pdfRunLength

RunLengthDecode ISO 32000-2 §7.4.5 — run-length coding.

Module pdfRunLength | Source packages/front/office/pdf/src/syntax/filters/runLength.js | Deps pdfErrors | Worker-safe yes

Each control byte n is read as:

• 0..127 → copy the next n + 1 bytes literally (1..128 bytes);
• 128 → end of data (EOD), decoding stops;
• 129..255 → repeat the next byte 257 - n times (2..128 repetitions).

The encoder is greedy: it first tries a repeat run when 3 or more identical bytes are available, otherwise it accumulates up to 128 literal bytes, stopping just before a productive run.

Resolve

const rl = runtime.resolve('pdfRunLength');
// Returns: { decode, encode }

API

Examples

Decode a repeat run

const rl = runtime.resolve('pdfRunLength');
// 0xFB = 251 → repeat the next byte (257-251 = 6) times; 128 = EOD
const src = new Uint8Array([0xFB, 0x41, 0x80]);
rl.decode(src);   // [0x41, 0x41, 0x41, 0x41, 0x41, 0x41]  ('AAAAAA')

Decode a literal run

// 0x02 → copy (2+1=3) literal bytes
rl.decode(new Uint8Array([0x02, 0x48, 0x69, 0x21, 0x80]));   // 'Hi!'

Round trip

const bin = new Uint8Array([1, 1, 1, 1, 1, 2, 3, 4]);
const enc = rl.encode(bin);
const back = rl.decode(enc);
// back equals bin.

Errors

See also

• pdfFilterDispatch
• Filters index