// from https://github.com/floodyberry/poly1305-donna.git

/*
	poly1305 implementation using 32 bit * 32 bit = 64 bit multiplication and 64 bit addition
*/

#if defined(_MSC_VER)
	#define POLY1305_NOINLINE __declspec(noinline)
#elif defined(__GNUC__)
	#define POLY1305_NOINLINE __attribute__((noinline))
#else
	#define POLY1305_NOINLINE
#endif

#define poly1305_block_size 16

/* 17 + sizeof(size_t) + 14*sizeof(unsigned long) */
typedef struct poly1305_state_internal_t {
	unsigned long r[5];
	unsigned long h[5];
	unsigned long pad[4];
	size_t leftover;
	unsigned char buffer[poly1305_block_size];
	unsigned char final;
} poly1305_state_internal_t;

/* interpret four 8 bit unsigned integers as a 32 bit unsigned integer in little endian */
static unsigned long
U8TO32(const unsigned char *p) {
	return
		(((unsigned long)(p[0] & 0xff)      ) |
	     ((unsigned long)(p[1] & 0xff) <<  8) |
         ((unsigned long)(p[2] & 0xff) << 16) |
         ((unsigned long)(p[3] & 0xff) << 24));
}

/* store a 32 bit unsigned integer as four 8 bit unsigned integers in little endian */
static void
U32TO8(unsigned char *p, unsigned long v) {
	p[0] = (v      ) & 0xff;
	p[1] = (v >>  8) & 0xff;
	p[2] = (v >> 16) & 0xff;
	p[3] = (v >> 24) & 0xff;
}

static void
poly1305_init(poly1305_context *ctx, const unsigned char key[32]) {
	poly1305_state_internal_t *st = (poly1305_state_internal_t *)ctx;

	/* r &= 0xffffffc0ffffffc0ffffffc0fffffff */
	st->r[0] = (U8TO32(&key[ 0])     ) & 0x3ffffff;
	st->r[1] = (U8TO32(&key[ 3]) >> 2) & 0x3ffff03;
	st->r[2] = (U8TO32(&key[ 6]) >> 4) & 0x3ffc0ff;
	st->r[3] = (U8TO32(&key[ 9]) >> 6) & 0x3f03fff;
	st->r[4] = (U8TO32(&key[12]) >> 8) & 0x00fffff;

	/* h = 0 */
	st->h[0] = 0;
	st->h[1] = 0;
	st->h[2] = 0;
	st->h[3] = 0;
	st->h[4] = 0;

	/* save pad for later */
	st->pad[0] = U8TO32(&key[16]);
	st->pad[1] = U8TO32(&key[20]);
	st->pad[2] = U8TO32(&key[24]);
	st->pad[3] = U8TO32(&key[28]);

	st->leftover = 0;
	st->final = 0;
}

static void
poly1305_blocks(poly1305_state_internal_t *st, const unsigned char *m, size_t bytes) {
	const unsigned long hibit = (st->final) ? 0 : (1UL << 24); /* 1 << 128 */
	unsigned long r0,r1,r2,r3,r4;
	unsigned long s1,s2,s3,s4;
	unsigned long h0,h1,h2,h3,h4;
	unsigned long long d0,d1,d2,d3,d4;
	unsigned long c;

	r0 = st->r[0];
	r1 = st->r[1];
	r2 = st->r[2];
	r3 = st->r[3];
	r4 = st->r[4];

	s1 = r1 * 5;
	s2 = r2 * 5;
	s3 = r3 * 5;
	s4 = r4 * 5;

	h0 = st->h[0];
	h1 = st->h[1];
	h2 = st->h[2];
	h3 = st->h[3];
	h4 = st->h[4];

	while (bytes >= poly1305_block_size) {
		/* h += m[i] */
		h0 += (U8TO32(m+ 0)     ) & 0x3ffffff;
		h1 += (U8TO32(m+ 3) >> 2) & 0x3ffffff;
		h2 += (U8TO32(m+ 6) >> 4) & 0x3ffffff;
		h3 += (U8TO32(m+ 9) >> 6) & 0x3ffffff;
		h4 += (U8TO32(m+12) >> 8) | hibit;

		/* h *= r */
		d0 = ((unsigned long long)h0 * r0) + ((unsigned long long)h1 * s4) + ((unsigned long long)h2 * s3) + ((unsigned long long)h3 * s2) + ((unsigned long long)h4 * s1);
		d1 = ((unsigned long long)h0 * r1) + ((unsigned long long)h1 * r0) + ((unsigned long long)h2 * s4) + ((unsigned long long)h3 * s3) + ((unsigned long long)h4 * s2);
		d2 = ((unsigned long long)h0 * r2) + ((unsigned long long)h1 * r1) + ((unsigned long long)h2 * r0) + ((unsigned long long)h3 * s4) + ((unsigned long long)h4 * s3);
		d3 = ((unsigned long long)h0 * r3) + ((unsigned long long)h1 * r2) + ((unsigned long long)h2 * r1) + ((unsigned long long)h3 * r0) + ((unsigned long long)h4 * s4);
		d4 = ((unsigned long long)h0 * r4) + ((unsigned long long)h1 * r3) + ((unsigned long long)h2 * r2) + ((unsigned long long)h3 * r1) + ((unsigned long long)h4 * r0);

		/* (partial) h %= p */
		              c = (unsigned long)(d0 >> 26); h0 = (unsigned long)d0 & 0x3ffffff;
		d1 += c;      c = (unsigned long)(d1 >> 26); h1 = (unsigned long)d1 & 0x3ffffff;
		d2 += c;      c = (unsigned long)(d2 >> 26); h2 = (unsigned long)d2 & 0x3ffffff;
		d3 += c;      c = (unsigned long)(d3 >> 26); h3 = (unsigned long)d3 & 0x3ffffff;
		d4 += c;      c = (unsigned long)(d4 >> 26); h4 = (unsigned long)d4 & 0x3ffffff;
		h0 += c * 5;  c =                (h0 >> 26); h0 =                h0 & 0x3ffffff;
		h1 += c;

		m += poly1305_block_size;
		bytes -= poly1305_block_size;
	}

	st->h[0] = h0;
	st->h[1] = h1;
	st->h[2] = h2;
	st->h[3] = h3;
	st->h[4] = h4;
}

POLY1305_NOINLINE static void
poly1305_finish(poly1305_context *ctx, unsigned char mac[16]) {
	poly1305_state_internal_t *st = (poly1305_state_internal_t *)ctx;
	unsigned long h0,h1,h2,h3,h4,c;
	unsigned long g0,g1,g2,g3,g4;
	unsigned long long f;
	unsigned long mask;

	/* process the remaining block */
	if (st->leftover) {
		size_t i = st->leftover;
		st->buffer[i++] = 1;
		for (; i < poly1305_block_size; i++)
			st->buffer[i] = 0;
		st->final = 1;
		poly1305_blocks(st, st->buffer, poly1305_block_size);
	}

	/* fully carry h */
	h0 = st->h[0];
	h1 = st->h[1];
	h2 = st->h[2];
	h3 = st->h[3];
	h4 = st->h[4];

	             c = h1 >> 26; h1 = h1 & 0x3ffffff;
	h2 +=     c; c = h2 >> 26; h2 = h2 & 0x3ffffff;
	h3 +=     c; c = h3 >> 26; h3 = h3 & 0x3ffffff;
	h4 +=     c; c = h4 >> 26; h4 = h4 & 0x3ffffff;
	h0 += c * 5; c = h0 >> 26; h0 = h0 & 0x3ffffff;
	h1 +=     c;

	/* compute h + -p */
	g0 = h0 + 5; c = g0 >> 26; g0 &= 0x3ffffff;
	g1 = h1 + c; c = g1 >> 26; g1 &= 0x3ffffff;
	g2 = h2 + c; c = g2 >> 26; g2 &= 0x3ffffff;
	g3 = h3 + c; c = g3 >> 26; g3 &= 0x3ffffff;
	g4 = h4 + c - (1UL << 26);

	/* select h if h < p, or h + -p if h >= p */
	mask = (g4 >> ((sizeof(unsigned long) * 8) - 1)) - 1;
	g0 &= mask;
	g1 &= mask;
	g2 &= mask;
	g3 &= mask;
	g4 &= mask;
	mask = ~mask;
	h0 = (h0 & mask) | g0;
	h1 = (h1 & mask) | g1;
	h2 = (h2 & mask) | g2;
	h3 = (h3 & mask) | g3;
	h4 = (h4 & mask) | g4;

	/* h = h % (2^128) */
	h0 = ((h0      ) | (h1 << 26)) & 0xffffffff;
	h1 = ((h1 >>  6) | (h2 << 20)) & 0xffffffff;
	h2 = ((h2 >> 12) | (h3 << 14)) & 0xffffffff;
	h3 = ((h3 >> 18) | (h4 <<  8)) & 0xffffffff;

	/* mac = (h + pad) % (2^128) */
	f = (unsigned long long)h0 + st->pad[0]            ; h0 = (unsigned long)f;
	f = (unsigned long long)h1 + st->pad[1] + (f >> 32); h1 = (unsigned long)f;
	f = (unsigned long long)h2 + st->pad[2] + (f >> 32); h2 = (unsigned long)f;
	f = (unsigned long long)h3 + st->pad[3] + (f >> 32); h3 = (unsigned long)f;

	U32TO8(mac +  0, h0);
	U32TO8(mac +  4, h1);
	U32TO8(mac +  8, h2);
	U32TO8(mac + 12, h3);

	/* zero out the state */
	st->h[0] = 0;
	st->h[1] = 0;
	st->h[2] = 0;
	st->h[3] = 0;
	st->h[4] = 0;
	st->r[0] = 0;
	st->r[1] = 0;
	st->r[2] = 0;
	st->r[3] = 0;
	st->r[4] = 0;
	st->pad[0] = 0;
	st->pad[1] = 0;
	st->pad[2] = 0;
	st->pad[3] = 0;
}

