Demystified the "state" variable in LZMA code. Use the

word literal instead of char for better consistency.
There are still some names with _char instead of _literal
in lzma_optimum, these may be changed later.

Renamed length coder variables.

This commit doesn't change the program logic.

src/liblzma/lzma/lzma_common.h

@@ -31,8 +31,6 @@
 ///////////////
 
 #define REP_DISTANCES 4
-#define STATES 12
-#define LIT_STATES 7
 
 #define POS_SLOT_BITS 6
 #define DICT_LOG_SIZE_MAX 30
@@ -105,25 +103,62 @@
 // State //
 ///////////
 
-// Used for updating strm->data->state in both encoder and decoder.
+/// This enum is used to track which events have occurred most recently and
+/// in which order. This information is used to predict the next event.
+///
+/// Events:
+///  - Literal: One 8-bit byte
+///  - Match: Repeat a chunk of data at some distance
+///  - Long repeat: Multi-byte match at a recently seen distance
+///  - Short repeat: One-byte repeat at a recently seen distance
+///
+/// The event names are in from STATE_oldest_older_previous. REP means
+/// either short or long repeated match, and NONLIT means any non-literal.
+typedef enum {
+	STATE_LIT_LIT,
+	STATE_MATCH_LIT_LIT,
+	STATE_REP_LIT_LIT,
+	STATE_SHORTREP_LIT_LIT,
+	STATE_MATCH_LIT,
+	STATE_REP_LIT,
+	STATE_SHORTREP_LIT,
+	STATE_LIT_MATCH,
+	STATE_LIT_LONGREP,
+	STATE_LIT_SHORTREP,
+	STATE_NONLIT_MATCH,
+	STATE_NONLIT_REP,
+} lzma_lzma_state;
 
-#define update_char(index) \
-	index = ((index) < 4 \
-			? 0 \
-			: ((index) < 10 \
-				? (index) - 3 \
-				: (index) - 6))
 
-#define update_match(index) \
+/// Total number of states
-	index = ((index) < LIT_STATES ? 7 : 10)
+#define STATES 12
 
-#define update_rep(index) \
+/// The lowest 7 states indicate that the previous state was a literal.
-	index = ((index) < LIT_STATES ? 8 : 11)
+#define LIT_STATES 7
 
-#define update_short_rep(index) \
-	index = ((index) < LIT_STATES ? 9 : 11)
 
-#define is_char_state(index) \
+/// Indicate that the latest state was a literal.
-	((index) < LIT_STATES)
+#define update_literal(state) \
+	state = ((state) <= STATE_SHORTREP_LIT_LIT \
+			? STATE_LIT_LIT \
+			: ((state) <= STATE_LIT_SHORTREP \
+				? (state) - 3 \
+				: (state) - 6))
+
+/// Indicate that the latest state was a match.
+#define update_match(state) \
+	state = ((state) < LIT_STATES ? STATE_LIT_MATCH : STATE_NONLIT_MATCH)
+
+/// Indicate that the latest state was a long repeated match.
+#define update_long_rep(state) \
+	state = ((state) < LIT_STATES ? STATE_LIT_LONGREP : STATE_NONLIT_REP)
+
+/// Indicate that the latest state was a short match.
+#define update_short_rep(state) \
+	state = ((state) < LIT_STATES ? STATE_LIT_SHORTREP : STATE_NONLIT_REP)
+
+/// Test if the previous state was a literal.
+#define is_literal_state(state) \
+	((state) < LIT_STATES)
 
 #endif

src/liblzma/lzma/lzma_decoder.c

@@ -106,7 +106,7 @@ struct lzma_coder_s {
 	lzma_range_decoder rc;
 
 	// State
-	uint32_t state;
+	lzma_lzma_state state;
 	uint32_t rep0;      ///< Distance of the latest match
 	uint32_t rep1;      ///< Distance of second latest match
 	uint32_t rep2;      ///< Distance of third latest match
@@ -143,10 +143,10 @@ struct lzma_coder_s {
 	probability pos_align_decoder[1 << ALIGN_BITS];
 
 	/// Length of a match
-	lzma_length_decoder len_decoder;
+	lzma_length_decoder match_len_decoder;
 
 	/// Length of a repeated match.
-	lzma_length_decoder rep_match_len_decoder;
+	lzma_length_decoder rep_len_decoder;
 
 	/// True when we have produced at least one byte of output since the
 	/// beginning of the stream or the latest flush marker.
@@ -179,7 +179,7 @@ decode_dummy(const lzma_coder *restrict coder,
 					coder->literal_coder, now_pos, lz_get_byte(coder->lz, 0));
 			uint32_t symbol = 1;
 
-			if (is_char_state(state)) {
+			if (is_literal_state(state)) {
 				// Decode literal without match byte.
 				do {
 					if_bit_0(subcoder[symbol]) {
@@ -222,8 +222,7 @@ decode_dummy(const lzma_coder *restrict coder,
 
 		if_bit_0(coder->is_rep[state]) {
 			update_bit_0_dummy();
-			length_decode_dummy(len, coder->len_decoder, pos_state);
+			length_decode_dummy(len, coder->match_len_decoder, pos_state);
-			update_match(state);
 
 			const uint32_t len_to_pos_state = get_len_to_pos_state(len);
 			uint32_t pos_slot = 0;
@@ -291,7 +290,7 @@ decode_dummy(const lzma_coder *restrict coder,
 				}
 			}
 
-			length_decode_dummy(len, coder->rep_match_len_decoder, pos_state);
+			length_decode_dummy(len, coder->rep_len_decoder, pos_state);
 		}
 	} while (0);
 
@@ -364,7 +363,7 @@ decode_real(lzma_coder *restrict coder, const uint8_t *restrict in,
 					now_pos, lz_get_byte(coder->lz, 0));
 			uint32_t symbol = 1;
 
-			if (is_char_state(state)) {
+			if (is_literal_state(state)) {
 				// Decode literal without match byte.
 				do {
 					if_bit_0(subcoder[symbol]) {
@@ -408,7 +407,7 @@ decode_real(lzma_coder *restrict coder, const uint8_t *restrict in,
 			// decoder state, and start a new decoding loop.
 			coder->lz.dict[coder->lz.pos++] = (uint8_t)(symbol);
 			++now_pos;
-			update_char(state);
+			update_literal(state);
 			has_produced_output = true;
 			continue;
 		}
@@ -429,7 +428,7 @@ decode_real(lzma_coder *restrict coder, const uint8_t *restrict in,
 			// the value to distance.
 
 			// Decode the length of the match.
-			length_decode(len, coder->len_decoder, pos_state);
+			length_decode(len, coder->match_len_decoder, pos_state);
 
 			update_match(state);
 
@@ -594,10 +593,10 @@ decode_real(lzma_coder *restrict coder, const uint8_t *restrict in,
 				rep0 = distance;
 			}
 
-			// Decode the length of the repeated match.
+			update_long_rep(state);
-			length_decode(len, coder->rep_match_len_decoder, pos_state);
 
-			update_rep(state);
+			// Decode the length of the repeated match.
+			length_decode(len, coder->rep_len_decoder, pos_state);
 		}
 
 
@@ -746,23 +745,25 @@ lzma_lzma_decoder_init(lzma_next_coder *next, lzma_allocator *allocator,
 
 	// Len decoders (also bit/bittree)
 	const uint32_t num_pos_states = 1 << next->coder->pos_bits;
-	bit_reset(next->coder->len_decoder.choice);
+	bit_reset(next->coder->match_len_decoder.choice);
-	bit_reset(next->coder->len_decoder.choice2);
+	bit_reset(next->coder->match_len_decoder.choice2);
-	bit_reset(next->coder->rep_match_len_decoder.choice);
+	bit_reset(next->coder->rep_len_decoder.choice);
-	bit_reset(next->coder->rep_match_len_decoder.choice2);
+	bit_reset(next->coder->rep_len_decoder.choice2);
 
 	for (uint32_t pos_state = 0; pos_state < num_pos_states; ++pos_state) {
-		bittree_reset(next->coder->len_decoder.low[pos_state], LEN_LOW_BITS);
+		bittree_reset(next->coder->match_len_decoder.low[pos_state],
-		bittree_reset(next->coder->len_decoder.mid[pos_state], LEN_MID_BITS);
-
-		bittree_reset(next->coder->rep_match_len_decoder.low[pos_state],
 				LEN_LOW_BITS);
-		bittree_reset(next->coder->rep_match_len_decoder.mid[pos_state],
+		bittree_reset(next->coder->match_len_decoder.mid[pos_state],
+				LEN_MID_BITS);
+
+		bittree_reset(next->coder->rep_len_decoder.low[pos_state],
+				LEN_LOW_BITS);
+		bittree_reset(next->coder->rep_len_decoder.mid[pos_state],
 				LEN_MID_BITS);
 	}
 
-	bittree_reset(next->coder->len_decoder.high, LEN_HIGH_BITS);
+	bittree_reset(next->coder->match_len_decoder.high, LEN_HIGH_BITS);
-	bittree_reset(next->coder->rep_match_len_decoder.high, LEN_HIGH_BITS);
+	bittree_reset(next->coder->rep_len_decoder.high, LEN_HIGH_BITS);
 
 	next->coder->has_produced_output = false;
 

src/liblzma/lzma/lzma_encoder.c

@@ -170,7 +170,7 @@ lzma_lzma_encode(lzma_coder *coder, uint8_t *restrict out,
 			lzma_read_match_distances(coder, &len, &num_distance_pairs);
 
 			bit_encode_0(coder->is_match[coder->state][0]);
-			update_char(coder->state);
+			update_literal(coder->state);
 
 			const uint8_t cur_byte = coder->lz.buffer[
 					coder->lz.read_pos - coder->additional_offset];
@@ -244,7 +244,7 @@ lzma_lzma_encode(lzma_coder *coder, uint8_t *restrict out,
 			probability *subcoder = literal_get_subcoder(coder->literal_coder,
 					coder->now_pos, coder->previous_byte);
 
-			if (is_char_state(coder->state)) {
+			if (is_literal_state(coder->state)) {
 				literal_encode(subcoder, cur_byte);
 			} else {
 				const uint8_t match_byte = coder->lz.buffer[
@@ -254,7 +254,7 @@ lzma_lzma_encode(lzma_coder *coder, uint8_t *restrict out,
 				literal_encode_matched(subcoder, match_byte, cur_byte);
 			}
 
-			update_char(coder->state);
+			update_literal(coder->state);
 			coder->previous_byte = cur_byte;
 
 		} else {
@@ -294,16 +294,16 @@ lzma_lzma_encode(lzma_coder *coder, uint8_t *restrict out,
 				if (len == 1) {
 					update_short_rep(coder->state);
 				} else {
-					length_encode(coder->rep_match_len_encoder,
+					length_encode(coder->rep_len_encoder,
 							len - MATCH_MIN_LEN, pos_state,
 							best_compression);
-					update_rep(coder->state);
+					update_long_rep(coder->state);
 				}
 
 			} else {
 				bit_encode_0(coder->is_rep[coder->state]);
 				update_match(coder->state);
-				length_encode(coder->len_encoder, len - MATCH_MIN_LEN,
+				length_encode(coder->match_len_encoder, len - MATCH_MIN_LEN,
 						pos_state, best_compression);
 				pos -= REP_DISTANCES;
 
@@ -364,7 +364,7 @@ lzma_lzma_encode(lzma_coder *coder, uint8_t *restrict out,
 
 			const uint32_t len = coder->lz.sequence == SEQ_FLUSH
 					? LEN_SPECIAL_FLUSH : LEN_SPECIAL_EOPM;
-			length_encode(coder->len_encoder, len - MATCH_MIN_LEN,
+			length_encode(coder->match_len_encoder, len - MATCH_MIN_LEN,
 					pos_state, best_compression);
 
 			const uint32_t pos_slot = (1 << POS_SLOT_BITS) - 1;

src/liblzma/lzma/lzma_encoder_getoptimum.c

@@ -63,7 +63,7 @@ do { \
 #define get_rep_price(price_target, rep_index, len, state, pos_state) \
 do { \
 	get_pure_rep_price(price_target, rep_index, state, pos_state); \
-	price_target += length_get_price(coder->rep_match_len_encoder, \
+	price_target += length_get_price(coder->rep_len_encoder, \
 			(len) - MATCH_MIN_LEN, pos_state); \
 } while (0)
 
@@ -80,7 +80,7 @@ do { \
 				+ align_prices[(pos) & ALIGN_MASK]; \
 	} \
 	price_target += length_get_price( \
-			coder->len_encoder, (len) - MATCH_MIN_LEN, pos_state); \
+			coder->match_len_encoder, (len) - MATCH_MIN_LEN, pos_state); \
 } while (0)
 
 
@@ -368,7 +368,7 @@ lzma_get_optimum(lzma_coder *restrict coder,
 			+ literal_get_price(
 				literal_get_subcoder(coder->literal_coder,
 					position, coder->previous_byte),
-				!is_char_state(coder->state), match_byte, current_byte);
+				!is_literal_state(coder->state), match_byte, current_byte);
 
 	make_as_char(coder->optimum[1]);
 
@@ -424,7 +424,7 @@ lzma_get_optimum(lzma_coder *restrict coder,
 		do {
 			const uint32_t cur_and_len_price = price
 					+ length_get_price(
-					coder->rep_match_len_encoder,
+					coder->rep_len_encoder,
 					rep_len - 2, pos_state);
 
 			if (cur_and_len_price < coder->optimum[rep_len].price) {
@@ -513,7 +513,7 @@ lzma_get_optimum(lzma_coder *restrict coder,
 			state = coder->optimum[coder->optimum[cur].pos_prev_2].state;
 
 			if (coder->optimum[cur].back_prev_2 < REP_DISTANCES)
-				update_rep(state);
+				update_long_rep(state);
 			else
 				update_match(state);
 
@@ -521,7 +521,7 @@ lzma_get_optimum(lzma_coder *restrict coder,
 			state = coder->optimum[pos_prev].state;
 		}
 
-		update_char(state);
+		update_literal(state);
 
 	} else {
 		state = coder->optimum[pos_prev].state;
@@ -531,17 +531,17 @@ lzma_get_optimum(lzma_coder *restrict coder,
 		if (is_short_rep(coder->optimum[cur]))
 			update_short_rep(state);
 		else
-			update_char(state);
+			update_literal(state);
 	} else {
 		uint32_t pos;
 		if (coder->optimum[cur].prev_1_is_char && coder->optimum[cur].prev_2) {
 			pos_prev = coder->optimum[cur].pos_prev_2;
 			pos = coder->optimum[cur].back_prev_2;
-			update_rep(state);
+			update_long_rep(state);
 		} else {
 			pos = coder->optimum[cur].back_prev;
 			if (pos < REP_DISTANCES)
-				update_rep(state);
+				update_long_rep(state);
 			else
 				update_match(state);
 		}
@@ -582,7 +582,7 @@ lzma_get_optimum(lzma_coder *restrict coder,
 			+ literal_get_price(
 				literal_get_subcoder(coder->literal_coder,
 					position, buf[-1]),
-        		!is_char_state(state), match_byte, current_byte);
+        		!is_literal_state(state), match_byte, current_byte);
 
 	bool next_is_char = false;
 
@@ -638,7 +638,7 @@ lzma_get_optimum(lzma_coder *restrict coder,
 
 		if (len_test_2 >= 2) {
 			uint32_t state_2 = state;
-			update_char(state_2);
+			update_literal(state_2);
 
 			const uint32_t pos_state_next = (position + 1) & pos_mask;
 			const uint32_t next_rep_match_price = cur_and_1_price
@@ -689,7 +689,7 @@ lzma_get_optimum(lzma_coder *restrict coder,
 
 		do {
 			const uint32_t cur_and_len_price = price
-					+ length_get_price(coder->rep_match_len_encoder,
+					+ length_get_price(coder->rep_len_encoder,
 							len_test - 2, pos_state);
 
 			if (cur_and_len_price < coder->optimum[cur + len_test].price) {
@@ -717,12 +717,12 @@ lzma_get_optimum(lzma_coder *restrict coder,
 
 		if (len_test_2 >= 2) {
 			uint32_t state_2 = state;
-			update_rep(state_2);
+			update_long_rep(state_2);
 
 			uint32_t pos_state_next = (position + len_test) & pos_mask;
 
 			const uint32_t cur_and_len_char_price = price
-					+ length_get_price(coder->rep_match_len_encoder,
+					+ length_get_price(coder->rep_len_encoder,
 						len_test - 2, pos_state)
 					+ bit_get_price_0(coder->is_match[state_2][pos_state_next])
 					+ literal_get_price(
@@ -730,7 +730,7 @@ lzma_get_optimum(lzma_coder *restrict coder,
 							position + len_test, buf[len_test - 1]),
 						true, *(buf + len_test - back_offset), buf[len_test]);
 
-			update_char(state_2);
+			update_literal(state_2);
 
 			pos_state_next = (position + len_test + 1) & pos_mask;
 
@@ -801,7 +801,7 @@ lzma_get_optimum(lzma_coder *restrict coder,
 						len_to_pos_state][pos_slot]
 						+ align_prices[cur_back & ALIGN_MASK];
 
-			cur_and_len_price += length_get_price(coder->len_encoder,
+			cur_and_len_price += length_get_price(coder->match_len_encoder,
 					len_test - MATCH_MIN_LEN, pos_state);
 
 			if (cur_and_len_price < coder->optimum[cur + len_test].price) {
@@ -843,7 +843,7 @@ lzma_get_optimum(lzma_coder *restrict coder,
 								*(buf + len_test - back_offset),
 								buf[len_test]);
 
-					update_char(state_2);
+					update_literal(state_2);
 					pos_state_next = (pos_state_next + 1) & pos_mask;
 
 					const uint32_t next_rep_match_price

src/liblzma/lzma/lzma_encoder_init.c

@@ -174,10 +174,11 @@ lzma_lzma_encoder_init(lzma_next_coder *next, lzma_allocator *allocator,
 	bittree_reset(next->coder->pos_align_encoder, ALIGN_BITS);
 
 	// Length encoders
-	length_encoder_reset(&next->coder->len_encoder, 1U << options->pos_bits,
+	length_encoder_reset(&next->coder->match_len_encoder,
+			1U << options->pos_bits,
 			options->fast_bytes + 1 - MATCH_MIN_LEN);
 
-	length_encoder_reset(&next->coder->rep_match_len_encoder,
+	length_encoder_reset(&next->coder->rep_len_encoder,
 			1U << options->pos_bits,
 			next->coder->fast_bytes + 1 - MATCH_MIN_LEN);
 

src/liblzma/lzma/lzma_encoder_private.h

@@ -60,7 +60,7 @@ typedef struct {
 
 
 typedef struct {
-	uint32_t state;
+	lzma_lzma_state state;
 
 	bool prev_1_is_char;
 	bool prev_2;
@@ -88,7 +88,7 @@ struct lzma_coder_s {
 	lzma_range_encoder rc;
 
 	// State
-	uint32_t state;
+	lzma_lzma_state state;
 	uint8_t previous_byte;
 	uint32_t rep_distances[REP_DISTANCES];
 
@@ -117,8 +117,8 @@ struct lzma_coder_s {
 	probability pos_align_encoder[1 << ALIGN_BITS];
 
 	// Length encoders
-	lzma_length_encoder len_encoder;
+	lzma_length_encoder match_len_encoder;
-	lzma_length_encoder rep_match_len_encoder;
+	lzma_length_encoder rep_len_encoder;
 
 	// Optimal
 	lzma_optimal optimum[OPTS];