From ba241b12e05071f26da09f0d28a6f840a79122f0 Mon Sep 17 00:00:00 2001
From: Andreas Huber <andreas.huber@lucares.de>
Date: Mon, 20 Dec 2021 20:09:29 +0100
Subject: [PATCH] use cache to store char-to-column mapping
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This is faster by O(n) to O(n²) compared to the previous algorithm
that would re-compute the column for each call.
---
 highlight_selection.py |  5 ++++-
 line.py                | 17 ++++++++++++++---
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/highlight_selection.py b/highlight_selection.py
index cd4b5c4..9f0b6d5 100644
--- a/highlight_selection.py
+++ b/highlight_selection.py
@@ -45,7 +45,10 @@ class HighlightSelection(Highlight):
 
             start_column = line.char_to_column(start_char)
             end_column = line.char_to_column(end_char)
-            length_in_columns = end_column - start_column
+            if end_column >= 0:
+                length_in_columns = end_column - start_column
+            else:
+                length_in_columns = 4096
 
             return [HighlightedRange(start_column, length_in_columns, brush=QBrush(QColor(156, 215, 255, 192)),
                                      pen=Qt.PenStyle.NoPen)]
diff --git a/line.py b/line.py
index a738970..f2f4e64 100644
--- a/line.py
+++ b/line.py
@@ -1,3 +1,5 @@
+from typing import Dict
+
 import constants
 
 
@@ -6,6 +8,7 @@ class Line:
         self._byte_offset = byte_offset
         self._byte_end = byte_end
         self._line = line
+        self._char_to_column_cache = self._cache_char_to_column()
 
     def byte_offset(self) -> int:
         return self._byte_offset
@@ -63,17 +66,25 @@ class Line:
 
         return result
 
-    # todo this method is slow
     def char_to_column(self, char_in_line: int) -> int:
+        if not char_in_line in self._char_to_column_cache:
+            # print("%d in %s" % (char_in_line, self._char_to_column_cache))
+            return -1
+        return self._char_to_column_cache[char_in_line]
+
+    def _cache_char_to_column(self) -> Dict[int, int]:
+        char_to_column_cache = {}
         result = 0
         i = 0
-        while i < char_in_line:
+        char_to_column_cache[0] = 0
+        while i < len(self._line):
             if i < len(self._line) and self._line[i] == "\t":
                 result = result + constants.tab_width - result % constants.tab_width
             else:
                 result = result + 1
             i = i + 1
-        return result
+            char_to_column_cache[i] = result
+        return char_to_column_cache
 
     def includes_byte(self, byte: int) -> bool:
         return self._byte_offset <= byte <= self._byte_end