Initial data commit

Task/Descending-primes/Go/descending-primes.go

@@ -0,0 +1,63 @@
+package main
+
+import (
+    "fmt"
+    "rcu"
+    "sort"
+    "strconv"
+)
+
+func combinations(a []int, k int) [][]int {
+    n := len(a)
+    c := make([]int, k)
+    var combs [][]int
+    var combine func(start, end, index int)
+    combine = func(start, end, index int) {
+        if index == k {
+            t := make([]int, len(c))
+            copy(t, c)
+            combs = append(combs, t)
+            return
+        }
+        for i := start; i <= end && end-i+1 >= k-index; i++ {
+            c[index] = a[i]
+            combine(i+1, end, index+1)
+        }
+    }
+    combine(0, n-1, 0)
+    return combs
+}
+
+func powerset(a []int) (res [][]int) {
+    if len(a) == 0 {
+        return
+    }
+    for i := 1; i <= len(a); i++ {
+        res = append(res, combinations(a, i)...)
+    }
+    return
+}
+
+func main() {
+    ps := powerset([]int{9, 8, 7, 6, 5, 4, 3, 2, 1})
+    var descPrimes []int
+    for i := 1; i < len(ps); i++ {
+        s := ""
+        for _, e := range ps[i] {
+            s += string(e + '0')
+        }
+        p, _ := strconv.Atoi(s)
+        if rcu.IsPrime(p) {
+            descPrimes = append(descPrimes, p)
+        }
+    }
+    sort.Ints(descPrimes)
+    fmt.Println("There are", len(descPrimes), "descending primes, namely:")
+    for i := 0; i < len(descPrimes); i++ {
+        fmt.Printf("%8d ", descPrimes[i])
+        if (i+1)%10 == 0 {
+            fmt.Println()
+        }
+    }
+    fmt.Println()
+}