diff --git a/logic/c.go b/logic/c.go
index d08091e..e60f5a1 100644
--- a/logic/c.go
+++ b/logic/c.go
@@ -266,7 +266,7 @@ func (p *C) And(a, b z.Lit) z.Lit {
 		return b
 	}
 	c := strashCode(a, b)
-	l := uint32(cap(p.nodes))
+	l := uint32(cap(p.nodes) - 1)
 	i := c % l
 	si := p.strash[i]
 	for {
@@ -282,7 +282,7 @@ func (p *C) And(a, b z.Lit) z.Lit {
 	m, j := p.newNode()
 	m.a = a
 	m.b = b
-	k := c % uint32(cap(p.nodes))
+	k := c % uint32(cap(p.nodes) - 1)
 	m.n = p.strash[k]
 	p.strash[k] = j
 	return z.Var(j).Pos()
@@ -367,7 +367,7 @@ func (p *C) grow() {
 			continue
 		}
 		c := strashCode(n.a, n.b)
-		j := c % ucap
+		j := c % (ucap - 1)
 		n.n = strash[j]
 		strash[j] = uint32(i)
 	}
diff --git a/sudoku_test.go b/sudoku_test.go
index a260390..2e08bbe 100644
--- a/sudoku_test.go
+++ b/sudoku_test.go
@@ -2,11 +2,18 @@ package gini_test
 
 import (
 	"fmt"
+	"testing"
 
 	"github.com/go-air/gini"
 	"github.com/go-air/gini/z"
 )
 
+func BenchmarkSudoku(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		Example_sudoku()
+	}
+}
+
 func Example_sudoku() {
 	g := gini.New()
 	// 9 rows, 9 cols, 9 boxes, 9 numbers