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.

1 files changed, 92 insertions, 35 deletions

diff --git a/src/liblzma/lzma/lzma_encoder.c b/src/liblzma/lzma/lzma_encoder.c
index 07d2b87b..62bb6343 100644
--- a/src/liblzma/lzma/lzma_encoder.c
+++ b/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;
+	}
 
-	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;
-		}
+	// 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) {
 		// 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);
-
-		encode_symbol(coder, mf, back, len, position);
-
-		position += len;
-	}
+			lzma_lzma_optimum_normal(coder, mf, &back, &len,
+					(uint32_t)(coder->uncomp_size));
+
+		encode_symbol(coder, mf, back, len,
+				(uint32_t)(coder->uncomp_size));
+
+		// 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);
+
+			// 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.
 
-	if (!coder->is_flushed) {
-		coder->is_flushed = true;
-
-		// 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);
 }