boa/docs/snapshot.md

Snapshot File format

This docoment describes the binary file format of the boa snapshot files.

Header

The header composes the first part of the snapshot.

Field	Description
signature : [u8; 4]	This is used to quickly check if this file is a snapshot file (.boa)
guid	Guid generated in compile time and backed into the binary, that is used to check if snapshot is compatibile.
checksum	Checksum that is used to check that the snapshot is not corrupted.

JsString Table

After the Header the table containing JsStrings each entry contains

static? : u8	length: : usize	JsString elements : [u16]
0	5	'H', 'e', 'l', 'l', 'o'
1	-	3
...	...	...

If it's a static string then it's elements comprise the index into the STATIC_STRINGs.

JsSymbol Table

JsSymbol hash : u64	Description (index into JsString table) : usize
200	0
...	...

JsBigInt Table

Length in bytes : u64	Content
32	...

Shapes (Hidden classes) Table

Unique Shapes

[[prototype]] : u32 (index into JsObject table)	property count : u32	key-value pairs
	0
	...

Shared Shapes

previous : u32	flags	transition
MAX (root)	...	x

JsObject Table

length	Content
200	...
...	...

JsValue Encoding

type : u8 (JsValue discriminant, Boolean, Null, etc) followed by the value if it applied for the type (Null and Undefined does not have a value part). If following the JsValue is an JsString, JsSymbol, JsBigInt, JsObject then the following value will be an index into the appropriate tables.