Source code in the yabasic programming language

clear screen

guesses = 0

void = ran()

while(len(secret$) < 4)    //    zero not allowed
    n$ = chr$(int(ran(1) * 9) + 49)
    if not(instr(secret$, n$)) secret$ = secret$ + n$
wend

print " Secretly, my opponent just chose a number. But she didn't tell anyone!\n\t\t\t\t", secret$, "."
print "     I can however be given a score for my guesses."

for i = 1234 to 9876
    if check(str$(i)) = 0 then
    	available$ = available$ + " " + str$(i)
    	k = k +1
    end if
next i

available$ = trim$(available$)  //   remove the surplus, leading space

while(true)
    print
    print "Currently holding ", k, " possible numbers. "

    guess$ =word$(available$, 1 + int(k * ran(1)), " ")
    print "Computer guessed ", guess$, " & got ";

    bulls = 0
    cows = 0
    guesses = guesses + 1

    r$ = score$(guess$, secret$)

    bulls = val(word$(r$, 1, ","))
    cows  = val(word$(r$, 2, ","))

    print bulls, " bull(s), and ", cows, " cow(s)."

    if guess$ = secret$ then
        print "\nComputer won after ", guesses, " guesses!";
        print " That took ", right$(time$, 1), " seconds. ENDED!"
        break
    end if
    kk = 0
    new$ = ""

    for j = 1 to k
        bullsT = 0
        cowsT = 0

        possible$ = word$(available$, j, " ")

        r$ = score$(guess$, possible$)

        bullsT = val(word$(r$, 1, ","))
        cowsT = val(word$(r$, 2, ","))

        if (bullsT = bulls) and ( cowsT = cows)  then
            new$ = new$ + " " + possible$    //    keep those with same score
            kk = kk + 1
        end if
    next j

    available$ = trim$(new$)
    k = kk
    
wend

sub score$(a$, b$)    //   return as a csv string the number of bulls & cows.
    local i, c$, bulls, cows
	
    bulls = 0 : cows = 0
    for i = 1 to 4
        c$ = mid$(a$, i, 1)
        if mid$(b$, i, 1) = c$ then
            bulls = bulls + 1
        else
            if (instr(b$, c$) <> 0) and (instr(b$, c$) <> i) cows = cows + 1
        end if
    next i
    return str$(bulls) + "," + str$(cows)
end sub

sub check(i$)
    local t, i, j
	
    t = 0    //    zero flags available: 1 means not available
    for i = 1 to 3
        for j = i + 1 to 4
            if mid$(i$, i, 1) = mid$(i$, j, 1) t = 1
        next j
    next i
    if instr(i$, "0") t = 1
    return t
end sub

sub word$(l$, i, d$)
    local c$(1), n
	
    n = token(l$, c$(), d$)
    return c$(i)
end sub