Data commit

Task/Dutch-national-flag-problem/Go/dutch-national-flag-problem.go

@@ -0,0 +1,88 @@
+package main
+
+import (
+    "fmt"
+    "math/rand"
+    "time"
+)
+
+// constants define order of colors in Dutch national flag
+const (
+    red = iota
+    white
+    blue
+    nColors
+)
+
+// zero object of type is valid red ball.
+type ball struct {
+    color int
+}
+
+// order of balls based on DNF
+func (b1 ball) lt(b2 ball) bool {
+    return b1.color < b2.color
+}
+
+// type for arbitrary ordering of balls
+type ordering []ball
+
+// predicate tells if balls are ordered by DNF
+func (o ordering) ordered() bool {
+    var b0 ball
+    for _, b := range o {
+        if b.lt(b0) {
+            return false
+        }
+        b0 = b
+    }
+    return true
+}
+
+func init() {
+    rand.Seed(time.Now().Unix())
+}
+
+// constructor returns new ordering of balls which is randomized but
+// guaranteed to be not in DNF order.  function panics for n < 2.
+func outOfOrder(n int) ordering {
+    if n < 2 {
+        panic(fmt.Sprintf("%d invalid", n))
+    }
+    r := make(ordering, n)
+    for {
+        for i, _ := range r {
+            r[i].color = rand.Intn(nColors)
+        }
+        if !r.ordered() {
+            break
+        }
+    }
+    return r
+}
+
+// O(n) algorithm
+// http://www.csse.monash.edu.au/~lloyd/tildeAlgDS/Sort/Flag/
+func (a ordering) sort3() {
+    lo, mid, hi := 0, 0, len(a)-1
+    for mid <= hi {
+        switch a[mid].color {
+        case red:
+            a[lo], a[mid] = a[mid], a[lo]
+            lo++
+            mid++
+        case white:
+            mid++
+        default:
+            a[mid], a[hi] = a[hi], a[mid]
+            hi--
+        }
+    }
+}
+
+func main() {
+    f := outOfOrder(12)
+    fmt.Println(f)
+    f.sort3()
+    fmt.Println(f)
+}