AK: Cache all the line positions in LineTrackingLexer

Also updates a LibWeb text test that used to report the wrong line
number.

AK/GenericLexer.cpp

@@ -175,27 +175,32 @@ ErrorOr<T> GenericLexer::consume_decimal_integer()
 
 LineTrackingLexer::Position LineTrackingLexer::position_for(size_t index) const
 {
-    auto& [cached_index, cached_line, cached_column] = m_cached_position;
+    // Sad case: we have no idea where the nearest newline is, so we have to
+    //           scan ahead a bit.
+    while (index > m_largest_known_line_start_position) {
+        auto next_newline = m_input.find('\n', m_largest_known_line_start_position);
+        if (!next_newline.has_value()) {
+            // No more newlines, add the end of the input as a line start to avoid searching again.
+            m_line_start_positions->insert(m_input.length(), m_line_start_positions->size());
+            m_largest_known_line_start_position = m_input.length();
+            break;
+        }
+        m_line_start_positions->insert(next_newline.value() + 1, m_line_start_positions->size());
+        m_largest_known_line_start_position = next_newline.value() + 1;
+    }
+    // We should always have at least the first line start position.
+    auto previous_line_it = m_line_start_positions->find_largest_not_above_iterator(index);
+    auto previous_line_index = previous_line_it.key();
 
-    if (cached_index <= index) {
-        for (size_t i = cached_index; i < index; ++i) {
-            if (m_input[i] == '\n')
-                ++cached_line, cached_column = 0;
-            else
-                ++cached_column;
+    auto line = *previous_line_it;
+    auto column = index - previous_line_index;
+    if (line == 0) {
+        // First line, take into account the start position.
+        column += m_first_line_start_position.column;
     }
-    } else {
-        auto lines_backtracked = m_input.substring_view(index, cached_index - index).count('\n');
-        cached_line -= lines_backtracked;
-        if (lines_backtracked == 0) {
-            cached_column -= cached_index - index;
-        } else {
-            auto current_line_start = m_input.substring_view(0, index).find_last('\n').value_or(0);
-            cached_column = index - current_line_start;
-        }
-    }
-    cached_index = index;
-    return m_cached_position;
+
+    line += m_first_line_start_position.line;
+    return { index, line, column };
 }
 
 template ErrorOr<u8> GenericLexer::consume_decimal_integer<u8>();

AK/GenericLexer.h

@@ -6,6 +6,8 @@
 
 #pragma once
 
+#include <AK/NonnullOwnPtr.h>
+#include <AK/RedBlackTree.h>
 #include <AK/Result.h>
 #include <AK/String.h>
 #include <AK/StringView.h>
@@ -227,8 +229,6 @@ protected:
 
 class LineTrackingLexer : public GenericLexer {
 public:
-    using GenericLexer::GenericLexer;
-
     struct Position {
         size_t offset { 0 };
         size_t line { 0 };
@@ -237,20 +237,27 @@ public:
 
     LineTrackingLexer(StringView input, Position start_position)
         : GenericLexer(input)
-        , m_cached_position {
-            .line = start_position.line,
-            .column = start_position.column,
+        , m_first_line_start_position(start_position)
+        , m_line_start_positions(make<RedBlackTree<size_t, size_t>>())
+    {
+        m_line_start_positions->insert(0, 0);
+        auto first_newline = input.find('\n').map([](auto x) { return x + 1; }).value_or(input.length());
+        m_line_start_positions->insert(first_newline, 1);
+        m_largest_known_line_start_position = first_newline;
     }
+
+    LineTrackingLexer(StringView input)
+        : LineTrackingLexer(input, { 0, 1, 1 })
     {
     }
 
-    Position cached_position() const { return m_cached_position; }
-    void restore_cached_offset(Position cached_position) { m_cached_position = cached_position; }
     Position position_for(size_t) const;
     Position current_position() const { return position_for(m_index); }
 
 protected:
-    mutable Position m_cached_position;
+    Position m_first_line_start_position;
+    mutable NonnullOwnPtr<RedBlackTree<size_t, size_t>> m_line_start_positions; // offset -> line index
+    mutable size_t m_largest_known_line_start_position { 0 };
 };
 
 constexpr auto is_any_of(StringView values)

Tests/LibWeb/Text/expected/XML/error-page.txt

@@ -1,3 +1,3 @@
 Got load event
 [object HTMLDocument]
-Failed to parse XML document: Expected '>' at line: 1, col: 20 (offset 59)
+Failed to parse XML document: Expected '>' at line: 2, col: 20 (offset 59)

Userland/Libraries/LibXML/Parser/Parser.h

@@ -147,9 +147,8 @@ private:
     [[nodiscard]] auto rollback_point(SourceLocation location = SourceLocation::current())
     {
         return ArmedScopeGuard {
-            [this, position = m_lexer.tell(), cached_position = m_lexer.cached_position(), location] {
+            [this, position = m_lexer.tell(), location] {
                 m_lexer.retreat(m_lexer.tell() - position);
-                m_lexer.restore_cached_offset(cached_position);
                 (void)location;
                 dbgln_if(XML_PARSER_DEBUG, "{:->{}}FAIL @ {} -- \x1b[31m{}\x1b[0m", " ", s_debug_indent_level * 2, location, m_lexer.remaining().substring_view(0, min(16, m_lexer.tell_remaining())).replace("\n"sv, "\\n"sv, ReplaceMode::All));
             }