Class: BitStructure

Layout~BitStructure(word, msbopt, propertyopt)

Contain a sequence of bit fields as an unsigned integer.

Factory: bits

This is a container element; within it there are BitField instances that provide the extracted properties. The container simply defines the aggregate representation and its bit ordering. The representation is an object containing properties with numeric or Boolean values.

BitFields are added with the addField and addBoolean methods.

Constructor

new BitStructure(word, msbopt, propertyopt)

Parameters:
Name Type Attributes Description
word module:Layout~Layout

initializer for word. The parameter must be an instance of UInt (or UIntBE) that is no more than 4 bytes wide.
msb bool <optional>

true if the bit numbering starts at the most significant bit of the containing word; false (default) if it starts at the least significant bit of the containing word. If the parameter at this position is a string and property is undefined the value of this argument will instead be used as the value of property.
property string <optional>

initializer for property.
Source:

Extends

Members

fields

The sequence of BitField layouts that comprise the packed structure.

NOTE The array remains mutable to allow fields to be added after construction. Users should not manipulate the content of this property.

Source:

msb

Whether the bit sequences are packed starting at the most significant bit growing down (true), or the least significant bit growing up (false).

NOTE Regardless of this value, the least significant bit of any BitField value is the least significant bit of the corresponding section of the packed value.

Source:

property

The property name used when this layout is represented in an Object.

Used only for layouts that decode to Object instances. If left undefined the span of the unnamed layout will be treated as padding: it will not be mutated by encode nor represented as a property in the decoded Object.

Inherited From:
Source:

span

The span of the layout in bytes.

Positive values are generally expected.

Zero will only appear in Constants and in Sequences where the count is zero.

A negative value indicates that the span is value-specific, and must be obtained using getSpan.

Inherited From:
Source:

word

The layout used for the packed value. BitField instances are packed sequentially depending on msb.

Source:

Methods

addBoolean(property) → {module:Layout~Boolean}

As with addField for single-bit fields with boolean value representation.

Parameters:
Name Type Description
property string

initializer for property.
Source:
Returns:
Type
module:Layout~Boolean

addField(bits, property) → {module:Layout~BitField}

Register a new bitfield with a containing bit structure. The resulting bitfield is returned.

Parameters:
Name Type Description
bits Number

initializer for bits.
property string

initializer for property.
Source:
Returns:
Type
module:Layout~BitField

decode(b, offsetopt) → {Number|Array|Object}

Decode from a Buffer into an JavaScript value.

Parameters:
Name Type Attributes Description
b Buffer

the buffer from which encoded data is read.
offset Number <optional>

the offset at which the encoded data starts. If absent a zero offset is inferred.
Overrides:
Source:
Returns:
  • the value of the decoded data.
Type
Number | Array | Object

encode()

Implement encode for BitStructure.

If src is missing a property for a member with a defined property the corresponding region of the packed value is left unmodified. Unused bits are also left unmodified.

Overrides:
Source:

fieldFor(property) → {module:Layout~BitField}

Get access to the bit field for a given property.

Parameters:
Name Type Description
property String

the bit field of interest.
Source:
Returns:
  • the field associated with property, or undefined if there is no such property.
Type
module:Layout~BitField

fromArray(values) → {Object|undefined}

Create an object from layout properties and an array of values.

NOTE This function returns undefined if invoked on a layout that does not return its value as an Object. Objects are returned for things that are a Structure, which includes variant layouts if they are structures, and excludes Unions. If you want this feature for a union you must use getVariant to select the desired layout.

Parameters:
Name Type Description
values Array

an array of values that correspond to the default order for properties. As with decode layout elements that have no property name are skipped when iterating over the array values. Only the top-level properties are assigned; arguments are not assigned to properties of contained layouts. Any unused values are ignored.
Inherited From:
Source:
Returns:
Type
Object | undefined

getSpan(b, offsetopt) → {Number}

Calculate the span of a specific instance of a layout.

Parameters:
Name Type Attributes Description
b Buffer

the buffer that contains an encoded instance.
offset Number <optional>

the offset at which the encoded instance starts. If absent a zero offset is inferred.
Inherited From:
Source:
Throws:
  • if the length of the value cannot be determined.
Type
RangeError
Returns:
  • the number of bytes covered by the layout instance. If this method is not overridden in a subclass the definition-time constant span will be returned.
Type
Number

makeDestinationObject()

Function to create an Object into which decoded properties will be written.

Used only for layouts that decode to Object instances, which means:

If left undefined the JavaScript representation of these layouts will be Object instances.

See bindConstructorLayout.

Inherited From:
Source:

replicate(property) → {module:Layout~Layout}

Replicate the layout using a new property.

This function must be used to get a structurally-equivalent layout with a different name since all Layout instances are immutable.

NOTE This is a shallow copy. All fields except property are strictly equal to the origin layout.

Parameters:
Name Type Description
property String

the value for property in the replica.
Inherited From:
Source:
Returns:
  • the copy with property set to property.
Type
module:Layout~Layout