69 lines
1.2 KiB
Swift
69 lines
1.2 KiB
Swift
func isPrime(_ n: Int) -> Bool {
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if n < 2 {
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return false
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}
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if n % 2 == 0 {
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return n == 2
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}
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if n % 3 == 0 {
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return n == 3
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}
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var p = 5
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while p * p <= n {
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if n % p == 0 {
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return false
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}
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p += 2
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if n % p == 0 {
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return false
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}
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p += 4
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}
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return true
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}
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func isLeftTruncatable(_ p: Int) -> Bool {
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var n = 10
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var q = p
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while p > n {
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if !isPrime(p % n) || q == p % n {
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return false
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}
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q = p % n
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n *= 10
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}
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return true
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}
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func isRightTruncatable(_ p: Int) -> Bool {
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var q = p / 10
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while q > 0 {
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if !isPrime(q) {
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return false
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}
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q /= 10
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}
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return true
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}
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let limit = 1000000
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var largestLeft = 0
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var largestRight = 0
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var p = limit
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while p >= 2 {
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if isPrime(p) && isLeftTruncatable(p) {
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largestLeft = p
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break
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}
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p -= 1
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}
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print("Largest left truncatable prime is \(largestLeft)")
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p = limit
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while p >= 2 {
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if isPrime(p) && isRightTruncatable(p) {
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largestRight = p
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break
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}
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p -= 1
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}
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print("Largest right truncatable prime is \(largestRight)")
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