ai-app/app/androidApp/src/main/kotlin/com/example/aiapp/ServerConfig.kt

package com.example.aiapp

import android.content.Context
import android.net.Uri
import android.security.keystore.KeyGenParameterSpec
import android.security.keystore.KeyProperties
import android.util.Base64
import java.security.KeyStore
import javax.crypto.Cipher
import javax.crypto.KeyGenerator
import javax.crypto.SecretKey
import javax.crypto.spec.GCMParameterSpec

/**
 * Where the backend is and how to authenticate to it. Absent until the
 * phone is enrolled -- by scanning the server's terminal QR (an
 * `aiapp://enroll` URI the camera app hands to MainActivity) or by typing
 * the fields into the settings screen.
 */
data class ServerSettings(val host: String, val port: Int, val token: String) {
    val baseUrl: String get() = "https://$host:$port"
}

private const val PREFS_NAME = "server"
private const val KEY_HOST = "host"
private const val KEY_PORT = "port"
private const val KEY_TOKEN = "token"

fun loadServerSettings(context: Context): ServerSettings? {
    val prefs = context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE)
    val host = prefs.getString(KEY_HOST, null) ?: return null
    val port = prefs.getInt(KEY_PORT, 0)
    val sealed = prefs.getString(KEY_TOKEN, null) ?: return null
    val token = unseal(sealed) ?: return null
    if (port == 0 || token.isEmpty()) return null
    return ServerSettings(host, port, token)
}

fun saveServerSettings(context: Context, settings: ServerSettings) {
    context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE)
        .edit()
        .putString(KEY_HOST, settings.host)
        .putInt(KEY_PORT, settings.port)
        .putString(KEY_TOKEN, seal(settings.token))
        .apply()
}

/**
 * Parses the enrollment URI the server's QR carries:
 * `aiapp://enroll?host=10.66.0.1&port=8443&token=...`. Null if any part is
 * missing -- a malformed scan shouldn't clobber a working enrollment.
 */
fun parseEnrollmentUri(uri: Uri): ServerSettings? {
    if (uri.scheme != "aiapp" || uri.host != "enroll") return null
    val host = uri.getQueryParameter("host") ?: return null
    val port = uri.getQueryParameter("port")?.toIntOrNull() ?: return null
    val token = uri.getQueryParameter("token") ?: return null
    if (host.isEmpty() || token.isEmpty()) return null
    return ServerSettings(host, port, token)
}

// ---------------------------------------------------------------------------
// Token sealing. The token is the credential for remote code execution on
// the backend, so it is stored AES-GCM-encrypted under an Android Keystore
// key (hardware-backed where the device has it) rather than in plain
// preferences. Hand-rolled (~40 lines) instead of Jetpack's
// EncryptedSharedPreferences because that library is deprecated with no
// drop-in successor -- Google's own guidance is now "use Keystore directly".

private const val KEYSTORE = "AndroidKeyStore"
private const val KEY_ALIAS = "aiapp-token-key"
private const val GCM_TAG_BITS = 128

private fun tokenKey(): SecretKey {
    val keyStore = KeyStore.getInstance(KEYSTORE).apply { load(null) }
    (keyStore.getKey(KEY_ALIAS, null) as? SecretKey)?.let { return it }
    val generator = KeyGenerator.getInstance(KeyProperties.KEY_ALGORITHM_AES, KEYSTORE)
    generator.init(
        KeyGenParameterSpec.Builder(
            KEY_ALIAS,
            KeyProperties.PURPOSE_ENCRYPT or KeyProperties.PURPOSE_DECRYPT,
        )
            .setBlockModes(KeyProperties.BLOCK_MODE_GCM)
            .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_NONE)
            .build(),
    )
    return generator.generateKey()
}

/** iv:ciphertext, both base64 -- the stored form of the token. */
private fun seal(token: String): String {
    val cipher = Cipher.getInstance("AES/GCM/NoPadding")
    cipher.init(Cipher.ENCRYPT_MODE, tokenKey())
    val ciphertext = cipher.doFinal(token.encodeToByteArray())
    return Base64.encodeToString(cipher.iv, Base64.NO_WRAP) + ":" +
        Base64.encodeToString(ciphertext, Base64.NO_WRAP)
}

/**
 * Null on any failure -- e.g. the Keystore key was lost to a device reset
 * or the app's data was restored onto another device, where the key never
 * travels. The caller treats that as "not enrolled"; re-scanning the QR
 * (or `--rotate-token`) is the recovery, so failing soft here is right.
 */
private fun unseal(sealed: String): String? = try {
    val (ivB64, dataB64) = sealed.split(":", limit = 2).let {
        if (it.size != 2) return null else it[0] to it[1]
    }
    val cipher = Cipher.getInstance("AES/GCM/NoPadding")
    cipher.init(
        Cipher.DECRYPT_MODE,
        tokenKey(),
        GCMParameterSpec(GCM_TAG_BITS, Base64.decode(ivB64, Base64.NO_WRAP)),
    )
    cipher.doFinal(Base64.decode(dataB64, Base64.NO_WRAP)).decodeToString()
} catch (_: Exception) {
    null
}