//
//  TokenAmount.swift
//  OS Vault
//
//  Author: Simon-Pierre Boucher
//  Mail: contact@spboucher.ai
//

import Foundation
import BigInt

/// Integer-only conversion between base units and display strings.
/// `Double` is banned for money: 6- and 18-decimal tokens both exceed the
/// 53-bit mantissa long before real-world balances do.
public enum TokenAmount {

    /// "12.5" with decimals 6 → 12_500_000. Returns nil on malformed input or
    /// more fraction digits than the token supports. Accepts "," as separator.
    public static func parse(_ input: String, decimals: Int) -> BigUInt? {
        let normalized = input
            .trimmingCharacters(in: .whitespacesAndNewlines)
            .replacingOccurrences(of: ",", with: ".")
        guard !normalized.isEmpty, normalized != "." else { return nil }

        let parts = normalized.split(separator: ".", omittingEmptySubsequences: false)
        guard parts.count <= 2 else { return nil }

        let wholePart = String(parts[0])
        let fracPart = parts.count == 2 ? String(parts[1]) : ""
        guard wholePart.allSatisfy(\.isNumber), fracPart.allSatisfy(\.isNumber) else { return nil }
        guard !(wholePart.isEmpty && fracPart.isEmpty) else { return nil }
        guard fracPart.count <= decimals else { return nil }

        let whole = BigUInt(wholePart.isEmpty ? "0" : wholePart, radix: 10) ?? 0
        let paddedFrac = fracPart.padding(toLength: decimals, withPad: "0", startingAt: 0)
        let frac = paddedFrac.isEmpty ? BigUInt(0) : (BigUInt(paddedFrac, radix: 10) ?? 0)
        return whole * BigUInt(10).power(decimals) + frac
    }

    /// 12_500_000 with decimals 6 → "12.5". Trailing zeros trimmed,
    /// optionally capped to `maxFractionDigits` (truncated, never rounded up —
    /// a wallet must not display more than the user owns).
    public static func format(_ units: BigUInt, decimals: Int, maxFractionDigits: Int? = nil) -> String {
        let divisor = BigUInt(10).power(decimals)
        let whole = units / divisor
        var frac = String(units % divisor)
        frac = String(repeating: "0", count: max(0, decimals - frac.count)) + frac
        if let cap = maxFractionDigits, frac.count > cap {
            frac = String(frac.prefix(cap))
        }
        while frac.hasSuffix("0") { frac.removeLast() }
        return frac.isEmpty ? String(whole) : "\(whole).\(frac)"
    }

    /// Wei → ETH display string (18 decimals), for gas costs.
    public static func formatWei(_ wei: BigUInt, maxFractionDigits: Int = 8) -> String {
        format(wei, decimals: 18, maxFractionDigits: maxFractionDigits)
    }
}