package com.wytehat.btlogger;

import android.content.Context;
import android.os.Build;
import android.provider.Settings;
import java.util.UUID;

/**
 * The identifier this phone reports itself as, sent in X-Device-ID on every
 * upload so the server can attribute sightings to the device that saw them.
 *
 * Settings.Secure.ANDROID_ID is preferred: it survives app reinstalls, costs
 * nothing to read, and needs no permission. It is not always usable, though -
 * it is null before the device finishes first boot, and a well-known family of
 * buggy builds all report the same constant. When it cannot be trusted, a
 * random UUID is generated once and kept.
 *
 * That fallback is stored encrypted. The plain identifier would otherwise sit
 * in a readable preferences file, and on a rooted or backed-up device that is
 * a stable handle on the user that anything able to read the file could lift.
 * Encryption uses an AES-GCM key held in the AndroidKeyStore, so the key
 * material itself never enters the app process and cannot be pulled out of a
 * backup.
 *
 * androidx.security's EncryptedSharedPreferences does exactly this, but it
 * needs the AndroidX support libraries and API 23; this project is built in
 * AIDE with no dependency resolution and minSdk 14, so the same construction
 * is done directly against the platform Keystore. Below API 23 there is no
 * Keystore AES support and the value is stored in the clear - see
 * {@link #isFallbackEncrypted}.
 *
 * Values produced here match what the server accepts: 16 lower-case hex digits
 * (ANDROID_ID) or a canonical UUID (the fallback).
 */
public final class DeviceIdentity {

    /** Name the fallback identifier is filed under in {@link SecureStore}. */
    private static final String KEY_DEVICE_UUID = "device_uuid";

    /**
     * Shipped in a number of early builds, where every unit returned it. It is
     * a real ANDROID_ID value, so it passes a format check, but it identifies a
     * model rather than a phone and must not be used.
     */
    private static final String KNOWN_BAD_ANDROID_ID = "9774d56d682e549c";

    /** Resolved once per process; the value cannot change while we are running. */
    private static volatile String cached;

    private DeviceIdentity() {
    }

    /**
     * The identifier to send in X-Device-ID. Never null, never empty.
     *
     * Safe to call from any thread, including the upload worker.
     */
    public static String get(Context context) {

        String local = cached;

        if (local != null) {
            return local;
        }

        synchronized (DeviceIdentity.class) {

            if (cached == null) {
                cached = resolve(context.getApplicationContext());
            }

            return cached;
        }
    }

    /** True when the stored fallback is actually encrypted at rest. */
    public static boolean isFallbackEncrypted() {
        return SecureStore.isAvailable();
    }

    // ------------------------------------------------------------------
    // Resolution
    // ------------------------------------------------------------------

    private static String resolve(Context context) {

        String androidId = readAndroidId(context);

        if (androidId != null) {
            return androidId;
        }

        return storedUuid(context);
    }

    /**
     * ANDROID_ID, lower-cased, or null when it cannot be trusted.
     *
     * Rejects the value outright rather than trying to repair it: a short or
     * non-hex reading means the platform gave us something we do not
     * understand, and guessing at it would produce an identifier that changes
     * between reads.
     */
    private static String readAndroidId(Context context) {

        String value;

        try {
            value = Settings.Secure.getString(
                context.getContentResolver(), Settings.Secure.ANDROID_ID);
        } catch (Exception error) {
            // Some heavily modified builds throw rather than returning null.
            return null;
        }

        if (value == null) {
            return null;
        }

        value = value.trim().toLowerCase();

        if (value.length() != 16 || !isHex(value)) {
            return null;
        }

        if (KNOWN_BAD_ANDROID_ID.equals(value)) {
            return null;
        }

        // All-zero shows up on emulators and on devices read before the ID has
        // been generated. It is not unique to anything.
        if (value.equals("0000000000000000")) {
            return null;
        }

        return value;
    }

    /**
     * The persisted fallback UUID, generating and storing one on first use.
     *
     * A stored value that cannot be read back - because the Keystore key was
     * cleared, or the app was restored onto a different device where the key
     * did not come with it - is replaced rather than treated as fatal. The
     * device gets a new identity in that case, which is the honest outcome:
     * the old one is genuinely unrecoverable.
     */
    private static String storedUuid(Context context) {

        String stored = SecureStore.get(context, KEY_DEVICE_UUID);

        if (isUuid(stored)) {
            return stored;
        }

        String fresh = UUID.randomUUID().toString();

        // A false return means this device cannot encrypt at rest. Writing the
        // identifier in the clear is worse than not persisting it, so it lives
        // for the life of the process only and the next launch makes another.
        SecureStore.put(context, KEY_DEVICE_UUID, fresh);

        return fresh;
    }

    // ------------------------------------------------------------------
    // Validation
    // ------------------------------------------------------------------

    private static boolean isHex(String value) {

        for (int index = 0; index < value.length(); index++) {

            char character = value.charAt(index);

            boolean digit = character >= '0' && character <= '9';
            boolean letter = character >= 'a' && character <= 'f';

            if (!digit && !letter) {
                return false;
            }
        }

        return true;
    }

    /** Canonical 8-4-4-4-12 lower-case UUID, the shape the server accepts. */
    private static boolean isUuid(String value) {

        if (value == null || value.length() != 36) {
            return false;
        }

        for (int index = 0; index < 36; index++) {

            char character = value.charAt(index);

            if (index == 8 || index == 13 || index == 18 || index == 23) {
                if (character != '-') {
                    return false;
                }
                continue;
            }

            boolean digit = character >= '0' && character <= '9';
            boolean letter = character >= 'a' && character <= 'f';

            if (!digit && !letter) {
                return false;
            }
        }

        return true;
    }
}