Potential answer for Rotation Point question in Interview Cake.

rotation-point.swift

func findRotationPoint(in words: [String]) -> Int {
    let firstWord = words[0]

    var floorIndex = 0
    var ceilingIndex = words.count - 1

    while floorIndex < ceilingIndex {

        // guess a point halfway between floor and ceiling
        let guessIndex = floorIndex + ((ceilingIndex - floorIndex) / 2)

        // if guess comes after first word or is the first word
        if words[guessIndex] >= firstWord {
            // go right
            floorIndex = guessIndex
        } else {
            // go left
            ceilingIndex = guessIndex
        }

        // if floor and ceiling have converged
        if (floorIndex + 1) == ceilingIndex {

            // between floor and ceiling is where we flipped to the beginning
            // so ceiling is alphabetically first
            break
        }
    }

    return ceilingIndex
}

// returns 2
findRotationPoint(in: ["a", "b", "c"])

func rotationPoint<T: Comparable>(from array: [T]) -> Int? {
    guard let first = array.first else {
        return nil
    }

    var leftIndex = 0
    var rightIndex = array.count - 1

    while leftIndex + 1 < rightIndex {

        let distance = rightIndex - leftIndex
        let middleIndex = (distance / 2) + leftIndex
        let middleValue = array[middleIndex]

        if middleValue == first || middleValue < array[middleIndex - 1] {
            // Found the rotation point.
            return middleIndex
        }

        if middleValue < first {
            // Rotation point is to left.
            rightIndex = middleIndex
        } else if middleValue > first {
            // Rotation point is to the right.
            leftIndex = middleIndex
        }
    }

    // This is optional. You could also say there is no rotation point,
    // and return `nil`.
    if leftIndex + 1 == rightIndex {
        // This array is in order. The rotation point is at the beginning.
        return 0
    }

    return nil
}

// returns Optional(0)
rotationPoint(from: ["a", "b", "c"])