use cache to store char-to-column mapping

This is faster by O(n) to O(n²) compared to the previous algorithm
that would re-compute the column for each call.

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