Refactor find_diff_rec and detect kept prefix/suffixes early

Certain cases, like diffing an empty buffer with a big buffer, were
very slow, now this should get better as we will directly detect the
matching eol at the end of both buffers, end then immediatly detect
we need to add the rest of the big buffer.

We still are too slow on some general diff when there is a lot of
differences.

1 files changed, 23 insertions, 11 deletions

diff --git a/src/diff.hh b/src/diff.hh
index 3535dadc..306ed301 100644
--- a/src/diff.hh
+++ b/src/diff.hh
@@ -96,6 +96,9 @@ struct Diff
 
 inline void append_diff(Vector<Diff>& diffs, Diff diff)
 {
+    if (diff.len == 0)
+        return;
+
     if (not diffs.empty() and diffs.back().mode == diff.mode
         and (diff.mode != Diff::Add or
              diffs.back().posB + diffs.back().len == diff.posB))
@@ -109,7 +112,21 @@ void find_diff_rec(Iterator a, int offA, int lenA,
                    Iterator b, int offB, int lenB,
                    int* V1, int* V2, Equal eq, Vector<Diff>& diffs)
 {
-    if (lenA > 0 and lenB > 0)
+    int prefix_len = 0;
+    while (lenA > 0 and lenB > 0 and a[offA] == b[offB])
+         ++offA, ++offB, --lenA, --lenB, ++prefix_len;
+
+    int suffix_len = 0;
+    while (lenA != 0 and lenB != 0 and a[offA + lenA - 1] == b[offB + lenB - 1])
+        --lenA, --lenB, ++suffix_len;
+
+    append_diff(diffs, {Diff::Keep, prefix_len, 0});
+
+    if (lenA == 0)
+        append_diff(diffs, {Diff::Add, lenB, offB});
+    else if (lenB == 0)
+        append_diff(diffs, {Diff::Remove, lenA, 0});
+    else
     {
         auto middle_snake = find_middle_snake(a + offA, lenA, b + offB, lenB, V1, V2, eq);
         kak_assert(middle_snake.u <= lenA and middle_snake.v <= lenB);
@@ -119,8 +136,7 @@ void find_diff_rec(Iterator a, int offA, int lenA,
                           b, offB, middle_snake.y,
                           V1, V2, eq, diffs);
 
-            if (int len = middle_snake.u - middle_snake.x)
-                append_diff(diffs, {Diff::Keep, len, 0});
+            append_diff(diffs, {Diff::Keep, middle_snake.u - middle_snake.x, 0});
 
             find_diff_rec(a, offA + middle_snake.u, lenA - middle_snake.u,
                           b, offB + middle_snake.v, lenB - middle_snake.v,
@@ -131,22 +147,18 @@ void find_diff_rec(Iterator a, int offA, int lenA,
             if (middle_snake.d == 1)
             {
                 const int diag = middle_snake.x - (middle_snake.add ? 0 : 1);
-                if (diag != 0)
-                    append_diff(diffs, {Diff::Keep, diag, 0});
+                append_diff(diffs, {Diff::Keep, diag, 0});
 
                 if (middle_snake.add)
                     append_diff(diffs, {Diff::Add, 1, offB + diag});
                 else
                     append_diff(diffs, {Diff::Remove, 1, 0});
             }
-            if (int len = middle_snake.u - middle_snake.x)
-                append_diff(diffs, {Diff::Keep, len, 0});
+            append_diff(diffs, {Diff::Keep, middle_snake.u - middle_snake.x, 0});
         }
     }
-    else if (lenB > 0)
-        append_diff(diffs, {Diff::Add, lenB, offB});
-    else if (lenA > 0)
-        append_diff(diffs, {Diff::Remove, lenA, 0});
+
+    append_diff(diffs, {Diff::Keep, suffix_len, 0});
 }
 
 template<typename Iterator, typename Equal = std::equal_to<typename std::iterator_traits<Iterator>::value_type>>