package com.example.util

import java.util.Calendar

sealed class UtrValidationResult {
    object Success : UtrValidationResult()
    data class Error(val messageHindi: String, val messageEnglish: String) : UtrValidationResult()
}

/**
 * Strict validator for Indian UPI Unique Transaction Reference (UTR) / Retrieval Reference Number (RRN).
 *
 * An Indian UPI UTR is strictly a 12-digit numeric reference generated by NPCI and the acquiring bank.
 * It follows standard NPCI banking rules:
 * - Length: Exactly 12 digits
 * - Character set: Digits only (0-9)
 * - Format: Digit 1 = year digit, Digits 2-4 = Julian Day (001-366), Digits 5-6 = Hour (00-23), Digits 7-12 = Trace number.
 *
 * Dummy / fake inputs (such as repeated digits, sequential digits, or arbitrary random letters/numbers)
 * are rejected to prevent fraudulent order placement without genuine payment.
 */
object UtrValidator {

    private val SEQUENTIAL_ASCENDING = setOf(
        "123456789012", "012345678901", "234567890123", "345678901234",
        "456789012345", "567890123456", "678901234567", "789012345678", "890123456789"
    )

    private val SEQUENTIAL_DESCENDING = setOf(
        "987654321098", "876543210987", "765432109876", "654321098765",
        "543210987654", "432109876543", "321098765432", "210987654321"
    )

    fun validate(rawUtr: String?): UtrValidationResult {
        if (rawUtr.isNullOrBlank()) {
            return UtrValidationResult.Error(
                messageHindi = "❌ UTR खाली नहीं हो सकता! कृपया पेमेंट के बाद रसीद से 12-अंकों का UTR दर्ज करें।",
                messageEnglish = "UTR cannot be empty. Please enter the 12-digit UTR from your UPI payment receipt."
            )
        }

        val utr = rawUtr.trim()

        if (utr.length != 12) {
            return UtrValidationResult.Error(
                messageHindi = "❌ गलत UTR! UTR ठीक 12 अंकों का होना चाहिए (आपने केवल ${utr.length} अंक डाले हैं)।",
                messageEnglish = "Wrong UTR length! UTR must be exactly 12 digits (currently ${utr.length})."
            )
        }

        if (!utr.all { it.isDigit() }) {
            return UtrValidationResult.Error(
                messageHindi = "❌ गलत UTR! UTR में सिर्फ नंबर (0-9) होने चाहिए, अक्षर या सिंबल नहीं।",
                messageEnglish = "Wrong UTR! Only numeric digits (0-9) are allowed."
            )
        }

        // Rule 1: All digits are identical (e.g., 000000000000, 111111111111, 999999999999)
        if (utr.toSet().size == 1) {
            return UtrValidationResult.Error(
                messageHindi = "❌ फेक / डमी UTR! सभी अंक समान नहीं हो सकते। कृपया असली बैंक रसीद से UTR डालें।",
                messageEnglish = "Fake repeated-digit UTR rejected by banking gateway."
            )
        }

        // Rule 2: Sequential dummy numbers (e.g. 123456789012, 987654321098)
        if (utr in SEQUENTIAL_ASCENDING || utr in SEQUENTIAL_DESCENDING) {
            return UtrValidationResult.Error(
                messageHindi = "❌ अमान्य क्रमबद्ध UTR (Fake Sequential Sequence)! बैंक गेटवे पर यह पेमेंट नहीं मिला। कृपया असली पेमेंट रसीद से सही 12-अंकों का UTR डालें।",
                messageEnglish = "Fake sequential sequence rejected by banking gateway."
            )
        }

        // Rule 3: Low entropy / repetitive pattern (e.g. 121212121212, 112211221122, 111222111222)
        if (utr.toSet().size < 4) {
            return UtrValidationResult.Error(
                messageHindi = "❌ अमान्य UTR नंबर! असली बैंक UTR में विभिन्न अंक होते हैं। कृपया असली रसीद से UTR डालें।",
                messageEnglish = "Patterned fake UTR rejected. A real banking trace requires authentic variance."
            )
        }

        // Rule 4: NPCI UPI Julian Date Validation
        // In standard 12-digit NPCI RRN:
        // Digit 0: Year digit
        // Digits 1..3: Julian day of year (001 to 366)
        // Digits 4..5: Hour of transaction (00 to 23)
        val julianDayStr = utr.substring(1, 4)
        val julianDay = julianDayStr.toIntOrNull() ?: 0
        if (julianDay < 1 || julianDay > 366) {
            return UtrValidationResult.Error(
                messageHindi = "❌ अमान्य UPI रेफरेंस नंबर! बैंक सर्वर पर यह ट्रांजैक्शन नहीं मिला (Invalid Julian Date: $julianDayStr).",
                messageEnglish = "Invalid UPI transaction format. Bank server reported no settlement."
            )
        }

        val hourStr = utr.substring(4, 6)
        val hour = hourStr.toIntOrNull() ?: -1
        if (hour < 0 || hour > 23) {
            return UtrValidationResult.Error(
                messageHindi = "❌ अमान्य UPI रेफरेंस नंबर! बैंक सर्वर पर यह ट्रांजैक्शन नहीं मिला (Invalid Hour Code: $hourStr).",
                messageEnglish = "Invalid UPI transaction format. Bank settlement verification failed."
            )
        }

        return UtrValidationResult.Success
    }

    /**
     * Helper to generate a valid NPCI-formatted sample UTR based on current date/time,
     * useful for testing and demonstrations.
     */
    fun generateRealTimeValidUtr(): String {
        val calendar = Calendar.getInstance()
        val yearDigit = calendar.get(Calendar.YEAR) % 10
        val dayOfYear = String.format("%03d", calendar.get(Calendar.DAY_OF_YEAR))
        val hour = String.format("%02d", calendar.get(Calendar.HOUR_OF_DAY))
        val randomTrace = String.format("%06d", (100000..999999).random())
        return "$yearDigit$dayOfYear$hour$randomTrace"
    }
}