implement encode part

reimplement/newset.c

@@ -11,7 +11,6 @@ struct set {
 };
 
 struct set* set_new() {
-  // should we use x___ funcs?
   struct set* set = xmalloc(sizeof *set);
   set->cnt = 0;
   set->symbols_v = NULL;
@@ -48,17 +47,20 @@ struct set* set_free(struct set* set) {
 
 // ---
 
-static unsigned int hash(const char* str) {
-  unsigned int hash = 0x9e3779b9;
+static unsigned hash(const char* str) {
+  unsigned hash = 0x9e3779b9;
   const unsigned char* p = (const unsigned char*)str;
+
   while (*p) {
     hash += *p++;
     hash += (hash << 10);
     hash ^= (hash >> 6);
   }
+
   hash += (hash << 3);
   hash ^= (hash >> 11);
   hash += (hash << 15);
+
   return hash;
 }
 
@@ -72,6 +74,171 @@ int cmp(const void* arg1, const void* arg2) {
   return 0;
 }
 
+// ---
+
+static int log2i(int n) {
+  int m = 0;
+  while (n / 2) m++;
+
+  return m;
+}
+
+// Calculate Mshift paramter for encoding.
+static int encode_golomb_Mshift(int cnt, int bpp) {
+  // XXX Slightly better Mshift estimations are probably possible.
+  // Recheck "Compression and coding algorithms" by Moffat & Turpin.
+  int Mshift = bpp - log2i(cnt) - 1;
+
+  // Adjust out-of-range values.
+  Mshift = (Mshift < 7) ? 7 : Mshift;
+  Mshift = (Mshift > 31) ? 31 : Mshift;
+  assert(Mshift < bpp);
+
+  return Mshift;
+}
+
+// Estimate how many bits can be filled up.
+static inline int encode_golomb_size(int cnt, int Mshift) {
+  // XXX No precise estimation.  However, we do not expect unary-encoded bits
+  // to take more than binary-encoded Mshift bits.
+  return Mshift * 2 * cnt + 16;
+}
+
+// Estimate base62 buffer size required to encode a given number of bits.
+static inline int encode_base62_size(int bit_cnt) {
+  // In the worst case, which is ZxZxZx..., five bits can make a character;
+  // the remaining bits can make a character, too.  And the string must be
+  // null-terminated.
+  return bit_cnt / 5 + 2;
+}
+
+static int encode_set_size(int cnt, int bpp) {
+  int Mshift = encode_golomb_Mshift(cnt, bpp);
+  int bit_cnt = encode_golomb_size(cnt, Mshift);
+  // two leading characters are special
+  return 2 + encode_base62_size(bit_cnt);
+}
+
+// ---
+
+static void encode_delta(int cnt, unsigned* hash_arr_pt) {
+  assert(cnt > 0);
+
+  unsigned* end_pt = hash_arr_pt + cnt;
+  unsigned prev_hash = *hash_arr_pt++;
+
+  while (hash_arr_pt < end_pt) {
+    *hash_arr_pt -= prev_hash;
+    prev_hash += *hash_arr_pt++;
+  }
+
+  return;
+}
+
+// Main golomb encoding routine: package integers into bits.
+// http://algo2.iti.uni-karlsruhe.de/singler/publications/cacheefficientbloomfilters-wea2007.pdf
+// The first integer is then stored in unary coding (which is a variable-length sequence of '0'
+// followed by a terminating '1'); the second part is stored in normal binary coding (using Mshift
+// bits).
+static int encode_golomb(int cnt, const unsigned* delta_arr_pt, int Mshift, char* bit_arr_pt) {
+  char* start_pt = bit_arr_pt;
+  const unsigned mask = (1 << Mshift) - 1;
+
+  for (int i = 0; i < cnt; ++i) {
+    unsigned elem = *delta_arr_pt++;
+
+    // first part: variable-length sequence
+    unsigned q = elem >> Mshift;
+    for (int j = 0; j < (int)q; ++j) {
+      *bit_arr_pt++ = 0;
+    }
+
+    *bit_arr_pt++ = 1;
+
+    // second part: lower Mshift bits
+    unsigned r = elem & mask;
+    for (int j = 0; j < Mshift; ++j) {
+      *bit_arr_pt++ = r & 1;
+      r >>= 1;
+    }
+  }
+
+  return bit_arr_pt - start_pt;
+}
+
+// Main base62 encoding routine: pack bit_arr into base62 string.
+/*
+ * Base62 routines - encode bits with alnum characters.
+ *
+ * This is a base64-based base62 implementation.  Values 0..61 are encoded
+ * with '0'..'9', 'a'..'z', and 'A'..'Z'.  However, 'Z' is special: it will
+ * also encode 62 and 63.  To achieve this, 'Z' will occupy two high bits in
+ * the next character.  Thus 'Z' can be interpreted as an escape character
+ * (which indicates that the next character must be handled specially).
+ * Note that setting high bits to "00", "01" or "10" cannot contribute
+ * to another 'Z' (which would require high bits set to "11").  This is
+ * how multiple escapes are avoided.
+ */
+static int encode_base62(int bit_cnt, const char* bit_arr_pt, char* base62_str_pt) {
+  char* base62_start = base62_str_pt;
+
+  // int bits_Z = 0;  // bits from Z-escape;
+  int bits_cnt = 0;
+  unsigned bits_buf = 1;  // 1 bit as marker
+
+  while (bit_cnt-- > 0) {
+    bits_buf <<= 1;
+    bits_buf |= *bit_arr_pt++;
+
+    if (!(bits_buf & (1 << 6))) continue;
+
+    bits_buf &= ~(1 << 6);  // remove flag
+
+    if (bits_buf >= 61) {  // 61 62 63 cases
+      base62_str_pt = bits_to_char(61, base62_str_pt);
+      bits_buf = (1 << 2) | (bits_buf - 61);  // 1 (00|01|10)
+    } else {
+      base62_str_pt = bits_to_char(bits_buf, base62_str_pt);
+      bits_buf = 1;
+    }
+  }
+
+  // flush buffer
+  if (bits_buf != 1) {
+    unsigned sliding_one = 1 << 6;
+    while (sliding_one > bits_buf) sliding_one >>= 1;
+    bits_buf &= ~sliding_one;  // remove flag
+
+    assert(bits_buf < 61);  // should not be 61 62 63 cases
+    base62_str_pt = bits_to_char(bits_buf, base62_str_pt);
+  }
+
+  *base62_str_pt = '\0';
+  return base62_str_pt - base62_start;
+}
+
+// ---
+
+static int encode_set(int cnt, unsigned* hash_arr, int bpp, char* base62_str) {
+  int Mshift = encode_golomb_Mshift(cnt, bpp);
+  int bit_cnt = encode_golomb_size(cnt, Mshift);
+
+  char bit_arr[bit_cnt];
+
+  *base62_str++ = bpp - 7 + 'a';
+  *base62_str++ = Mshift - 7 + 'a';
+
+  // hash_arr -> delta_arr
+  encode_delta(cnt, hash_arr);
+  bit_cnt = encode_golomb(cnt, hash_arr, Mshift, bit_arr);
+  assert(bit_cnt >= 0);
+
+  size_t base62_len = encode_base62(bit_cnt, bit_arr, base62_str);
+  assert(base62_len > 0);
+
+  return 2 + base62_len;
+}
+
 const char* set_fini(struct set* set, int bpp) {
   // Implementation for finalizing the set
 
@@ -107,5 +274,19 @@ const char* set_fini(struct set* set, int bpp) {
     unique_hash[unique_cnt++] = set->symbols_v[i].hash;
   }
 
-  return NULL;
+  char base62_str[encode_set_size(unique_cnt, bpp)];
+  encode_set(unique_cnt, unique_hash, bpp, base62_str);
+
+  return xstrdup(base62_str);
+}
+
+int main(void) {
+  struct set* set1 = set_new();
+  set_add(set1, "mama");
+  set_add(set1, "myla");
+  set_add(set1, "ramu");
+  const char* str10 = set_fini(set1, 16);
+  fprintf(stderr, "set10=%s\n", str10);
+
+  return 0;
 }