import Foundation

/// Exact decimal <-> drops conversion (1 XRP = 1_000_000 drops) without
/// floating point rounding errors.
enum XRPAmount {
  static let dropsPerXRP: UInt64 = 1_000_000
  static let maxDrops: UInt64 = 100_000_000_000 * dropsPerXRP  // 1e11 XRP

  static func drops(fromXRP value: String) throws -> UInt64 {
    let text = value.trimmingCharacters(in: .whitespacesAndNewlines)
    guard !text.isEmpty, !text.hasPrefix("-"), !text.hasPrefix("+") else {
      throw XRPError.rpc("Enter a valid amount")
    }
    let parts = text.split(separator: ".", omittingEmptySubsequences: false)
    guard parts.count <= 2 else { throw XRPError.rpc("Enter a valid amount") }

    let wholeText = parts[0].isEmpty ? "0" : String(parts[0])
    guard let whole = UInt64(wholeText) else { throw XRPError.rpc("Enter a valid amount") }

    var fraction: UInt64 = 0
    if parts.count == 2 {
      let fractionText = String(parts[1])
      guard fractionText.count <= 6, fractionText.allSatisfy({ $0.isNumber }) else {
        throw XRPError.rpc("XRP supports at most 6 decimal places")
      }
      let padded = fractionText.padding(toLength: 6, withPad: "0", startingAt: 0)
      guard let parsed = UInt64(padded) else { throw XRPError.rpc("Enter a valid amount") }
      fraction = parsed
    }

    let (wholeDrops, overflow) = whole.multipliedReportingOverflow(by: dropsPerXRP)
    guard !overflow else { throw XRPError.rpc("Amount is too large") }
    let (total, carry) = wholeDrops.addingReportingOverflow(fraction)
    guard !carry, total <= maxDrops else { throw XRPError.rpc("Amount is too large") }
    return total
  }

  static func xrp(fromDrops drops: String) -> String {
    guard let value = UInt64(drops.trimmingCharacters(in: .whitespacesAndNewlines)) else {
      return "0"
    }
    return xrp(fromDrops: value)
  }

  static func xrp(fromDrops value: UInt64) -> String {
    let whole = value / dropsPerXRP
    let fraction = value % dropsPerXRP
    if fraction == 0 { return String(whole) }
    var fractionText = String(format: "%06llu", fraction)
    while fractionText.hasSuffix("0") { fractionText.removeLast() }
    return "\(whole).\(fractionText)"
  }
}