2 files changed, 43 insertions, 3 deletions
diff --git a/Userland/Libraries/LibJS/SourceCode.cpp b/Userland/Libraries/LibJS/SourceCode.cpp
index 89a844f3e3..f0700a1c2d 100644
--- a/Userland/Libraries/LibJS/SourceCode.cpp
+++ b/Userland/Libraries/LibJS/SourceCode.cpp
@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <AK/BinarySearch.h>
 #include <AK/Utf8View.h>
 #include <LibJS/SourceCode.h>
 #include <LibJS/SourceRange.h>
@@ -32,18 +33,52 @@ String const& SourceCode::code() const
     return m_code;
 }
 
+void SourceCode::compute_line_break_offsets() const
+{
+    m_line_break_offsets = Vector<size_t> {};
+
+    if (m_code.is_empty())
+        return;
+
+    bool previous_code_point_was_carriage_return = false;
+    Utf8View view(m_code.view());
+    for (auto it = view.begin(); it != view.end(); ++it) {
+        u32 code_point = *it;
+        bool is_line_terminator = code_point == '\r' || (code_point == '\n' && !previous_code_point_was_carriage_return) || code_point == LINE_SEPARATOR || code_point == PARAGRAPH_SEPARATOR;
+        previous_code_point_was_carriage_return = code_point == '\r';
+
+        if (is_line_terminator)
+            m_line_break_offsets->append(view.byte_offset_of(it));
+    }
+}
+
 SourceRange SourceCode::range_from_offsets(u32 start_offset, u32 end_offset) const
 {
+    if (m_code.is_empty())
+        return { *this, {}, {} };
+
+    if (!m_line_break_offsets.has_value())
+        compute_line_break_offsets();
+
+    size_t line = 1;
+    size_t nearest_line_break_index = 0;
+    size_t nearest_preceding_line_break_offset = 0;
+
+    if (!m_line_break_offsets->is_empty()) {
+        binary_search(*m_line_break_offsets, start_offset, &nearest_line_break_index);
+        line = 1 + nearest_line_break_index;
+        nearest_preceding_line_break_offset = (*m_line_break_offsets)[nearest_line_break_index];
+    }
+
     Position start;
     Position end;
 
-    size_t line = 1;
     size_t column = 1;
 
     bool previous_code_point_was_carriage_return = false;
 
-    Utf8View view(m_code);
-    for (auto it = view.begin(); it != view.end(); ++it) {
+    Utf8View view(m_code.view());
+    for (auto it = view.iterator_at_byte_offset_without_validation(nearest_preceding_line_break_offset); it != view.end(); ++it) {
 
         if (start_offset == view.byte_offset_of(it)) {
             start = Position {
diff --git a/Userland/Libraries/LibJS/SourceCode.h b/Userland/Libraries/LibJS/SourceCode.h
index 3980efed69..1550c50c89 100644
--- a/Userland/Libraries/LibJS/SourceCode.h
+++ b/Userland/Libraries/LibJS/SourceCode.h
@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/String.h>
+#include <AK/Vector.h>
 #include <LibJS/Forward.h>
 
 namespace JS {
@@ -23,8 +24,12 @@ public:
 private:
     SourceCode(String filename, String code);
 
+    void compute_line_break_offsets() const;
+
     String m_filename;
     String m_code;
+
+    Optional<Vector<size_t>> mutable m_line_break_offsets;
 };
 
 }