Fix Prev() to behave like Next() in reverse

Enumerator.Prev() would return the next item if there was no hit.
This fix instead changes Prev to return previous item as expected.
Resolving issue https://github.com/cznic/b/issues/6.

CONTRIBUTORS

@@ -6,6 +6,7 @@
 #
 # Please keep the list sorted.
 
+Andrey Elenskiy <andrey.elenskiy@gmail.com>
 Brian Fallik <bfallik@gmail.com>
 Dan Kortschak <dan.kortschak@adelaide.edu.au>
 Jan Mercl <0xjnml@gmail.com>

all_test.go

@@ -800,13 +800,13 @@ func TestEnumeratorPrev(t *testing.T) {
 		hit  bool
 		keys []int
 	}{
-		{5, false, []int{10}},
+		{5, false, []int{}},
 		{10, true, []int{10}},
-		{15, false, []int{20, 10}},
+		{15, false, []int{10}},
 		{20, true, []int{20, 10}},
-		{25, false, []int{30, 20, 10}},
+		{25, false, []int{20, 10}},
 		{30, true, []int{30, 20, 10}},
-		{35, false, []int{}},
+		{35, false, []int{30, 20, 10}},
 	}
 
 	for i, test := range table {
@@ -863,6 +863,51 @@ func TestEnumeratorPrev(t *testing.T) {
 	}
 }
 
+func TestEnumeratorPrevSanity(t *testing.T) {
+	// seeking within 3 keys: 10, 20, 30
+	table := []struct {
+		k      int
+		hit    bool
+		kOut   interface{}
+		vOut   interface{}
+		errOut error
+	}{
+		{10, true, 10, 100, nil},
+		{20, true, 20, 200, nil},
+		{30, true, 30, 300, nil},
+		{35, false, 30, 300, nil},
+		{25, false, 20, 200, nil},
+		{15, false, 10, 100, nil},
+		{5, false, nil, nil, io.EOF},
+	}
+
+	for i, test := range table {
+		r := TreeNew(cmp)
+
+		r.Set(10, 100)
+		r.Set(20, 200)
+		r.Set(30, 300)
+
+		en, hit := r.Seek(test.k)
+
+		if g, e := hit, test.hit; g != e {
+			t.Fatal(i, g, e)
+		}
+
+		k, v, err := en.Prev()
+
+		if g, e := err, test.errOut; g != e {
+			t.Fatal(i, g, e)
+		}
+		if g, e := k, test.kOut; g != e {
+			t.Fatal(i, g, e)
+		}
+		if g, e := v, test.vOut; g != e {
+			t.Fatal(i, g, e)
+		}
+	}
+}
+
 func BenchmarkSeekSeq1e3(b *testing.B) {
 	benchmarkSeekSeq(b, 1e3)
 }

btree.go

@@ -826,13 +826,7 @@ func (e *Enumerator) Next() (k interface{} /*K*/, v interface{} /*V*/, err error
 	}
 
 	if e.ver != e.t.ver {
-		f, hit := e.t.Seek(e.k)
-		if !e.hit && hit {
-			if err = f.next(); err != nil {
-				return
-			}
-		}
-
+		f, _ := e.t.Seek(e.k)
 		*e = *f
 		f.Close()
 	}
@@ -849,7 +843,7 @@ func (e *Enumerator) Next() (k interface{} /*K*/, v interface{} /*V*/, err error
 
 	i := e.q.d[e.i]
 	k, v = i.k, i.v
-	e.k, e.hit = k, false
+	e.k, e.hit = k, true
 	e.next()
 	return
 }
@@ -880,13 +874,7 @@ func (e *Enumerator) Prev() (k interface{} /*K*/, v interface{} /*V*/, err error
 	}
 
 	if e.ver != e.t.ver {
-		f, hit := e.t.Seek(e.k)
-		if !e.hit && hit {
-			if err = f.prev(); err != nil {
-				return
-			}
-		}
-
+		f, _ := e.t.Seek(e.k)
 		*e = *f
 		f.Close()
 	}
@@ -895,15 +883,22 @@ func (e *Enumerator) Prev() (k interface{} /*K*/, v interface{} /*V*/, err error
 		return
 	}
 
+	if !e.hit {
+		// move to previous because Seek overshoots if there's no hit
+		if err = e.prev(); err != nil {
+			return
+		}
+	}
+
 	if e.i >= e.q.c {
-		if err = e.next(); err != nil {
+		if err = e.prev(); err != nil {
 			return
 		}
 	}
 
 	i := e.q.d[e.i]
 	k, v = i.k, i.v
-	e.k, e.hit = k, false
+	e.k, e.hit = k, true
 	e.prev()
 	return
 }