Data commit

Task/Giuga-numbers/Go/giuga-numbers.go

@@ -0,0 +1,74 @@
+package main
+
+import "fmt"
+
+var factors []int
+var inc = []int{4, 2, 4, 2, 4, 6, 2, 6}
+
+// Assumes n is even with exactly one factor of 2.
+// Empties 'factors' if any other prime factor is repeated.
+func primeFactors(n int) {
+    factors = factors[:0]
+    factors = append(factors, 2)
+    last := 2
+    n /= 2
+    for n%3 == 0 {
+        if last == 3 {
+            factors = factors[:0]
+            return
+        }
+        last = 3
+        factors = append(factors, 3)
+        n /= 3
+    }
+    for n%5 == 0 {
+        if last == 5 {
+            factors = factors[:0]
+            return
+        }
+        last = 5
+        factors = append(factors, 5)
+        n /= 5
+    }
+    for k, i := 7, 0; k*k <= n; {
+        if n%k == 0 {
+            if last == k {
+                factors = factors[:0]
+                return
+            }
+            last = k
+            factors = append(factors, k)
+            n /= k
+        } else {
+            k += inc[i]
+            i = (i + 1) % 8
+        }
+    }
+    if n > 1 {
+        factors = append(factors, n)
+    }
+}
+
+func main() {
+    const limit = 5
+    var giuga []int
+    // n can't be 2 or divisible by 4
+    for n := 6; len(giuga) < limit; n += 4 {
+        primeFactors(n)
+        // can't be prime or semi-prime
+        if len(factors) > 2 {
+            isGiuga := true
+            for _, f := range factors {
+                if (n/f-1)%f != 0 {
+                    isGiuga = false
+                    break
+                }
+            }
+            if isGiuga {
+                giuga = append(giuga, n)
+            }
+        }
+    }
+    fmt.Println("The first", limit, "Giuga numbers are:")
+    fmt.Println(giuga)
+}