Scroll Editing: An On-Line Algorithm for Manipulating Long Character Strings

Abstract

An algorithm that runs on a 2048-word LINC provides efficient on-line editing of character strings virtually unlimited in length. Fixed-address LINC tape holds the character sequence in the manner of a scroll. Edited characters are spliced directly in or out of the scroll as it moves across a display scope under the viewer's control. A 512-character "playground" created at the splice point provides sufficient ease to permit changing the scroll contents dynamically, and thereby simplifies several problems commonly associated with on-line editing. Compensatory inserting and deleting are practical. Inserted characters require no special identification and scroll maintenance is automatic. Editing commands for simple editorial functions and editorial text identifiers are eliminated, and the number of characters that can be inserted anywhere is limited only by the total length of the scroll. Line numbers, if provided, are resequenced automatically as the scroll contents change. As little as 2 percent of the scroll is manipulated in the memory at a time. Despite the relatively slow transfer characteristics of the tape, performance is satisfactory on a LINC for scrolls up to 23 040 characters and is not strongly dependent on the size of the playground.