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This project is part of the @thi.ng/umbrella monorepo.


ES6 iterator based read/write bit streams with support for variable word widths.


STABLE - used in production

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yarn add @thi.ng/bitstream
// ES module
<script type="module" src="https://unpkg.com/@thi.ng/bitstream?module" crossorigin></script>

// UMD
<script src="https://unpkg.com/@thi.ng/bitstream/lib/index.umd.js" crossorigin></script>

Package sizes (gzipped, pre-treeshake): ESM: 1.09 KB / CJS: 1.15 KB / UMD: 1.22 KB



Generated API docs


Uint8Array backed, bitwise output stream abstraction (big endian order). Individual word sizes can range between 1-52 bits (in practice) and are not fixed (each word can have a different size).

The constructor accepts an optional initial Uint8Array buffer or buffer size (in bytes) and an optional write start position (in bits). The buffer will only be written to starting from the given bit position (even if in the middle of a byte). Default buffer size is 16 bytes, but the array is resized (x2) automatically each time capacity is reached.

Note: The max. word size of 52 bits is not enforced by the library, but JS can only represent integers (w/o loss of precision) up to 2^53-1. If you're willing to accept lossy precision for larger values, technically the max. supported word width is 64 bits.

out = new BitOutputStream();
// write 3-bit number (only the lowest 3 bits are used, here 0x05)
out.write(0xf5, 3);
// write 7-bit number
out.write(0x66, 7);
// write 32-bit number
out.write(0xdecafbad, 32);

// write values from given iterable (w/ fixed word size, here 16 bits)
out.writeWords([0xaaaa, 0x5555], 16);

// get bytes (only up to current write position)
// Uint8Array [ 185, 183, 178, 190, 235, 106, 170, 149, 85, 64 ]

In addition to the generic write() method, there's also the slightly faster writeBit() for writing single bits (the arg MUST be 0 or 1 only).

Using seek(pos), the write position can be repositioned within current limits (does not attempt to resize backing buffer).


Uint8Array backed bitwise input stream abstraction (big endian order) with optional start position and read limit (both in bits). All readers are independent instances, but if obtained from BitOutputStream will share the same backing buffer as the writer. An auto-configured input stream can also be obtained via output.reader(). The class too implements the ES6 Iterator API for bitwise read access, as well as a read() method to read bitfields.

Note: Attempting to read beyond capacity will throw an EOF error.

Using input.seek(pos), the read position can be repositioned within stream limits.

// get input from output stream...
// (for reference, the ^ indicate the start of each bit field)
// "10111001101101111011001010111110111010110110101010101010100101010101010101"
//  ^  ^      ^                               ^               ^

// obtain new reader
input = out.reader();
// sequentially read bit fields of varying sizes
out.reader().readFields([3, 7, 32, 16, 16]).map(x => x.toString(16))
// [ "5", "66", "decafbad", "aaaa", "5555" ]

// or read fields into object
out.reader().readStruct([["a", 3], ["b", 7], ["c", 32], ["d", 16], ["e", 16]]);
// { a: 5, b: 102, c: 3737844653, d: 43690, e: 21845 }

// or read a number of fixed size words (here also from given pos)
out.reader().seek(10).readWords(4, 16).map(x=>x.toString(16));
// [ 'deca', 'fbad', 'aaaa', '5555' ]

src = new Uint8Array([0xf1,0xe2,0xd3,0xc4,0xb5,0xa6,0x97,0x88]);
// create stream from bit 36
input = new BitInputStream(src, 36);
// 5a6
// 9
// 7
input.read(1) // or use input.readBit()
// 1 => msb of last byte (0x88)
// 8 => low nibble of last byte

In addition to the generic read() method, there's also the slightly faster readBit() for reading single bits.


Karsten Schmidt

If this project contributes to an academic publication, please cite it as:

  title = "@thi.ng/bitstream",
  author = "Karsten Schmidt",
  note = "https://thi.ng/bitstream",
  year = 2016


© 2016 - 2021 Karsten Schmidt // Apache Software License 2.0

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