global tt$
tt$="-0+"   '-1 0 1; +2 -> 1 2 3, instr

'Test case:
'With balanced ternaries a from string "+-0++0+", b from native integer -436, c "+-++-":
'* write out a, b and c in decimal notation;
'* calculate a * (b - c), write out the result in both ternary and decimal notations.

a$="+-0++0+"
a=deci(a$)
print "a",a, a$

b=-436
b$=ternary$(b)
print "b",b, b$

c$="+-++-"
c=deci(c$)
print "c",c, c$

'calculate in ternary

res$=multTernary$(a$, subTernary$(b$, c$))
print "a * (b - c)", res$
print "In decimal:",deci(res$)

print "Check:"
print "a * (b - c)", a * (b - c)
end

function deci(s$)
    pow = 1
    for i = len(s$) to 1 step -1
        c$ = mid$(s$,i,1)
        'select case c$
        '    case "+":sign= 1
        '    case "-":sign=-1
        '    case "0":sign= 0
        'end select
        sign = instr(tt$,c$)-2
        deci = deci+pow*sign
        pow = pow*3
    next
end function

function ternary$(n)
    while abs(n)>3^k/2
        k=k+1
    wend
    k=k-1

    pow = 3^k
    for i = k to 0 step -1
        sign = (n>0) - (n<0)
        sign = sign * (abs(n)>pow/2)
        ternary$ = ternary$+mid$(tt$,sign+2,1)
        n = n - sign*pow
        pow = pow/3
    next
    if  ternary$ = "" then  ternary$ ="0"
end function

function multTernary$(a$, b$)

    c$ = ""
    t$ = ""
    shift$ = ""
    for i = len(a$) to 1 step -1

        select case mid$(a$,i,1)
        case "+": t$ = b$
        case "0": t$ = "0"
        case "-": t$ = negate$(b$)
        end select

        c$ = addTernary$(c$, t$+shift$)

        shift$ = shift$ +"0"
    'print d, t$, c$
    next
    multTernary$ = c$
end function

function subTernary$(a$, b$)
     subTernary$ = addTernary$(a$, negate$(b$))
end function

function negate$(s$)
    negate$=""
    for i = 1 to len(s$)
        'print mid$(s$,i,1), instr(tt$, mid$(s$,i,1)), 4-instr(tt$, mid$(s$,i,1))
        negate$=negate$+mid$(tt$, 4-instr(tt$, mid$(s$,i,1)), 1)
    next
end function

function addTernary$(a$, b$)
'add a$ + b$, for now only positive
    l = max(len(a$), len(b$))
    a$=pad$(a$,l)
    b$=pad$(b$,l)
    c$ = "" 'result
    carry = 0
    for i = l to 1 step -1
        a = instr(tt$,mid$(a$,i,1))-2
        b = instr(tt$,mid$(b$,i,1))-2     '-1 0 1
        c = a+b+carry

        select case
        case abs(c)<2
            carry = 0
        case c>0
            carry =1: c=c-3
        case c<0
            carry =-1: c=c+3
        end select

        'print a, b, c
        c$ = mid$(tt$,c+2,1)+c$
    next
    if carry<>0 then c$ = mid$(tt$,carry+2,1) +c$
    'print c$
    'have to trim leading 0's
    i=0
    while mid$(c$,i+1,1)="0"
        i=i+1
    wend
    c$=mid$(c$,i+1)
    if c$="" then c$="0"
    addTernary$ = c$
end function

function pad$(a$,n)  'pad from right with 0 to length n
     pad$ = a$
     while len(pad$)<n
        pad$ = "0"+pad$
     wend
end function