Add single-element cache for DirIV lookup

Another 3x performance boost for applications that walk the
directory tree.

Excerpt from performance.txt:

VERSION         UNTAR    LS     RM
v0.4               48     1.5    5
v0.5-rc1           56     7     19
v0.5-rc1-1         54     4.1    9
v0.5-rc1-2         45     1.7	 3.4  <---- THIS VERSION

3 files changed, 62 insertions, 5 deletions

diff --git a/cryptfs/cryptfs.go b/cryptfs/cryptfs.go
index 63febc3..df04973 100644
--- a/cryptfs/cryptfs.go
+++ b/cryptfs/cryptfs.go
@@ -26,6 +26,8 @@ type CryptFS struct {
 	// Stores an all-zero block of size cipherBS
 	allZeroBlock   []byte
 	plaintextNames bool
+	// DirIV cache for filename encryption
+	DirIVCacheEnc DirIVCache
 }
 
 func NewCryptFS(key []byte, useOpenssl bool, plaintextNames bool) *CryptFS {
diff --git a/cryptfs/kdf.go b/cryptfs/kdf.go
index 73be0bb..db08500 100644
--- a/cryptfs/kdf.go
+++ b/cryptfs/kdf.go
@@ -1,10 +1,10 @@
 package cryptfs
 
 import (
-	"os"
-	"math"
 	"fmt"
 	"golang.org/x/crypto/scrypt"
+	"math"
+	"os"
 )
 
 const (
@@ -50,5 +50,5 @@ func (s *scryptKdf) DeriveKey(pw string) []byte {
 // LogN - N is saved as 2^LogN, but LogN is much easier to work with.
 // This function gives you LogN = Log2(N).
 func (s *scryptKdf) LogN() int {
-	return int(math.Log2(float64(s.N))+0.5)
+	return int(math.Log2(float64(s.N)) + 0.5)
 }
diff --git a/cryptfs/names_diriv.go b/cryptfs/names_diriv.go
index ae8545b..9012e48 100644
--- a/cryptfs/names_diriv.go
+++ b/cryptfs/names_diriv.go
@@ -5,8 +5,47 @@ import (
 	"io/ioutil"
 	"path/filepath"
 	"strings"
+	"sync"
 )
 
+// A simple one-entry DirIV cache
+type DirIVCache struct {
+	// Invalidated?
+	cleared       bool
+	// The DirIV
+	iv            []byte
+	// Directory the DirIV belongs to
+	dir           string
+	// Ecrypted version of "dir"
+	translatedDir string
+	// Synchronisation
+	lock          sync.RWMutex
+}
+
+func (c *DirIVCache) lookup(dir string) (bool, []byte, string) {
+	c.lock.RLock()
+	defer c.lock.RUnlock()
+	if !c.cleared && c.dir == dir {
+		return true, c.iv, c.translatedDir
+	}
+	return false, nil, ""
+}
+
+func (c *DirIVCache) store(dir string, iv []byte, translatedDir string) {
+	c.lock.Lock()
+	defer c.lock.Unlock()
+	c.cleared = false
+	c.iv = iv
+	c.dir = dir
+	c.translatedDir = translatedDir
+}
+
+func (c *DirIVCache) Clear() {
+	c.lock.Lock()
+	defer c.lock.Unlock()
+	c.cleared = true
+}
+
 // readDirIV - read the "gocryptfs.diriv" file from "dir" (absolute ciphertext path)
 func (be *CryptFS) ReadDirIV(dir string) (iv []byte, err error) {
 	ivfile := filepath.Join(dir, DIRIV_FILENAME)
@@ -41,11 +80,23 @@ func (be *CryptFS) EncryptPathDirIV(plainPath string, rootDir string) (string, e
 	if plainPath == "" {
 		return plainPath, nil
 	}
+	// Check if the DirIV is cached
+	parentDir := filepath.Dir(plainPath)
+	found, iv, cParentDir := be.DirIVCacheEnc.lookup(parentDir)
+	if found {
+		//fmt.Print("h")
+		baseName := filepath.Base(plainPath)
+		cBaseName := be.encryptName(baseName, iv)
+		cPath := cParentDir + "/" + cBaseName
+		return cPath, nil
+	}
+	// Walk the directory tree
 	var wd = rootDir
 	var encryptedNames []string
+	var err error
 	plainNames := strings.Split(plainPath, "/")
 	for _, plainName := range plainNames {
-		iv, err := be.ReadDirIV(wd)
+		iv, err = be.ReadDirIV(wd)
 		if err != nil {
 			return "", err
 		}
@@ -53,7 +104,11 @@ func (be *CryptFS) EncryptPathDirIV(plainPath string, rootDir string) (string, e
 		encryptedNames = append(encryptedNames, encryptedName)
 		wd = filepath.Join(wd, encryptedName)
 	}
-	return filepath.Join(encryptedNames...), nil
+	// Cache the final DirIV
+	cPath := strings.Join(encryptedNames, "/")
+	cParentDir = filepath.Dir(cPath)
+	be.DirIVCacheEnc.store(parentDir, iv, cParentDir)
+	return cPath, nil
 }
 
 // DecryptPathDirIV - encrypt path using CBC with DirIV