Re: Lua and text-representation

[David Given <dg@...>]

On 2010-03-01 17:35, Gavin Wraith wrote:
> If Lua is to be embedded in such an application
> it would be nice to use the memory allocation that comes with Lua.
> In that case one might use two lists, one indexing forward from the first
> character to the last before the cursor, the other indexing backwards
> from the last character of the text to the one immediately following the
> cursor. Moving the cursor then means sloshing top elements from one
> list to the other.

For WordGrinder I stored a simple array of paragraphs, each paragraph 
being an array of words. The cursor was a (paragraph, word, offset) tuple.

Redrawing the screen involves flowing the current paragraph, drawing it 
in the right place, and then working up and down until we reach the 
edges of the screen (yes, that's why the cursor is always in the 
middle). As paragraph caches its flowed data and only updates it on 
demand, to avoid the cost of having to reflow the entire document if the 
width of the screen changes.

Performance is adequate but not brilliant --- it's not usable on low-end 
machines (266MHz ARM, 300MHz MIPS). I was really aiming at machines on 
the order of my 600MHz eee, which it works fine on.

Why did I do it like that? I decided against lists because in Lua, being 
interpreted, you can consider any built-in operation to be O(1). So 
insertion into an array is cheap. (Particularly since the paragraph/word 
structure means the array sizes are quite small.) Plus, arrays are far 
more natural in Lua than lists, and the navigation code was going to ba 
a huge mess of special cases anyway, so I wanted to keep things 
straightforward.

Well, that's my excuse. The *real* reason is that I violated my cardinal 
rule and failed to read the literature on how to do it properly. But 
don't tell anyone...

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