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/*
 * Copyright 2015-2018 The OpenZipkin Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */
package zipkin.internal;

/**
 * Adapted from okio.Base64 as JRE 6 doesn't have a base64Url encoder
 *
 * @author Alexander Y. Kleymenov
 */
final class Base64 {
  private Base64() {
  }

  public static byte[] decode(String in) {
    // Ignore trailing '=' padding and whitespace from the input.
    int limit = in.length();
    for (; limit > 0; limit--) {
      char c = in.charAt(limit - 1);
      if (c != '=' && c != '\n' && c != '\r' && c != ' ' && c != '\t') {
        break;
      }
    }

    // If the input includes whitespace, this output array will be longer than necessary.
    byte[] out = new byte[(int) (limit * 6L / 8L)];
    int outCount = 0;
    int inCount = 0;

    int word = 0;
    for (int pos = 0; pos < limit; pos++) {
      char c = in.charAt(pos);

      int bits;
      if (c >= 'A' && c <= 'Z') {
        // char ASCII value
        //  A    65    0
        //  Z    90    25 (ASCII - 65)
        bits = c - 65;
      } else if (c >= 'a' && c <= 'z') {
        // char ASCII value
        //  a    97    26
        //  z    122   51 (ASCII - 71)
        bits = c - 71;
      } else if (c >= '0' && c <= '9') {
        // char ASCII value
        //  0    48    52
        //  9    57    61 (ASCII + 4)
        bits = c + 4;
      } else if (c == '+' || c == '-') {
        bits = 62;
      } else if (c == '/' || c == '_') {
        bits = 63;
      } else if (c == '\n' || c == '\r' || c == ' ' || c == '\t') {
        continue;
      } else {
        return null;
      }

      // Append this char's 6 bits to the word.
      word = (word << 6) | (byte) bits;

      // For every 4 chars of input, we accumulate 24 bits of output. Emit 3 bytes.
      inCount++;
      if (inCount % 4 == 0) {
        out[outCount++] = (byte) (word >> 16);
        out[outCount++] = (byte) (word >> 8);
        out[outCount++] = (byte) word;
      }
    }

    int lastWordChars = inCount % 4;
    if (lastWordChars == 1) {
      // We read 1 char followed by "===". But 6 bits is a truncated byte! Fail.
      return null;
    } else if (lastWordChars == 2) {
      // We read 2 chars followed by "==". Emit 1 byte with 8 of those 12 bits.
      word = word << 12;
      out[outCount++] = (byte) (word >> 16);
    } else if (lastWordChars == 3) {
      // We read 3 chars, followed by "=". Emit 2 bytes for 16 of those 18 bits.
      word = word << 6;
      out[outCount++] = (byte) (word >> 16);
      out[outCount++] = (byte) (word >> 8);
    }

    // If we sized our out array perfectly, we're done.
    if (outCount == out.length) return out;

    // Copy the decoded bytes to a new, right-sized array.
    byte[] prefix = new byte[outCount];
    System.arraycopy(out, 0, prefix, 0, outCount);
    return prefix;
  }
}