec<DiffOp>
+where
+    T: Eq + Hash + Ord,
+{
+    capture_diff_slices_deadline(alg, old, new, None)
+}
+
+/// Creates a diff between old and new with the given algorithm capturing the ops.
+///
+/// Works like [`capture_diff_slices`] but with an optional deadline.
+pub fn capture_diff_slices_deadline<T>(
+    alg: Algorithm,
+    old: &[T],
+    new: &[T],
+    deadline: Option<Instant>,
+) -> Vec<DiffOp>
 where
     T: Eq + Hash + Ord,
 {
     let mut d = Replace::new(Capture::new());
-    diff_slices(alg, &mut d, old, new).unwrap();
+    diff_slices_deadline(alg, &mut d, old, new, deadline).unwrap();
     d.into_inner().into_ops()
 }
 

src/lib.rs

@@ -106,6 +106,22 @@
 //! As the [`TextDiff::grouped_ops`] method can isolate clusters of changes
 //! this even works for very long files if paired with this method.
 //!
+//! # Deadlines and Performance
+//!
+//! For large and very distinct inputs the algorithms as implemented can take
+//! a very, very long time to execute.  Too long to make sense in practice.
+//! To work around this issue all diffing algorithms also provide a version
+//! that accepts a deadline which is the point in time as defined by an
+//! [`Instant`](std::time::Instant) after which the algorithm should give up.
+//! What giving up means depends on the algorithm.  For instance due to the
+//! recursive, divide and conquer nature of Myer's diff you will still get a
+//! pretty decent diff in many cases when a deadline is reached.  Whereas on the
+//! other hand the LCS diff is unlikely to give any decent results in such a
+//! situation.
+//!
+//! The [`TextDiff`] type also lets you configure a deadline and/or timeout
+//! when performing a text diff.
+//!
 //! # Feature Flags
 //!
 //! The crate by default does not have any dependencies however for some use

src/text/mod.rs

@@ -2,6 +2,7 @@
 use std::borrow::Cow;
 use std::cmp::Reverse;
 use std::collections::BinaryHeap;
+use std::time::{Duration, Instant};
 
 mod abstraction;
 #[cfg(feature = "inline")]
@@ -15,7 +16,22 @@ pub use self::inline::InlineChange;
 use self::utils::{upper_seq_ratio, QuickSeqRatio};
 use crate::iter::{AllChangesIter, ChangesIter};
 use crate::udiff::UnifiedDiff;
-use crate::{capture_diff_slices, get_diff_ratio, group_diff_ops, Algorithm, DiffOp};
+use crate::{capture_diff_slices_deadline, get_diff_ratio, group_diff_ops, Algorithm, DiffOp};
+
+#[derive(Debug, Clone, Copy)]
+enum Deadline {
+    Absolute(Instant),
+    Relative(Duration),
+}
+
+impl Deadline {
+    fn into_instant(self) -> Instant {
+        match self {
+            Deadline::Absolute(instant) => instant,
+            Deadline::Relative(duration) => Instant::now() + duration,
+        }
+    }
+}
 
 /// A builder type config for more complex uses of [`TextDiff`].
 ///
@@ -24,6 +40,7 @@ use crate::{capture_diff_slices, get_diff_ratio, group_diff_ops, Algorithm, Diff
 pub struct TextDiffConfig {
     algorithm: Algorithm,
     newline_terminated: Option<bool>,
+    deadline: Option<Deadline>,
 }
 
 impl Default for TextDiffConfig {
@@ -31,6 +48,7 @@ impl Default for TextDiffConfig {
         TextDiffConfig {
             algorithm: Algorithm::default(),
             newline_terminated: None,
+            deadline: None,
         }
     }
 }
@@ -44,6 +62,24 @@ impl TextDiffConfig {
         self
     }
 
+    /// Sets a deadline for the diff operation.
+    ///
+    /// By default a diff will take as long as it takes.  For certain diff
+    /// algorthms like Myer's and Patience a maximum running time can be
+    /// defined after which the algorithm gives up and approximates.
+    pub fn deadline(&mut self, deadline: Instant) -> &mut Self {
+        self.deadline = Some(Deadline::Absolute(deadline));
+        self
+    }
+
+    /// Sets a timeout for thediff operation.
+    ///
+    /// This is like [`deadline`](Self::deadline) but accepts a duration.
+    pub fn timeout(&mut self, timeout: Duration) -> &mut Self {
+        self.deadline = Some(Deadline::Relative(timeout));
+        self
+    }
+
     /// Changes the newline termination flag.
     ///
     /// The default is automatic based on input.  This flag controls the
@@ -291,7 +327,12 @@ impl TextDiffConfig {
         new: Cow<'bufs, [&'new T]>,
         newline_terminated: bool,
     ) -> TextDiff<'old, 'new, 'bufs, T> {
-        let ops = capture_diff_slices(self.algorithm, &old, &new);
+        let ops = capture_diff_slices_deadline(
+            self.algorithm,
+            &old,
+            &new,
+            self.deadline.map(|x| x.into_instant()),
+        );
         TextDiff {
             old,
             new,