galaga: Manually apply pull request #3532: star field lfsr

implementation

Manually applied the lfsr part of #3532 and verifying against static
star field. Also added documentation and my findings about the known
star field decoding algorithms.

src/mame/includes/galaga.h

@@ -63,6 +63,8 @@ public:
 	void galaga_mem4(address_map &map);
 	void gatsbee_main_map(address_map &map);
 
+	void starfield_init();
+
 protected:
 	virtual void machine_start() override;
 	virtual void machine_reset() override;

src/mame/video/galaga.cpp

@@ -11,6 +11,12 @@
 #include "emu.h"
 #include "includes/galaga.h"
 
+#define LOG_DEBUG           (1 << 0U)
+#define LOG_ALL             (LOG_DEBUG)
+
+#define VERBOSE             (LOG_ALL)
+
+#include "logmacro.h"
 
 #define MAX_STARS 252
 #define STARS_COLOR_BASE (64*4+64*4)
@@ -376,7 +382,99 @@ void galaga_state::galaga_palette(palette_device &palette) const
 		palette.set_pen_indirect(64*4 + 64*4 + i, 32 + i);
 }
 
+/***************************************************************************
 
+	Star field documentation
+
+	Couriersud, May 2019:
+
+	The code below was manually applied based on a pull request from
+	Jindřich Makovička. He based his work on information published by
+	Wolfgang Scherr about the 05XX on pin4.at.
+
+	Wolfgang shared is VHDL implementation with the MAME team. I have
+	created a spreadsheet implementation for his decode logic.
+
+	Both implementations use the same Galois type LFSR( tap 15,12,10,5).
+	Using the same seed value, the following holds true:
+
+	Decode_W(lfsr[t-4]) = Decode_J(lfsr[t])
+
+	The two decoding algorithm (Wolfgang, Jindřich) thus deliver the same
+	results but with a 4 clock difference.
+
+	Jindřich's code filters out stars with y<4. This matches the starfield
+	measurements documented above. Wolfgang states that his code matches his
+	measurements and there are stars with y<4.
+	We need to have a closer look at this.
+
+	Both implementations are complex compared to other star field gnerators
+	used in the industry. We thus now have two decoding solutions matching
+	the output. I wonder if there is a simpler one.
+
+***************************************************************************/
+
+void galaga_state::starfield_init()
+{
+	const uint16_t feed = 0x9420;
+
+	int idx = 0;
+	for (uint16_t sf = 0; sf < 4; ++sf)
+	{
+		// starfield select flags
+		uint16_t sf1 = (sf >> 1) & 1;
+		uint16_t sf2 = sf & 1;
+
+		uint16_t i = 0x70cc;
+		for (int cnt = 0; cnt < 65535; ++cnt)
+		{
+			// output enable lookup
+			uint16_t xor1 = i ^ (i >> 3);
+			uint16_t xor2 = xor1 ^ (i >> 2);
+			uint16_t oe = (sf1 ? 0 : 0x4000) | ((sf1 ^ sf2) ? 0 : 0x1000);
+			if ((i & 0x8007) == 0x8007
+			    && (~i & 0x2008) == 0x2008
+			    && (xor1 & 0x0100) == (sf1 ? 0 : 0x0100)
+			    && (xor2 & 0x0040) == (sf2 ? 0 : 0x0040)
+			    && (i & 0x5000) == oe
+			    && cnt >= 256 * 4)
+			{
+				// color lookup
+				uint16_t xor3 = (i >> 1) ^ (i >> 6);
+				uint16_t clr =
+					(((i >> 9) & 0x07)
+					 | ((xor3 ^ (i >> 4) ^ (i >> 7)) & 0x08)
+					 | (~xor3 & 0x10)
+					 | (((i >> 2) ^ (i >> 5)) & 0x20))
+					^ ((i & 0x4000) ? 0 : 0x24)
+					^ ((((i >> 2) ^ i) & 0x1000) ? 0x21 : 0);
+#if 0
+				m_star_seed_tab[idx].x = cnt % 256;
+				m_star_seed_tab[idx].y = cnt / 256;
+				m_star_seed_tab[idx].col = clr;
+				m_star_seed_tab[idx].set = sf;
+#else
+				int x = cnt % 256;
+				int y = cnt / 256;
+				int col = clr;
+				int set = sf;
+
+				if ((x != s_star_seed_tab[idx].x) || (y != s_star_seed_tab[idx].y)
+					|| (col != s_star_seed_tab[idx].col) || (s_star_seed_tab[idx].set != set))
+					LOGMASKED(LOG_DEBUG, "Mismatch: %d %d %d %d %d %d %d %d\n", x, y, col, set, s_star_seed_tab[idx].x,
+						s_star_seed_tab[idx].y, s_star_seed_tab[idx].col, s_star_seed_tab[idx].set);
+#endif
+				++idx;
+			}
+
+			// update the LFSR
+			if (i & 1)
+				i = (i >> 1) ^ feed;
+			else
+				i = (i >> 1);
+		}
+	}
+}
 
 /***************************************************************************
 
@@ -433,6 +531,8 @@ VIDEO_START_MEMBER(galaga_state,galaga)
 	save_item(NAME(m_stars_scrollx));
 	save_item(NAME(m_stars_scrolly));
 	save_item(NAME(m_galaga_gfxbank));
+
+	starfield_init();
 }