import Foundation

// MARK: - 1. UnitCategory Enum
// Defines the broad categories of units. This is crucial for preventing
// invalid conversions (e.g., mass to length).
public enum UnitCategory: String, CaseIterable, CustomStringConvertible {
	case mass
	case length
	case volume
	case time
	case temperature
	case speed
	// Add more categories as needed

	public var description: String {
		rawValue.capitalized
	}
}

// MARK: - 2. Unit Enum
// Defines all the specific units, grouped by their category.
// Each unit has a 'rawValue' for its common abbreviation and a 'toBaseFactor'
// that converts its value to the base unit of its category.
//
// Base Units:
// - Mass: Gram (g)
// - Length: Meter (m)
// - Volume: Liter (L)
// - Time: Second (s)
// - Temperature: Celsius (C) (Note: Temperature conversions are more complex,
//   this example uses a simple factor relative to Celsius for demonstration.
//   For real-world, a dedicated temperature conversion function is needed.)
// - Speed: Meters per second (mps)
public enum Unit: String, CaseIterable, CustomStringConvertible {
	// MARK: Mass Units
	case kilogram = "kg"
	case gram = "g"
	case milligram = "mg"
	case microgram = "µg"
	case pound = "lbs"
	case ounce = "oz"
	case metricTon = "ton"  // Metric ton (1000 kg)

	// MARK: Length Units
	case meter = "m"
	case centimeter = "cm"
	case millimeter = "mm"
	case kilometer = "km"
	case inch = "in"
	case foot = "ft"
	case yard = "yd"
	case mile = "mi"
	case nauticalMile = "nmi"

	// MARK: Volume Units
	case liter = "L"
	case milliliter = "mL"
	case cubicMeter = "m³"
	case cubicCentimeter = "cm³"
	case gallon = "gal"  // US Liquid Gallon
	case quart = "qt"
	case pint = "pt"
	case fluidOunce = "fl oz"

	// MARK: Time Units
	case second = "s"
	case millisecond = "ms"
	case microsecond = "µs"
	case minute = "min"
	case hour = "hr"
	case day = "day"
	case week = "wk"

	// MARK: Temperature Units (Note: Simplified for factor-based conversion)
	case celsius = "°C"
	case fahrenheit = "°F"
	case kelvin = "K"

	// MARK: Speed Units
	case metersPerSecond = "m/s"
	case kilometersPerHour = "km/h"
	case milesPerHour = "mph"
	case knots = "kn"

	// Add more units as needed

	// Returns the category for the current unit.
	public var category: UnitCategory {
		switch self {
		case .kilogram, .gram, .milligram, .microgram, .pound, .ounce,
			.metricTon:
			return .mass
		case .meter, .centimeter, .millimeter, .kilometer, .inch, .foot, .yard, .mile,
			.nauticalMile:
			return .length
		case .liter, .milliliter, .cubicMeter, .cubicCentimeter, .gallon, .quart, .pint,
			.fluidOunce:
			return .volume
		case .second, .millisecond, .microsecond, .minute, .hour, .day, .week:
			return .time
		case .celsius, .fahrenheit, .kelvin: return .temperature
		case .metersPerSecond, .kilometersPerHour, .milesPerHour, .knots: return .speed
		}
	}

	// Returns the conversion factor to the base unit of its category.
	// All conversions happen via the base unit.
	public var toBaseFactor: Double {
		switch self {
		// Mass (Base: Gram)
		case .kilogram: 1000.0
		case .gram: 1.0
		case .milligram: 0.001
		case .microgram: 0.000001
		case .pound: 453.59237
		case .ounce: 28.349523125
		case .metricTon: 1_000_000.0  // 1000 kg = 1,000,000 g

		// Length (Base: Meter)
		case .meter: 1.0
		case .centimeter: 0.01
		case .millimeter: 0.001
		case .kilometer: 1000.0
		case .inch: 0.0254
		case .foot: 0.3048
		case .yard: 0.9144
		case .mile: 1609.344
		case .nauticalMile: 1852.0

		// Volume (Base: Liter)
		case .liter: 1.0
		case .milliliter: 0.001
		case .cubicMeter: 1000.0  // 1 m³ = 1000 L
		case .cubicCentimeter: 0.001  // 1 cm³ = 1 mL = 0.001 L
		case .gallon: 3.78541
		case .quart: 0.946353
		case .pint: 0.473176
		case .fluidOunce: 0.0295735

		// Time (Base: Second)
		case .second: 1.0
		case .millisecond: 0.001
		case .microsecond: 0.000001
		case .minute: 60.0
		case .hour: 3600.0
		case .day: 86400.0
		case .week: 604800.0

		case .celsius: 1.0
		case .fahrenheit: 5.0 / 9.0  // Factor to Celsius, offset handled separately
		case .kelvin: 1.0  // Factor to Celsius, offset handled separately

		// Speed (Base: Meters per second)
		case .metersPerSecond: 1.0
		case .kilometersPerHour: 1000.0 / 3600.0  // km/h to m/s
		case .milesPerHour: 1609.344 / 3600.0  // mph to m/s
		case .knots: 0.514444  // nautical miles per hour to m/s
		}
	}

	public var description: String { rawValue }

	// MARK: - Subscripts for Unit Enum
	/// Allows retrieving a Unit by its raw string value.
	/// Example: `Unit(rawValue: "kg")` or `Unit["kg"]`
	public static subscript(rawValue: String) -> Unit? {
		.init(rawValue: rawValue)
	}

	/// Allows retrieving all Units belonging to a specific UnitCategory.
	/// Example: `Unit[.mass]` will return `[.kilogram, .gram, ...]`
	public static subscript(category: UnitCategory) -> [Unit] {
		Unit.allCases.filter { $0.category == category }
	}

}

// MARK: - 3. UnitValue Struct
// This struct holds a numeric value and its associated unit.
// It provides the core logic for converting between units.
public struct UnitValue<T: BinaryFloatingPoint>: CustomStringConvertible, Equatable {
	public let value: T
	public let unit: Unit

	// MARK: Initialization
	public init(value: T, unit: Unit) {
		self.value = value
		self.unit = unit
	}

	// MARK: Conversion Logic
	/// Converts the current UnitValue to a specified target unit.
	/// - Parameter targetUnit: The unit to convert to.
	/// - Returns: A new `UnitValue` in the target unit, or `nil` if the units
	///   are of different categories (e.g., mass to length).
	func converted(to targetUnit: Unit) -> UnitValue<T>? {
		// Ensure units are of the same category for valid conversion
		guard self.unit.category == targetUnit.category else {
			print(
				"Conversion Error: Cannot convert \(self.unit.rawValue) (Category: \(self.unit.category)) to \(targetUnit.rawValue) (Category: \(targetUnit.category)). Units are of different categories."
			)
			return nil
		}

		// Special handling for Temperature, as it's not a simple multiplicative factor.
		// This demonstrates how to handle more complex conversions.
		if self.unit.category == .temperature {
			return convertTemperature(to: targetUnit)
		}

		// For other categories, convert via the base unit
		let valueInBaseUnit = value * T(self.unit.toBaseFactor)
		let convertedValue = valueInBaseUnit / T(targetUnit.toBaseFactor)
		return .init(value: convertedValue, unit: targetUnit)
	}

	// MARK: Temperature Conversion Helper
	private func convertTemperature(to targetUnit: Unit) -> UnitValue<T>? {
		// Convert current value to Celsius first
		var celsiusValue: T
		switch self.unit {
		case .celsius:
			celsiusValue = self.value
		case .fahrenheit:
			celsiusValue = (self.value - 32) * (5 / 9)
		case .kelvin:
			celsiusValue = self.value - 273.15
		default:
			// This case should ideally not be reached if category check is correct
			print("Error: Unknown temperature unit \(self.unit.rawValue)")
			return nil
		}

		// Convert from Celsius to target unit
		var finalValue: T
		switch targetUnit {
		case .celsius:
			finalValue = celsiusValue
		case .fahrenheit:
			finalValue = (celsiusValue * (9 / 5)) + 32
		case .kelvin:
			finalValue = celsiusValue + 273.15
		default:
			print("Error: Target unit \(targetUnit.rawValue) is not a valid temperature unit.")
			return nil
		}
		return .init(value: finalValue, unit: targetUnit)
	}

	// MARK: CustomStringConvertible Conformance
	public var description: String {
		// Format the value to avoid excessive decimal places for readability
		let formatter = NumberFormatter()
		formatter.maximumFractionDigits = 4  // Adjust as needed
		formatter.minimumFractionDigits = 0
		formatter.numberStyle = .decimal

		if let formattedValue = formatter.string(from: NSNumber(value: Double(value))) {
			return "\(formattedValue) \(unit.rawValue)"
		} else {
			return "\(value) \(unit.rawValue)"
		}
	}

	// MARK: Equatable Conformance
	// Allows comparing two UnitValue instances for equality.
	// Considers both value and unit.
	public static func == (lhs: UnitValue<T>, rhs: UnitValue<T>) -> Bool {
		lhs.value == rhs.value && lhs.unit == rhs.unit
	}

	public static func >= (lhs: UnitValue<T>, rhs: UnitValue<T>) -> Bool {
		lhs.value >= rhs.value && lhs.unit == rhs.unit
	}

	public static func > (lhs: UnitValue<T>, rhs: UnitValue<T>) -> Bool {
		lhs.value > rhs.value && lhs.unit == rhs.unit
	}

	public static func <= (lhs: UnitValue<T>, rhs: UnitValue<T>) -> Bool {
		lhs.value <= rhs.value && lhs.unit == rhs.unit
	}

	public static func < (lhs: UnitValue<T>, rhs: UnitValue<T>) -> Bool {
		lhs.value < rhs.value && lhs.unit == rhs.unit
	}

	// MARK: - Subscripts for UnitValue
	/// Allows converting the UnitValue to another Unit using subscript syntax.
	/// Example: `tenKilos[.pound]`
	public subscript(targetUnit: Unit) -> UnitValue<T>? {
		converted(to: targetUnit)
	}

	/// Allows converting the UnitValue to another Unit using its raw string value.
	/// Example: `tenKilos["lbs"]`
	public subscript(targetUnitString: String) -> UnitValue<T>? {
		guard let targetUnit = Unit(rawValue: targetUnitString) else {
			print("Conversion Error: Unknown unit string '\(targetUnitString)'")
			return nil
		}
		return converted(to: targetUnit)
	}

}

// MARK: - 4. Extension on BinaryFloatingPoint
// This extension adds computed properties to numeric types (like Double, Float, CGFloat)
// allowing you to write `10.kg` or `5.5.m`.
extension BinaryFloatingPoint {
	// MARK: Mass Initializers
	public var kg: UnitValue<Self> { .init(value: self, unit: .kilogram) }
	public var g: UnitValue<Self> { .init(value: self, unit: .gram) }
	public var mg: UnitValue<Self> { .init(value: self, unit: .milligram) }
	public var µg: UnitValue<Self> { .init(value: self, unit: .microgram) }
	public var lbs: UnitValue<Self> { .init(value: self, unit: .pound) }
	public var oz: UnitValue<Self> { .init(value: self, unit: .ounce) }
	public var ton: UnitValue<Self> { .init(value: self, unit: .metricTon) }

	// MARK: Length Initializers
	public var m: UnitValue<Self> { .init(value: self, unit: .meter) }
	public var cm: UnitValue<Self> { .init(value: self, unit: .centimeter) }
	public var mm: UnitValue<Self> { .init(value: self, unit: .millimeter) }
	public var km: UnitValue<Self> { .init(value: self, unit: .kilometer) }
	public var `in`: UnitValue<Self> { .init(value: self, unit: .inch) }  // 'in' is a keyword, so use backticks
	public var ft: UnitValue<Self> { .init(value: self, unit: .foot) }
	public var yd: UnitValue<Self> { .init(value: self, unit: .yard) }
	public var mi: UnitValue<Self> { .init(value: self, unit: .mile) }
	public var nmi: UnitValue<Self> { .init(value: self, unit: .nauticalMile) }

	// MARK: Volume Initializers
	public var L: UnitValue<Self> { .init(value: self, unit: .liter) }
	public var mL: UnitValue<Self> { .init(value: self, unit: .milliliter) }
	public var m3: UnitValue<Self> { .init(value: self, unit: .cubicMeter) }
	public var cm3: UnitValue<Self> { .init(value: self, unit: .cubicCentimeter) }
	public var gal: UnitValue<Self> { .init(value: self, unit: .gallon) }
	public var qt: UnitValue<Self> { .init(value: self, unit: .quart) }
	public var pt: UnitValue<Self> { .init(value: self, unit: .pint) }
	public var fl_oz: UnitValue<Self> { .init(value: self, unit: .fluidOunce) }

	// MARK: Time Initializers
	public var s: UnitValue<Self> { .init(value: self, unit: .second) }
	public var ms: UnitValue<Self> { .init(value: self, unit: .millisecond) }
	public var µs: UnitValue<Self> { .init(value: self, unit: .microsecond) }
	public var min: UnitValue<Self> { .init(value: self, unit: .minute) }
	public var hr: UnitValue<Self> { .init(value: self, unit: .hour) }
	public var day: UnitValue<Self> { .init(value: self, unit: .day) }
	public var wk: UnitValue<Self> { .init(value: self, unit: .week) }

	// MARK: Temperature Initializers
	public var C: UnitValue<Self> { .init(value: self, unit: .celsius) }
	public var F: UnitValue<Self> { .init(value: self, unit: .fahrenheit) }
	public var K: UnitValue<Self> { .init(value: self, unit: .kelvin) }

	// MARK: Speed Initializers
	public var mps: UnitValue<Self> { .init(value: self, unit: .metersPerSecond) }
	public var kmh: UnitValue<Self> { .init(value: self, unit: .kilometersPerHour) }
	public var mph: UnitValue<Self> { .init(value: self, unit: .milesPerHour) }
	public var kn: UnitValue<Self> { .init(value: self, unit: .knots) }

	// MARK: - Subscripts for BinaryFloatingPoint
	/// Allows creating a UnitValue directly from a numeric literal using a Unit enum case.
	/// Example: `10[.kg]`
	public subscript(unit: Unit) -> UnitValue<Self> {
		.init(value: self, unit: unit)
	}

	/// Allows creating a UnitValue directly from a numeric literal using a unit's raw string value.
	/// Example: `10["kg"]`
	public subscript(unitString: String) -> UnitValue<Self>? {
		guard let unit = Unit(rawValue: unitString) else {
			print("Initialization Error: Unknown unit string '\(unitString)' for value \(self)")
			return nil
		}
		return .init(value: self, unit: unit)
	}

}

// MARK: - 5. Extension on UnitValue for Direct Conversions
// This extension adds computed properties to `UnitValue` instances,
// allowing you to write `myValue.lbs` or `myValue.g`.
// Each property attempts to convert the value to the specified unit.
extension UnitValue {
	// MARK: Mass Conversions
	public var kg: UnitValue<T>? { converted(to: .kilogram) }
	public var g: UnitValue<T>? { converted(to: .gram) }
	public var mg: UnitValue<T>? { converted(to: .milligram) }
	public var µg: UnitValue<T>? { converted(to: .microgram) }
	public var lbs: UnitValue<T>? { converted(to: .pound) }
	public var oz: UnitValue<T>? { converted(to: .ounce) }
	public var ton: UnitValue<T>? { converted(to: .metricTon) }

	// MARK: Length Conversions
	public var m: UnitValue<T>? { converted(to: .meter) }
	public var cm: UnitValue<T>? { converted(to: .centimeter) }
	public var mm: UnitValue<T>? { converted(to: .millimeter) }
	public var km: UnitValue<T>? { converted(to: .kilometer) }
	public var `in`: UnitValue<T>? { converted(to: .inch) }
	public var ft: UnitValue<T>? { converted(to: .foot) }
	public var yd: UnitValue<T>? { converted(to: .yard) }
	public var mi: UnitValue<T>? { converted(to: .mile) }
	public var nmi: UnitValue<T>? { converted(to: .nauticalMile) }

	// MARK: Volume Conversions
	public var L: UnitValue<T>? { converted(to: .liter) }
	public var mL: UnitValue<T>? { converted(to: .milliliter) }
	public var m3: UnitValue<T>? { converted(to: .cubicMeter) }
	public var cm3: UnitValue<T>? { converted(to: .cubicCentimeter) }
	public var gal: UnitValue<T>? { converted(to: .gallon) }
	public var qt: UnitValue<T>? { converted(to: .quart) }
	public var pt: UnitValue<T>? { converted(to: .pint) }
	public var fl_oz: UnitValue<T>? { converted(to: .fluidOunce) }

	// MARK: Time Conversions
	public var s: UnitValue<T>? { converted(to: .second) }
	public var ms: UnitValue<T>? { converted(to: .millisecond) }
	public var µs: UnitValue<T>? { converted(to: .microsecond) }
	public var min: UnitValue<T>? { converted(to: .minute) }
	public var hr: UnitValue<T>? { converted(to: .hour) }
	public var day: UnitValue<T>? { converted(to: .day) }
	public var wk: UnitValue<T>? { converted(to: .week) }

	// MARK: Temperature Conversions
	public var C: UnitValue<T>? { converted(to: .celsius) }
	public var F: UnitValue<T>? { converted(to: .fahrenheit) }
	public var K: UnitValue<T>? { converted(to: .kelvin) }

	// MARK: Speed Conversions
	public var mps: UnitValue<T>? { converted(to: .metersPerSecond) }
	public var kmh: UnitValue<T>? { converted(to: .kilometersPerHour) }
	public var mph: UnitValue<T>? { converted(to: .milesPerHour) }
	public var kn: UnitValue<T>? { converted(to: .knots) }
}