diff --git a/libs/utils/file_backup_helper.cpp b/libs/utils/file_backup_helper.cpp
new file mode 100644
index 0000000..453084a
--- /dev/null
+++ b/libs/utils/file_backup_helper.cpp
@@ -0,0 +1,685 @@
+#define LOG_TAG "file_backup_helper"
+
+#include <utils/backup_helpers.h>
+
+#include <utils/KeyedVector.h>
+#include <utils/ByteOrder.h>
+#include <utils/String8.h>
+
+#include <errno.h>
+#include <sys/types.h>
+#include <sys/uio.h>
+#include <sys/stat.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <utime.h>
+#include <fcntl.h>
+#include <zlib.h>
+
+#include <cutils/log.h>
+
+using namespace android;
+
+#define MAGIC0 0x70616e53 // Snap
+#define MAGIC1 0x656c6946 // File
+
+#define LOGP(x...) LOGD(x)
+//#define LOGP(x...) printf(x)
+
+struct SnapshotHeader {
+    int magic0;
+    int fileCount;
+    int magic1;
+    int totalSize;
+};
+
+struct FileState {
+    int modTime_sec;
+    int modTime_nsec;
+    int size;
+    int crc32;
+    int nameLen;
+};
+
+const static int ROUND_UP[4] = { 0, 3, 2, 1 };
+
+static inline int
+round_up(int n)
+{
+    return n + ROUND_UP[n % 4];
+}
+
+static int
+read_snapshot_file(int fd, KeyedVector<String8,FileState>* snapshot)
+{
+    int bytesRead = 0;
+    int amt;
+    SnapshotHeader header;
+
+    amt = read(fd, &header, sizeof(header));
+    if (amt != sizeof(header)) {
+        return errno;
+    }
+    bytesRead += amt;
+
+    if (header.magic0 != MAGIC0 || header.magic1 != MAGIC1) {
+        LOGW("read_snapshot_file header.magic0=0x%08x magic1=0x%08x", header.magic0, header.magic1);
+        return 1;
+    }
+
+    for (int i=0; i<header.fileCount; i++) {
+        FileState file;
+        char filenameBuf[128];
+
+        amt = read(fd, &file, sizeof(file));
+        if (amt != sizeof(file)) {
+            LOGW("read_snapshot_file FileState truncated/error with read at %d bytes\n", bytesRead);
+            return 1;
+        }
+        bytesRead += amt;
+
+        // filename is not NULL terminated, but it is padded
+        int nameBufSize = round_up(file.nameLen);
+        char* filename = nameBufSize <= (int)sizeof(filenameBuf)
+                ? filenameBuf
+                : (char*)malloc(nameBufSize);
+        amt = read(fd, filename, nameBufSize);
+        if (amt == nameBufSize) {
+            snapshot->add(String8(filename, file.nameLen), file);
+        }
+        bytesRead += amt;
+        if (filename != filenameBuf) {
+            free(filename);
+        }
+        if (amt != nameBufSize) {
+            LOGW("read_snapshot_file filename truncated/error with read at %d bytes\n", bytesRead);
+            return 1;
+        }
+    }
+
+    if (header.totalSize != bytesRead) {
+        LOGW("read_snapshot_file length mismatch: header.totalSize=%d bytesRead=%d\n",
+                header.totalSize, bytesRead);
+        return 1;
+    }
+
+    return 0;
+}
+
+static int
+write_snapshot_file(int fd, const KeyedVector<String8,FileState>& snapshot)
+{
+    int bytesWritten = sizeof(SnapshotHeader);
+    // preflight size
+    const int N = snapshot.size();
+    for (int i=0; i<N; i++) {
+        const String8& name = snapshot.keyAt(i);
+        bytesWritten += sizeof(FileState) + round_up(name.length());
+    }
+
+    int amt;
+    SnapshotHeader header = { MAGIC0, N, MAGIC1, bytesWritten };
+
+    amt = write(fd, &header, sizeof(header));
+    if (amt != sizeof(header)) {
+        LOGW("write_snapshot_file error writing header %s", strerror(errno));
+        return errno;
+    }
+
+    for (int i=0; i<header.fileCount; i++) {
+        const String8& name = snapshot.keyAt(i);
+        FileState file = snapshot.valueAt(i);
+        int nameLen = file.nameLen = name.length();
+
+        amt = write(fd, &file, sizeof(file));
+        if (amt != sizeof(file)) {
+            LOGW("write_snapshot_file error writing header %s", strerror(errno));
+            return 1;
+        }
+
+        // filename is not NULL terminated, but it is padded
+        amt = write(fd, name.string(), nameLen);
+        if (amt != nameLen) {
+            LOGW("write_snapshot_file error writing filename %s", strerror(errno));
+            return 1;
+        }
+        int paddingLen = ROUND_UP[nameLen % 4];
+        if (paddingLen != 0) {
+            int padding = 0xabababab;
+            amt = write(fd, &padding, paddingLen);
+            if (amt != paddingLen) {
+                LOGW("write_snapshot_file error writing %d bytes of filename padding %s",
+                        paddingLen, strerror(errno));
+                return 1;
+            }
+        }
+    }
+
+    return 0;
+}
+
+static int
+write_delete_file(const String8& key)
+{
+    LOGP("write_delete_file %s\n", key.string());
+    return 0;
+}
+
+static int
+write_update_file(const String8& realFilename, const String8& key)
+{
+    LOGP("write_update_file %s (%s)\n", realFilename.string(), key.string());
+    return 0;
+}
+
+static int
+compute_crc32(const String8& filename)
+{
+    const int bufsize = 4*1024;
+    int amt;
+
+    int fd = open(filename.string(), O_RDONLY);
+    if (fd == -1) {
+        return -1;
+    }
+
+    char* buf = (char*)malloc(bufsize);
+    int crc = crc32(0L, Z_NULL, 0);
+
+    while ((amt = read(fd, buf, bufsize)) != 0) {
+        crc = crc32(crc, (Bytef*)buf, amt);
+    }
+
+    close(fd);
+    free(buf);
+
+    return crc;
+}
+
+int
+back_up_files(int oldSnapshotFD, int oldDataStream, int newSnapshotFD,
+        char const* fileBase, char const* const* files, int fileCount)
+{
+    int err;
+    const String8 base(fileBase);
+    KeyedVector<String8,FileState> oldSnapshot;
+    KeyedVector<String8,FileState> newSnapshot;
+
+    if (oldSnapshotFD != -1) {
+        err = read_snapshot_file(oldSnapshotFD, &oldSnapshot);
+        if (err != 0) {
+            // On an error, treat this as a full backup.
+            oldSnapshot.clear();
+        }
+    }
+
+    for (int i=0; i<fileCount; i++) {
+        String8 name(files[i]);
+        FileState s;
+        struct stat st;
+        String8 realFilename(base);
+        realFilename.appendPath(name);
+
+        err = stat(realFilename.string(), &st);
+        if (err != 0) {
+            LOGW("Error stating file %s", realFilename.string());
+            continue;
+        }
+
+        s.modTime_sec = st.st_mtime;
+        s.modTime_nsec = 0; // workaround sim breakage
+        //s.modTime_nsec = st.st_mtime_nsec;
+        s.size = st.st_size;
+        s.crc32 = compute_crc32(realFilename);
+
+        newSnapshot.add(name, s);
+    }
+
+    int n = 0;
+    int N = oldSnapshot.size();
+    int m = 0;
+
+    while (n<N && m<fileCount) {
+        const String8& p = oldSnapshot.keyAt(n);
+        const String8& q = newSnapshot.keyAt(m);
+        int cmp = p.compare(q);
+        if (cmp > 0) {
+            // file added
+            String8 realFilename(base);
+            realFilename.appendPath(q);
+            write_update_file(realFilename, q);
+            m++;
+        }
+        else if (cmp < 0) {
+            // file removed
+            write_delete_file(p);
+            n++;
+        }
+        else {
+            // both files exist, check them
+            String8 realFilename(base);
+            realFilename.appendPath(q);
+            const FileState& f = oldSnapshot.valueAt(n);
+            const FileState& g = newSnapshot.valueAt(m);
+
+            LOGP("%s\n", q.string());
+            LOGP("  new: modTime=%d,%d size=%-3d crc32=0x%08x\n",
+                    f.modTime_sec, f.modTime_nsec, f.size, f.crc32);
+            LOGP("  old: modTime=%d,%d size=%-3d crc32=0x%08x\n",
+                    g.modTime_sec, g.modTime_nsec, g.size, g.crc32);
+            if (f.modTime_sec != g.modTime_sec || f.modTime_nsec != g.modTime_nsec
+                    || f.size != g.size || f.crc32 != g.crc32) {
+                write_update_file(realFilename, p);
+            }
+            n++;
+            m++;
+        }
+    }
+
+    // these were deleted
+    while (n<N) {
+        write_delete_file(oldSnapshot.keyAt(n));
+        n++;
+    }
+
+    // these were added
+    while (m<fileCount) {
+        const String8& q = newSnapshot.keyAt(m);
+        String8 realFilename(base);
+        realFilename.appendPath(q);
+        write_update_file(realFilename, q);
+        m++;
+    }
+
+    err = write_snapshot_file(newSnapshotFD, newSnapshot);
+
+    return 0;
+}
+
+#if TEST_BACKUP_HELPERS
+
+#define SCRATCH_DIR "/data/backup_helper_test/"
+
+static int
+write_text_file(const char* path, const char* data)
+{
+    int amt;
+    int fd;
+    int len;
+
+    fd = creat(path, 0666);
+    if (fd == -1) {
+        fprintf(stderr, "creat %s failed\n", path);
+        return errno;
+    }
+
+    len = strlen(data);
+    amt = write(fd, data, len);
+    if (amt != len) {
+        fprintf(stderr, "error (%s) writing to file %s\n", strerror(errno), path);
+        return errno;
+    }
+
+    close(fd);
+
+    return 0;
+}
+
+static int
+compare_file(const char* path, const unsigned char* data, int len)
+{
+    int fd;
+    int amt;
+
+    fd = open(path, O_RDONLY);
+    if (fd == -1) {
+        fprintf(stderr, "compare_file error (%s) opening %s\n", strerror(errno), path);
+        return errno;
+    }
+
+    unsigned char* contents = (unsigned char*)malloc(len);
+    if (contents == NULL) {
+        fprintf(stderr, "malloc(%d) failed\n", len);
+        return ENOMEM;
+    }
+
+    bool sizesMatch = true;
+    amt = lseek(fd, 0, SEEK_END);
+    if (amt != len) {
+        fprintf(stderr, "compare_file file length should be %d, was %d\n", len, amt);
+        sizesMatch = false;
+    }
+    lseek(fd, 0, SEEK_SET);
+
+    int readLen = amt < len ? amt : len;
+    amt = read(fd, contents, readLen);
+    if (amt != readLen) {
+        fprintf(stderr, "compare_file read expected %d bytes but got %d\n", len, amt);
+    }
+
+    bool contentsMatch = true;
+    for (int i=0; i<readLen; i++) {
+        if (data[i] != contents[i]) {
+            if (contentsMatch) {
+                fprintf(stderr, "compare_file contents are different: (index, expected, actual)\n");
+                contentsMatch = false;
+            }
+            fprintf(stderr, "  [%-2d] %02x %02x\n", i, data[i], contents[i]);
+        }
+    }
+
+    return contentsMatch && sizesMatch ? 0 : 1;
+}
+
+int
+backup_helper_test_empty()
+{
+    int err;
+    int fd;
+    KeyedVector<String8,FileState> snapshot;
+    const char* filename = SCRATCH_DIR "backup_helper_test_empty.snap";
+
+    system("rm -r " SCRATCH_DIR);
+    mkdir(SCRATCH_DIR, 0777);
+
+    // write
+    fd = creat(filename, 0666);
+    if (fd == -1) {
+        fprintf(stderr, "error creating %s\n", filename);
+        return 1;
+    }
+
+    err = write_snapshot_file(fd, snapshot);
+
+    close(fd);
+
+    if (err != 0) {
+        fprintf(stderr, "write_snapshot_file reported error %d (%s)\n", err, strerror(err));
+        return err;
+    }
+
+    static const unsigned char correct_data[] = {
+        0x53, 0x6e, 0x61, 0x70,  0x00, 0x00, 0x00, 0x00,
+        0x46, 0x69, 0x6c, 0x65,  0x10, 0x00, 0x00, 0x00
+    };
+
+    err = compare_file(filename, correct_data, sizeof(correct_data));
+    if (err != 0) {
+        return err;
+    }
+
+    // read
+    fd = open(filename, O_RDONLY);
+    if (fd == -1) {
+        fprintf(stderr, "error opening for read %s\n", filename);
+        return 1;
+    }
+
+    KeyedVector<String8,FileState> readSnapshot;
+    err = read_snapshot_file(fd, &readSnapshot);
+    if (err != 0) {
+        fprintf(stderr, "read_snapshot_file failed %d\n", err);
+        return err;
+    }
+
+    if (readSnapshot.size() != 0) {
+        fprintf(stderr, "readSnapshot should be length 0\n");
+        return 1;
+    }
+
+    return 0;
+}
+
+int
+backup_helper_test_four()
+{
+    int err;
+    int fd;
+    KeyedVector<String8,FileState> snapshot;
+    const char* filename = SCRATCH_DIR "backup_helper_test_four.snap";
+
+    system("rm -r " SCRATCH_DIR);
+    mkdir(SCRATCH_DIR, 0777);
+
+    // write
+    fd = creat(filename, 0666);
+    if (fd == -1) {
+        fprintf(stderr, "error opening %s\n", filename);
+        return 1;
+    }
+
+    String8 filenames[4];
+    FileState states[4];
+
+    states[0].modTime_sec = 0xfedcba98;
+    states[0].modTime_nsec = 0xdeadbeef;
+    states[0].size = 0xababbcbc;
+    states[0].crc32 = 0x12345678;
+    states[0].nameLen = -12;
+    filenames[0] = String8("bytes_of_padding");
+    snapshot.add(filenames[0], states[0]);
+
+    states[1].modTime_sec = 0x93400031;
+    states[1].modTime_nsec = 0xdeadbeef;
+    states[1].size = 0x88557766;
+    states[1].crc32 = 0x22334422;
+    states[1].nameLen = -1;
+    filenames[1] = String8("bytes_of_padding3");
+    snapshot.add(filenames[1], states[1]);
+
+    states[2].modTime_sec = 0x33221144;
+    states[2].modTime_nsec = 0xdeadbeef;
+    states[2].size = 0x11223344;
+    states[2].crc32 = 0x01122334;
+    states[2].nameLen = 0;
+    filenames[2] = String8("bytes_of_padding_2");
+    snapshot.add(filenames[2], states[2]);
+
+    states[3].modTime_sec = 0x33221144;
+    states[3].modTime_nsec = 0xdeadbeef;
+    states[3].size = 0x11223344;
+    states[3].crc32 = 0x01122334;
+    states[3].nameLen = 0;
+    filenames[3] = String8("bytes_of_padding__1");
+    snapshot.add(filenames[3], states[3]);
+
+    err = write_snapshot_file(fd, snapshot);
+
+    close(fd);
+
+    if (err != 0) {
+        fprintf(stderr, "write_snapshot_file reported error %d (%s)\n", err, strerror(err));
+        return err;
+    }
+
+    static const unsigned char correct_data[] = {
+        // header
+        0x53, 0x6e, 0x61, 0x70,  0x04, 0x00, 0x00, 0x00,
+        0x46, 0x69, 0x6c, 0x65,  0xac, 0x00, 0x00, 0x00,
+
+        // bytes_of_padding
+        0x98, 0xba, 0xdc, 0xfe,  0xef, 0xbe, 0xad, 0xde,
+        0xbc, 0xbc, 0xab, 0xab,  0x78, 0x56, 0x34, 0x12,
+        0x10, 0x00, 0x00, 0x00,  0x62, 0x79, 0x74, 0x65,
+        0x73, 0x5f, 0x6f, 0x66,  0x5f, 0x70, 0x61, 0x64,
+        0x64, 0x69, 0x6e, 0x67,
+
+        // bytes_of_padding3
+        0x31, 0x00, 0x40, 0x93,  0xef, 0xbe, 0xad, 0xde,
+        0x66, 0x77, 0x55, 0x88,  0x22, 0x44, 0x33, 0x22,
+        0x11, 0x00, 0x00, 0x00,  0x62, 0x79, 0x74, 0x65,
+        0x73, 0x5f, 0x6f, 0x66,  0x5f, 0x70, 0x61, 0x64,
+        0x64, 0x69, 0x6e, 0x67,  0x33, 0xab, 0xab, 0xab,
+        
+        // bytes of padding2
+        0x44, 0x11, 0x22, 0x33,  0xef, 0xbe, 0xad, 0xde,
+        0x44, 0x33, 0x22, 0x11,  0x34, 0x23, 0x12, 0x01,
+        0x12, 0x00, 0x00, 0x00,  0x62, 0x79, 0x74, 0x65,
+        0x73, 0x5f, 0x6f, 0x66,  0x5f, 0x70, 0x61, 0x64,
+        0x64, 0x69, 0x6e, 0x67,  0x5f, 0x32, 0xab, 0xab,
+        
+        // bytes of padding3
+        0x44, 0x11, 0x22, 0x33,  0xef, 0xbe, 0xad, 0xde,
+        0x44, 0x33, 0x22, 0x11,  0x34, 0x23, 0x12, 0x01,
+        0x13, 0x00, 0x00, 0x00,  0x62, 0x79, 0x74, 0x65,
+        0x73, 0x5f, 0x6f, 0x66,  0x5f, 0x70, 0x61, 0x64,
+        0x64, 0x69, 0x6e, 0x67,  0x5f, 0x5f, 0x31, 0xab
+    };
+
+    err = compare_file(filename, correct_data, sizeof(correct_data));
+    if (err != 0) {
+        return err;
+    }
+    
+    // read
+    fd = open(filename, O_RDONLY);
+    if (fd == -1) {
+        fprintf(stderr, "error opening for read %s\n", filename);
+        return 1;
+    }
+
+
+    KeyedVector<String8,FileState> readSnapshot;
+    err = read_snapshot_file(fd, &readSnapshot);
+    if (err != 0) {
+        fprintf(stderr, "read_snapshot_file failed %d\n", err);
+        return err;
+    }
+
+    if (readSnapshot.size() != 4) {
+        fprintf(stderr, "readSnapshot should be length 4 is %d\n", readSnapshot.size());
+        return 1;
+    }
+
+    bool matched = true;
+    for (size_t i=0; i<readSnapshot.size(); i++) {
+        const String8& name = readSnapshot.keyAt(i);
+        const FileState state = readSnapshot.valueAt(i);
+
+        if (name != filenames[i] || states[i].modTime_sec != state.modTime_sec
+                || states[i].modTime_nsec != state.modTime_nsec
+                || states[i].size != state.size || states[i].crc32 != states[i].crc32) {
+            fprintf(stderr, "state %d expected={%d/%d, 0x%08x, 0x%08x, %3d} '%s'\n"
+                            "          actual={%d/%d, 0x%08x, 0x%08x, %3d} '%s'\n", i,
+                    states[i].modTime_sec, states[i].modTime_nsec, states[i].size, states[i].crc32,
+                    name.length(), filenames[i].string(),
+                    state.modTime_sec, state.modTime_nsec, state.size, state.crc32, state.nameLen,
+                    name.string());
+            matched = false;
+        }
+    }
+    
+    return matched ? 0 : 1;
+}
+
+static int
+get_mod_time(const char* filename, struct timeval times[2])
+{
+    int err;
+    struct stat64 st;
+    err = stat64(filename, &st);
+    if (err != 0) {
+        fprintf(stderr, "stat '%s' failed: %s\n", filename, strerror(errno));
+        return errno;
+    }
+    times[0].tv_sec = st.st_atime;
+    times[0].tv_usec = st.st_atime_nsec / 1000;
+    times[1].tv_sec = st.st_mtime;
+    times[1].tv_usec = st.st_mtime_nsec / 1000;
+    return 0;
+}
+
+int
+backup_helper_test_files()
+{
+    int err;
+    int newSnapshotFD;
+    int oldSnapshotFD;
+
+    system("rm -r " SCRATCH_DIR);
+    mkdir(SCRATCH_DIR, 0777);
+    mkdir(SCRATCH_DIR "data", 0777);
+
+    write_text_file(SCRATCH_DIR "data/b", "b\nbb\n");
+    write_text_file(SCRATCH_DIR "data/c", "c\ncc\n");
+    write_text_file(SCRATCH_DIR "data/d", "d\ndd\n");
+    write_text_file(SCRATCH_DIR "data/e", "e\nee\n");
+    write_text_file(SCRATCH_DIR "data/f", "f\nff\n");
+    write_text_file(SCRATCH_DIR "data/h", "h\nhh\n");
+
+    char const* files_before[] = {
+        "data/b",
+        "data/c",
+        "data/d",
+        "data/e",
+        "data/f"
+    };
+
+    newSnapshotFD = creat(SCRATCH_DIR "before.snap", 0666);
+    if (newSnapshotFD == -1) {
+        fprintf(stderr, "error creating: %s\n", strerror(errno));
+        return errno;
+    }
+
+    err = back_up_files(-1, newSnapshotFD, 0, SCRATCH_DIR, files_before, 5);
+    if (err != 0) {
+        return err;
+    }
+
+    close(newSnapshotFD);
+
+    sleep(3);
+
+    struct timeval d_times[2];
+    struct timeval e_times[2];
+
+    err = get_mod_time(SCRATCH_DIR "data/d", d_times);
+    err |= get_mod_time(SCRATCH_DIR "data/e", e_times);
+    if (err != 0) {
+        return err;
+    }
+
+    write_text_file(SCRATCH_DIR "data/a", "a\naa\n");
+    unlink(SCRATCH_DIR "data/c");
+    write_text_file(SCRATCH_DIR "data/c", "c\ncc\n");
+    write_text_file(SCRATCH_DIR "data/d", "dd\ndd\n");
+    utimes(SCRATCH_DIR "data/d", d_times);
+    write_text_file(SCRATCH_DIR "data/e", "z\nzz\n");
+    utimes(SCRATCH_DIR "data/e", e_times);
+    write_text_file(SCRATCH_DIR "data/g", "g\ngg\n");
+    unlink(SCRATCH_DIR "data/f");
+    
+    char const* files_after[] = {
+        "data/a", // added
+        "data/b", // same
+        "data/c", // different mod time
+        "data/d", // different size (same mod time)
+        "data/e", // different contents (same mod time, same size)
+        "data/g"  // added
+    };
+
+    oldSnapshotFD = open(SCRATCH_DIR "before.snap", O_RDONLY);
+    if (oldSnapshotFD == -1) {
+        fprintf(stderr, "error opening: %s\n", strerror(errno));
+        return errno;
+    }
+
+    newSnapshotFD = creat(SCRATCH_DIR "after.snap", 0666);
+    if (newSnapshotFD == -1) {
+        fprintf(stderr, "error creating: %s\n", strerror(errno));
+        return errno;
+    }
+
+    err = back_up_files(oldSnapshotFD, newSnapshotFD, 0, SCRATCH_DIR, files_after, 6);
+    if (err != 0) {
+        return err;
+    }
+
+    close(oldSnapshotFD);
+    close(newSnapshotFD);
+    
+    return 0;
+}
+
+#endif // TEST_BACKUP_HELPERS