/* 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/. */

/**
 * Random number generator.
 *
 * Copyright: Eugene Wissner 2016-2020.
 * 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/source/tanya/math/random.d,
 *                 tanya/math/random.d)
 */
module tanya.math.random;

import std.typecons;
import tanya.memory.allocator;
import tanya.typecons;

/// Maximum amount gathered from the entropy sources.
enum maxGather = 128;

/**
 * Exception thrown if random number generating fails.
 */
class EntropyException : Exception
{
    /**
     * Params:
     *  msg  = Message to output.
     *  file = The file where the exception occurred.
     *  line = The line number where the exception occurred.
     *  next = The previous exception in the chain of exceptions, if any.
     */
    this(string msg,
         string file = __FILE__,
         size_t line = __LINE__,
         Throwable next = null) const @nogc nothrow pure @safe
    {
        super(msg, file, line, next);
    }
}

/**
 * Interface for implementing entropy sources.
 */
abstract class EntropySource
{
    /// Amount of already generated entropy.
    protected ushort size_;

    /**
     * Returns: Minimum bytes required from the entropy source.
     */
    @property ubyte threshold() const @nogc nothrow pure @safe;

    /**
     * Returns: Whether this entropy source is strong.
     */
    @property bool strong() const @nogc nothrow pure @safe;

    /**
     * Returns: Amount of already generated entropy.
     */
    @property ushort size() const @nogc nothrow pure @safe
    {
        return size_;
    }

    /**
     * Params:
     *  size = Amount of already generated entropy. Cannot be smaller than the
     *         already set value.
     */
    @property void size(ushort size) @nogc nothrow pure @safe
    {
        size_ = size;
    }

    /**
     * Poll the entropy source.
     *
     * Params:
     *  output = Buffer to save the generate random sequence (the method will
     *           to fill the buffer).
     *
     * Returns: Number of bytes that were copied to the $(D_PARAM output)
     *          or nothing on error.
     *
     * Postcondition: Returned length is less than or equal to
     *                $(D_PARAM output) length.
     */
    Nullable!ubyte poll(out ubyte[maxGather] output) @nogc;
}

version (CRuntime_Bionic)
{
    version = SecureARC4Random;
}
else version (OSX)
{
    version = SecureARC4Random;
}
else version (OpenBSD)
{
    version = SecureARC4Random;
}
else version (NetBSD)
{
    version = SecureARC4Random;
}
else version (Solaris)
{
    version = SecureARC4Random;
}

version (linux)
{
    import core.stdc.config : c_long;
    private extern(C) c_long syscall(c_long number, ...) @nogc nothrow @system;

    /**
     * Uses getrandom system call.
     */
    class PlatformEntropySource : EntropySource
    {
        /**
         * Returns: Minimum bytes required from the entropy source.
         */
        override @property ubyte threshold() const @nogc nothrow pure @safe
        {
            return 32;
        }

        /**
         * Returns: Whether this entropy source is strong.
         */
        override @property bool strong() const @nogc nothrow pure @safe
        {
            return true;
        }

        /**
         * Poll the entropy source.
         *
         * Params:
         *  output = Buffer to save the generate random sequence (the method will
         *           to fill the buffer).
         *
         * Returns: Number of bytes that were copied to the $(D_PARAM output)
         *          or nothing on error.
         */
        override Nullable!ubyte poll(out ubyte[maxGather] output) @nogc nothrow
        out (length)
        {
            assert(length.isNull || length.get <= maxGather);
        }
        do
        {
            // int getrandom(void *buf, size_t buflen, unsigned int flags);
            import mir.linux._asm.unistd : NR_getrandom;
            auto length = syscall(NR_getrandom, output.ptr, output.length, 0);
            Nullable!ubyte ret;

            if (length >= 0)
            {
                ret = cast(ubyte) length;
            }
            return ret;
        }
    }
}
else version (SecureARC4Random)
{
    private extern(C) void arc4random_buf(scope void* buf, size_t nbytes)
    @nogc nothrow @system;

    /**
     * Uses arc4random_buf.
     */
    class PlatformEntropySource : EntropySource
    {
        /**
         * Returns: Minimum bytes required from the entropy source.
         */
        override @property ubyte threshold() const @nogc nothrow pure @safe
        {
            return 32;
        }

        /**
         * Returns: Whether this entropy source is strong.
         */
        override @property bool strong() const @nogc nothrow pure @safe
        {
            return true;
        }

        /**
         * Poll the entropy source.
         *
         * Params:
         *  output = Buffer to save the generate random sequence (the method will
         *           to fill the buffer).
         *
         * Returns: Number of bytes that were copied to the $(D_PARAM output)
         *          or nothing on error.
         */
        override Nullable!ubyte poll(out ubyte[maxGather] output)
        @nogc nothrow @safe
        out (length)
        {
            assert(length.isNull || length.get <= maxGather);
        }
        do
        {
            (() @trusted => arc4random_buf(output.ptr, output.length))();
            return Nullable!ubyte(cast(ubyte) (output.length));
        }
    }
}
else version (Windows)
{
    import core.sys.windows.basetsd : ULONG_PTR;
    import core.sys.windows.winbase : GetLastError;
    import core.sys.windows.wincrypt;
    import core.sys.windows.windef : BOOL, DWORD, PBYTE;
    import core.sys.windows.winerror : NTE_BAD_KEYSET;
    import core.sys.windows.winnt : LPCSTR, LPCWSTR;

    private extern(Windows) @nogc nothrow
    {
        BOOL CryptGenRandom(HCRYPTPROV, DWORD, PBYTE);
        BOOL CryptAcquireContextA(HCRYPTPROV*, LPCSTR, LPCSTR, DWORD, DWORD);
        BOOL CryptAcquireContextW(HCRYPTPROV*, LPCWSTR, LPCWSTR, DWORD, DWORD);
        BOOL CryptReleaseContext(HCRYPTPROV, ULONG_PTR);
    }

    private bool initCryptGenRandom(scope ref HCRYPTPROV hProvider)
    @nogc nothrow @trusted
    {
        // https://msdn.microsoft.com/en-us/library/windows/desktop/aa379886(v=vs.85).aspx
        // For performance reasons, we recommend that you set the pszContainer
        // parameter to NULL and the dwFlags parameter to CRYPT_VERIFYCONTEXT
        // in all situations where you do not require a persisted key.
        // CRYPT_SILENT is intended for use with applications for which the UI
        // cannot be displayed by the CSP.
        if (!CryptAcquireContextW(&hProvider,
                                  null,
                                  null,
                                  PROV_RSA_FULL,
                                  CRYPT_VERIFYCONTEXT | CRYPT_SILENT))
        {
            if (GetLastError() != NTE_BAD_KEYSET)
            {
                return false;
            }
            // Attempt to create default container
            if (!CryptAcquireContextA(&hProvider,
                                      null,
                                      null,
                                      PROV_RSA_FULL,
                                      CRYPT_NEWKEYSET | CRYPT_SILENT))
            {
                return false;
            }
        }

        return true;
    }

    class PlatformEntropySource : EntropySource
    {
        private HCRYPTPROV hProvider;

        /**
         * Uses CryptGenRandom.
         */
        this() @nogc
        {
            if (!initCryptGenRandom(hProvider))
            {
                throw defaultAllocator.make!EntropyException("CryptAcquireContextW failed.");
            }
            assert(hProvider > 0, "hProvider not properly initialized.");
        }

        ~this() @nogc nothrow @safe
        {
            if (hProvider > 0)
            {
                (() @trusted => CryptReleaseContext(hProvider, 0))();
            }
        }

        /**
         * Returns: Minimum bytes required from the entropy source.
         */
        override @property ubyte threshold() const @nogc nothrow pure @safe
        {
            return 32;
        }

        /**
         * Returns: Whether this entropy source is strong.
         */
        override @property bool strong() const @nogc nothrow pure @safe
        {
            return true;
        }

        /**
         * Poll the entropy source.
         *
         * Params:
         *  output = Buffer to save the generate random sequence (the method will
         *           to fill the buffer).
         *
         * Returns: Number of bytes that were copied to the $(D_PARAM output)
         *          or nothing on error.
         */
        override Nullable!ubyte poll(out ubyte[maxGather] output)
        @nogc nothrow @safe
        out (length)
        {
            assert(length.isNull || length.get <= maxGather);
        }
        do
        {
            Nullable!ubyte ret;

            assert(hProvider > 0, "hProvider not properly initialized");
            if ((() @trusted => CryptGenRandom(hProvider, output.length, cast(PBYTE) output.ptr))())
            {
                ret = cast(ubyte) (output.length);
            }
            return ret;
        }
    }
}