Cleanup

Author: korthaj

euler.go

@@ -4,15 +4,19 @@ package graph
 // If no such walk exists, it returns an empty walk and sets ok to false.
 func EulerDirected(g Iterator) (walk []int, ok bool) {
 	n := g.Order()
-	// Compute outdegree - indegree for each vertex.
-	degree := make([]int, n)
+	degree := make([]int, n) // outdegree - indegree for each vertex
+	edgeCount := 0
 	for v := range degree {
 		g.Visit(v, func(w int, _ int64) (skip bool) {
+			edgeCount++
 			degree[v]++
 			degree[w]--
 			return
 		})
 	}
+	if edgeCount == 0 {
+		return []int{}, true
+	}
 
 	start, end := -1, -1
 	for v := range degree {
@@ -28,42 +32,38 @@ func EulerDirected(g Iterator) (walk []int, ok bool) {
 	}
 
 	// Make a copy of g
-	edgeCount := 0
 	h := make([][]int, n)
 	for v := range h {
 		g.Visit(v, func(w int, _ int64) (skip bool) {
 			h[v] = append(h[v], w)
-			edgeCount++
 			return
 		})
 	}
-	if edgeCount == 0 {
-		return []int{}, true
-	}
 
 	// Find a starting point with neighbors.
-	for v := 0; v < n && start == -1; v++ {
-		if len(h[v]) > 0 {
-			start = v
+	if start == -1 {
+		for v, neighbors := range h {
+			if len(neighbors) > 0 {
+				start = v
+				break
+			}
 		}
 	}
 
-	stack := []int{start}
-	for len(stack) > 0 {
-		v := stack[len(stack)-1]
-		stack = stack[:len(stack)-1]
+	for stack := []int{start}; len(stack) > 0; {
+		n := len(stack)
+		v := stack[n-1]
+		stack = stack[:n-1]
 		for len(h[v]) > 0 {
 			stack = append(stack, v)
 			v, h[v] = h[v][0], h[v][1:]
 			edgeCount--
 		}
 		walk = append(walk, v)
 	}
-
-	if edgeCount != 0 {
+	if edgeCount > 0 {
 		return []int{}, false
 	}
-
 	for i, j := 0, len(walk)-1; i < j; i, j = i+1, j-1 {
 		walk[i], walk[j] = walk[j], walk[i]
 	}
@@ -75,16 +75,20 @@ func EulerDirected(g Iterator) (walk []int, ok bool) {
 // and sets ok to false.
 func EulerUndirected(g Iterator) (walk []int, ok bool) {
 	n := g.Order()
-	// Compute outdegree for each vertex.
-	out := make([]int, n)
+	out := make([]int, n) // outdegree for each vertex
+	edgeCount := 0
 	for v := range out {
 		g.Visit(v, func(w int, _ int64) (skip bool) {
+			edgeCount++
 			if v != w {
 				out[v]++
 			}
 			return
 		})
 	}
+	if edgeCount == 0 {
+		return []int{}, true
+	}
 
 	start, oddDeg := -1, 0
 	for v := range out {
@@ -97,52 +101,40 @@ func EulerUndirected(g Iterator) (walk []int, ok bool) {
 		return []int{}, false
 	}
 
-	// Make a copy of g.
-	edgeCount := 0
-	h := New(n)
-	for v := 0; v < n; v++ {
-		g.Visit(v, func(w int, _ int64) (skip bool) {
-			h.Add(v, w)
-			edgeCount++
-			return
-		})
-	}
-	if edgeCount == 0 {
-		return []int{}, true
-	}
-
 	// Find a starting point with neighbors.
-	for v := 0; v < n && start == -1; v++ {
-		h.Visit(v, func(w int, _ int64) (skip bool) {
-			start = w
-			return true
-		})
+	if start == -1 {
+		for v := 0; v < n; v++ {
+			if g.Visit(v, func(w int, _ int64) (skip bool) {
+				start = w
+				return true
+			}) {
+				break
+			}
+		}
 	}
 
-	stack := []int{start}
-	for len(stack) > 0 {
-		v := stack[len(stack)-1]
-		stack = stack[:len(stack)-1]
+	h := Copy(g)
+	for stack := []int{start}; len(stack) > 0; {
+		n := len(stack)
+		v := stack[n-1]
+		stack = stack[:n-1]
 		for h.Degree(v) > 0 {
 			stack = append(stack, v)
 			var w int
 			h.Visit(v, func(u int, _ int64) (skip bool) {
 				w = u
 				return true
 			})
+			h.DeleteBoth(v, w)
+			edgeCount--
 			if v != w {
-				h.DeleteBoth(v, w)
-				edgeCount -= 2
-			} else {
-				h.Delete(v, v)
 				edgeCount--
 			}
 			v = w
 		}
 		walk = append(walk, v)
 	}
-
-	if edgeCount != 0 {
+	if edgeCount > 0 {
 		return []int{}, false
 	}
 	return walk, true