Ошибка: local variable 'index' referenced before assignment

f = open("input.txt", encoding='utf8')
text = f.read().lower()
f.close()
d = {}
for i in text:
    if i not in '!—,.-?...()—:;«»\nъь':
        d[i] = d.get(i, 0) + 1
sum_sim = sum(d.values())
for i in d:
    d[i] = round(d[i] / sum_sim, 3)
d = sorted(d.items(), key=lambda x: x[1], reverse=True)
arr = []
for i in d:
    arr.append(list(i) + [''])
d = {}
 
def func(arr):    
    half = sum(map(lambda x: x[1], arr))
    sum1 = 0
    for i, j in enumerate(arr):
        sum1 += j[1]
        if sum1 * 2 >= half:
            index = i + (abs(2 * sum1 - half) < abs(2 * (sum1 - j[1]) - half))
            break
 
    arr0, arr1 = [], []
    for i in arr[:index]:
        i[2] += '0'        
        arr0.append(i)
    for i in arr[index:]:
        i[2] += '1'
        arr1.append(i)
 
    if len(arr1) == 1:        
        arr3 = arr1[0]
        d[arr3[0]] = arr3[2]
    else:
        func(arr1)
    if len(arr0) == 1:        
        arr4 = arr0[0]
        d[arr4[0]] = arr4[2]
    else:
        func(arr0)
 
func(arr)
print(d)
for i in text:
    if i not in '!—,.-?...()—:;«»\nъь':
        text = text.replace(i, d[i])
print(text)
f2 = open('output.txt', 'w+', encoding='utf8')
f2.write(text)