tools: Add decompression mode to codectest

When the vertex count is specified, instead of compressing the input we
decompress it; it's also possible to pipe the output through codectest
again to re-compress it.

This makes it easier to experiment with already-compressed data,
including intermediate measurements with other compresors and
prefiltering.

tools/codectest.cpp

@@ -17,9 +17,9 @@ int main(int argc, char** argv)
 	_setmode(_fileno(stdout), _O_BINARY);
 #endif
 
-	if (argc != 2 || atoi(argv[1]) <= 0)
+	if (argc < 2 || argc > 3 || atoi(argv[1]) <= 0)
 	{
-		fprintf(stderr, "Usage: %s <stride>\n", argv[0]);
+		fprintf(stderr, "Usage: %s <stride> [<count>]\n", argv[0]);
 		return 1;
 	}
 
@@ -32,10 +32,29 @@ int main(int argc, char** argv)
 	while ((bytes_read = fread(buffer, 1, sizeof(buffer), stdin)) > 0)
 		input.insert(input.end(), buffer, buffer + bytes_read);
 
-	size_t vertex_count = input.size() / stride;
-	std::vector<unsigned char> output(meshopt_encodeVertexBufferBound(vertex_count, stride));
-	size_t output_size = meshopt_encodeVertexBuffer(output.data(), output.size(), input.data(), vertex_count, stride);
+	if (argc == 3)
+	{
+		// if count is specified, we assume input is meshopt-encoded and decode it first
+		size_t count = atoi(argv[2]);
 
-	fwrite(output.data(), 1, output_size, stdout);
-	return 0;
+		std::vector<unsigned char> decoded(count * stride);
+		int res = meshopt_decodeVertexBuffer(&decoded[0], count, stride, &input[0], input.size());
+		if (res != 0)
+		{
+			fprintf(stderr, "Error decoding input: %d\n", res);
+			return 2;
+		}
+
+		fwrite(decoded.data(), 1, decoded.size(), stdout);
+		return 0;
+	}
+	else
+	{
+		size_t vertex_count = input.size() / stride;
+		std::vector<unsigned char> output(meshopt_encodeVertexBufferBound(vertex_count, stride));
+		size_t output_size = meshopt_encodeVertexBuffer(output.data(), output.size(), input.data(), vertex_count, stride);
+
+		fwrite(output.data(), 1, output_size, stdout);
+		return 0;
+	}
 }