Serialization¶ ↑

Prism ships with the ability to serialize a syntax tree to a single string. The string can then be deserialized back into a syntax tree using a language other than C. This is useful for using the parsing logic in other tools without having to write a parser in that language. The syntax tree still requires a copy of the original source, as for the most part it just contains byte offsets into the source string.

Types¶ ↑

Let us define some simple types for readability.

varuint¶ ↑

A variable-length unsigned integer with the value fitting in uint32_t using between 1 and 5 bytes, using the LEB128 encoding. This drastically cuts down on the size of the serialized string, especially when the source file is large.

varsint¶ ↑

A variable-length signed integer with the value fitting in int32_t using between 1 and 5 bytes, using ZigZag encoding into [LEB128].

string¶ ↑

location¶ ↑

comment¶ ↑

The comment type is one of:

• 0=INLINE (# comment)

• 1=EMBEDDED_DOCUMENT (=begin/=end)

magic comment¶ ↑

error¶ ↑

warning¶ ↑

integer¶ ↑

Structure¶ ↑

The serialized string representing the syntax tree is composed of three parts: the header, the body, and the constant pool. The header contains information like the version of prism that serialized the tree. The body contains the actual nodes in the tree. The constant pool contains constants that were interned while parsing.

The header is structured like the following table:

After the header comes the body of the serialized string. The body consists of a sequence of nodes that is built using a prefix traversal order of the syntax tree. Each node is structured like the following table:

Every field on the node is then appended to the serialized string. The fields can be determined by referencing config.yml. Depending on the type of field, it could take a couple of different forms, described below:

• double - A field that is a double. This is structured as a sequence of 8 bytes in native endian order.

• node - A field that is a node. This is structured just as like parent node.

• node? - A field that is a node that is optionally present. If the node is not present, then a single 0 byte will be written in its place. If it is present, then it will be structured just as like parent node.

• node[] - A field that is an array of nodes. This is structured as a variable-length integer length, followed by the child nodes themselves.

• string - A field that is a string. For example, this is used as the name of the method in a call node, since it cannot directly reference the source string (as in @- or foo=). This is structured as a variable-length integer byte length, followed by the string itself (without a trailing null byte).

• constant - A variable-length integer that represents an index in the constant pool.

• constant? - An optional variable-length integer that represents an index in the constant pool. If it’s not present, then a single 0 byte will be written in its place.

• integer - A field that represents an arbitrary-sized integer. The structure is listed above.

• location - A field that is a location. This is structured as a variable-length integer start followed by a variable-length integer length.

• location? - A field that is a location that is optionally present. If the location is not present, then a single 0 byte will be written in its place. If it is present, then it will be structured just like the location child node.

• uint8 - A field that is an 8-bit unsigned integer. This is structured as a single byte.

• uint32 - A field that is a 32-bit unsigned integer. This is structured as a variable-length integer.

After the syntax tree, the content pool is serialized. This is a list of constants that were referenced from within the tree. The content pool begins at the offset specified in the header. Constants can be either “owned” (in which case their contents are embedded in the serialization) or “shared” (in which case their contents represent a slice of the source string). The most significant bit of the constant indicates whether it is owned or shared.

In the case that it is owned, the constant is structured as follows:

Note that you will need to mask off the most significant bit for the byte offset in the serialization. In the case that it is shared, the constant is structured as follows:

After the constant pool, the contents of the owned constants are serialized. This is just a sequence of bytes that represent the contents of the constants. At the end of the serialization, the buffer is null terminated.

APIs¶ ↑

The relevant APIs and struct definitions are listed below:

// A pm_buffer_t is a simple memory buffer that stores data in a contiguous
// block of memory. It is used to store the serialized representation of a
// prism tree.
typedef struct {
  char *value;
  size_t length;
  size_t capacity;
} pm_buffer_t;

// Free the memory associated with the buffer.
void pm_buffer_free(pm_buffer_t *);

// Parse and serialize the AST represented by the given source to the given
// buffer.
void pm_serialize_parse(pm_buffer_t *buffer, const uint8_t *source, size_t length, const char *data);

Typically you would use a stack-allocated pm_buffer_t and call pm_serialize_parse, as in:

void
serialize(const uint8_t *source, size_t length) {
  pm_buffer_t buffer = { 0 };
  pm_serialize_parse(&buffer, source, length, NULL);

  // Do something with the serialized string.

  pm_buffer_free(&buffer);
}

The final argument to pm_serialize_parse is an optional string that controls the options to the parse function. This includes all of the normal options that could be passed to pm_parser_init through a pm_options_t struct, but serialized as a string to make it easier for callers through FFI. Note that no varuint are used here to make it easier to produce the data for the caller, and also serialized size is less important here. The format of the data is structured as follows:

Command line flags are a bitset. By default every flag is 0. It includes the following values:

• 0x1 - the -a option

• 0x2 - the -e option

• 0x4 - the -l option

• 0x8 - the -n option

• 0x10 - the -p option

• 0x20 - the -x option

Scopes are ordered from the outermost scope to the innermost one.

Each scope is laid out as follows:

Each local is laid out as follows:

The data can be NULL (as seen in the example above).