package combinatorics;

import java.util.*;

import java.util.function.BiFunction;

import java.util.stream.IntStream;

public class NumberIterator {

public static long count(long to, BiFunction<int[], Integer, Long> countWithPrefix) {

char[] s = String.valueOf(to).toCharArray();

int n = s.length;

int[] digits = IntStream.range(0, n).map(i -> s[i] - '0').toArray();

long res = 0;

for (int i = 0; i < n; i++) {

int[] prefix = Arrays.copyOf(digits, i + 1);

for (prefix[i] = 0; prefix[i] < digits[i] + (i == n - 1 ? 1 : 0); ++prefix[i]) {

res += countWithPrefix.apply(prefix, n - 1 - i);

}

}

return res;

}

// Usage example

public static void main(String[] args) {

// count numbers consisting only of digits 2 and 5

long num_2_5 = count(222, (prefix, remainingDigits) -> {

if (Arrays.equals(prefix, new int[] {0}))

return (1L << (remainingDigits + 1)) - 2;

boolean all_2_5 = Arrays.stream(prefix).allMatch(v -> v == 2 || v == 5);

return all_2_5 ? 1L << remainingDigits : 0;

});

System.out.println(7 == num_2_5);

}

}