figura-tools/lib/tiny_lz4.js

const MIN_MATCH = 3;
const MAX_OFFSET = 0xffff;

function writeExtLen(out, n) {
  while (n >= 255) {
    out.push(255);
    n -= 255;
  }
  out.push(n);
}

function readExtLen(input, pos, base) {
  let add = 0;
  while (true) {
    let b = input[pos++];
    add += b;
    if (b < 255) break;
  }
  return [base + add, pos];
}

function findLongestMatch(data, pos, window) {
  let n = data.length;
  let max_off = Math.min(window, pos);
  let best_len = 0;
  let best_off = 0;
  let max_match_len = n - pos;
  if (max_match_len < MIN_MATCH) return [0, 0];

  let start = pos - max_off;
  for (let j = start; j < pos; j++) {
    if (data[j] === data[pos]) {
      let k = 0;
      while (k < max_match_len && data[j + k] === data[pos + k]) {
        k++;
      }
      if (k >= MIN_MATCH && k > best_len) {
        best_len = k;
        best_off = pos - j;
        if (best_len >= 255 + MIN_MATCH) break;
      }
    }
  }
  return [best_off, best_len];
}

export function compress(input) {
  if (!(input instanceof Uint8Array))
    throw new Error("compress expects Uint8Array");

  let n = input.length;
  let i = 0;
  let anchor = 0;
  let out = [];
  let window = MAX_OFFSET;

  while (i < n) {
    let [off, match_len] = findLongestMatch(input, i, window);

    if (match_len >= MIN_MATCH) {
      let lit_len = i - anchor;
      let lit_nibble = Math.min(lit_len, 15);
      let match_nibble = Math.min(match_len - MIN_MATCH, 15);
      let token = lit_nibble * 16 + match_nibble;
      out.push(token);

      if (lit_len >= 15) {
        writeExtLen(out, lit_len - 15);
      }

      for (let k = anchor; k < i; k++) out.push(input[k]);

      out.push(off & 0xff, (off >> 8) & 0xff);

      let rem = match_len - MIN_MATCH;
      if (rem >= 15) {
        writeExtLen(out, rem - 15);
      }

      i += match_len;
      anchor = i;
    } else {
      i++;
    }
  }

  let final_lit = n - anchor;
  let tok_lit = Math.min(final_lit, 15);
  let token = tok_lit * 16;
  out.push(token);
  if (final_lit >= 15) {
    writeExtLen(out, final_lit - 15);
  }
  for (let k = anchor; k < n; k++) out.push(input[k]);

  return Uint8Array.from(out);
}

export function decompress(input) {
  if (!(input instanceof Uint8Array))
    throw new Error("decompress expects Uint8Array");

  let pos = 0;
  let n = input.length;
  let out = [];

  while (pos < n) {
    let token = input[pos++];
    if (token === undefined) break;

    let lit_nibble = token >> 4;
    let match_nibble = token & 0xf;

    let lit_len = lit_nibble;
    if (lit_len === 15) {
      [lit_len, pos] = readExtLen(input, pos, 15);
    }

    for (let k = 0; k < lit_len; k++) {
      out.push(input[pos++]);
    }

    if (pos >= n) break;

    let off = input[pos] | (input[pos + 1] << 8);
    pos += 2;

    let match_len = match_nibble + MIN_MATCH;
    if (match_nibble === 15) {
      let extra;
      [extra, pos] = readExtLen(input, pos, 0);
      match_len += extra;
    }

    let sofarLen = out.length;
    let match_start = sofarLen - off;
    if (match_start < 0) throw new Error("Invalid offset");

    for (let k = 0; k < match_len; k++) {
      out.push(out[match_start + k]);
    }
  }

  return Uint8Array.from(out);
}