Formats bytes into a more human-readable form (e.g. MB).

formatBytes.swift

import Foundation

func format(bytes: Double) -> String {
    guard bytes > 0 else {
        return "0 bytes"
    }

    // Adapted from http://stackoverflow.com/a/18650828
    let suffixes = ["bytes", "KB", "MB", "GB", "TB", "PB", "EB", "ZB", "YB"]
    let k: Double = 1000
    let i = floor(log(bytes) / log(k))

    // Format number with thousands separator and everything below 1 GB with no decimal places.
    let numberFormatter = NumberFormatter()
    numberFormatter.maximumFractionDigits = i < 3 ? 0 : 1
    numberFormatter.numberStyle = .decimal

    let numberString = numberFormatter.string(from: NSNumber(value: bytes / pow(k, i))) ?? "Unknown"
    let suffix = suffixes[Int(i)]
    return "\(numberString) \(suffix)"
}

@kmi4k4k: There's some discussion of when to use 1024 vs. 1000 in the comments of the Stack Overflow answer I linked to.

It's more correct to use the "Kib", etc. abbreviations when using base 1024, because "KB", etc. actually refers to base 1000.

macOS's Get Info window, for example, reports file sizes in base 1000.