Re: [TUHS] Data structures in Unix editors

This is actually what ed, ex, and vi do as well, albeit not with a standard interface. The buffer is an array of integer offsets into a file. Line n in the buffer is pointed at by buffer[n], whose contents are an offset into the temp file. That offset points to a null terminated line of buffer text. When a new line of text is created (or changed) the new line is appended to the temp file and the corresponding offset goes into the buffer array. One big reason for this was that ed was originally written for a 16 bit machine, and buffers didn't fit in memory. This works very well to insert or delete lines, you only have to copy the line references in the buffer array. It's also convenient for undo, since putting things back just means restoring the original array. My enhanced ed (hed at Wisconsin, Portable ed at Bell Labs) had a second array for undo, and a full copy of the array was made before a change. Bill Joy wrote a fancier implementation in vi, where only lines added or deleted were saved, command-specific. Undo knew which command it had to undo and special-cased each one. The big disadvantage to this structure is that ends of lines are not just newline characters. You can't backspace over a newline, or change newlines to something else.     Mary Ann On 4/1/21 5:56 AM, Tony Finch wrote: