Bit shifting with overflow protection using overflow operator "&"

shift_bits_with_truncation

//
//  IntExtension.swift
//  CryptoSwift
//
//  Created by Marcin Krzyzanowski on 12/08/14.
//  Copyright (C) 2014 Marcin Krzyżanowski <[email protected]>
//  This software is provided 'as-is', without any express or implied warranty.
//
//  In no event will the authors be held liable for any damages arising from the use of this software.
//
//  Permission is granted to anyone to use this software for any purpose,including commercial applications, and to alter it and redistribute it freely, subject to the following restrictions:
//
//  - The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation is required.
//  - Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
//  - This notice may not be removed or altered from any source or binary distribution.

/*
    Bit shifting with overflow protection using overflow operator "&".
    Approach is consistent with standard overflow operators &+, &-, &*, &/
    and introduce new overflow operators for shifting: &<<, &>>

    Note: Works with unsigned integers values only

    Usage

    var i = 1       // init
    var j = i &<< 2 //shift left
    j &<<= 2        //shift left and assign
    
    
    @see: https://medium.com/@krzyzanowskim/swiftly-shift-bits-and-protect-yourself-be33016ce071
*/

extension Int {
    
    /** Shift bits to the left. All bits are shifted (including sign bit) */
    private mutating func shiftLeft(count: Int) -> Int {
        if (self == 0) {
            return self;
        }
        
        let bitsCount = sizeof(Int) * 8
        let shiftCount = Swift.min(count, bitsCount - 1)
        var shiftedValue:Int = 0;
        
        for bitIdx in 0..<bitsCount {
            // if bit is set then copy to result and shift left 1
            let bit = 1 << bitIdx
            if ((self & bit) == bit) {
                shiftedValue = shiftedValue | (bit << shiftCount)
            }
        }
        self = shiftedValue
        return self
    }
    
    /** Shift bits to the right. All bits are shifted (including sign bit) */
    private mutating func shiftRight(count: UInt32) -> UInt32 {
        if (self == 0) {
            return self;
        }
        
        var bitsCount = UInt32(sizeofValue(self) * 8)

        if (count >= bitsCount) {
            return 0
        }

        var maxBitsForValue = UInt32(floor(log2(Double(self)) + 1))
        var shiftCount = Swift.min(count, maxBitsForValue - 1)
        var shiftedValue:UInt32 = 0;
        
        for bitIdx in 0..<bitsCount {
            // if bit is set then copy to result and shift left 1
            var bit = 1 << bitIdx
            if ((self & bit) == bit) {
                shiftedValue = shiftedValue | (bit >> shiftCount)
            }
        }
        self = shiftedValue
        return self
    }
}

// Left operator

infix operator &<<= {
    associativity none
    precedence 160
}


/** shift left and assign with bits truncation */
func &<<= (inout lhs: Int, rhs: Int) {
    lhs.shiftLeft(rhs)
}

infix operator &<< {
    associativity none
    precedence 160
}

/** shift left with bits truncation */
func &<< (lhs: Int, rhs: Int) -> Int {
    var l = lhs;
    l.shiftLeft(rhs)
    return l
}

// Right operator

infix operator &>>= {
    associativity none
    precedence 160
}

/** shift right and assign with bits truncation */
func &>>= (inout lhs: Int, rhs: Int) {
    lhs.shiftRight(rhs)
}

infix operator &>> {
associativity none
precedence 160
}

/** shift right and assign with bits truncation */
func &>> (lhs: Int, rhs: Int) -> Int {
    var l = lhs;
    l.shiftRight(rhs)
    return l
}