import Foundation
// Checks whether a string contains only binary digits 0 or 1
func isBinary(_ s: String) -> Bool {
// Swift's modern regex literal
let binary = try! Regex("^[01]+$")
return s.wholeMatch(of: binary) != nil
}
/*
In decimal: reading "123" means
remainder = remainder * 10 + digit
In binary: reading "1011" means
remainder = remainder * 2 + bit
*/
/*
0 (0*2 + 0) % 7 0
0 (0*2 + 0) % 7 0
0 (0*2 + 0) % 7 0
1 (0*2 + 1) % 7 1
1 (1*2 + 1) % 7 3
1 (3*2 + 1) % 7 0
0 (0*2 + 0) % 7 0
0 (0*2 + 0) % 7 0
*/
// Computes whether a binary string represents a number divisible by 7
func divisibleBy7(_ s: String) -> Bool {
var remainder = 0
for c in s {
remainder = (remainder * 2 + (c == "1" ? 1 : 0)) % 7
}
return remainder == 0
}
func main() {
let s = "00011100" // 28
if !isBinary(s) {
print("Not a binary string")
return
}
if divisibleBy7(s) {
print("Divisible by 7")
} else {
print("Not divisible by 7")
}
}
main()
/*
run:
Divisible by 7
*/
