diff --git a/cmds/idmap/scan.cpp b/cmds/idmap/scan.cpp
new file mode 100644
index 0000000..c5fc941
--- /dev/null
+++ b/cmds/idmap/scan.cpp
@@ -0,0 +1,244 @@
+#include "idmap.h"
+
+#include <UniquePtr.h>
+#include <androidfw/ResourceTypes.h>
+#include <androidfw/StreamingZipInflater.h>
+#include <androidfw/ZipFileRO.h>
+#include <private/android_filesystem_config.h> // for AID_SYSTEM
+#include <utils/SortedVector.h>
+#include <utils/String16.h>
+#include <utils/String8.h>
+
+#include <dirent.h>
+
+#define NO_OVERLAY_TAG (-1000)
+
+using namespace android;
+
+namespace {
+    struct Overlay {
+        Overlay() {}
+        Overlay(const String8& a, const String8& i, int p) :
+            apk_path(a), idmap_path(i), priority(p) {}
+
+        bool operator<(Overlay const& rhs) const
+        {
+            // Note: order is reversed by design
+            return rhs.priority < priority;
+        }
+
+        String8 apk_path;
+        String8 idmap_path;
+        int priority;
+    };
+
+    bool writePackagesList(const char *filename, const SortedVector<Overlay>& overlayVector)
+    {
+        FILE* fout = fopen(filename, "w");
+        if (fout == NULL) {
+            return false;
+        }
+
+        for (size_t i = 0; i < overlayVector.size(); ++i) {
+            const Overlay& overlay = overlayVector[i];
+            fprintf(fout, "%s %s\n", overlay.apk_path.string(), overlay.idmap_path.string());
+        }
+
+        fclose(fout);
+
+        // Make file world readable since Zygote (running as root) will read
+        // it when creating the initial AssetManger object
+        const mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH; // 0644
+        if (chmod(filename, mode) == -1) {
+            unlink(filename);
+            return false;
+        }
+
+        return true;
+    }
+
+    String8 flatten_path(const char *path)
+    {
+        String16 tmp(path);
+        tmp.replaceAll('/', '@');
+        return String8(tmp);
+    }
+
+    int mkdir_p(const String8& path, uid_t uid, gid_t gid)
+    {
+        static const mode_t mode =
+            S_IRUSR | S_IWUSR | S_IXUSR | S_IRGRP | S_IWGRP | S_IXGRP | S_IROTH | S_IXOTH;
+        struct stat st;
+
+        if (stat(path.string(), &st) == 0) {
+            return 0;
+        }
+        if (mkdir_p(path.getPathDir(), uid, gid) < 0) {
+            return -1;
+        }
+        if (mkdir(path.string(), 0755) != 0) {
+            return -1;
+        }
+        if (chown(path.string(), uid, gid) == -1) {
+            return -1;
+        }
+        if (chmod(path.string(), mode) == -1) {
+            return -1;
+        }
+        return 0;
+    }
+
+    int parse_overlay_tag(const ResXMLTree& parser, const char *target_package_name)
+    {
+        const size_t N = parser.getAttributeCount();
+        String16 target;
+        int priority = -1;
+        for (size_t i = 0; i < N; ++i) {
+            size_t len;
+            String16 key(parser.getAttributeName(i, &len));
+            if (key == String16("targetPackage")) {
+                const uint16_t *p = parser.getAttributeStringValue(i, &len);
+                if (p) {
+                    target = String16(p, len);
+                }
+            } else if (key == String16("priority")) {
+                Res_value v;
+                if (parser.getAttributeValue(i, &v) == sizeof(Res_value)) {
+                    priority = v.data;
+                    if (priority < 0 || priority > 9999) {
+                        return -1;
+                    }
+                }
+            }
+        }
+        if (target == String16(target_package_name)) {
+            return priority;
+        }
+        return NO_OVERLAY_TAG;
+    }
+
+    int parse_manifest(const void *data, size_t size, const char *target_package_name)
+    {
+        ResXMLTree parser(data, size);
+        if (parser.getError() != NO_ERROR) {
+            ALOGD("%s failed to init xml parser, error=0x%08x\n", __FUNCTION__, parser.getError());
+            return -1;
+        }
+
+        ResXMLParser::event_code_t type;
+        do {
+            type = parser.next();
+            if (type == ResXMLParser::START_TAG) {
+                size_t len;
+                String16 tag(parser.getElementName(&len));
+                if (tag == String16("overlay")) {
+                    return parse_overlay_tag(parser, target_package_name);
+                }
+            }
+        } while (type != ResXMLParser::BAD_DOCUMENT && type != ResXMLParser::END_DOCUMENT);
+
+        return NO_OVERLAY_TAG;
+    }
+
+    int parse_apk(const char *path, const char *target_package_name)
+    {
+        UniquePtr<ZipFileRO> zip(ZipFileRO::open(path));
+        if (zip.get() == NULL) {
+            ALOGW("%s: failed to open zip %s\n", __FUNCTION__, path);
+            return -1;
+        }
+        ZipEntryRO entry;
+        if ((entry = zip->findEntryByName("AndroidManifest.xml")) == NULL) {
+            ALOGW("%s: failed to find entry AndroidManifest.xml\n", __FUNCTION__);
+            return -1;
+        }
+        size_t uncompLen = 0;
+        int method;
+        if (!zip->getEntryInfo(entry, &method, &uncompLen, NULL, NULL, NULL, NULL)) {
+            ALOGW("%s: failed to read entry info\n", __FUNCTION__);
+            return -1;
+        }
+        if (method != ZipFileRO::kCompressDeflated) {
+            ALOGW("%s: cannot handle zip compression method %d\n", __FUNCTION__, method);
+            return -1;
+        }
+        FileMap *dataMap = zip->createEntryFileMap(entry);
+        if (!dataMap) {
+            ALOGW("%s: failed to create FileMap\n", __FUNCTION__);
+            return -1;
+        }
+        char *buf = new char[uncompLen];
+        if (NULL == buf) {
+            ALOGW("%s: failed to allocate %d byte\n", __FUNCTION__, uncompLen);
+            dataMap->release();
+            return -1;
+        }
+        StreamingZipInflater inflater(dataMap, uncompLen);
+        if (inflater.read(buf, uncompLen) < 0) {
+            ALOGW("%s: failed to inflate %d byte\n", __FUNCTION__, uncompLen);
+            delete[] buf;
+            dataMap->release();
+            return -1;
+        }
+
+        int priority = parse_manifest(buf, uncompLen, target_package_name);
+        delete[] buf;
+        dataMap->release();
+        return priority;
+    }
+}
+
+int idmap_scan(const char *overlay_dir, const char *target_package_name,
+        const char *target_apk_path, const char *idmap_dir)
+{
+    String8 filename = String8(idmap_dir);
+    filename.appendPath("overlays.list");
+    if (unlink(filename.string()) != 0 && errno != ENOENT) {
+        return EXIT_FAILURE;
+    }
+
+    DIR *dir = opendir(overlay_dir);
+    if (dir == NULL) {
+        return EXIT_FAILURE;
+    }
+
+    SortedVector<Overlay> overlayVector;
+    struct dirent *dirent;
+    while ((dirent = readdir(dir)) != NULL) {
+        struct stat st;
+        char overlay_apk_path[PATH_MAX + 1];
+        snprintf(overlay_apk_path, PATH_MAX, "%s/%s", overlay_dir, dirent->d_name);
+        if (stat(overlay_apk_path, &st) < 0) {
+            continue;
+        }
+        if (!S_ISREG(st.st_mode)) {
+            continue;
+        }
+
+        int priority = parse_apk(overlay_apk_path, target_package_name);
+        if (priority < 0) {
+            continue;
+        }
+
+        String8 idmap_path(idmap_dir);
+        idmap_path.appendPath(flatten_path(overlay_apk_path + 1));
+        idmap_path.append("@idmap");
+
+        if (idmap_create_path(target_apk_path, overlay_apk_path, idmap_path.string()) != 0) {
+            ALOGE("error: failed to create idmap for target=%s overlay=%s idmap=%s\n",
+                    target_apk_path, overlay_apk_path, idmap_path.string());
+            continue;
+        }
+
+        Overlay overlay(String8(overlay_apk_path), idmap_path, priority);
+        overlayVector.add(overlay);
+    }
+
+    closedir(dir);
+
+    if (!writePackagesList(filename.string(), overlayVector)) {
+        return EXIT_FAILURE;
+    }
+
+    return EXIT_SUCCESS;
+}