Swift implementation of the classic Find a peak problem

FindAPeak.swift

func findAPeak(arr: [Int], low: Int, high: Int, size: Int) -> Int {
    
    //Find index of the middle element
    let mid = low + (high - low)/2
    
    // Compare the middle element with its neighbours if they exist
    if (mid == 0 || arr[mid - 1] <= arr[mid]) && (mid == size - 1 || arr[mid + 1] <= arr[mid]) {
        return mid
    }
    
    // If the middle neighbour is not the peak and the left neighbour is greater than it, then the left half must have the peak element
    else if (mid > 0 && arr[mid - 1] > arr[mid]) {
        return findAPeak(arr: arr, low: low, high: mid - 1, size: size)
    }
    
    // If middle element is not the peal and its right neighbour is greater than it, then the right half must have the peak element
    else {
        return findAPeak(arr: arr, low: (mid + 1), high: high, size: size)
    }
}

let array = [10, 20, 15, 60, 90]
let size = array.count
let peak = findAPeak(arr: array, low: 0, high: size - 1, size: size)