Base58 encoding in Lua.

lbase58.c

#define LUA_LIB
#include <lua.h>
#include <lauxlib.h>

#include <assert.h>
#include <string.h>

static size_t b58_posrelatI(lua_Integer pos, size_t len) {
    if (pos > 0)
        return (size_t)pos;
    else if (pos == 0)
        return 1;
    else if (pos < -(lua_Integer)len)
        return 1;
    else return len + (size_t)pos + 1;
}

static size_t b58_getendpos(lua_State *L, int arg, lua_Integer def, size_t len) {
    lua_Integer pos = luaL_optinteger(L, arg, def);
    if (pos > (lua_Integer)len)
        return len;
    else if (pos >= 0)
        return (size_t)pos;
    else if (pos < -(lua_Integer)len)
        return 0;
    else return len + (size_t)pos + 1;
}

static const char *b58_checklstring(lua_State *L, int idx, size_t *plen) {
    size_t l;
    const char *s = luaL_checklstring(L, idx, &l);
    size_t start = b58_posrelatI(luaL_optinteger(L, idx+1, 1), l);
    size_t end = b58_getendpos(L, idx+2, -1, l);
    if (start <= end) {
        *plen = end - start + 1;
        return s + start - 1;
    }
    *plen = 0;
    return "";
}

static char b58_enctbl[58];
static char b58_dectbl[256];

static int Lsettable(lua_State *L) {
    size_t i, len;
    const char *tbl = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz";
    const char *s = luaL_optlstring(L, 1, tbl, &len);
    if (len != 58) return luaL_argerror(L, 1, "length invalid");
    memcpy(b58_enctbl, s, len);
    memset(b58_dectbl, 0xFF, 256);
    for (i = 0; i < len; ++i)
        b58_dectbl[s[i] & 0xFF] = i;
    return 0;
}

static int Lencode(lua_State *L) {
    size_t len, buflen = 0;
    const char *s = b58_checklstring(L, 1, &len), *e = s + len;
    size_t i, j, carry, size = len * 138 / 100 + 1;
    char *buf;
    luaL_Buffer B;
    luaL_buffinit(L, &B);
    for (; s < e && *s == 0; ++s, --len)
        luaL_addchar(&B, b58_enctbl[0]);
    buf = memset(luaL_prepbuffsize(&B, size), 0, size);
    for (; s < e; ++s) {
        for (carry = *s, i = 0; (carry != 0 || i < buflen) && i < size; ++i) {
            carry += buf[size - i - 1] << 8;
            buf[size - i - 1] = carry % 58;
            carry /= 58;
        }
        assert(carry == 0);
        buflen = i;
    }
    i = size - buflen;
    while (i < size && buf[i] == 0)
        ++i, --buflen;
    for (j = 0; i < size; ++i, ++j)
        buf[j] = b58_enctbl[buf[i] & 0xFF];
    luaL_addsize(&B, buflen);
    luaL_pushresult(&B);
    return 1;
}

static int Ldecode(lua_State *L) {
    size_t len, buflen = 0;
    const char *s = b58_checklstring(L, 1, &len), *e = s + len;
    size_t i, carry, size = len * 733 / 1000 + 1;
    char *buf;
    luaL_Buffer B;
    luaL_buffinit(L, &B);
    for (; s < e && *s == b58_enctbl[0]; ++s, --len)
        luaL_addchar(&B, 0);
    buf = memset(luaL_prepbuffsize(&B, size), 0, size);
    for (; s < e; ++s) {
        carry = b58_dectbl[*s & 0xFF] & 0xFF;
        if (carry >= 58)
            return luaL_error(L, "invalid character '%c' in input", *s);
        for (i = 0; (carry != 0 || i < buflen) && i < size; ++i) {
            carry += (buf[size - 1 - i] & 0xFF) * 58;
            buf[size - 1 - i] = carry & 0xFF;
            carry >>= 8;
        }
        assert(carry == 0);
        buflen = i;
    }
    memmove(buf, buf + size - buflen, buflen);
    luaL_addsize(&B, buflen);
    luaL_pushresult(&B);
    return 1;
}

LUALIB_API int luaopen_base58(lua_State *L) {
    luaL_Reg libs[] = {
#define ENTRY(name) { #name, L##name }
        ENTRY(encode),
        ENTRY(decode),
        ENTRY(settable),
#undef  ENTRY
        { NULL, NULL }
    };
    lua_pushcfunction(L, Lsettable);
    lua_call(L, 0, 0);
    luaL_newlib(L, libs);
    return 1;
}

/* cc: flags+='-s -O2 -mdll -DLUA_BUILD_AS_DLL'
 * cc: libs+='-llua54' output='base58.dll' */