Add stripe-based RLE encoder

legal_screen_nametable compresses to 294 bytes, vs 221 with Konami RLE.
If twoplayer wasn't also including the two-player playfield, it would be
a good choice.

nes.mk

@@ -37,6 +37,11 @@ build/%.rle: % rle-enc.awk | build
 	# basenc isn't as widely available as xxd since it was added in
 	# coreutils 8.31
 	xxd -c1 -p $< | LC_ALL=C awk -f rle-enc.awk | xxd -r -p > $@
+build/%.rle.stripe: % stripe-enc.awk | build
+	# 'basenc --base16 -w2' and 'basenc --base16 -d' would also work, but
+	# basenc isn't as widely available as xxd since it was added in
+	# coreutils 8.31
+	xxd -c1 -p $< | LC_ALL=C awk -f stripe-enc.awk | xxd -r -p > $@
 
 build/%.s: %.bin %.info Makefile | build
 	# Strip off the first two lines of header, which contain variable

stripe-enc.awk

@@ -0,0 +1,71 @@
+# Encode input using Stripe RLE compression (Tetris variation)
+#
+# Intended to be run with LC_ALL=C and 'basenc --base16 -w2' as input and
+# 'basenc --base16 -d' on output
+
+# variable addr can be set to change the destination PPU address
+# input is pairs of hex digits, each followed by newline
+# output is chunks of hex digits, with newlines between each chunk
+
+BEGIN {
+	runlen=0
+	if (addr == "") {
+		addr=0x2000
+	}
+}
+
+runlen != 0 && lastbyte == $0 {
+	runlen++
+	if (runlen == 0x40) {
+		printf("%04X%02X%s\n", addr, 0x40, lastbyte)
+		addr+=runlen
+		runlen=0
+	}
+	next
+}
+
+runlen != 0 && lastbyte != $0 {
+	printf("%04X%02X%s\n", addr, 0x40+runlen, lastbyte)
+	addr+=runlen
+	runlen=0
+}
+
+{
+	literal=$0
+	runlen=1
+	lastbyte=$0
+	while (getline) {
+		literal=literal $0
+		if (lastbyte != $0) {
+			runlen=1
+		} else {
+			runlen++
+			# Optimal is 4 if next stripe is a run, and 7 if
+			# literal. Assume a literal typically follows a run.
+			if ((runlen == 4 && runlen*2 == length(literal)) \
+			     || runlen == 7) {
+				literal=substr(literal, 1, length(literal)-runlen*2)
+				if (literal == "") {
+					next
+				} else {
+					break
+				}
+			}
+		}
+		if (length(literal)/2 == 0x40) {
+			runlen=0
+			break
+		}
+		lastbyte=$0
+	}
+	printf("%04X%02X%s\n", addr, (length(literal)/2)%0x40, literal)
+	addr+=length(literal)/2
+}
+
+END {
+	if (runlen != 0) {
+		printf("%04X%02X%s\n", addr, 0x40+runlen, lastbyte)
+		addr+=runlen
+	}
+	print "FF"
+}

tetris-PRG.info

@@ -972,7 +972,7 @@ LABEL { ADDR $AA8A; NAME "copyCurrentScrollAndCtrlToPPU"; };
 LABEL { ADDR $AA98; NAME "bulkCopyToPpu"; COMMENT "Return address should contain data address; ret addr+2 is actual return. $80-$FF end. $4C recurse, 2 byte cpu addr. $60 unrecurse. Otherwise NES data stripe, 2 byte ppu addr, 1 byte length, data. length controls RLE and write direction"; };
 LABEL { ADDR $AA9E; NAME "@processStripe"; };
 LABEL { ADDR $AAB5; NAME "@setDirection"; COMMENT "Bit 7 of length: 0-right, 1-down"; };
-LABEL { ADDR $AAC2; NAME "@checkForZeroLength"; COMMENT "Bit 6 of length: 0-literal, 1-run. If bits 0-5 are zero, length is 128"; };
+LABEL { ADDR $AAC2; NAME "@checkForZeroLength"; COMMENT "Bit 6 of length: 0-literal, 1-run. If bits 0-5 are zero, length is 64"; };
 LABEL { ADDR $AAC7; NAME "@clearLengthFlagBits"; };
 LABEL { ADDR $AACA; NAME "@copyByteLoop"; };
 LABEL { ADDR $AACD; NAME "@copyByte"; };