LibJS: Make Error stack trace generation faster with a line break cache

Generating Error objects got a lot slower after the introduction of
SourceCode in b0b022507b2f73be2f4cef17deb8ece91dae6822.

This was noticeable with `test-js` which generates a lot of errors,
so walking the source code over and over to compute (line, column)
was eating a ton of time.

This patch makes repeated lookups a lot faster by building a cache
of line break offsets in the source code. The cache is built on first
offset lookup, so we only pay for this in code that actually throws.

On my machine, this takes `test-js` runtime from 6.7 sec to 4.3 sec.

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;
 };
 
 }