Daily Coding Problem: Problem #691

/**
 * This problem was asked by Yahoo.
 * Write a function that returns the bitwise AND of all integers between M and N, inclusive.
 * */
class Problem_691 {
    /*
    * solution: Most Significant Bit (msb), 最高有效位, 在大端序中，msb就是左最边的位
    * 1. get msb of two number
    * 2. if two msb are equals, return 0; else add 2^msp_value into result
    * 3. subtract msp_value from m and n
    * */
    fun bitwiseAND(m_: Int, n_: Int): Int {
        var m = m_
        var n = n_
        var result = 0
        while (m > 0 && n > 0) {
            val msbM = msbPosition(m)
            val msbN = msbPosition(n)
            if (msbM != msbN) {
                break
            }
            val msbValue = 1 shl msbM//1 shl msbM = 2^msbM = 2的msbM次方
            result += msbValue
            m -= msbValue
            n -= msbValue
        }
        return result
    }

    private fun msbPosition(n_: Int): Int {
        var msb = -1
        var n = n_
        //position++ when n shift right until n==0 (mean reach n's most highest bit)
        while (n > 0) {
            n = n shr 1
            msb++
        }
        return msb
    }
}