/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/**
 * Set of operations on memory blocks.
 *
 * Copyright: Eugene Wissner 2017-2020.
 * License: $(LINK2 https://www.mozilla.org/en-US/MPL/2.0/,
 *                  Mozilla Public License, v. 2.0).
 * Authors: $(LINK2 mailto:info@caraus.de, Eugene Wissner)
 * Source: $(LINK2 https://github.com/caraus-ecms/tanya/blob/master/middle/tanya/memory/op.d,
 *                 tanya/memory/op.d)
 */
module tanya.memory.op;

import core.stdc..string;

private enum alignMask = size_t.sizeof - 1;

/**
 * Copies $(D_PARAM source) into $(D_PARAM target).
 *
 * $(D_PARAM source) and $(D_PARAM target) shall not overlap so that
 * $(D_PARAM source) points ahead of $(D_PARAM target).
 *
 * $(D_PARAM target) shall have enough space for $(D_INLINECODE source.length)
 * elements.
 *
 * Params:
 *  source = Memory to copy from.
 *  target = Destination memory.
 *
 * See_Also: $(D_PSYMBOL copyBackward).
 *
 * Precondition: $(D_INLINECODE source.length <= target.length).
 */
void copy(const void[] source, void[] target) @nogc nothrow pure @trusted
in
{
    assert(source.length <= target.length);
    assert(source.length == 0 || source.ptr !is null);
    assert(target.length == 0 || target.ptr !is null);
}
do
{
    memcpy(target.ptr, source.ptr, source.length);
}

///
@nogc nothrow pure @safe unittest
{
    ubyte[9] source = [1, 2, 3, 4, 5, 6, 7, 8, 9];
    ubyte[9] target;
    source.copy(target);
    assert(equal(source, target));
}

/*
 * size_t value each of which bytes is set to `Byte`.
 */
private template filledBytes(ubyte Byte, ubyte I = 0)
{
    static if (I == size_t.sizeof)
    {
        enum size_t filledBytes = Byte;
    }
    else
    {
        enum size_t filledBytes = (filledBytes!(Byte, I + 1) << 8) | Byte;
    }
}

/**
 * Fills $(D_PARAM memory) with the single byte $(D_PARAM c).
 *
 * Param:
 *  c      = The value to fill $(D_PARAM memory) with.
 *  memory = Memory block.
 */
void fill(ubyte c = 0)(void[] memory) @trusted
in
{
    assert(memory.length == 0 || memory.ptr !is null);
}
do
{
    memset(memory.ptr, c, memory.length);
}

///
@nogc nothrow pure @safe unittest
{
    ubyte[9] memory = [1, 2, 3, 4, 5, 6, 7, 8, 9];
    memory.fill!0();
    foreach (ubyte v; memory)
    {
        assert(v == 0);
    }
}

/**
 * Copies starting from the end of $(D_PARAM source) into the end of
 * $(D_PARAM target).
 *
 * $(D_PSYMBOL copyBackward) copies the elements in reverse order, but the
 * order of elements in the $(D_PARAM target) is exactly the same as in the
 * $(D_PARAM source).
 *
 * $(D_PARAM source) and $(D_PARAM target) shall not overlap so that
 * $(D_PARAM target) points ahead of $(D_PARAM source).
 *
 * $(D_PARAM target) shall have enough space for $(D_INLINECODE source.length)
 * elements.
 *
 * Params:
 *  source = Memory to copy from.
 *  target = Destination memory.
 *
 * See_Also: $(D_PSYMBOL copy).
 *
 * Precondition: $(D_INLINECODE source.length <= target.length).
 */
void copyBackward(const void[] source, void[] target) @nogc nothrow pure @trusted
in
{
    assert(source.length <= target.length);
    assert(source.length == 0 || source.ptr !is null);
    assert(target.length == 0 || target.ptr !is null);
}
do
{
    memmove(target.ptr, source.ptr, source.length);
}

///
@nogc nothrow pure @safe unittest
{
    ubyte[6] mem = [ 'a', 'a', 'b', 'b', 'c', 'c' ];
    ubyte[6] expected = [ 'a', 'a', 'a', 'a', 'b', 'b' ];

    copyBackward(mem[0 .. 4], mem[2 .. $]);
    assert(equal(expected, mem));
}

/**
 * Finds the first occurrence of $(D_PARAM needle) in $(D_PARAM haystack) if
 * any.
 *
 * Params:
 *  haystack = Memory block.
 *  needle   = A byte.
 *
 * Returns: The subrange of $(D_PARAM haystack) whose first element is the
 *          first occurrence of $(D_PARAM needle). If $(D_PARAM needle)
 *          couldn't be found, an empty `inout void[]` is returned.
 */
inout(void[]) find(return inout void[] haystack, ubyte needle)
@nogc nothrow pure @trusted
in
{
    assert(haystack.length == 0 || haystack.ptr !is null);
}
do
{
    auto length = haystack.length;
    const size_t needleWord = size_t.max * needle;
    enum size_t highBits = filledBytes!(0x01, 0);
    enum size_t mask = filledBytes!(0x80, 0);

    // Align
    auto bytes = cast(inout(ubyte)*) haystack;
    while (length > 0 && ((cast(size_t) bytes) & 3) != 0)
    {
        if (*bytes == needle)
        {
            return bytes[0 .. length];
        }
        ++bytes;
        --length;
    }

    // Check if some of the words has the needle
    auto words = cast(inout(size_t)*) bytes;
    while (length >= size_t.sizeof)
    {
        if ((((*words ^ needleWord) - highBits) & (~*words) & mask) != 0)
        {
            break;
        }
        ++words;
        length -= size_t.sizeof;
    }

    // Find the exact needle position in the word
    bytes = cast(inout(ubyte)*) words;
    while (length > 0)
    {
        if (*bytes == needle)
        {
            return bytes[0 .. length];
        }
        ++bytes;
        --length;
    }

    return haystack[$ .. $];
}

///
@nogc nothrow pure @safe unittest
{
    const ubyte[9] haystack = ['a', 'b', 'c', 'd', 'e', 'f', 'b', 'g', 'h'];

    assert(equal(find(haystack, 'a'), haystack[]));
    assert(equal(find(haystack, 'b'), haystack[1 .. $]));
    assert(equal(find(haystack, 'c'), haystack[2 .. $]));
    assert(equal(find(haystack, 'd'), haystack[3 .. $]));
    assert(equal(find(haystack, 'e'), haystack[4 .. $]));
    assert(equal(find(haystack, 'f'), haystack[5 .. $]));
    assert(equal(find(haystack, 'h'), haystack[8 .. $]));
    assert(find(haystack, 'i').length == 0);

    assert(find(null, 'a').length == 0);
}

/**
 * Looks for `\0` in the $(D_PARAM haystack) and returns the part of the
 * $(D_PARAM haystack) ahead of it.
 *
 * Returns $(D_KEYWORD null) if $(D_PARAM haystack) doesn't contain a null
 * character.
 *
 * Params:
 *  haystack = Memory block.
 *
 * Returns: The subrange that spans all bytes before the null character or
 *          $(D_KEYWORD null) if the $(D_PARAM haystack) doesn't contain any.
 */
inout(char[]) findNullTerminated(return inout char[] haystack)
@nogc nothrow pure @trusted
in
{
    assert(haystack.length == 0 || haystack.ptr !is null);
}
do
{
    auto length = haystack.length;
    enum size_t highBits = filledBytes!(0x01, 0);
    enum size_t mask = filledBytes!(0x80, 0);

    // Align
    auto bytes = cast(inout(ubyte)*) haystack;
    while (length > 0 && ((cast(size_t) bytes) & 3) != 0)
    {
        if (*bytes == '\0')
        {
            return haystack[0 .. haystack.length - length];
        }
        ++bytes;
        --length;
    }

    // Check if some of the words contains 0
    auto words = cast(inout(size_t)*) bytes;
    while (length >= size_t.sizeof)
    {
        if (((*words - highBits) & (~*words) & mask) != 0)
        {
            break;
        }
        ++words;
        length -= size_t.sizeof;
    }

    // Find the exact 0 position in the word
    bytes = cast(inout(ubyte)*) words;
    while (length > 0)
    {
        if (*bytes == '\0')
        {
            return haystack[0 .. haystack.length - length];
        }
        ++bytes;
        --length;
    }

    return null;
}

///
@nogc nothrow pure @safe unittest
{
    assert(equal(findNullTerminated("abcdef\0gh"), "abcdef"));
    assert(equal(findNullTerminated("\0garbage"), ""));
    assert(equal(findNullTerminated("\0"), ""));
    assert(equal(findNullTerminated("cstring\0"), "cstring"));
    assert(findNullTerminated(null) is null);
    assert(findNullTerminated("abcdef") is null);
}

/**
 * Compares two memory areas $(D_PARAM r1) and $(D_PARAM r2) for equality.
 *
 * Params:
 *  r1 = First memory block.
 *  r2 = Second memory block.
 *
 * Returns: $(D_KEYWORD true) if $(D_PARAM r1) and $(D_PARAM r2) are equal,
 *          $(D_KEYWORD false) otherwise.
 */
bool equal(const void[] r1, const void[] r2) @nogc nothrow pure @trusted
in
{
    assert(r1.length == 0 || r1.ptr !is null);
    assert(r2.length == 0 || r2.ptr !is null);
}
do
{
    return r1.length == r2.length && memcmp(r1.ptr, r2.ptr, r1.length) == 0;
}

///
@nogc nothrow pure @safe unittest
{
    assert(equal("asdf", "asdf"));
    assert(!equal("asd", "asdf"));
    assert(!equal("asdf", "asd"));
    assert(!equal("asdf", "qwer"));
}