liblzma: Add rough support for output-size-limited encoding in LZMA1.

With this it is possible to encode LZMA1 data without EOPM so that
the encoder will encode as much input as it can without exceeding
the specified output size limit. The resulting LZMA1 stream will
be a normal LZMA1 stream without EOPM. The actual uncompressed size
will be available to the caller via the uncomp_size pointer.

One missing thing is that the LZMA layer doesn't inform the LZ layer
when the encoding is finished and thus the LZ may read more input
when it won't be used. However, this doesn't matter if encoding is
done with a single call (which is the planned use case for now).
For proper multi-call encoding this should be improved.

This commit only adds the functionality for internal use.
Nothing uses it yet.

src/liblzma/common/common.h

@@ -172,6 +172,16 @@ struct lzma_next_coder_s {
 	lzma_ret (*update)(void *coder, const lzma_allocator *allocator,
 			const lzma_filter *filters,
 			const lzma_filter *reversed_filters);
+
+	/// Set how many bytes of output this coder may produce at maximum.
+	/// On success LZMA_OK must be returned.
+	/// If the filter chain as a whole cannot support this feature,
+	/// this must return LZMA_OPTIONS_ERROR.
+	/// If no input has been given to the coder and the requested limit
+	/// is too small, this must return LZMA_BUF_ERROR. If input has been
+	/// seen, LZMA_OK is allowed too.
+	lzma_ret (*set_out_limit)(void *coder, uint64_t *uncomp_size,
+			uint64_t out_limit);
 };
 
 
@@ -187,6 +197,7 @@ struct lzma_next_coder_s {
 		.get_check = NULL, \
 		.memconfig = NULL, \
 		.update = NULL, \
+		.set_out_limit = NULL, \
 	}
 
 

src/liblzma/lz/lz_encoder.c

@@ -521,6 +521,21 @@ lz_encoder_update(void *coder_ptr, const lzma_allocator *allocator,
 }
 
 
+static lzma_ret
+lz_encoder_set_out_limit(void *coder_ptr, uint64_t *uncomp_size,
+		uint64_t out_limit)
+{
+	lzma_coder *coder = coder_ptr;
+
+	// This is supported only if there are no other filters chained.
+	if (coder->next.code == NULL && coder->lz.set_out_limit != NULL)
+		return coder->lz.set_out_limit(
+				coder->lz.coder, uncomp_size, out_limit);
+
+	return LZMA_OPTIONS_ERROR;
+}
+
+
 extern lzma_ret
 lzma_lz_encoder_init(lzma_next_coder *next, const lzma_allocator *allocator,
 		const lzma_filter_info *filters,
@@ -544,6 +559,7 @@ lzma_lz_encoder_init(lzma_next_coder *next, const lzma_allocator *allocator,
 		next->code = &lz_encode;
 		next->end = &lz_encoder_end;
 		next->update = &lz_encoder_update;
+		next->set_out_limit = &lz_encoder_set_out_limit;
 
 		coder->lz.coder = NULL;
 		coder->lz.code = NULL;

src/liblzma/lz/lz_encoder.h

@@ -204,6 +204,10 @@ typedef struct {
 	/// Update the options in the middle of the encoding.
 	lzma_ret (*options_update)(void *coder, const lzma_filter *filter);
 
+	/// Set maximum allowed output size
+	lzma_ret (*set_out_limit)(void *coder, uint64_t *uncomp_size,
+			uint64_t out_limit);
+
 } lzma_lz_encoder;
 
 

src/liblzma/lzma/lzma_encoder.c

@@ -268,6 +268,7 @@ static bool
 encode_init(lzma_lzma1_encoder *coder, lzma_mf *mf)
 {
 	assert(mf_position(mf) == 0);
+	assert(coder->uncomp_size == 0);
 
 	if (mf->read_pos == mf->read_limit) {
 		if (mf->action == LZMA_RUN)
@@ -283,6 +284,7 @@ encode_init(lzma_lzma1_encoder *coder, lzma_mf *mf)
 		mf->read_ahead = 0;
 		rc_bit(&coder->rc, &coder->is_match[0][0], 0);
 		rc_bittree(&coder->rc, coder->literal[0], 8, mf->buffer[0]);
+		++coder->uncomp_size;
 	}
 
 	// Initialization is done (except if empty file).
@@ -317,21 +319,28 @@ lzma_lzma_encode(lzma_lzma1_encoder *restrict coder, lzma_mf *restrict mf,
 	if (!coder->is_initialized && !encode_init(coder, mf))
 		return LZMA_OK;
 
-	// Get the lowest bits of the uncompressed offset from the LZ layer.
-	uint32_t position = mf_position(mf);
+	// Encode pending output bytes from the range encoder.
+	// At the start of the stream, encode_init() encodes one literal.
+	// Later there can be pending output only with LZMA1 because LZMA2
+	// ensures that there is always enough output space. Thus when using
+	// LZMA2, rc_encode() calls in this function will always return false.
+	if (rc_encode(&coder->rc, out, out_pos, out_size)) {
+		// We don't get here with LZMA2.
+		assert(limit == UINT32_MAX);
+		return LZMA_OK;
+	}
+
+	// If the range encoder was flushed in an earlier call to this
+	// function but there wasn't enough output buffer space, those
+	// bytes would have now been encoded by the above rc_encode() call
+	// and the stream has now been finished. This can only happen with
+	// LZMA1 as LZMA2 always provides enough output buffer space.
+	if (coder->is_flushed) {
+		assert(limit == UINT32_MAX);
+		return LZMA_STREAM_END;
+	}
 
 	while (true) {
-		// Encode pending bits, if any. Calling this before encoding
-		// the next symbol is needed only with plain LZMA, since
-		// LZMA2 always provides big enough buffer to flush
-		// everything out from the range encoder. For the same reason,
-		// rc_encode() never returns true when this function is used
-		// as part of LZMA2 encoder.
-		if (rc_encode(&coder->rc, out, out_pos, out_size)) {
-			assert(limit == UINT32_MAX);
-			return LZMA_OK;
-		}
-
 		// With LZMA2 we need to take care that compressed size of
 		// a chunk doesn't get too big.
 		// FIXME? Check if this could be improved.
@@ -365,37 +374,64 @@ lzma_lzma_encode(lzma_lzma1_encoder *restrict coder, lzma_mf *restrict mf,
 		if (coder->fast_mode)
 			lzma_lzma_optimum_fast(coder, mf, &back, &len);
 		else
-			lzma_lzma_optimum_normal(
-					coder, mf, &back, &len, position);
+			lzma_lzma_optimum_normal(coder, mf, &back, &len,
+					(uint32_t)(coder->uncomp_size));
 
-		encode_symbol(coder, mf, back, len, position);
+		encode_symbol(coder, mf, back, len,
+				(uint32_t)(coder->uncomp_size));
 
-		position += len;
-	}
+		// If output size limiting is active (out_limit != 0), check
+		// if encoding this LZMA symbol would make the output size
+		// exceed the specified limit.
+		if (coder->out_limit != 0 && rc_encode_dummy(
+				&coder->rc, coder->out_limit)) {
+			// The most recent LZMA symbol would make the output
+			// too big. Throw it away.
+			rc_forget(&coder->rc);
 
-	if (!coder->is_flushed) {
-		coder->is_flushed = true;
+			// FIXME: Tell the LZ layer to not read more input as
+			// it would be waste of time. This doesn't matter if
+			// output-size-limited encoding is done with a single
+			// call though.
 
-		// We don't support encoding plain LZMA streams without EOPM,
-		// and LZMA2 doesn't use EOPM at LZMA level.
-		if (limit == UINT32_MAX)
-			encode_eopm(coder, position);
+			break;
+		}
 
-		// Flush the remaining bytes from the range encoder.
-		rc_flush(&coder->rc);
+		// This symbol will be encoded so update the uncompressed size.
+		coder->uncomp_size += len;
 
-		// Copy the remaining bytes to the output buffer. If there
-		// isn't enough output space, we will copy out the remaining
-		// bytes on the next call to this function by using
-		// the rc_encode() call in the encoding loop above.
+		// Encode the LZMA symbol.
 		if (rc_encode(&coder->rc, out, out_pos, out_size)) {
+			// Once again, this can only happen with LZMA1.
 			assert(limit == UINT32_MAX);
 			return LZMA_OK;
 		}
 	}
 
-	// Make it ready for the next LZMA2 chunk.
-	coder->is_flushed = false;
+	// Make the uncompressed size available to the application.
+	if (coder->uncomp_size_ptr != NULL)
+		*coder->uncomp_size_ptr = coder->uncomp_size;
+
+	// LZMA2 doesn't use EOPM at LZMA level.
+	//
+	// Plain LZMA streams without EOPM aren't supported except when
+	// output size limiting is enabled.
+	if (limit == UINT32_MAX && coder->out_limit == 0)
+		encode_eopm(coder, (uint32_t)(coder->uncomp_size));
+
+	// Flush the remaining bytes from the range encoder.
+	rc_flush(&coder->rc);
+
+	// Copy the remaining bytes to the output buffer. If there
+	// isn't enough output space, we will copy out the remaining
+	// bytes on the next call to this function.
+	if (rc_encode(&coder->rc, out, out_pos, out_size)) {
+		// This cannot happen with LZMA2.
+		assert(limit == UINT32_MAX);
+
+		coder->is_flushed = true;
+		return LZMA_OK;
+	}
 
 	return LZMA_STREAM_END;
 }
@@ -414,6 +450,22 @@ lzma_encode(void *coder, lzma_mf *restrict mf,
 }
 
 
+static lzma_ret
+lzma_lzma_set_out_limit(
+		void *coder_ptr, uint64_t *uncomp_size, uint64_t out_limit)
+{
+	// Minimum output size is 5 bytes but that cannot hold any output
+	// so we use 6 bytes.
+	if (out_limit < 6)
+		return LZMA_BUF_ERROR;
+
+	lzma_lzma1_encoder *coder = coder_ptr;
+	coder->out_limit = out_limit;
+	coder->uncomp_size_ptr = uncomp_size;
+	return LZMA_OK;
+}
+
+
 ////////////////////
 // Initialization //
 ////////////////////
@@ -598,6 +650,10 @@ lzma_lzma_encoder_create(void **coder_ptr,
 	coder->is_initialized = options->preset_dict != NULL
 			&& options->preset_dict_size > 0;
 	coder->is_flushed = false;
+	coder->uncomp_size = 0;
+
+	// Output size limitting is disabled by default.
+	coder->out_limit = 0;
 
 	set_lz_options(lz_options, options);
 
@@ -610,6 +666,7 @@ lzma_encoder_init(lzma_lz_encoder *lz, const lzma_allocator *allocator,
 		const void *options, lzma_lz_options *lz_options)
 {
 	lz->code = &lzma_encode;
+	lz->set_out_limit = &lzma_lzma_set_out_limit;
 	return lzma_lzma_encoder_create(
 			&lz->coder, allocator, options, lz_options);
 }

src/liblzma/lzma/lzma_encoder_private.h

@@ -72,6 +72,18 @@ struct lzma_lzma1_encoder_s {
 	/// Range encoder
 	lzma_range_encoder rc;
 
+	/// Uncompressed size (doesn't include possible preset dictionary)
+	uint64_t uncomp_size;
+
+	/// If non-zero, produce at most this much output.
+	/// Some input may then be missing from the output.
+	uint64_t out_limit;
+
+	/// If the above out_limit is non-zero, *uncomp_size_ptr is set to
+	/// the amount of uncompressed data that we were able to fit
+	/// in the output buffer.
+	uint64_t *uncomp_size_ptr;
+
 	/// State
 	lzma_lzma_state state;
 

src/liblzma/rangecoder/range_encoder.h

@@ -30,6 +30,9 @@ typedef struct {
 	uint32_t range;
 	uint8_t cache;
 
+	/// Number of bytes written out by rc_encode() -> rc_shift_low()
+	uint64_t out_total;
+
 	/// Number of symbols in the tables
 	size_t count;
 
@@ -58,11 +61,21 @@ rc_reset(lzma_range_encoder *rc)
 	rc->cache_size = 1;
 	rc->range = UINT32_MAX;
 	rc->cache = 0;
+	rc->out_total = 0;
 	rc->count = 0;
 	rc->pos = 0;
 }
 
 
+static inline void
+rc_forget(lzma_range_encoder *rc)
+{
+	// This must not be called when rc_encode() is partially done.
+	assert(rc->pos == 0);
+	rc->count = 0;
+}
+
+
 static inline void
 rc_bit(lzma_range_encoder *rc, probability *prob, uint32_t bit)
 {
@@ -132,6 +145,7 @@ rc_shift_low(lzma_range_encoder *rc,
 
 			out[*out_pos] = rc->cache + (uint8_t)(rc->low >> 32);
 			++*out_pos;
+			++rc->out_total;
 			rc->cache = 0xFF;
 
 		} while (--rc->cache_size != 0);
@@ -146,6 +160,31 @@ rc_shift_low(lzma_range_encoder *rc,
 }
 
 
+static inline bool
+rc_shift_low_dummy(uint64_t *low, uint64_t *cache_size, uint8_t *cache,
+		size_t *out_pos, size_t out_size)
+{
+	if ((uint32_t)(*low) < (uint32_t)(0xFF000000)
+			|| (uint32_t)(*low >> 32) != 0) {
+		do {
+			if (*out_pos == out_size)
+				return true;
+
+			++*out_pos;
+			*cache = 0xFF;
+
+		} while (--*cache_size != 0);
+
+		*cache = (*low >> 24) & 0xFF;
+	}
+
+	++*cache_size;
+	*low = (*low & 0x00FFFFFF) << RC_SHIFT_BITS;
+
+	return false;
+}
+
+
 static inline bool
 rc_encode(lzma_range_encoder *rc,
 		uint8_t *out, size_t *out_pos, size_t out_size)
@@ -222,6 +261,78 @@ rc_encode(lzma_range_encoder *rc,
 }
 
 
+static inline bool
+rc_encode_dummy(const lzma_range_encoder *rc, size_t out_size)
+{
+	assert(rc->count <= RC_SYMBOLS_MAX);
+
+	uint64_t low = rc->low;
+	uint64_t cache_size = rc->cache_size;
+	uint32_t range = rc->range;
+	uint8_t cache = rc->cache;
+	uint64_t out_pos = rc->out_total;
+
+	size_t pos = rc->pos;
+
+	while (pos < rc->count) {
+		// Normalize
+		if (range < RC_TOP_VALUE) {
+			if (rc_shift_low_dummy(&low, &cache_size, &cache,
+					&out_pos, out_size))
+				return true;
+
+			range <<= RC_SHIFT_BITS;
+		}
+
+		// Encode a bit
+		switch (rc->symbols[pos]) {
+		case RC_BIT_0: {
+			probability prob = *rc->probs[pos];
+			range = (range >> RC_BIT_MODEL_TOTAL_BITS)
+					* prob;
+			break;
+		}
+
+		case RC_BIT_1: {
+			probability prob = *rc->probs[pos];
+			const uint32_t bound = prob * (range
+					>> RC_BIT_MODEL_TOTAL_BITS);
+			low += bound;
+			range -= bound;
+			break;
+		}
+
+		case RC_DIRECT_0:
+			range >>= 1;
+			break;
+
+		case RC_DIRECT_1:
+			range >>= 1;
+			low += range;
+			break;
+
+		case RC_FLUSH:
+		default:
+			assert(0);
+			break;
+		}
+
+		++pos;
+	}
+
+	// Flush the last bytes. This isn't in rc->symbols[] so we do
+	// it after the above loop to take into account the size of
+	// the flushing that will be done at the end of the stream.
+	for (pos = 0; pos < 5; ++pos) {
+		if (rc_shift_low_dummy(&low, &cache_size,
+				&cache, &out_pos, out_size))
+			return true;
+	}
+
+	return false;
+}
+
+
 static inline uint64_t
 rc_pending(const lzma_range_encoder *rc)
 {