forum-msg-id-133749-edit

Original date:2019-04-14 08:01:15 Edited by: petelomax Subject: Wrap-around search algorithm

This proved far harder than I ever imagined possible, any improvements welcome.

The following is a simplified testbed.
First simplification: text is "0000".."1111" and "find" is whether or not text[1..4] is '1'.
Second simplification: we set "start" explicitly in the testing loop, maybe unlike real use.
(Technically we shouldn't need to reset wrappable in this testbed, but abs. rqd. in real use)
Starting from 1..4, with N=sum of 1s, we want f3find() to get N/eof/N/eof, iyswim.
The challenge is(was) to get all 64 test cases to work, then repeat searching backwards.

Anyway, a second set of eyes, before I try applying this to Edix/Edita, anything that simplifies it or makes it any easier to understand, or OE-compatible, thankx.

sequence text 
integer start, current, wrappable = 1 
 
function f3find(integer direction) 
-- 
-- direction should be +/-1. [existing code uses 0 to mean "from line 1"] 
-- return "next/prev" '1' in text, wrapping around relative to start. 
-- return -1 at "eof", aka "start", allowing restart. 
-- 
integer limit = iff(direction<0?1:length(text)), 
        begin = iff(direction>0?1:length(text)) 
 
    current += direction 
    bool low = compare(start,current)=direction or not wrappable 
    limit = iff(low ? start-(wrappable=0)*direction : limit) 
    for i=current to limit by direction do 
        if text[i]='1' then current=i return i end if 
    end for 
    if wrappable and compare(start,current)!=direction then 
        wrappable = 0 
        limit = start-direction 
        for i=begin to limit by direction do 
            if text[i]='1' then current=i return i end if 
        end for 
    end if 
    current = start-direction 
    wrappable = 1 
    return -1 
end function 
 
constant TRIES=4 
integer fails = 0, total = 0 
for i=0 to 15 do 
    text = sprintf("%04b",i) 
    integer N = sum(sq_eq(text,'1')) 
    ?{text,N} 
    for direction=+1 to -1 by -2 do 
        for j=1 to 4 do 
            start = j 
            current = j-direction 
            wrappable = 1 
            sequence s = {} 
            for t=1 to (N+1)*TRIES do 
                s &= f3find(direction) 
            end for 
            total += 1 
            if s[$]!=-1 or sum(sq_eq(s,-1))!=TRIES then 
                s &= {"9/0"} 
                fails += 1 
                ?{"s=",s} 
            end if 
--          ?s 
        end for          
    end for          
end for 
printf(1,"fails: %d, pass: %d/%d\n",{fails,total-fails,total}) 

output:

{"0000",0} 
{"0001",1} 
{"0010",1} 
{"0011",2} 
{"0100",1} 
{"0101",2} 
{"0110",2} 
{"0111",3} 
{"1000",1} 
{"1001",2} 
{"1010",2} 
{"1011",3} 
{"1100",2} 
{"1101",3} 
{"1110",3} 
{"1111",4} 
fails: 0, pass: 128/128 

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