/*
 * base64.h — Standalone RFC 4648 Base64 encode/decode, header-only (static inline).
 * No external dependencies.
 */
#ifndef BASE64_H
#define BASE64_H

#include <stdint.h>
#include <stddef.h>
#include <string.h>

static const char B64_ENC[] =
    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

/*
 * base64_encode - Encode src_len bytes to a NUL-terminated Base64 string.
 * dst must be at least ((src_len + 2) / 3) * 4 + 1 bytes.
 */
static inline void base64_encode(const uint8_t *src, size_t src_len, char *dst)
{
    size_t i = 0, j = 0;

    while (i + 2 < src_len) {
        uint32_t v = ((uint32_t)src[i] << 16)
                   | ((uint32_t)src[i+1] << 8)
                   |  (uint32_t)src[i+2];
        dst[j++] = B64_ENC[(v >> 18) & 0x3F];
        dst[j++] = B64_ENC[(v >> 12) & 0x3F];
        dst[j++] = B64_ENC[(v >>  6) & 0x3F];
        dst[j++] = B64_ENC[ v        & 0x3F];
        i += 3;
    }

    if (i < src_len) {
        uint32_t v = (uint32_t)src[i] << 16;
        if (i + 1 < src_len) v |= (uint32_t)src[i+1] << 8;
        dst[j++] = B64_ENC[(v >> 18) & 0x3F];
        dst[j++] = B64_ENC[(v >> 12) & 0x3F];
        dst[j++] = (i + 1 < src_len) ? B64_ENC[(v >> 6) & 0x3F] : '=';
        dst[j++] = '=';
    }

    dst[j] = '\0';
}

/*
 * base64_decode - Decode a Base64 string to bytes.
 * Returns number of decoded bytes, or -1 on invalid input.
 * dst must be at least (strlen(src) / 4) * 3 bytes.
 */
static inline int base64_decode(const char *src, uint8_t *dst)
{
    /* Lookup table: 0-63=value, 64='=' padding, -1=invalid */
    static const int8_t LUT[256] = {
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,  /* 0x00 */
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,  /* 0x10 */
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63,  /* 0x20 */
        52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, 64, -1, -1,  /* 0x30 */
        -1,  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14,  /* 0x40 */
        15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1,  /* 0x50 */
        -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,  /* 0x60 */
        41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1,  /* 0x70 */
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,  /* 0x80 */
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,  /* 0x90 */
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,  /* 0xa0 */
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,  /* 0xb0 */
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,  /* 0xc0 */
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,  /* 0xd0 */
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,  /* 0xe0 */
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1   /* 0xf0 */
    };

    int len = (int)strlen(src);
    if (len % 4 != 0) return -1;

    int out = 0;
    for (int i = 0; i < len; i += 4) {
        int8_t a = LUT[(uint8_t)src[i]];
        int8_t b = LUT[(uint8_t)src[i+1]];
        int8_t c = LUT[(uint8_t)src[i+2]];
        int8_t d = LUT[(uint8_t)src[i+3]];
        if (a < 0 || b < 0 || c < 0 || d < 0) return -1;

        uint32_t v = ((uint32_t)a << 18)
                   | ((uint32_t)b << 12)
                   | ((c == 64) ? 0 : ((uint32_t)c << 6))
                   | ((d == 64) ? 0 :  (uint32_t)d);

        dst[out++] = (uint8_t)(v >> 16);
        if (c != 64) dst[out++] = (uint8_t)(v >> 8);
        if (d != 64) dst[out++] = (uint8_t)v;
    }
    return out;
}

#endif /* BASE64_H */