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/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(" @@");
+    }
+};