A real delete often spans more than one piece - and deleting a newline is how two lines become one. Today you make Delete correct across piece boundaries, including joining lines.
Delete a range that spans several pieces, and delete a newline so two lines join.
Deletes rarely respect piece boundaries. After a few inserts the text is a patchwork
of pieces, and a single backspace-and-hold can sweep across several of them. The
walk from last lesson already has the right shape; it just needs to handle the
range covering whole pieces in the middle (dropped entirely) as well as
trimming the first and last pieces it touches. Deleting "bXYc" out of "abXYcd"
drops the middle add-buffer piece completely and trims one character off each
original piece on either side, leaving "ad".
The line-joining case is the one that matters most for an editor, and it needs no
special code at all: a newline is just a byte living in some piece, so deleting
it merges the line before with the line after. "ab\ncd" with its \n removed is
simply "abcd" - one line where there were two. This is exactly what backspace at
the start of a line will do later, and it works today because the buffer treats
newlines as ordinary content and lets the line structure fall out of where they sit.
// the same trim walk, but now several pieces fall inside the range:// pieces fully covered by [pos, pos+count) are dropped entirely,// the first and last overlapping pieces keep their outside remainders.// a newline is just a byte in a piece - deleting it joins two lines.