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Bitops: u64_log2() and encoding variable-length bitstrings into u64
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parent
3a22a6e858
commit
6cc547a13d
12
lib/bitops.c
12
lib/bitops.c
@ -68,3 +68,15 @@ u32_log2(u32 v)
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return r;
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}
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u64
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u64_log2(u64 v)
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{
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u64 r, shift;
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r = (v > 0xFFFFFFFFLL) << 5; v >>= r;
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shift = (v > 0xFFFF ) << 4; v >>= shift; r |= shift;
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shift = (v > 0xFF ) << 3; v >>= shift; r |= shift;
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shift = (v > 0xF ) << 2; v >>= shift; r |= shift;
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shift = (v > 0x3 ) << 1; v >>= shift; r |= shift;
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r |= (v >> 1);
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return r;
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}
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38
lib/bitops.h
38
lib/bitops.h
@ -18,15 +18,53 @@
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* from the left and the rest filled with zeroes.
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* E.g., u32_mkmask(5) = 0xf8000000.
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* u32_masklen Inverse operation to u32_mkmask, -1 if not a bitmask.
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*
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* u32_log2
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* u64_log2 Find the first 1 in the number
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*
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* u32_hash Compute a common hash
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*
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* u64_var_encode Encode a variable-length bitstring into fixed-length u64
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* u64_var_decode Decode the bitstring
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*/
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u32 u32_mkmask(uint n);
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uint u32_masklen(u32 x);
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u32 u32_log2(u32 v);
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u64 u64_log2(u64 v);
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static inline u32 u32_hash(u32 v) { return v * 2902958171u; }
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static inline u8 u32_popcount(u32 v) { return __builtin_popcount(v); }
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static inline u64 u64_var_encode(u64 data, uint padlen)
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{
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ASSERT(padlen > 0);
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/* Append the other bit than the last */
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if (data & 1)
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return data << padlen;
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else
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return (data << padlen) | ((1ULL << padlen) - 1);
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}
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static inline u64 u64_var_decode(u64 enc, uint *padlen)
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{
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/* If enc is ....|100..00, then cpl is ....|011..11
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* If enc is ....|011..11, then cpl is ....|100..00
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*
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* In both cases, enc ^ cpl is then 0...0|111..11
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* so u64_log2((enc ^ cpl) + 1) is the number of bits to shift right.
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* */
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u64 cpl = (enc & 1) ? (enc + 1) : (enc - 1);
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if ((~enc == 0) || (~cpl == 0)) {
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*padlen = 64;
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return 0;
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} else {
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*padlen = u64_log2(enc ^ cpl);
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return enc >> *padlen;
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}
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}
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#endif
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@ -82,11 +82,14 @@ t_masklen(void)
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}
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static void
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check_log2(u32 n)
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check_log2(u64 n)
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{
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u32 log = u32_log2(n);
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u32 low = bt_naive_pow(2, log);
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u32 high = bt_naive_pow(2, log+1);
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u64 log = u64_log2(n);
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u64 low = bt_naive_pow(2, log);
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u64 high = bt_naive_pow(2, log+1);
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if (n <= 0xffffffff)
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bt_assert(u32_log2(n) == log);
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bt_assert_msg(n >= low && n < high,
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"u32_log2(%u) = %u, %u should be in the range <%u, %u)",
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@ -101,15 +104,47 @@ t_log2(void)
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for (i = 0; i < 31; i++)
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bt_assert(u32_log2(bt_naive_pow(2, i+1)) == i+1);
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for (i = 0; i < 63; i++)
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bt_assert(u64_log2(bt_naive_pow(2, i+1)) == i+1);
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for (i = 1; i < MAX_NUM; i++)
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check_log2(i);
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for (i = 1; i < MAX_NUM; i++)
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check_log2(((u32) bt_random()) % 0x0fffffff);
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check_log2((unsigned long int) bt_random());
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return 1;
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}
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static void
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var_enc_dec(u64 data, uint padlen)
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{
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uint olen = ~0;
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u64 enc = u64_var_encode(data, padlen);
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u64 odata = u64_var_decode(enc, &olen);
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bt_assert_msg(
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(odata == data) && (olen == padlen),
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"u64_var_encode(0x%llx, %u) == 0x%llx; u64_var_decode(0x%llx, %u) == 0x%llx",
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data, padlen, enc, enc, olen, odata
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);
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}
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static int
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t_var(void)
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{
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for (uint i = 0; i < 63; i++)
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for (uint j = 1; j+i < 64; j++) {
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var_enc_dec(1ULL << i, j);
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var_enc_dec((1ULL << i) - 1, j);
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var_enc_dec(((unsigned long int) bt_random()) & ((1ULL << (64-j)) - 1), j);
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}
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return 1;
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}
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int
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main(int argc, char *argv[])
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{
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@ -118,6 +153,7 @@ main(int argc, char *argv[])
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bt_test_suite(t_mkmask, "u32_mkmask()");
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bt_test_suite(t_masklen, "u32_masklen()");
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bt_test_suite(t_log2, "u32_log2()");
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bt_test_suite(t_var, "u64_var_(en|de)code()");
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return bt_exit_value();
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}
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@ -195,10 +195,10 @@ bt_config_file_parse(const char *filepath)
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/*
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* Returns @base raised to the power of @power.
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*/
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uint
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u64
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bt_naive_pow(uint base, uint power)
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{
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uint result = 1;
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u64 result = 1;
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uint i;
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for (i = 0; i < power; i++)
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result *= base;
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@ -24,7 +24,7 @@
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#define BT_CONFIG_PARSE_STATIC_PROTO "protocol static { ipv4; } \n"
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#define BT_CONFIG_SIMPLE BT_CONFIG_PARSE_ROUTER_ID BT_CONFIG_PARSE_STATIC_PROTO
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uint bt_naive_pow(uint base, uint power);
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u64 bt_naive_pow(uint base, uint power);
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void bt_bytes_to_hex(char *buf, const byte *in_data, size_t size);
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void bt_bird_init(void);
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