Add Document.get_index(location)

src/textual/document/_document.py

@@ -90,6 +90,18 @@ def text(self) -> str:
     def newline(self) -> Newline:
         """Return the line separator used in the document."""
 
+    @abstractmethod
+    def get_index(self, at: Location) -> int:
+        """Given a location in the document, returns the index
+        in the document seen as a flat string.
+
+        Args:
+            at: The location in the document.
+
+        Returns:
+            The index in the document seen as a flat string.
+        """
+
     @abstractmethod
     def get_line(self, index: int) -> str:
         """Returns the line with the given index from the document.
@@ -320,6 +332,22 @@ def line_count(self) -> int:
         """Returns the number of lines in the document."""
         return len(self._lines)
 
+    def get_index(self, at: Location) -> int:
+        """Given a location in the document, returns the index
+        in the document seen as a flat string.
+
+        Args:
+            at: The location in the document.
+
+        Returns:
+            The index in the document seen as a flat string.
+        """
+        row, col = at
+        idx = row * len(self.newline) + col
+        for i in range(row):
+            idx += len(self.get_line(i))
+        return idx
+
     def get_line(self, index: int) -> str:
         """Returns the line with the given index from the document.