Row replacement algorithms for screen editors

Abstract

Interactive screen editors repeatedly determine terminal command sequences to update a screen row. Computing an optimal command sequence differs from the traditional sequence comparison problem in that there is a cost for moving the cursor over unedited characters and the cost of an n -character command is not always the cost of n one-character commands. For example, on an ANSI-standard terminal, it takes nine bytes to insert one character, ten to insert two, eleven to insert three, and so on. This paper presents an O ( MN ) dynamic programming algorithm for row replacement where an n -character command costs α n + β for constants α and β. M is the length of the original row and N is the length of its replacement. Also given is an O ( Cost × ( M + N )) “greedy” algorithm for optimal row replacement. Here Cost is the optimal cost (in bytes) of the replacement, so the algorithm is fast when the require d update is small. Though the algorithm is rather complicated, it is fast enough to be useful in practice.