Algorithm for compress string

CompressString.swift

import Cocoa

let str = "aabbaaccccdaeeeebb"

func compressString(_ value: String) -> String {
    var resultStr = ""
    var currentChar: Character?
    var currentCount: Int = 0
    for c in value {
        guard let char = currentChar else {
            currentChar = c
            currentCount += 1
            continue
        }
        if char == c {
            currentCount += 1
        } else {
            resultStr.append(char)
            resultStr.append("\(currentCount)")
            currentChar = c
            currentCount = 1
        }
    }
    if let c  = currentChar {
        resultStr.append(c)
        resultStr.append("\(currentCount)")
    }
    return resultStr
}
print(compressString(str)) //a2b2a2c4d1a1e4b2