Git delta parser in pure TypeScript

delta.ts

// A git delta parser in TypeScript for deno
// Author: Anthony Som
// MIT License.

function copy(source, target, targetOffset, sourcePos, length) {
  for (let i = 0; i < length; i++) {
    target[targetOffset + i] = source[sourcePos + i]
  }
}

function readHeader(buffer) {
  let offset = 0;

  function readSize() {
    let byte = buffer[offset++],
      rv = byte & 0x7F,ts
      shift = 7;
    
    while (byte & 0x80) {
      byte = buffer[offset++]
      rv |= (byte & 0x7F) << shift;
      shift += 7;
    }

    return rv;
  }

  return [readSize(), readSize(), offset];
}

export function applyDelta(base: Uint8Array, delta: Uint8Array) {
  let rv, opcode, baseOffset, copyLength;
  let rvOffset = 0;
  const header = readHeader(delta);
  let offset = header[2];

  if (header[0] != base.length) {
    throw new Error("Invalid base buffer length in header")
  }

  rv = new Uint8Array(header[1]);

  while (offset < delta.length) {
    opcode = delta[offset++]
    if (opcode & 0x80) {
      // Copy
      baseOffset = 0;
      copyLength = 0;

      // Base offset:
      if (opcode & 0x01) baseOffset = delta[offset++];
      if (opcode & 0x02) baseOffset |= delta[offset++] << 8;
      if (opcode & 0x04) baseOffset |= delta[offset++] << 16;
      if (opcode & 0x08) baseOffset |= delta[offset++] << 24

      // Copy length
      if (opcode & 0x10) copyLength = delta[offset++];
      if (opcode & 0x20) copyLength |= delta[offset++] << 8;
      if (opcode & 0x40) copyLength |= delta[offset++] << 16;
      if (copyLength === 0) copyLength = 1 << 16;

      copy(base, rv, rvOffset, baseOffset, copyLength);
    } else if (opcode) {
      copyLength = opcode;
      copy(delta, rv, rvOffset, offset, offset + copyLength);
      offset += copyLength;
    } else {
      throw new Error("Invalid opcode in delta")
    }

    rvOffset += copyLength;
  }

  if (rvOffset !== rv.length) {
    throw new Error("Error patching the base buffer")
  }

  return rv
}