How to Build an AI Writing Assistant for Android

enum class WritingAction { FIX_GRAMMAR, FRIENDLIER, MORE_PROFESSIONAL, SHORTER }

data class Suggestion(val text: String, val source: Source) {
    enum class Source { ON_DEVICE, CLOUD }
}

interface WritingAssistant {
    suspend fun supports(action: WritingAction): Boolean
    suspend fun suggest(action: WritingAction, text: String): List<Suggestion>
}

class CloudWritingAssistant(private val consent: () -> Boolean) : WritingAssistant {
    private val model = Firebase.ai(backend = GenerativeBackend.googleAI()).generativeModel(
        modelName = "gemini-3.5-flash-lite",                 // small, fast model is enough; check current names
        generationConfig = generationConfig { temperature = 0.3f; maxOutputTokens = 400 },
        systemInstruction = content {
            text(
                "You edit user text. Return ONLY the rewritten text, no explanations or quotes. " +
                "Preserve meaning, names, numbers and the original language. Never add new facts."
            )
        },
    )

    override suspend fun supports(action: WritingAction) = consent()

    override suspend fun suggest(action: WritingAction, text: String): List<Suggestion> {
        val instruction = when (action) {
            WritingAction.FIX_GRAMMAR -> "Fix grammar, spelling and punctuation only."
            WritingAction.FRIENDLIER -> "Make the tone warmer and friendlier."
            WritingAction.MORE_PROFESSIONAL -> "Make the tone polite and professional."
            WritingAction.SHORTER -> "Make it about half as long."
        }
        // The user's text is clearly delimited as data, not instructions.
        val prompt = "$instruction\n\n<text>\n${text.take(2_000)}\n</text>"
        val out = model.generateContent(prompt).text?.trim().orEmpty()
        return if (out.isBlank()) emptyList() else listOf(Suggestion(out, Suggestion.Source.CLOUD))
    }
}

data class EditorState(
    val text: String = "",
    val busy: WritingAction? = null,
    val suggestions: List<Suggestion> = emptyList(),
    val error: String? = null,
)

class ComposeMessageViewModel(private val assistant: WritingAssistant) : ViewModel() {
    private val _state = MutableStateFlow(EditorState())
    val state = _state.asStateFlow()

    fun onTextChange(t: String) = _state.update { it.copy(text = t, suggestions = emptyList()) }

    fun request(action: WritingAction) = viewModelScope.launch {
        val text = _state.value.text
        if (text.isBlank() || !assistant.supports(action)) return@launch
        _state.update { it.copy(busy = action, error = null) }
        try {
            _state.update { it.copy(suggestions = assistant.suggest(action, text), busy = null) }
        } catch (e: Exception) {
            if (e is CancellationException) throw e
            _state.update { it.copy(busy = null, error = "Couldn't get a suggestion. Try again.") }
        }
    }

    fun accept(s: Suggestion) = _state.update { it.copy(text = s.text, suggestions = emptyList()) }
    fun dismiss() = _state.update { it.copy(suggestions = emptyList()) }
}

@Composable
fun SuggestionCard(s: Suggestion, onAccept: () -> Unit, onDismiss: () -> Unit) {
    OutlinedCard(Modifier.fillMaxWidth()) {
        Column(Modifier.padding(12.dp)) {
            Text(
                if (s.source == Suggestion.Source.ON_DEVICE) "Suggestion · processed on your device"
                else "Suggestion · AI-generated (cloud)",
                style = MaterialTheme.typography.labelSmall,
            )
            Text(s.text, Modifier.padding(vertical = 8.dp))
            Row(horizontalArrangement = Arrangement.spacedBy(8.dp)) {
                Button(onClick = onAccept) { Text("Use this") }
                TextButton(onClick = onDismiss) { Text("Keep mine") }
            }
        }
    }
}