base58.h

#pragma once
#include <cstdint>

namespace Base58 {

    static const char b58digits_ordered[] = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz";

    static const int8_t b58digits_map[] = {
        -1,-1,-1,-1,-1,-1,-1,-1, -1,-1,-1,-1,-1,-1,-1,-1,
        -1,-1,-1,-1,-1,-1,-1,-1, -1,-1,-1,-1,-1,-1,-1,-1,
        -1,-1,-1,-1,-1,-1,-1,-1, -1,-1,-1,-1,-1,-1,-1,-1,
        -1, 0, 1, 2, 3, 4, 5, 6,  7, 8,-1,-1,-1,-1,-1,-1,
        -1, 9,10,11,12,13,14,15, 16,-1,17,18,19,20,21,-1,
        22,23,24,25,26,27,28,29, 30,31,32,-1,-1,-1,-1,-1,
        -1,33,34,35,36,37,38,39, 40,41,42,43,-1,44,45,46,
        47,48,49,50,51,52,53,54, 55,56,57,-1,-1,-1,-1,-1,
    };

    inline bool Decode(void* bin, size_t& binszp, const char* b58, size_t b58sz)
    {
        size_t binsz = binszp;
        const uint8_t* b58u = (const uint8_t*)b58;
        uint8_t* binu = (uint8_t*)bin;
        size_t outisz = (binsz + 3) / 4;
        uint32_t outi[outisz];
        uint64_t t;
        uint32_t c;
        size_t i, j;
        uint8_t bytesleft = binsz % 4;
        uint32_t zeromask = bytesleft ? (0xffffffff << (bytesleft * 8)) : 0;
        unsigned zerocount = 0;

        if (!b58sz)
            b58sz = strlen(b58);

        memset(outi, 0, outisz * sizeof(*outi));

        // Leading zeros, just count
        for (i = 0; i < b58sz && b58u[i] == '1'; ++i)
            ++zerocount;

        for ( ; i < b58sz; ++i)
        {
            if (b58u[i] & 0x80)
                // High-bit set on invalid digit
                return false;
            if (b58digits_map[b58u[i]] == -1)
                // Invalid base58 digit
                return false;
            c = (unsigned)b58digits_map[b58u[i]];
            for (j = outisz; j--; )
            {
                t = ((uint64_t)outi[j]) * 58 + c;
                c = (t & 0x3f00000000) >> 32;
                outi[j] = t & 0xffffffff;
            }
            if (c)
                // Output number too big (carry to the next int32)
                return false;
            if (outi[0] & zeromask)
                // Output number too big (last int32 filled too far)
                return false;
        }

        j = 0;
        switch (bytesleft) {
            case 3:
                *(binu++) = (outi[0] &   0xff0000) >> 16;
            case 2:
                *(binu++) = (outi[0] &     0xff00) >>  8;
            case 1:
                *(binu++) = (outi[0] &       0xff);
                ++j;
            default:
                break;
        }

        for (; j < outisz; ++j)
        {
            *(binu++) = (outi[j] >> 0x18) & 0xff;
            *(binu++) = (outi[j] >> 0x10) & 0xff;
            *(binu++) = (outi[j] >>    8) & 0xff;
            *(binu++) = (outi[j] >>    0) & 0xff;
        }

        // Count canonical base58 byte count
        binu = (uint8_t*)bin;
        for (i = 0; i < binsz; ++i)
        {
            if (binu[i])
                break;
            --binszp;
        }
        binszp += zerocount;

        return true;
    }

    inline bool Encode(char* b58, size_t& b58sz, const void* data, size_t binsz)
    {
        const uint8_t *bin = (const uint8_t*)data;
        int carry;
        ssize_t i, j, high, zcount = 0;
        size_t size;

        while (zcount < (ssize_t)binsz && !bin[zcount])
            ++zcount;

        size = (binsz - zcount) * 138 / 100 + 1;
        uint8_t buf[size];
        memset(buf, 0, size);

        for (i = zcount, high = size - 1; i < (ssize_t)binsz; ++i, high = j)
        {
            for (carry = bin[i], j = size - 1; (j > high) || carry; --j)
            {
                carry += 256 * buf[j];
                buf[j] = carry % 58;
                carry /= 58;
            }
        }

        for (j = 0; j < (ssize_t)size && !buf[j]; ++j);

        if (b58sz <= zcount + size - j)
        {
            b58sz = zcount + size - j;
            return false;
        }

        if (zcount)
            memset(b58, '1', zcount);
        for (i = zcount; j < (ssize_t)size; ++i, ++j)
            b58[i] = b58digits_ordered[buf[j]];
        b58[i] = '\0';
        b58sz = i;

        return true;
    }
};