import { DATABASE_PAGE_HEADER_SIZE, HASH_INDEX_ENTRY_SIZE } from './types';
import { ParserError } from '../types';

/**
 * Extract the values from a hash index, which is stored in a Hash DB page.
 * The hash index contains key/value pairs. The key is usually some number
 * which is not relevant to the application. This function returns only the
 * values stored in the index.
 * @param data A database page.
 * @param entries How many entries are stored in the index.
 */
export function bufferToHashIndexValues(
  page: Buffer,
  entries: number,
): number[] {
  // Hash table entries are always stored in pairs of 2.
  if (entries % 2 !== 0) {
    const pageNumber = page.readUInt32LE(8);
    throw new ParserError('The number of entries must be a multiple of 2', {
      entries,
      pageNumber,
    });
  }

  // Every entry is a 2-byte offset that points somewhere in the current database page.
  const hashIndexSize = entries * HASH_INDEX_ENTRY_SIZE;
  const hashIndex = page.slice(
    DATABASE_PAGE_HEADER_SIZE,
    DATABASE_PAGE_HEADER_SIZE + hashIndexSize,
  );

  // We only want the values, not the keys. Data is stored in key/value pairs.
  // The following skips over all keys and stores only the values. An entry is 2 bytes long.
  const keyValuePairSize = 2 * HASH_INDEX_ENTRY_SIZE;
  const hashIndexValues = hashIndex.reduce((values, _, byteOffset) => {
    // Is the current byte the start of a value?
    if ((byteOffset - HASH_INDEX_ENTRY_SIZE) % keyValuePairSize === 0) {
      const value = hashIndex.readInt16LE(byteOffset);
      values.push(value);
    }
    return values;
  }, new Array<number>());

  return hashIndexValues;
}