diff --git a/cmds/idmap/create.cpp b/cmds/idmap/create.cpp
new file mode 100644
index 0000000..ae35f7b
--- /dev/null
+++ b/cmds/idmap/create.cpp
@@ -0,0 +1,222 @@
+#include "idmap.h"
+
+#include <UniquePtr.h>
+#include <androidfw/AssetManager.h>
+#include <androidfw/ResourceTypes.h>
+#include <androidfw/ZipFileRO.h>
+#include <utils/String8.h>
+
+#include <fcntl.h>
+#include <sys/stat.h>
+
+using namespace android;
+
+namespace {
+    int get_zip_entry_crc(const char *zip_path, const char *entry_name, uint32_t *crc)
+    {
+        UniquePtr<ZipFileRO> zip(ZipFileRO::open(zip_path));
+        if (zip.get() == NULL) {
+            return -1;
+        }
+        ZipEntryRO entry = zip->findEntryByName(entry_name);
+        if (entry == NULL) {
+            return -1;
+        }
+        if (!zip->getEntryInfo(entry, NULL, NULL, NULL, NULL, NULL, (long*)crc)) {
+            return -1;
+        }
+        zip->releaseEntry(entry);
+        return 0;
+    }
+
+    int open_idmap(const char *path)
+    {
+        int fd = TEMP_FAILURE_RETRY(open(path, O_WRONLY | O_CREAT | O_TRUNC, 0644));
+        if (fd == -1) {
+            ALOGD("error: open %s: %s\n", path, strerror(errno));
+            goto fail;
+        }
+        if (fchmod(fd, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH) < 0) {
+            ALOGD("error: fchmod %s: %s\n", path, strerror(errno));
+            goto fail;
+        }
+        if (TEMP_FAILURE_RETRY(flock(fd, LOCK_EX | LOCK_NB)) != 0) {
+            ALOGD("error: flock %s: %s\n", path, strerror(errno));
+            goto fail;
+        }
+
+        return fd;
+fail:
+        if (fd != -1) {
+            close(fd);
+            unlink(path);
+        }
+        return -1;
+    }
+
+    int write_idmap(int fd, const uint32_t *data, size_t size)
+    {
+        if (lseek(fd, SEEK_SET, 0) < 0) {
+            return -1;
+        }
+        size_t bytesLeft = size;
+        while (bytesLeft > 0) {
+            ssize_t w = TEMP_FAILURE_RETRY(write(fd, data + size - bytesLeft, bytesLeft));
+            if (w < 0) {
+                fprintf(stderr, "error: write: %s\n", strerror(errno));
+                return -1;
+            }
+            bytesLeft -= w;
+        }
+        return 0;
+    }
+
+    bool is_idmap_stale_fd(const char *target_apk_path, const char *overlay_apk_path, int idmap_fd)
+    {
+        static const size_t N = ResTable::IDMAP_HEADER_SIZE_BYTES;
+        struct stat st;
+        if (fstat(idmap_fd, &st) == -1) {
+            return true;
+        }
+        if (st.st_size < N) {
+            // file is empty or corrupt
+            return true;
+        }
+
+        char buf[N];
+        ssize_t bytesLeft = N;
+        if (lseek(idmap_fd, SEEK_SET, 0) < 0) {
+            return true;
+        }
+        for (;;) {
+            ssize_t r = TEMP_FAILURE_RETRY(read(idmap_fd, buf + N - bytesLeft, bytesLeft));
+            if (r < 0) {
+                return true;
+            }
+            bytesLeft -= r;
+            if (bytesLeft == 0) {
+                break;
+            }
+            if (r == 0) {
+                // "shouldn't happen"
+                return true;
+            }
+        }
+
+        uint32_t cached_target_crc, cached_overlay_crc;
+        String8 cached_target_path, cached_overlay_path;
+        if (!ResTable::getIdmapInfo(buf, N, &cached_target_crc, &cached_overlay_crc,
+                    &cached_target_path, &cached_overlay_path)) {
+            return true;
+        }
+
+        if (cached_target_path != target_apk_path) {
+            return true;
+        }
+        if (cached_overlay_path != overlay_apk_path) {
+            return true;
+        }
+
+        uint32_t actual_target_crc, actual_overlay_crc;
+        if (get_zip_entry_crc(target_apk_path, AssetManager::RESOURCES_FILENAME,
+				&actual_target_crc) == -1) {
+            return true;
+        }
+        if (get_zip_entry_crc(overlay_apk_path, AssetManager::RESOURCES_FILENAME,
+				&actual_overlay_crc) == -1) {
+            return true;
+        }
+
+        return cached_target_crc != actual_target_crc || cached_overlay_crc != actual_overlay_crc;
+    }
+
+    bool is_idmap_stale_path(const char *target_apk_path, const char *overlay_apk_path,
+            const char *idmap_path)
+    {
+        struct stat st;
+        if (stat(idmap_path, &st) == -1) {
+            // non-existing idmap is always stale; on other errors, abort idmap generation
+            return errno == ENOENT;
+        }
+
+        int idmap_fd = TEMP_FAILURE_RETRY(open(idmap_path, O_RDONLY));
+        if (idmap_fd == -1) {
+            return false;
+        }
+        bool is_stale = is_idmap_stale_fd(target_apk_path, overlay_apk_path, idmap_fd);
+        close(idmap_fd);
+        return is_stale;
+    }
+
+    int create_idmap(const char *target_apk_path, const char *overlay_apk_path,
+            uint32_t **data, size_t *size)
+    {
+        uint32_t target_crc, overlay_crc;
+        if (get_zip_entry_crc(target_apk_path, AssetManager::RESOURCES_FILENAME,
+				&target_crc) == -1) {
+            return -1;
+        }
+        if (get_zip_entry_crc(overlay_apk_path, AssetManager::RESOURCES_FILENAME,
+				&overlay_crc) == -1) {
+            return -1;
+        }
+
+        AssetManager am;
+        bool b = am.createIdmap(target_apk_path, overlay_apk_path, target_crc, overlay_crc,
+                data, size);
+        return b ? 0 : -1;
+    }
+
+    int create_and_write_idmap(const char *target_apk_path, const char *overlay_apk_path,
+            int fd, bool check_if_stale)
+    {
+        if (check_if_stale) {
+            if (!is_idmap_stale_fd(target_apk_path, overlay_apk_path, fd)) {
+                // already up to date -- nothing to do
+                return 0;
+            }
+        }
+
+        uint32_t *data = NULL;
+        size_t size;
+
+        if (create_idmap(target_apk_path, overlay_apk_path, &data, &size) == -1) {
+            return -1;
+        }
+
+        if (write_idmap(fd, data, size) == -1) {
+            free(data);
+            return -1;
+        }
+
+        free(data);
+        return 0;
+    }
+}
+
+int idmap_create_path(const char *target_apk_path, const char *overlay_apk_path,
+        const char *idmap_path)
+{
+    if (!is_idmap_stale_path(target_apk_path, overlay_apk_path, idmap_path)) {
+        // already up to date -- nothing to do
+        return EXIT_SUCCESS;
+    }
+
+    int fd = open_idmap(idmap_path);
+    if (fd == -1) {
+        return EXIT_FAILURE;
+    }
+
+    int r = create_and_write_idmap(target_apk_path, overlay_apk_path, fd, false);
+    close(fd);
+    if (r != 0) {
+        unlink(idmap_path);
+    }
+    return r == 0 ? EXIT_SUCCESS : EXIT_FAILURE;
+}
+
+int idmap_create_fd(const char *target_apk_path, const char *overlay_apk_path, int fd)
+{
+    return create_and_write_idmap(target_apk_path, overlay_apk_path, fd, true) == 0 ?
+        EXIT_SUCCESS : EXIT_FAILURE;
+}