npm package: @gemachain/buffer-layout
Description: Translation between JavaScript values and Buffers
Author: Peter A. Bigot
Latest version: 3.0.0
Deprecated: false

# buffer-layout

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buffer-layout is a utility module implemented in pure JavaScript that
supports translations between JavaScript values and Buffers.  It is made
available through [github](https://github.com/pabigot/buffer-layout) and
released under the MIT license.

Layout support is provided for these types of data:

* Signed and unsigned integral values from 1 to 6 bytes in length, in
  little-endian or big-endian format;
* Signed and unsigned 64-bit integral values decoded as integral
  Numbers;
* Float and double values (also little-endian or big-endian);
* Sequences of instances of an arbitrary layout, with constant or
  data-dependent length;
* Structures with named fields containing arbitrary layouts;
* Unions of variant layouts where the type of data is recorded in a
  prefix value, another layout element, or provided externally;
* Bit fields within 8, 16, 24, or 32-bit unsigned integers, numbering
  from the least or most significant bit;
* NUL-terminated C strings;
* Blobs of fixed or variable-length raw data.

## Installation

Development and testing is done using Node.js, supporting versions 4.5
and later.  Install with `npm install buffer-layout`.

## Examples

All examples are from the `test/examples.js` unit test and assume the
following context:

    const assert = require('assert');
    const util = require('util');
    const lo = require('buffer-layout');

The examples give only a taste of what can be done.  Structures, unions,
and sequences can nest; [union
discriminators](http://pabigot.github.io/buffer-layout/module-Layout-UnionDiscriminator.html)
can be within the union or external to it; sequence and blob lengths may
be fixed or read from the buffer.

For full details see the [documentation](http://pabigot.github.io/buffer-layout/).

### Four-element array of 16-bit signed little-endian integers

The C definition:

    int16_t arr[4] = { 1, -1, 3, -3 };

The buffer-layout way:

    const ds = lo.seq(lo.s16(), 4);
    const b = Buffer.alloc(8);
    assert.equal(ds.encode([1, -1, 3, -3], b), 4 * 2);
    assert.equal(Buffer.from('0100ffff0300fdff', 'hex').compare(b), 0);
    assert.deepEqual(ds.decode(b), [1, -1, 3, -3]);

See [Int](http://pabigot.github.io/buffer-layout/module-Layout-Int.html)
and [Sequence](http://pabigot.github.io/buffer-layout/module-Layout-Sequence.html).

### A native C `struct` on a 32-bit little-endian machine

The C definition:

    struct ds {
      uint8_t v;
      uint32_t u32;
    } st;

The buffer-layout way:

    const ds = lo.struct([lo.u8('v'),
                        lo.seq(lo.u8(), 3), // alignment padding
                        lo.u32('u32')]);
    assert.equal(ds.offsetOf('u32'), 4);
    const b = Buffer.alloc(8);
    b.fill(0xbd);
    assert.equal(ds.encode({v: 1, u32: 0x12345678}, b), 1 + 3 + 4);
    assert.equal(Buffer.from('01bdbdbd78563412', 'hex').compare(b), 0);
    assert.deepEqual(ds.decode(b), {v: 1, u32: 0x12345678});

Note that the C language requires padding which must be explicitly added
in the buffer-layout structure definition.  Since the padding is not
accessible, the corresponding layout has no
[property](http://pabigot.github.io/buffer-layout/module-Layout-Layout.html#property).

See [Structure](http://pabigot.github.io/buffer-layout/module-Layout-Structure.html).

### A packed C `struct` on a 32-bit little-endian machine

The C definition:

    struct ds {
      uint8_t v;
      uint32_t u32;
    } __attribute__((__packed__)) st;

The buffer-layout way:

    const ds = lo.struct([lo.u8('v'),
                        lo.u32('u32')]);
    assert.equal(ds.offsetOf('u32'), 1);
    const b = Buffer.alloc(5);
    b.fill(0xbd);
    assert.equal(ds.encode({v: 1, u32: 0x12345678}, b), 1 + 4);
    assert.equal(Buffer.from('0178563412', 'hex').compare(b), 0);
    assert.deepEqual(ds.decode(b), {v: 1, u32: 0x12345678});

### A tagged union of 4-byte values

Assume a 5-byte packed structure where the interpretation of the last
four bytes depends on the first byte.  The C definition:

    struct {
      uint8_t t;
      union ds {
        uint8_t u8[4];  // default interpretation
        int16_t s16[2]; // when t is 'h'
        uint32_t u32;   // when t is 'w'
        float f32;      // when t is 'f'
      } u;
    } __attribute__((__packed__)) un;

The buffer-layout way:

    const t = lo.u8('t');
    const un = lo.union(t, lo.seq(lo.u8(), 4, 'u8'));
    const nul = un.addVariant('n'.charCodeAt(0), 'nul');
    const u32 = un.addVariant('w'.charCodeAt(0), lo.u32(), 'u32');
    const s16 = un.addVariant('h'.charCodeAt(0), lo.seq(lo.s16(), 2), 's16');
    const f32 = un.addVariant('f'.charCodeAt(0), lo.f32(), 'f32');
    const b = Buffer.alloc(un.span);
    assert.deepEqual(un.decode(b), {t: 0, u8: [0, 0, 0, 0]});
    assert.deepEqual(un.decode(Buffer.from('6e01020304', 'hex')),
                     {nul: true});
    assert.deepEqual(un.decode(Buffer.from('7778563412', 'hex')),
                     {u32: 0x12345678});
    assert.deepEqual(un.decode(Buffer.from('660000bd41', 'hex')),
                     {f32: 23.625});
    assert.deepEqual(un.decode(Buffer.from('a5a5a5a5a5', 'hex')),
                     {t: 0xa5, u8: [0xa5, 0xa5, 0xa5, 0xa5]});
    assert.equal(s16.encode({s16: [123, -123]}, b), 1 + 2 * 2);
    assert.equal(Buffer.from('687b0085ff', 'hex').compare(b), 0);

See [Union](http://pabigot.github.io/buffer-layout/module-Layout-Union.html).

### Decoding into class instances

Using the same 5-byte packet structure but with JavaScript classes
representing the union and the variants:

    function Union() { }
    lo.bindConstructorLayout(Union,
                             lo.union(lo.u8('t'), lo.seq(lo.u8(), 4, 'u8')));

    function Vn() {}
    util.inherits(Vn, Union);
    lo.bindConstructorLayout(Vn,
                             Union.layout_.addVariant('n'.charCodeAt(0), 'nul'));

    function Vu32(v) { this.u32 = v; }
    util.inherits(Vu32, Union);
    lo.bindConstructorLayout(Vu32,
                             Union.layout_.addVariant('w'.charCodeAt(0), lo.u32(), 'u32'));

    function Vs16(v) { this.s16 = v; }
    util.inherits(Vs16, Union);
    lo.bindConstructorLayout(Vs16,
                             Union.layout_.addVariant('h'.charCodeAt(0), lo.seq(lo.s16(), 2), 's16'));

    function Vf32(v) { this.f32 = v; }
    util.inherits(Vf32, Union);
    lo.bindConstructorLayout(Vf32,
                             Union.layout_.addVariant('f'.charCodeAt(0), lo.f32(), 'f32'));

    let v = Union.decode(Buffer.from('7778563412', 'hex'));
    assert(v instanceof Vu32);
    assert(v instanceof Union);
    assert.equal(v.u32, 0x12345678);

    v = Union.decode(Buffer.from('a5a5a5a5a5', 'hex'));
    assert(v instanceof Union);
    assert.equal(v.t, 0xa5);
    assert.deepEqual(v.u8, [0xa5, 0xa5, 0xa5, 0xa5]);

    const b = Buffer.alloc(Union.layout_.span);
    v = new Vf32(23.625);
    v.encode(b);
    assert.equal(Buffer.from('660000bd41', 'hex').compare(b), 0);

    b.fill(0xFF);
    v = new Vn();
    v.encode(b);
    assert.equal(Buffer.from('6effffffff', 'hex').compare(b), 0);

Note that one variant (`'n'`) carries no data, leaving the remainder of
the buffer unchanged when stored.

See
[Layout.makeDestinationObject()](http://pabigot.github.io/buffer-layout/module-Layout-Layout.html#makeDestinationObject)
and
[bindConstructorLayout](http://pabigot.github.io/buffer-layout/module-Layout.html#.bindConstructorLayout).

### Packed bit fields on a little-endian machine

The C definition:

    struct ds {
      unsigned int b00l03: 3;
      unsigned int flg03: 1;
      unsigned int b04l18: 24;
      unsigned int b1Cl04: 4;
    } st;

The buffer-layout way:

    const ds = lo.bits(lo.u32());
    const b = Buffer.alloc(4);
    ds.addField(3, 'b00l03');
    ds.addBoolean('flg03');
    ds.addField(24, 'b04l18');
    ds.addField(4, 'b1Cl04');
    b.fill(0xff);
    assert.equal(ds.encode({b00l03: 3, b04l18: 24, b1Cl04: 4}, b), 4);
    assert.equal(Buffer.from('8b010040', 'hex').compare(b), 0);
    assert.deepEqual(ds.decode(b),
                     {b00l03: 3, flg03: true, b04l18: 24, b1Cl04: 4});

See [BitStructure](http://pabigot.github.io/buffer-layout/module-Layout-BitStructure.html).

### 64-bit values as Numbers

The C definition:

    uint64_t v = 0x0102030405060708ULL;

The buffer-layout way:

    const ds = lo.nu64be();
    const b = Buffer.from('0102030405060708', 'hex');
    const v = 72623859790382856;
    const nv = v - 6;
    assert.equal(v, nv);
    assert.equal(ds.decode(b), nv);

Note that because the exact value is not less than 2^53 it cannot be
represented as a JavaScript Number, and is instead approximated by a
nearby representable integer that is equivalent within Numbers.

See [NearUInt64](http://pabigot.github.io/buffer-layout/module-Layout-NearUInt64.html).

### A NUL-terminated C string

The C definition:

    const char str[] = "hi!";

The buffer-layout way:

    const ds = lo.cstr();
    const b = Buffer.alloc(8);
    assert.equal(ds.encode('hi!', b), 3 + 1);
    const slen = ds.getSpan(b);
    assert.equal(slen, 4);
    assert.equal(Buffer.from('68692100', 'hex').compare(b.slice(0, slen)), 0);
    assert.equal(ds.decode(b), 'hi!');

See [CString](http://pabigot.github.io/buffer-layout/module-Layout-CString.html).

### A fixed-length block of data offset within a buffer

The buffer-layout way:

    const ds = lo.blob(4);
    const b = Buffer.from('0102030405060708', 'hex');
    assert.equal(Buffer.from('03040506', 'hex').compare(ds.decode(b, 2)), 0);

See [Blob](http://pabigot.github.io/buffer-layout/module-Layout-Blob.html).

### A variable-length array of pairs of C strings

The buffer-layout way:

    const pr = lo.seq(lo.cstr(), 2);
    const n = lo.u8('n');
    const vla = lo.seq(pr, lo.offset(n, -1), 'a');
    const st = lo.struct([n, vla], 'st');
    const b = Buffer.alloc(32);
    const arr = [['k1', 'v1'], ['k2', 'v2'], ['k3', 'etc']];
    b.fill(0);
    assert.equal(st.encode({a: arr}, b),
                 1 + (2 * ((2 + 1) + (2 + 1)) + (2 + 1) + (3 + 1)));
    const span = st.getSpan(b);
    assert.equal(span, 20);
    assert.equal(Buffer.from('036b31007631006b32007632006b330065746300', 'hex')
                 .compare(b.slice(0, span)), 0);
    assert.deepEqual(st.decode(b), {n: 3, a: arr});

See [OffsetLayout](http://pabigot.github.io/buffer-layout/module-Layout-OffsetLayout.html).

### A C flexible array member with implicit length

When data is obtained over a packetized interface the length of the
packet can provide implicit limits on the last field.

The C definition:

    struct ds {
      uint8_t prop;
      uint16_t data[];
    };

The buffer-layout way:

    const st = lo.struct([lo.u8('prop'),
                        lo.seq(lo.u16(),
                               lo.greedy(lo.u16().span),
                               'data')],
                       'ds');
    const b = Buffer.from('21010002030405', 'hex');
    assert.deepEqual(st.decode(b), {prop: 33, data: [0x0001, 0x0302, 0x0504]});
    b.fill(0xFF);
    assert.equal(st.encode({prop: 9, data: [5, 6]}, b), 1 + 2 * 2);
    assert.equal(Buffer.from('0905000600FFFF', 'hex').compare(b), 0);

### Tagged values, or variable-length unions

Storing arbitrary data using a leading byte to identify the content then
a value that takes up only as much room as is necessary.

The example also shows how to extend the variant recognition API to
support abitrary constant without consuming space for them in the
encoded union.  This could be used to make something similar to
[BSON](http://bsonspec.org/spec.html).

Here's the code that defines the union, the variants, and the
recognition of `true` and `false` values for `b` as distinct variants:

    const un = lo.union(lo.u8('t'));
    const u8 = un.addVariant('B'.charCodeAt(0), lo.u8(), 'u8');
    const s16 = un.addVariant('h'.charCodeAt(0), lo.s16(), 's16');
    const s48 = un.addVariant('Q'.charCodeAt(0), lo.s48(), 's48');
    const cstr = un.addVariant('s'.charCodeAt(0), lo.cstr(), 'str');
    const tr = un.addVariant('T'.charCodeAt(0), lo.const(true), 'b');
    const fa = un.addVariant('F'.charCodeAt(0), lo.const(false), 'b');
    const b = Buffer.alloc(1 + 6);
    un.configGetSourceVariant(function(src) {
      if (src.hasOwnProperty('b')) {
        return src.b ? tr : fa;
      }
      return this.defaultGetSourceVariant(src);
    });

And here are examples of encoding, checking the encoded length, and
decoding each of the alternatives:

    b.fill(0xff);
    assert.equal(un.encode({u8: 1}, b), 1 + 1);
    assert.equal(un.getSpan(b), 2);
    assert.equal(Buffer.from('4201ffffffffff', 'hex').compare(b), 0);
    assert.equal(un.decode(b).u8, 1);

    b.fill(0xff);
    assert.equal(un.encode({s16: -32000}, b), 1 + 2);
    assert.equal(un.getSpan(b), 3);
    assert.equal(Buffer.from('680083ffffffff', 'hex').compare(b), 0);
    assert.equal(un.decode(b).s16, -32000);

    b.fill(0xff);
    const v48 = Math.pow(2, 47) - 1;
    assert.equal(un.encode({s48: v48}, b), 1 + 6);
    assert.equal(un.getSpan(b), 7);
    assert.equal(Buffer.from('51ffffffffff7f', 'hex').compare(b), 0);
    assert.equal(un.decode(b).s48, v48);

    b.fill(0xff);
    assert.equal(un.encode({b: true}, b), 1);
    assert.equal(un.getSpan(b), 1);
    assert.equal(Buffer.from('54ffffffffffff', 'hex').compare(b), 0);
    assert.strictEqual(un.decode(b).b, true);

    b.fill(0xff);
    assert.equal(un.encode({b: false}, b), 1);
    assert.equal(un.getSpan(b), 1);
    assert.equal(Buffer.from('46ffffffffffff', 'hex').compare(b), 0);
    assert.strictEqual(un.decode(b).b, false);

**NOTE** This code tickles a long-standing [bug in
Buffer.writeInt{L,B}E](https://github.com/nodejs/node/pull/3994); if you
are using Node prior to 4.2.4 or 5.2.0 you should update.