LibDiff: Add support for generating diffs with surrounding context

While not used in normal diffs due to limitations in the format, this
may be used in context and unified format diffs.

Userland/Libraries/LibDiff/Generator.cpp

@@ -9,7 +9,7 @@
 
 namespace Diff {
 
-ErrorOr<Vector<Hunk>> from_text(StringView old_text, StringView new_text)
+ErrorOr<Vector<Hunk>> from_text(StringView old_text, StringView new_text, size_t context)
 {
     auto old_lines = old_text.lines();
     auto new_lines = new_text.lines();
@@ -65,71 +65,99 @@ ErrorOr<Vector<Hunk>> from_text(StringView old_text, StringView new_text)
 
     Vector<Hunk> hunks;
     Hunk cur_hunk;
-    bool in_hunk = false;
-
-    auto update_hunk = [&](size_t i, size_t j, Direction direction) -> ErrorOr<void> {
-        if (!in_hunk) {
-            HunkLocation location;
-            location.old_range.start_line = i;
-            location.new_range.start_line = j;
-            in_hunk = true;
-            cur_hunk = { location, {} };
-        }
 
-        if (direction == Direction::Down) {
-            TRY(cur_hunk.lines.try_append(Line { Line::Operation::Addition, TRY(String::from_utf8(new_lines[j])) }));
-            cur_hunk.location.new_range.number_of_lines++;
-        } else if (direction == Direction::Right) {
-            TRY(cur_hunk.lines.try_append(Line { Line::Operation::Removal, TRY(String::from_utf8(old_lines[i])) }));
-            cur_hunk.location.old_range.number_of_lines++;
-        }
+    auto flush_hunk = [&]() -> ErrorOr<void> {
+        // A file with no content has a zero indexed start line.
+        if (cur_hunk.location.new_range.start_line != 0 || cur_hunk.location.new_range.number_of_lines != 0)
+            cur_hunk.location.new_range.start_line++;
+        if (cur_hunk.location.old_range.start_line != 0 || cur_hunk.location.old_range.number_of_lines != 0)
+            cur_hunk.location.old_range.start_line++;
+
+        TRY(hunks.try_append(cur_hunk));
+        cur_hunk.lines.clear();
 
         return {};
     };
 
-    auto flush_hunk = [&]() -> ErrorOr<void> {
-        if (in_hunk) {
-            // A file with no content has a zero indexed start line.
-            if (cur_hunk.location.new_range.start_line != 0 || cur_hunk.location.new_range.number_of_lines != 0)
-                cur_hunk.location.new_range.start_line++;
-            if (cur_hunk.location.old_range.start_line != 0 || cur_hunk.location.old_range.number_of_lines != 0)
-                cur_hunk.location.old_range.start_line++;
-
-            TRY(hunks.try_append(cur_hunk));
-            in_hunk = false;
+    size_t i = 0;
+    size_t j = 0;
+
+    auto set_up_hunk_prepended_with_context = [&](Hunk& hunk) -> ErrorOr<void> {
+        // Prefix the hunk with requested number context lines, and set the hunk location to where that context begins.
+        size_t available_context = min(i, context);
+        hunk.location.old_range = { i - available_context, available_context };
+        hunk.location.new_range = { j - available_context, available_context };
+
+        for (size_t offset = 0; offset < available_context; ++offset) {
+            size_t context_line = i + offset - available_context;
+            TRY(hunk.lines.try_append(Line { Line::Operation::Context, TRY(String::from_utf8(new_lines[context_line])) }));
         }
 
         return {};
     };
 
-    size_t i = 0;
-    size_t j = 0;
-
+    size_t current_context = 0;
     while (i < old_lines.size() && j < new_lines.size()) {
+
         auto& cell = dp(i, j);
         if (cell.direction == Direction::Down) {
-            TRY(update_hunk(i, j, cell.direction));
+            if (cur_hunk.lines.is_empty())
+                TRY(set_up_hunk_prepended_with_context(cur_hunk));
+            TRY(cur_hunk.lines.try_append(Line { Line::Operation::Addition, TRY(String::from_utf8(new_lines[j])) }));
+            cur_hunk.location.new_range.number_of_lines++;
+
             ++j;
+            current_context = 0;
         } else if (cell.direction == Direction::Right) {
-            TRY(update_hunk(i, j, cell.direction));
+            if (cur_hunk.lines.is_empty())
+                TRY(set_up_hunk_prepended_with_context(cur_hunk));
+            TRY(cur_hunk.lines.try_append(Line { Line::Operation::Removal, TRY(String::from_utf8(old_lines[i])) }));
+            cur_hunk.location.old_range.number_of_lines++;
+
             ++i;
+            current_context = 0;
         } else {
+            if (!cur_hunk.lines.is_empty()) {
+                // We're currently in the middle of generating a hunk and have found a context line. If we have already added
+                // the number of context lines that were requested then we have already finished with this hunk. Otherwise we
+                // need to continue looking through the hunk until we have located the requested number of context lines in a
+                // row.
+                if (current_context == context) {
+                    TRY(flush_hunk());
+                } else {
+                    TRY(cur_hunk.lines.try_append(Line { Line::Operation::Context, TRY(String::from_utf8(old_lines[i])) }));
+                    cur_hunk.location.new_range.number_of_lines++;
+                    cur_hunk.location.old_range.number_of_lines++;
+                }
+
+                ++current_context;
+            }
+
             ++i;
             ++j;
-            TRY(flush_hunk());
         }
     }
 
     while (i < old_lines.size()) {
-        TRY(update_hunk(i, new_lines.is_empty() ? 0 : new_lines.size() - 1, Direction::Right)); // Remove a line
+        if (cur_hunk.lines.is_empty())
+            TRY(set_up_hunk_prepended_with_context(cur_hunk));
+
+        TRY(cur_hunk.lines.try_append(Line { Line::Operation::Removal, TRY(String::from_utf8(old_lines[i])) }));
+        cur_hunk.location.old_range.number_of_lines++;
         ++i;
     }
+
     while (j < new_lines.size()) {
-        TRY(update_hunk(old_lines.is_empty() ? 0 : old_lines.size() - 1, j, Direction::Down)); // Add a line
+        if (cur_hunk.lines.is_empty())
+            TRY(set_up_hunk_prepended_with_context(cur_hunk));
+        TRY(cur_hunk.lines.try_append(Line { Line::Operation::Addition, TRY(String::from_utf8(new_lines[j])) }));
+        cur_hunk.location.new_range.number_of_lines++;
+
         ++j;
     }
 
-    TRY(flush_hunk());
+    if (!cur_hunk.lines.is_empty())
+        TRY(flush_hunk());
 
     return hunks;
 }

Userland/Libraries/LibDiff/Generator.h

@@ -11,6 +11,6 @@
 
 namespace Diff {
 
-ErrorOr<Vector<Hunk>> from_text(StringView old_text, StringView new_text);
+ErrorOr<Vector<Hunk>> from_text(StringView old_text, StringView new_text, size_t context = 0);
 
 }