/*

Gaelco video RAM encryption

Thanks to GAELCO SA for information on the algorithm.

*/

#include "emu.h"
#include "includes/gaelcrpt.h"

static int decrypt(int const param1, int const param2, int const enc_prev_word, int const dec_prev_word, int const enc_word)
{
	int const swap = (BIT(dec_prev_word, 8) << 1) | BIT(dec_prev_word, 7);
	int const type = (BIT(dec_prev_word,12) << 1) | BIT(dec_prev_word, 2);
	int res=0;
	int k=0;

	switch (swap)
	{
		case 0:	res = BITSWAP16(enc_word,  1, 2, 0,14,12,15, 4, 8,13, 7, 3, 6,11, 5,10, 9); break;
		case 1:	res = BITSWAP16(enc_word, 14,10, 4,15, 1, 6,12,11, 8, 0, 9,13, 7, 3, 5, 2); break;
		case 2:	res = BITSWAP16(enc_word,  2,13,15, 1,12, 8,14, 4, 6, 0, 9, 5,10, 7, 3,11); break;
		case 3:	res = BITSWAP16(enc_word,  3, 8, 1,13,14, 4,15, 0,10, 2, 7,12, 6,11, 9, 5); break;
	}

	res ^= param2;

	switch (type)
	{
		case 0:
			k =	(0 << 0) |
				(1 << 1) |
				(0 << 2) |
				(1 << 3) |
				(1 << 4) |
				(1 << 5);
			break;

		case 1:
			k =	(BIT(dec_prev_word, 0) << 0) |
				(BIT(dec_prev_word, 1) << 1) |
				(BIT(dec_prev_word, 1) << 2) |
				(BIT(enc_prev_word, 3) << 3) |
				(BIT(enc_prev_word, 8) << 4) |
				(BIT(enc_prev_word,15) << 5);
			break;

		case 2:
			k =	(BIT(enc_prev_word, 5) << 0) |
				(BIT(dec_prev_word, 5) << 1) |
				(BIT(enc_prev_word, 7) << 2) |
				(BIT(enc_prev_word, 3) << 3) |
				(BIT(enc_prev_word,13) << 4) |
				(BIT(enc_prev_word,14) << 5);
			break;

		case 3:
			k =	(BIT(enc_prev_word, 0) << 0) |
				(BIT(enc_prev_word, 9) << 1) |
				(BIT(enc_prev_word, 6) << 2) |
				(BIT(dec_prev_word, 4) << 3) |
				(BIT(enc_prev_word, 2) << 4) |
				(BIT(dec_prev_word,11) << 5);
			break;
	}

	k ^= param1;

	res = (res & 0xffc0) | ((res + k) & 0x003f);

	res ^= param1;

	switch (type)
	{
		case 0:
			k =	(BIT(enc_word, 9) << 0) |
				(BIT(res,2)       << 1) |
				(BIT(enc_word, 5) << 2) |
				(BIT(res,5)       << 3) |
				(BIT(res,4)       << 4);
			break;

		case 1:
			k =	(BIT(dec_prev_word, 2) << 0) |	// always 1
				(BIT(enc_prev_word, 4) << 1) |
				(BIT(dec_prev_word,14) << 2) |
				(BIT(res, 1)           << 3) |
				(BIT(dec_prev_word,12) << 4);	// always 0
			break;

		case 2:
			k =	(BIT(enc_prev_word, 6) << 0) |
				(BIT(dec_prev_word, 6) << 1) |
				(BIT(dec_prev_word,15) << 2) |
				(BIT(res,0)            << 3) |
				(BIT(dec_prev_word, 7) << 4);
			break;

		case 3:
			k =	(BIT(dec_prev_word, 2) << 0) |	// always 1
				(BIT(dec_prev_word, 9) << 1) |
				(BIT(enc_prev_word, 5) << 2) |
				(BIT(dec_prev_word, 1) << 3) |
				(BIT(enc_prev_word,10) << 4);

			break;
	}

	k ^= param1;

	res =	(res & 0x003f) |
			((res + (k <<  6)) & 0x07c0) |
			((res + (k << 11)) & 0xf800);

	res ^= (param1 << 6) | (param1 << 11);

	return BITSWAP16(res, 2,6,0,11,14,12,7,10,5,4,8,3,9,1,13,15);
}



UINT16 gaelco_decrypt(const address_space *space, int offset, int data, int param1, int param2)
{
	static int lastpc, lastoffset, lastencword, lastdecword;

	int thispc = cpu_get_pc(space->cpu);
//  int savedata = data;

	/* check if 2nd half of 32 bit */
	if(lastpc == thispc && offset == lastoffset + 1)
	{
		lastpc = 0;
		data = decrypt(param1, param2, lastencword, lastdecword, data);
	}
	else
	{
		/* code as 1st word */

		lastpc = thispc;
		lastoffset = offset;
		lastencword = data;

		/* high word returned */
		data = decrypt(param1, param2, 0, 0, data);

		lastdecword = data;

//      logerror("%s : data1 = %4x > %4x @ %8x\n",cpuexec_describe_context(space->machine),savedata,data,lastoffset);
	}
	return data;
}
