Converts between number and its abbreviation (1000 -> 1K -> 1000)

NumberAbbreviationFormatter.swift

import Foundation

@dynamicMemberLookup
class NumberAbbreviationFormatter {

    // MARK: - Internal properties

    var formatter = NumberFormatter()

    // MARK: - Static configurations

    private let units = "KMBTPE"
        .enumerated()
        .map { (symbol: $0.element, value: pow(10, ($0.offset + 1) * 3)) }

    // MARK: - Number to String conversion

    func format(_ value: Int) -> String {
        guard let unit = units.last(where: { $0.value.isLessThanOrEqualTo(Decimal(abs(value))) }) else {
            return formatter.string(from: NSNumber(value: value)) ?? value.description
        }

        let factor = Decimal(value) / unit.value
        let number = NSDecimalNumber(decimal: factor) as NSNumber
        let formattedNumber = formatter.string(from: number) ?? number.description
        return formattedNumber + String(unit.symbol).trimmingCharacters(in: .whitespaces)
    }

    // MARK: - String to Number conversion

    func number(from string: String) -> Int? {
        guard !string.isEmpty else { return nil }
        guard string.last?.isNumber == false else { return formatter.number(from: string)?.intValue }
        var string = string
        let symbol = string.removeLast()
        guard units.map(\.symbol).contains(symbol) else { return nil }
        guard let number = formatter.number(from: string.trimmingCharacters(in: .whitespaces)) else { return nil }
        let multiplier = units.first(where: { $0.symbol == symbol })?.value ?? 1
        let result = number.decimalValue * multiplier
        return NSDecimalNumber(decimal: result).intValue
    }
}

// MARK: - Dynamic member lookup conformance

/// Access to the internal formatter
extension NumberAbbreviationFormatter {
    subscript<T>(dynamicMember keyPath: KeyPath<NumberFormatter, T>) -> T {
        formatter[keyPath: keyPath]
    }

    subscript<T>(dynamicMember keyPath: WritableKeyPath<NumberFormatter, T>) -> T {
        get { formatter[keyPath: keyPath] }
        set { formatter[keyPath: keyPath] = newValue }
    }
}