// Copyright 2026 @quantus/crypto authors & contributors
// SPDX-License-Identifier: Apache-2.0

const CHARS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';

// An array indexer rather than a Map: the input is ASCII by construction, so it
// cannot overflow, and array access is measurably faster on the hot loop.
const MAP = new Array<number>(256);

for (let i = 0; i < CHARS.length; i++) {
  MAP[CHARS.charCodeAt(i)] = i;
}

/**
 * Decode base64 into a caller-supplied buffer.
 *
 * Deliberately not `atob` or `Buffer.from`: the first is browser-only, the second
 * node-only, and this runs in an MV3 service worker, a Worker, node tests and a
 * bundled extension page. The output length is known at build time, so the
 * caller provides the buffer and there is no growth or reallocation.
 *
 * This is a reimplementation of `@polkadot/wasm-util`'s base64Decode, which was
 * the dependency it replaced. That package's index re-exports `packageDetect`,
 * dragging in a `@polkadot/util` peer dependency for a side effect we do not
 * want, and being a workspace package it resolved through its own repo's
 * node_modules when consumed by symlink from another checkout. Fifteen lines is
 * cheaper than either problem.
 */
export function base64Decode (data: string, out: Uint8Array): Uint8Array {
  let byte = 0;
  let bits = 0;
  let pos = 0;

  for (let i = 0; i < data.length && pos < out.length; i++) {
    const value = MAP[data.charCodeAt(i)];

    if (value === undefined) {
      continue;
    }

    byte = (byte << 6) | value;
    bits += 6;

    if (bits >= 8) {
      bits -= 8;
      out[pos++] = (byte >>> bits) & 0xff;
    }
  }

  return out;
}