Initial commit

engines/glk/alan2/decode.cpp

@@ -0,0 +1,163 @@
+/* ScummVM - Graphic Adventure Engine
+ *
+ * ScummVM is the legal property of its developers, whose names
+ * are too numerous to list here. Please refer to the COPYRIGHT
+ * file distributed with this source distribution.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#include "glk/alan2/main.h"
+#include "glk/alan2/decode.h"
+
+namespace Glk {
+namespace Alan2 {
+
+/* Bit output */
+static int decodeBuffer;    /* Bits to be input */
+static int bitsToGo;        /* Bits still in buffer */
+static int garbageBits;     /* Bits past EOD */
+
+static int inputBit() {
+	int bit;
+
+	if (!bitsToGo) {      /* More bits available ? */
+		decodeBuffer = txtfil->readByte(); /* No, so get more */
+		if ((uint)decodeBuffer == EOD) {
+			garbageBits++;
+			if (garbageBits > VALUEBITS - 2)
+				syserr("Error in encoded data file.");
+		} else
+			bitsToGo = 8;     /* Another Char, 8 new bits */
+	}
+	bit = decodeBuffer & 1;   /* Get next bit */
+	decodeBuffer = decodeBuffer >> 1; /* and remove it */
+	bitsToGo--;
+	return bit;
+}
+
+
+/* Current state of decoding */
+
+static CodeValue value;         /* Currently seen code value */
+static CodeValue low, high;     /* Current code region */
+
+
+void startDecoding() {
+	int i;
+
+	bitsToGo = 0;
+	garbageBits = 0;
+
+	value = 0;
+	for (i = 0; i < VALUEBITS; i++)
+		value = 2 * value + inputBit();
+	low = 0;
+	high = TOPVALUE;
+}
+
+int decodeChar() {
+	long range;
+	int f;
+	int symbol;
+
+	range = (long)(high - low) + 1;
+	f = (((long)(value - low) + 1) * freq[0] - 1) / range;
+
+	/* Find the symbol */
+	for (symbol = 1; (int)freq[symbol] > f; symbol++);
+
+	high = low + range * freq[symbol - 1] / freq[0] - 1;
+	low = low + range * freq[symbol] / freq[0];
+
+	for (;;) {
+		if (high < HALF)
+			;
+		else if (low >= HALF) {
+			value = value - HALF;
+			low = low - HALF;
+			high = high - HALF;
+		} else if (low >= ONEQUARTER && high < THREEQUARTER) {
+			value = value - ONEQUARTER;
+			low = low - ONEQUARTER;
+			high = high - ONEQUARTER;
+		} else
+			break;
+
+		/* Scale up the range */
+		low = 2 * low;
+		high = 2 * high + 1;
+		value = 2 * value + inputBit();
+	}
+	return symbol - 1;
+}
+
+
+
+/* Structure for saved decode info */
+typedef struct DecodeInfo {
+	long fpos;
+	int buffer;
+	int bits;
+	CodeValue value;
+	CodeValue high;
+	CodeValue low;
+} DecodeInfo;
+
+
+/*======================================================================
+
+  pushDecode()
+
+  Save so much info about the decoding process so it is possible to
+  restore and continue later.
+
+ */
+void *pushDecode() {
+	DecodeInfo *info;
+
+	info = (DecodeInfo *) allocate(sizeof(DecodeInfo));
+	info->fpos = txtfil->pos();
+	info->buffer = decodeBuffer;
+	info->bits = bitsToGo;
+	info->value = value;
+	info->high = high;
+	info->low = low;
+	return (info);
+}
+
+
+/*======================================================================
+
+  popDecode()
+
+  Restore enough info about the decoding process so it is possible to
+  continue after having decoded something else.
+
+ */
+void popDecode(void *i) {
+	DecodeInfo *info = (DecodeInfo *) i;
+	fseek(txtfil, info->fpos, 0);
+	decodeBuffer = info->buffer;
+	bitsToGo = info->bits;
+	value = info->value;
+	high = info->high;
+	low = info->low;
+
+	free(info);
+}
+
+} // End of namespace Alan2
+} // End of namespace Glk