LibDiff: Change underlying representation of Hunk to allow context

The existing hunk data structure does not contain any way to easily
store information about context surrounding the additions and removals
in a hunk. While this does work fine for normal diffs (where there is
never any surrounding context) this data structure is quite limiting for
other use cases.

Without support for surrounding context it is not possible to:
 * Add support for unified or context format to the diff utility to
   output surrounding context.
 * Be able to implement a patch utility that uses the surrounding
   context to reliably locate where to apply a patch when a hunk range
   does not apply perfectly.

This patch changes Diff::Hunk such that its data structure more closely
resembles a unified diff. Each line in a hunk is now either a change,
removal, addition or context.

Allowing hunks to have context inside of them exposes that HackStudio
heavily relies on there being no context in the hunks that it uses for
its' git gutter implementation. The fix here is simple - ask git to
produce us a diff that has no context in it!

Userland/DevTools/HackStudio/Editor.cpp

@@ -831,12 +831,15 @@ ErrorOr<void> Editor::update_git_diff_indicators()
     for (auto i = 0u; i < document().line_count(); ++i)
         line_differences.unchecked_append(CodeDocument::DiffType::None);
 
-    for (auto& hunk : wrapper().hunks()) {
-        auto start_line = hunk.target_start_line;
-        auto finish_line = start_line + hunk.added_lines.size();
+    for (auto const& hunk : wrapper().hunks()) {
+        auto start_line = hunk.location.new_range.start_line;
+        // Account for 1 indexed hunk location
+        if (start_line != 0)
+            start_line--;
+        auto finish_line = start_line + hunk.location.new_range.number_of_lines;
 
-        auto additions = hunk.added_lines.size();
-        auto deletions = hunk.removed_lines.size();
+        auto additions = hunk.location.new_range.number_of_lines;
+        auto deletions = hunk.location.old_range.number_of_lines;
 
         for (size_t line_offset = 0; line_offset < additions; line_offset++) {
             auto line = start_line + line_offset;

Userland/DevTools/HackStudio/Git/DiffViewer.cpp

@@ -36,28 +36,32 @@ void DiffViewer::paint_event(GUI::PaintEvent& event)
     size_t y_offset = 10;
     size_t current_original_line_index = 0;
     for (auto const& hunk : m_hunks) {
-        for (size_t i = current_original_line_index; i < hunk.original_start_line; ++i) {
+        for (size_t i = current_original_line_index; i < hunk.location.old_range.start_line; ++i) {
             draw_line(painter, m_original_lines[i], y_offset, LinePosition::Both, LineType::Normal);
             y_offset += line_height();
         }
-        current_original_line_index = hunk.original_start_line + hunk.removed_lines.size();
+        current_original_line_index = hunk.location.old_range.start_line + hunk.location.old_range.number_of_lines;
 
         size_t left_y_offset = y_offset;
-        for (auto const& removed_line : hunk.removed_lines) {
-            draw_line(painter, removed_line, left_y_offset, LinePosition::Left, LineType::Diff);
+        for (auto const& line : hunk.lines) {
+            if (line.operation != Diff::Line::Operation::Removal)
+                continue;
+            draw_line(painter, line.content, left_y_offset, LinePosition::Left, LineType::Diff);
             left_y_offset += line_height();
         }
-        for (int i = 0; i < (int)hunk.added_lines.size() - (int)hunk.removed_lines.size(); ++i) {
+        for (int i = 0; i < (int)hunk.location.new_range.number_of_lines - (int)hunk.location.old_range.number_of_lines; ++i) {
             draw_line(painter, ""sv, left_y_offset, LinePosition::Left, LineType::Missing);
             left_y_offset += line_height();
         }
 
         size_t right_y_offset = y_offset;
-        for (auto const& added_line : hunk.added_lines) {
-            draw_line(painter, added_line, right_y_offset, LinePosition::Right, LineType::Diff);
+        for (auto const& line : hunk.lines) {
+            if (line.operation != Diff::Line::Operation::Addition)
+                continue;
+            draw_line(painter, line.content, right_y_offset, LinePosition::Right, LineType::Diff);
             right_y_offset += line_height();
         }
-        for (int i = 0; i < (int)hunk.removed_lines.size() - (int)hunk.added_lines.size(); ++i) {
+        for (int i = 0; i < (int)hunk.location.old_range.number_of_lines - (int)hunk.location.new_range.number_of_lines; ++i) {
             draw_line(painter, ""sv, right_y_offset, LinePosition::Right, LineType::Missing);
             right_y_offset += line_height();
         }
@@ -205,13 +209,13 @@ void DiffViewer::update_content_size()
     size_t num_lines = 0;
     size_t current_original_line_index = 0;
     for (auto const& hunk : m_hunks) {
-        num_lines += ((int)hunk.original_start_line - (int)current_original_line_index);
+        num_lines += (hunk.location.old_range.start_line - (int)current_original_line_index);
 
-        num_lines += hunk.removed_lines.size();
-        if (hunk.added_lines.size() > hunk.removed_lines.size()) {
-            num_lines += ((int)hunk.added_lines.size() - (int)hunk.removed_lines.size());
+        num_lines += hunk.location.old_range.number_of_lines;
+        if (hunk.location.new_range.number_of_lines > hunk.location.old_range.number_of_lines) {
+            num_lines += ((int)hunk.location.new_range.number_of_lines - (int)hunk.location.old_range.number_of_lines);
         }
-        current_original_line_index = hunk.original_start_line + hunk.removed_lines.size();
+        current_original_line_index = hunk.location.old_range.start_line + hunk.location.old_range.number_of_lines;
     }
     num_lines += ((int)m_original_lines.size() - (int)current_original_line_index);
 

Userland/DevTools/HackStudio/Git/GitRepo.cpp

@@ -118,7 +118,7 @@ Optional<DeprecatedString> GitRepo::original_file_content(DeprecatedString const
 
 Optional<DeprecatedString> GitRepo::unstaged_diff(DeprecatedString const& file) const
 {
-    return command({ "diff", file.characters() });
+    return command({ "diff", "-U0", file.characters() });
 }
 
 bool GitRepo::is_tracked(DeprecatedString const& file) const

Userland/Libraries/LibCpp/SemanticSyntaxHighlighter.cpp

@@ -63,14 +63,14 @@ void SemanticSyntaxHighlighter::rehighlight(Palette const& palette)
         size_t previous_token_index = 0;
         size_t current_token_index = 0;
         for (auto const& hunk : diff_hunks) {
-            for (; previous_token_index < hunk.original_start_line; ++previous_token_index) {
+            for (; previous_token_index < hunk.location.old_range.start_line; ++previous_token_index) {
                 new_tokens_info[current_token_index].type = m_tokens_info[previous_token_index].type;
                 ++current_token_index;
             }
-            for (size_t i = 0; i < hunk.added_lines.size(); ++i) {
+            for (size_t i = 0; i < hunk.location.new_range.number_of_lines; ++i) {
                 ++current_token_index;
             }
-            for (size_t i = 0; i < hunk.removed_lines.size(); ++i) {
+            for (size_t i = 0; i < hunk.location.old_range.number_of_lines; ++i) {
                 ++previous_token_index;
             }
         }

Userland/Libraries/LibDiff/Format.cpp

@@ -26,47 +26,40 @@ DeprecatedString generate_only_additions(StringView text)
 
 ErrorOr<void> write_normal(Hunk const& hunk, Stream& stream, ColorOutput color_output)
 {
-    auto original_start = hunk.original_start_line;
-    auto target_start = hunk.target_start_line;
-    auto num_added = hunk.added_lines.size();
-    auto num_removed = hunk.removed_lines.size();
-
     // Source line(s)
-    TRY(stream.write_formatted("{}", original_start));
-
-    if (num_removed > 1)
-        TRY(stream.write_formatted(",{}", original_start + num_removed - 1));
+    TRY(stream.write_formatted("{}", hunk.location.old_range.start_line));
+    if (hunk.location.old_range.number_of_lines > 1)
+        TRY(stream.write_formatted(",{}", (hunk.location.old_range.start_line + hunk.location.old_range.number_of_lines - 1)));
 
     // Action
-    if (num_added > 0 && num_removed > 0)
+    if (hunk.location.old_range.number_of_lines > 0 && hunk.location.new_range.number_of_lines > 0)
         TRY(stream.write_formatted("c"));
-    else if (num_added > 0)
+    else if (hunk.location.new_range.number_of_lines > 0)
         TRY(stream.write_formatted("a"));
     else
         TRY(stream.write_formatted("d"));
 
     // Target line(s)
-    TRY(stream.write_formatted("{}", target_start));
-    if (num_added > 1)
-        TRY(stream.write_formatted(",{}", target_start + num_added - 1));
+    TRY(stream.write_formatted("{}", hunk.location.new_range.start_line));
+    if (hunk.location.new_range.number_of_lines > 1)
+        TRY(stream.write_formatted(",{}", (hunk.location.new_range.start_line + hunk.location.new_range.number_of_lines - 1)));
 
     TRY(stream.write_formatted("\n"));
 
-    for (auto const& line : hunk.removed_lines) {
-        if (color_output == ColorOutput::Yes)
-            TRY(stream.write_formatted("\033[31;1m< {}\033[0m\n", line));
-        else
-            TRY(stream.write_formatted("< {}\n", line));
-    }
+    for (auto const& line : hunk.lines) {
+        VERIFY(line.operation == Line::Operation::Removal || line.operation == Line::Operation::Addition);
 
-    if (num_added > 0 && num_removed > 0)
-        TRY(stream.write_formatted("---\n"));
-
-    for (auto const& line : hunk.added_lines) {
-        if (color_output == ColorOutput::Yes)
-            TRY(stream.write_formatted("\033[32;1m> {}\033[0m\n", line));
-        else
-            TRY(stream.write_formatted("> {}\n", line));
+        if (line.operation == Line::Operation::Addition) {
+            if (color_output == ColorOutput::Yes)
+                TRY(stream.write_formatted("\033[32;1m> {}\033[0m\n", line.content));
+            else
+                TRY(stream.write_formatted("> {}\n", line.content));
+        } else {
+            if (color_output == ColorOutput::Yes)
+                TRY(stream.write_formatted("\033[31;1m< {}\033[0m\n", line.content));
+            else
+                TRY(stream.write_formatted("< {}\n", line.content));
+        }
     }
 
     return {};

Userland/Libraries/LibDiff/Generator.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2021, Mustafa Quraish <mustafa@serenityos.org>
+ * Copyright (c) 2023, Shannon Booth <shannon.ml.booth@gmail.com>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -68,13 +69,19 @@ ErrorOr<Vector<Hunk>> from_text(StringView old_text, StringView new_text)
 
     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 = { i, j, {}, {} };
+            cur_hunk = { location, {} };
         }
+
         if (direction == Direction::Down) {
-            TRY(cur_hunk.added_lines.try_append(TRY(String::from_utf8(new_lines[j]))));
+            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.removed_lines.try_append(TRY(String::from_utf8(old_lines[i]))));
+            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++;
         }
 
         return {};
@@ -82,10 +89,12 @@ ErrorOr<Vector<Hunk>> from_text(StringView old_text, StringView new_text)
 
     auto flush_hunk = [&]() -> ErrorOr<void> {
         if (in_hunk) {
-            if (cur_hunk.added_lines.size() > 0)
-                cur_hunk.target_start_line++;
-            if (cur_hunk.removed_lines.size() > 0)
-                cur_hunk.original_start_line++;
+            // 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;
         }

Userland/Libraries/LibDiff/Hunks.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2020, Itamar S. <itamar8910@gmail.com>
+ * Copyright (c) 2023, Shannon Booth <shannon.ml.booth@gmail.com>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -31,28 +32,21 @@ ErrorOr<Vector<Hunk>> parse_hunks(StringView diff)
             ++line_index;
             continue;
         }
-        if (diff_lines[line_index][0] == ' ') {
-            current_location.apply_offset(1, HunkLocation::LocationType::Both);
-            ++line_index;
-            continue;
-        }
+
         Hunk hunk {};
-        hunk.original_start_line = current_location.original_start_line;
-        hunk.target_start_line = current_location.target_start_line;
+        hunk.location = current_location;
 
-        while (line_index < diff_lines.size() && diff_lines[line_index][0] == '-') {
-            TRY(hunk.removed_lines.try_append(TRY(String::from_utf8(diff_lines[line_index].substring_view(1, diff_lines[line_index].length() - 1)))));
-            current_location.apply_offset(1, HunkLocation::LocationType::Original);
-            ++line_index;
-        }
-        while (line_index < diff_lines.size() && diff_lines[line_index][0] == '+') {
-            TRY(hunk.added_lines.try_append(TRY(String::from_utf8(diff_lines[line_index].substring_view(1, diff_lines[line_index].length() - 1)))));
-            current_location.apply_offset(1, HunkLocation::LocationType::Target);
-            ++line_index;
-        }
+        while (line_index < diff_lines.size()) {
+            auto const& line = diff_lines[line_index];
+
+            char const operation = line[0];
+            if (operation != ' ' && operation != '+' && operation != '-')
+                break;
+
+            auto const content = line.substring_view(1, line.length() - 1);
+
+            TRY(hunk.lines.try_append(Line { Line::operation_from_symbol(operation), TRY(String::from_utf8(content)) }));
 
-        while (line_index < diff_lines.size() && diff_lines[line_index][0] == ' ') {
-            current_location.apply_offset(1, HunkLocation::LocationType::Both);
             ++line_index;
         }
         TRY(hunks.try_append(hunk));
@@ -60,15 +54,9 @@ ErrorOr<Vector<Hunk>> parse_hunks(StringView diff)
 
     if constexpr (HUNKS_DEBUG) {
         for (auto const& hunk : hunks) {
-            dbgln("Hunk location:");
-            dbgln("  orig: {}", hunk.original_start_line);
-            dbgln("  target: {}", hunk.target_start_line);
-            dbgln("  removed:");
-            for (auto const& line : hunk.removed_lines)
-                dbgln("- {}", line);
-            dbgln("  added:");
-            for (auto const& line : hunk.added_lines)
-                dbgln("+ {}", line);
+            dbgln("{}", hunk.location);
+            for (auto const& line : hunk.lines)
+                dbgln("{}", line);
         }
     }
 
@@ -78,22 +66,21 @@ ErrorOr<Vector<Hunk>> parse_hunks(StringView diff)
 HunkLocation parse_hunk_location(StringView location_line)
 {
     size_t char_index = 0;
-    struct StartAndLength {
-        size_t start { 0 };
-        size_t length { 0 };
-    };
     auto parse_start_and_length_pair = [](StringView raw) {
-        auto index_of_separator = raw.find(',').value();
-        auto start = raw.substring_view(0, index_of_separator).to_uint().value();
-        auto length = raw.substring_view(index_of_separator + 1, raw.length() - index_of_separator - 1).to_uint().value();
+        auto maybe_index_of_separator = raw.find(',');
 
-        if (start != 0)
-            start--;
+        size_t start = 0;
+        size_t length = 0;
+        if (maybe_index_of_separator.has_value()) {
+            auto index_of_separator = maybe_index_of_separator.value();
+            start = raw.substring_view(0, index_of_separator).to_uint().value();
+            length = raw.substring_view(index_of_separator + 1, raw.length() - index_of_separator - 1).to_uint().value();
+        } else {
+            length = 1;
+            start = raw.to_uint().value();
+        }
 
-        if (length != 0)
-            length--;
-
-        return StartAndLength { start, length };
+        return Range { start, length };
     };
     while (char_index < location_line.length() && location_line[char_index++] != '-') {
     }
@@ -115,21 +102,9 @@ HunkLocation parse_hunk_location(StringView location_line)
 
     size_t target_location_end_index = char_index - 2;
 
-    auto original_pair = parse_start_and_length_pair(location_line.substring_view(original_location_start_index, original_location_end_index - original_location_start_index + 1));
-    auto target_pair = parse_start_and_length_pair(location_line.substring_view(target_location_start_index, target_location_end_index - target_location_start_index + 1));
-    return { original_pair.start, original_pair.length, target_pair.start, target_pair.length };
-}
-
-void HunkLocation::apply_offset(size_t offset, HunkLocation::LocationType type)
-{
-    if (type == LocationType::Original || type == LocationType::Both) {
-        original_start_line += offset;
-        original_length -= offset;
-    }
-    if (type == LocationType::Target || type == LocationType::Both) {
-        target_start_line += offset;
-        target_length -= offset;
-    }
+    auto old_range = parse_start_and_length_pair(location_line.substring_view(original_location_start_index, original_location_end_index - original_location_start_index + 1));
+    auto new_range = parse_start_and_length_pair(location_line.substring_view(target_location_start_index, target_location_end_index - target_location_start_index + 1));
+    return { old_range, new_range };
 }
 
 };

Userland/Libraries/LibDiff/Hunks.h

@@ -1,38 +1,96 @@
 /*
  * Copyright (c) 2020, Itamar S. <itamar8910@gmail.com>
+ * Copyright (c) 2023, Shannon Booth <shannon.ml.booth@gmail.com>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #pragma once
 
+#include <AK/Assertions.h>
+#include <AK/Format.h>
 #include <AK/String.h>
 #include <AK/StringView.h>
 #include <AK/Vector.h>
 
 namespace Diff {
 
-struct HunkLocation {
-    size_t original_start_line { 0 };
-    size_t original_length { 0 };
-    size_t target_start_line { 0 };
-    size_t target_length { 0 };
+struct Range {
+    size_t start_line { 0 };
+    size_t number_of_lines { 0 };
+};
+
+struct HunkLocation {
+    Range old_range;
+    Range new_range;
+};
+
+struct Line {
+    enum class Operation {
+        Addition = '+',
+        Removal = '-',
+        Context = ' ',
 
-    enum class LocationType {
-        Original,
-        Target,
-        Both
     };
-    void apply_offset(size_t offset, LocationType);
+
+    static constexpr Operation operation_from_symbol(char symbol)
+    {
+        switch (symbol) {
+        case '+':
+            return Operation::Addition;
+        case '-':
+            return Operation::Removal;
+        case ' ':
+            return Operation::Context;
+        default:
+            VERIFY_NOT_REACHED();
+        }
+    }
+
+    Operation operation;
+    String content;
 };
 
 struct Hunk {
-    size_t original_start_line { 0 };
-    size_t target_start_line { 0 };
-    Vector<String> removed_lines;
-    Vector<String> added_lines;
+    HunkLocation location;
+    Vector<Line> lines;
 };
 
 ErrorOr<Vector<Hunk>> parse_hunks(StringView diff);
 HunkLocation parse_hunk_location(StringView location_line);
 };
+
+template<>
+struct AK::Formatter<Diff::Line::Operation> : Formatter<FormatString> {
+    ErrorOr<void> format(FormatBuilder& builder, Diff::Line::Operation operation)
+    {
+        return Formatter<FormatString>::format(builder, "{}"sv, static_cast<char>(operation));
+    }
+};
+
+template<>
+struct AK::Formatter<Diff::Line> : Formatter<FormatString> {
+    ErrorOr<void> format(FormatBuilder& builder, Diff::Line const& line)
+    {
+        return Formatter<FormatString>::format(builder, "{}{}"sv, line.operation, line.content);
+    }
+};
+
+template<>
+struct AK::Formatter<Diff::HunkLocation> : Formatter<FormatString> {
+    static ErrorOr<void> format(FormatBuilder& format_builder, Diff::HunkLocation const& location)
+    {
+        auto& builder = format_builder.builder();
+        TRY(builder.try_appendff("@@ -{}"sv, location.old_range.start_line));
+
+        if (location.old_range.number_of_lines != 1)
+            TRY(builder.try_appendff(",{}", location.old_range.number_of_lines));
+
+        TRY(builder.try_appendff(" +{}", location.new_range.start_line));
+
+        if (location.new_range.number_of_lines != 1)
+            TRY(builder.try_appendff(",{}", location.new_range.number_of_lines));
+
+        return builder.try_appendff(" @@");
+    }
+};