Internals

Complete docstring documentation of the JLPeg internal API.

JLpeg.Instruction — Type

Instruction

Abstract type of JLPeg VM instructions.

The possible struct fields have consistent meanings:

Field	Type	Meaning
`op`	@enum `UInt8`	The `Opcode`
`l`	`Int32	16`
`n`	`UInt32`	A subject index or offset
`c`	`Char`	A character to match
`b`	`UInt8`	A test byte
`tag`	`UInt16`	Key in a capture/throw Dict
`check`	`UInt16`	Which check to perform
`vec`	`BitVector` (kinda)	A set's test `BitVector`

There are also fields moe, larry, and curly, which are struct padding, so that the op field (for the instructions which have it) are in a consistent location. These consist of all 1s and do not play a role in the semantics of the VM.

source

JLpeg.VMState — Type

VMState(patt::Pattern, subject::AbstractString)

Contains the state of a match on subject by program.

Fields

Other than the subject, program, and patt, we have:

top: the byte length of subject
i: Instruction counter
s: Subject pointer
ti, ts, tc, tm, tp: Stack top registers
t_on: flag for nonempty stack
running: Bool which is true when the VM is executing
matched: true if we matched the program on the subject
inpred: Bool is true inside predicates (PAnd, PNot)
sfar: furthest subject pointer we've failed at, for error reporting
failtag: tag for a labeled failure
stack: Contains stack frames for calls and backtracks
cap: A stack for captures
mark: A stack for marks

Implementation

Classic dispatch-driven VM with a program counter, opcodes, stack frames. I borrowed a page from Forth and made the top of the stack a register, allowing stack frames to be immutable while still permitting the PartialCommit optimization.

source