Bitops: u64_log2() and encoding variable-length bitstrings into u64 (6cc547a1) · Commits · 戴 / Bird

lib/bitops.c

+12 −0

Original line number	Diff line number	Diff line
		@@ -68,3 +68,15 @@ u32_log2(u32 v)
		return r;
		}

		u64
		u64_log2(u64 v)
		{
		u64 r, shift;
		r = (v > 0xFFFFFFFFLL) << 5; v >>= r;
		shift = (v > 0xFFFF ) << 4; v >>= shift; r \|= shift;
		shift = (v > 0xFF ) << 3; v >>= shift; r \|= shift;
		shift = (v > 0xF ) << 2; v >>= shift; r \|= shift;
		shift = (v > 0x3 ) << 1; v >>= shift; r \|= shift;
		r \|= (v >> 1);
		return r;
		}

lib/bitops.h

+38 −0

Original line number	Diff line number	Diff line
		@@ -18,15 +18,53 @@
		* from the left and the rest filled with zeroes.
		* E.g., u32_mkmask(5) = 0xf8000000.
		* u32_masklen Inverse operation to u32_mkmask, -1 if not a bitmask.
		*
		* u32_log2
		* u64_log2 Find the first 1 in the number
		*
		* u32_hash Compute a common hash
		*
		* u64_var_encode Encode a variable-length bitstring into fixed-length u64
		* u64_var_decode Decode the bitstring
		*/

		u32 u32_mkmask(uint n);
		uint u32_masklen(u32 x);

		u32 u32_log2(u32 v);
		u64 u64_log2(u64 v);

		static inline u32 u32_hash(u32 v) { return v * 2902958171u; }

		static inline u8 u32_popcount(u32 v) { return __builtin_popcount(v); }

		static inline u64 u64_var_encode(u64 data, uint padlen)
		{
		ASSERT(padlen > 0);

		/* Append the other bit than the last */
		if (data & 1)
		return data << padlen;
		else
		return (data << padlen) \| ((1ULL << padlen) - 1);
		}

		static inline u64 u64_var_decode(u64 enc, uint *padlen)
		{
		/* If enc is ....\|100..00, then cpl is ....\|011..11
		* If enc is ....\|011..11, then cpl is ....\|100..00
		*
		* In both cases, enc ^ cpl is then 0...0\|111..11
		* so u64_log2((enc ^ cpl) + 1) is the number of bits to shift right.
		* */
		u64 cpl = (enc & 1) ? (enc + 1) : (enc - 1);
		if ((~enc == 0) \|\| (~cpl == 0)) {
		*padlen = 64;
		return 0;
		} else {
		*padlen = u64_log2(enc ^ cpl);
		return enc >> *padlen;
		}
		}

		#endif

lib/bitops_test.c

+41 −5

Original line number	Diff line number	Diff line
		@@ -82,11 +82,14 @@ t_masklen(void)
		}

		static void
		check_log2(u32 n)
		check_log2(u64 n)
		{
		u32 log = u32_log2(n);
		u32 low = bt_naive_pow(2, log);
		u32 high = bt_naive_pow(2, log+1);
		u64 log = u64_log2(n);
		u64 low = bt_naive_pow(2, log);
		u64 high = bt_naive_pow(2, log+1);

		if (n <= 0xffffffff)
		bt_assert(u32_log2(n) == log);

		bt_assert_msg(n >= low && n < high,
		"u32_log2(%u) = %u, %u should be in the range <%u, %u)",
		@@ -101,15 +104,47 @@ t_log2(void)
		for (i = 0; i < 31; i++)
		bt_assert(u32_log2(bt_naive_pow(2, i+1)) == i+1);

		for (i = 0; i < 63; i++)
		bt_assert(u64_log2(bt_naive_pow(2, i+1)) == i+1);

		for (i = 1; i < MAX_NUM; i++)
		check_log2(i);

		for (i = 1; i < MAX_NUM; i++)
		check_log2(((u32) bt_random()) % 0x0fffffff);
		check_log2((unsigned long int) bt_random());

		return 1;
		}

		static void
		var_enc_dec(u64 data, uint padlen)
		{
		uint olen = ~0;
		u64 enc = u64_var_encode(data, padlen);
		u64 odata = u64_var_decode(enc, &olen);
		bt_assert_msg(
		(odata == data) && (olen == padlen),
		"u64_var_encode(0x%llx, %u) == 0x%llx; u64_var_decode(0x%llx, %u) == 0x%llx",
		data, padlen, enc, enc, olen, odata
		);
		}

		static int
		t_var(void)
		{
		for (uint i = 0; i < 63; i++)
		for (uint j = 1; j+i < 64; j++) {
		var_enc_dec(1ULL << i, j);
		var_enc_dec((1ULL << i) - 1, j);
		var_enc_dec(((unsigned long int) bt_random()) & ((1ULL << (64-j)) - 1), j);
		}

		return 1;
		}




		int
		main(int argc, char *argv[])
		{
		@@ -118,6 +153,7 @@ main(int argc, char *argv[])
		bt_test_suite(t_mkmask, "u32_mkmask()");
		bt_test_suite(t_masklen, "u32_masklen()");
		bt_test_suite(t_log2, "u32_log2()");
		bt_test_suite(t_var, "u64_var_(en\|de)code()");

		return bt_exit_value();
		}

test/bt-utils.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -195,10 +195,10 @@ bt_config_file_parse(const char *filepath)
		/*
		* Returns @base raised to the power of @power.
		*/
		uint
		u64
		bt_naive_pow(uint base, uint power)
		{
		uint result = 1;
		u64 result = 1;
		uint i;
		for (i = 0; i < power; i++)
		result *= base;

test/bt-utils.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -24,7 +24,7 @@
		#define BT_CONFIG_PARSE_STATIC_PROTO "protocol static { ipv4; } \n"
		#define BT_CONFIG_SIMPLE BT_CONFIG_PARSE_ROUTER_ID BT_CONFIG_PARSE_STATIC_PROTO

		uint bt_naive_pow(uint base, uint power);
		u64 bt_naive_pow(uint base, uint power);
		void bt_bytes_to_hex(char buf, const byte in_data, size_t size);

		void bt_bird_init(void);

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