Data commit

Task/Ludic-numbers/Go/ludic-numbers.go

@@ -0,0 +1,81 @@
+package main
+
+import "fmt"
+
+// Ludic returns a slice of Ludic numbers stopping after
+// either n entries or when max is exceeded.
+// Either argument may be <=0 to disable that limit.
+func Ludic(n int, max int) []uint32 {
+	const maxInt32 = 1<<31 - 1 // i.e. math.MaxInt32
+	if max > 0 && n < 0 {
+		n = maxInt32
+	}
+	if n < 1 {
+		return nil
+	}
+	if max < 0 {
+		max = maxInt32
+	}
+	sieve := make([]uint32, 10760) // XXX big enough for 2005 Ludics
+	sieve[0] = 1
+	sieve[1] = 2
+	if n > 2 {
+		// We start with even numbers already removed
+		for i, j := 2, uint32(3); i < len(sieve); i, j = i+1, j+2 {
+			sieve[i] = j
+		}
+		// We leave the Ludic numbers in place,
+		// k is the index of the next Ludic
+		for k := 2; k < n; k++ {
+			l := int(sieve[k])
+			if l >= max {
+				n = k
+				break
+			}
+			i := l
+			l--
+			// last is the last valid index
+			last := k + i - 1
+			for j := k + i + 1; j < len(sieve); i, j = i+1, j+1 {
+				last = k + i
+				sieve[last] = sieve[j]
+				if i%l == 0 {
+					j++
+				}
+			}
+			// Truncate down to only the valid entries
+			if last < len(sieve)-1 {
+				sieve = sieve[:last+1]
+			}
+		}
+	}
+	if n > len(sieve) {
+		panic("program error") // should never happen
+	}
+	return sieve[:n]
+}
+
+func has(x []uint32, v uint32) bool {
+	for i := 0; i < len(x) && x[i] <= v; i++ {
+		if x[i] == v {
+			return true
+		}
+	}
+	return false
+}
+
+func main() {
+	// Ludic() is so quick we just call it repeatedly
+	fmt.Println("First 25:", Ludic(25, -1))
+	fmt.Println("Numner of Ludics below 1000:", len(Ludic(-1, 1000)))
+	fmt.Println("Ludic 2000 to 2005:", Ludic(2005, -1)[1999:])
+
+	fmt.Print("Tripples below 250:")
+	x := Ludic(-1, 250)
+	for i, v := range x[:len(x)-2] {
+		if has(x[i+1:], v+2) && has(x[i+2:], v+6) {
+			fmt.Printf(", (%d %d %d)", v, v+2, v+6)
+		}
+	}
+	fmt.Println()
+}