BitCoin源碼研究(1)-Base58編碼

Base58編碼由58個數字和大小寫字母組成，BitCoin源碼中定義及註釋如下：

/** All alphanumeric characters except for "0", "I", "O", and "l" */

static const char* pszBase58 = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz";

如unsigned char ucData[4] = { 0x39, 0x3a, 0x3b, 0x3c };的base58編碼過程如下：

1、先計算ucData開頭爲0x00的個數 zeros ，這裏zeros = 0;

    while (pbegin != pend && *pbegin == 0) {

        pbegin++;

        zeroes++;

    }

2、跳過開頭的zeros個0x00，計算所需要的緩存

    int size = (pend - pbegin) * 138 / 100 + 1; // log(256) / log(58), rounded up.

3、256進制轉58進制的計算

std::vector<unsigned char> b58(size);

// Process the bytes.

while (pbegin != pend) {

    int carry = *pbegin;

    int i = 0;

    // Apply "b58 = b58 * 256 + ch".

    for (std::vector<unsigned char>::reverse_iterator it = b58.rbegin(); 

                    (carry != 0 || i < length) && (it != b58.rend()); it++, i++) {

        carry += 256 * (*it);

        *it = carry % 58;

        carry /= 58;

    }

    assert(carry == 0);

    length = i;

    pbegin++;

}

4、輸出編碼結果

先在字符串前補上zeros 個1 ，後面的依次綴加DstByte對應的 pszBase58 字符