/**
 * This module contains a collection of bit-level operations.
 *
 * Copyright: Copyright Don Clugston 2005 - 2013.
 * License:   $(LINK2 http://www.boost.org/LICENSE_1_0.txt, Boost License 1.0)
 * Authors:   Don Clugston, Sean Kelly, Walter Bright, Alex Rønne Petersen
 * Source:    $(DRUNTIMESRC core/_bitop.d)
 *
 * Some of the LDC-specific parts came »From GDC ... public domain!«
 */

module core.bitop;

nothrow:
@safe:
@nogc:

version( D_InlineAsm_X86_64 )
    version = AsmX86;
else version( D_InlineAsm_X86 )
    version = AsmX86;

version (X86_64)
    version = AnyX86;
else version (X86)
    version = AnyX86;

/**
 * Scans the bits in v starting with bit 0, looking
 * for the first set bit.
 * Returns:
 *      The bit number of the first bit set.
 *      The return value is undefined if v is zero.
 * Example:
 * ---
 * import core.bitop;
 *
 * int main()
 * {
 *     assert(bsf(0x21) == 0);
 *     return 0;
 * }
 * ---
 */
version (LDC)
{
    // KLUDGE: Need to adapt the return type.
    private pure pragma(LDC_intrinsic, "llvm.cttz.i#") T cttz(T)(T v, bool is_zero_undef);
    pure int bsf(size_t v)
    {
        return cast(int)cttz(v, true);
    }
}
else
{
    int bsf(size_t v) pure;
}

unittest
{
    assert(bsf(0x21) == 0);
}

/**
 * Scans the bits in v from the most significant bit
 * to the least significant bit, looking
 * for the first set bit.
 * Returns:
 *      The bit number of the first bit set.
 *      The return value is undefined if v is zero.
 * Example:
 * ---
 * import core.bitop;
 *
 * int main()
 * {
 *     assert(bsr(0x21) == 5);
 *     return 0;
 * }
 * ---
 */
version (LDC)
{
    private pure pragma(LDC_intrinsic, "llvm.ctlz.i#") T ctlz(T)(T v, bool is_zero_undef);
    pure int bsr(size_t v)
    {
        return cast(int)(size_t.sizeof * 8 - 1 - ctlz(v, true));
    }
}
else
{
    int bsr(size_t v) pure;
}

unittest
{
    assert(bsr(0x21) == 5);
}

/**
 * Tests the bit.
 * (No longer an intrisic - the compiler recognizes the patterns
 * in the body.)
 */
version (none)
{
    // Our implementation returns an arbitrary non-zero value if the bit was
    // set, which is not what std.bitmanip expects.
    pragma(LDC_intrinsic, "ldc.bitop.bt")
        int bt(in size_t* p, size_t bitnum) pure @system;
}
else
int bt(in size_t* p, size_t bitnum) pure @system
{
    static if (size_t.sizeof == 8)
        return ((p[bitnum >> 6] & (1L << (bitnum & 63)))) != 0;
    else static if (size_t.sizeof == 4)
        return ((p[bitnum >> 5] & (1  << (bitnum & 31)))) != 0;
    else
        static assert(0);
}
///
@system pure unittest
{
    size_t array[2];

    array[0] = 2;
    array[1] = 0x100;

    assert(bt(array.ptr, 1));
    assert(array[0] == 2);
    assert(array[1] == 0x100);
}

/**
 * Tests and complements the bit.
 */
version (LDC)
{
    pragma(LDC_intrinsic, "ldc.bitop.btc")
        int btc(size_t* p, size_t bitnum) pure @system;
}
else
int btc(size_t* p, size_t bitnum) pure @system;


/**
 * Tests and resets (sets to 0) the bit.
 */
version (LDC)
{
    pragma(LDC_intrinsic, "ldc.bitop.btr")
        int btr(size_t* p, size_t bitnum) pure @system;
}
else
int btr(size_t* p, size_t bitnum) pure @system;


/**
 * Tests and sets the bit.
 * Params:
 * p = a non-NULL pointer to an array of size_ts.
 * bitnum = a bit number, starting with bit 0 of p[0],
 * and progressing. It addresses bits like the expression:
---
p[index / (size_t.sizeof*8)] & (1 << (index & ((size_t.sizeof*8) - 1)))
---
 * Returns:
 *      A non-zero value if the bit was set, and a zero
 *      if it was clear.
 */
version (LDC)
{
    pragma(LDC_intrinsic, "ldc.bitop.bts")
        int bts(size_t* p, size_t bitnum) pure @system;
}
else
int bts(size_t* p, size_t bitnum) pure @system;

///
@system pure unittest
{
    size_t array[2];

    array[0] = 2;
    array[1] = 0x100;

    assert(btc(array.ptr, 35) == 0);
    if (size_t.sizeof == 8)
    {
        assert(array[0] == 0x8_0000_0002);
        assert(array[1] == 0x100);
    }
    else
    {
        assert(array[0] == 2);
        assert(array[1] == 0x108);
    }

    assert(btc(array.ptr, 35));
    assert(array[0] == 2);
    assert(array[1] == 0x100);

    assert(bts(array.ptr, 35) == 0);
    if (size_t.sizeof == 8)
    {
        assert(array[0] == 0x8_0000_0002);
        assert(array[1] == 0x100);
    }
    else
    {
        assert(array[0] == 2);
        assert(array[1] == 0x108);
    }

    assert(btr(array.ptr, 35));
    assert(array[0] == 2);
    assert(array[1] == 0x100);
}

/**
 * Swaps bytes in a 4 byte uint end-to-end, i.e. byte 0 becomes
 * byte 3, byte 1 becomes byte 2, byte 2 becomes byte 1, byte 3
 * becomes byte 0.
 */
version (LDC)
{
    pure pragma(LDC_intrinsic, "llvm.bswap.i32")
        uint bswap(uint v);
}
else
{
    uint bswap(uint v) pure;
}


version (DigitalMars) version (AnyX86) @system // not pure
{
    /**
     * Reads I/O port at port_address.
     */
    ubyte inp(uint port_address);


    /**
     * ditto
     */
    ushort inpw(uint port_address);


    /**
     * ditto
     */
    uint inpl(uint port_address);


    /**
     * Writes and returns value to I/O port at port_address.
     */
    ubyte outp(uint port_address, ubyte value);


    /**
     * ditto
     */
    ushort outpw(uint port_address, ushort value);


    /**
     * ditto
     */
    uint outpl(uint port_address, uint value);
}
version(LDC) @system // not pure
{
    /**
     * Reads I/O port at port_address.
     */
    uint inp(uint port_address)
    {
        assert(false, "inp not yet implemented for LDC.");
    }


    /**
     * ditto
     */
    uint inpw(uint port_address)
    {
        assert(false, "inpw not yet implemented for LDC.");
    }


    /**
     * ditto
     */
    uint inpl(uint port_address)
    {
        assert(false, "inpl not yet implemented for LDC.");
    }


    /**
     * Writes and returns value to I/O port at port_address.
     */
    ubyte outp(uint port_address, uint value)
    {
        assert(false, "outp not yet implemented for LDC.");
    }


    /**
     * ditto
     */
    ushort outpw(uint port_address, uint value)
    {
        assert(false, "outpw not yet implemented for LDC.");
    }


    /**
     * ditto
     */
    uint outpl(uint port_address, uint value)
    {
        assert(false, "outpl not yet implemented for LDC.");
    }
}


/**
 *  Calculates the number of set bits in a 32-bit integer.
 */
version (LDC)
{
    private pure pragma(LDC_intrinsic, "llvm.ctpop.i#") T ctpop(T)(T v);
    pure int popcnt(uint x)
    {
        return cast(int) ctpop(x);
    }
}
else
{
    int popcnt( uint x ) pure
    {
        // Avoid branches, and the potential for cache misses which
        // could be incurred with a table lookup.

        // We need to mask alternate bits to prevent the
        // sum from overflowing.
        // add neighbouring bits. Each bit is 0 or 1.
        x = x - ((x>>1) & 0x5555_5555);
        // now each two bits of x is a number 00,01 or 10.
        // now add neighbouring pairs
        x = ((x&0xCCCC_CCCC)>>2) + (x&0x3333_3333);
        // now each nibble holds 0000-0100. Adding them won't
        // overflow any more, so we don't need to mask any more

        // Now add the nibbles, then the bytes, then the words
        // We still need to mask to prevent double-counting.
        // Note that if we used a rotate instead of a shift, we
        // wouldn't need the masks, and could just divide the sum
        // by 8 to account for the double-counting.
        // On some CPUs, it may be faster to perform a multiply.

        x += (x>>4);
        x &= 0x0F0F_0F0F;
        x += (x>>8);
        x &= 0x00FF_00FF;
        x += (x>>16);
        x &= 0xFFFF;
        return x;
    }
}


unittest
{
    assert( popcnt( 0 ) == 0 );
    assert( popcnt( 7 ) == 3 );
    assert( popcnt( 0xAA )== 4 );
    assert( popcnt( 0x8421_1248 ) == 8 );
    assert( popcnt( 0xFFFF_FFFF ) == 32 );
    assert( popcnt( 0xCCCC_CCCC ) == 16 );
    assert( popcnt( 0x7777_7777 ) == 24 );
}


/**
 * Reverses the order of bits in a 32-bit integer.
 */
@trusted uint bitswap( uint x ) pure
{
    version (AsmX86)
    {
        asm { naked; }

        version (D_InlineAsm_X86_64)
        {
            version (Win64)
                asm { mov EAX, ECX; }
            else
                asm { mov EAX, EDI; }
        }

        asm
        {
            // Author: Tiago Gasiba.
            mov EDX, EAX;
            shr EAX, 1;
            and EDX, 0x5555_5555;
            and EAX, 0x5555_5555;
            shl EDX, 1;
            or  EAX, EDX;
            mov EDX, EAX;
            shr EAX, 2;
            and EDX, 0x3333_3333;
            and EAX, 0x3333_3333;
            shl EDX, 2;
            or  EAX, EDX;
            mov EDX, EAX;
            shr EAX, 4;
            and EDX, 0x0f0f_0f0f;
            and EAX, 0x0f0f_0f0f;
            shl EDX, 4;
            or  EAX, EDX;
            bswap EAX;
            ret;
        }
    }
    else
    {
        // swap odd and even bits
        x = ((x >> 1) & 0x5555_5555) | ((x & 0x5555_5555) << 1);
        // swap consecutive pairs
        x = ((x >> 2) & 0x3333_3333) | ((x & 0x3333_3333) << 2);
        // swap nibbles
        x = ((x >> 4) & 0x0F0F_0F0F) | ((x & 0x0F0F_0F0F) << 4);
        // swap bytes
        x = ((x >> 8) & 0x00FF_00FF) | ((x & 0x00FF_00FF) << 8);
        // swap 2-byte long pairs
        x = ( x >> 16              ) | ( x               << 16);
        return x;

    }
}


unittest
{
    assert( bitswap( 0x8000_0100 ) == 0x0080_0001 );
    foreach(i; 0 .. 32)
        assert(bitswap(1 << i) == 1 << 32 - i - 1);
}
