Make ziplist schema more efficient for strings with length > 15

diff --git a/src/ziplist.c b/src/ziplist.c
index 7a3a8b01b5cc47e955b9b0cc6f97194f7608d95e..a6383517d008152b1c591ea9dfedde38a47c085d 100644 (file)
--- a/src/ziplist.c
+++ b/src/ziplist.c
@@ -1,17 +1,63 @@
-/* Memory layout of a ziplist, containing "foo", "bar", "quux":
- * <zlbytes><zllen><len>"foo"<len>"bar"<len>"quux"
+/* The ziplist is a specially encoded dually linked list that is designed
+ * to be very memory efficient. It stores both strings and integer values,
+ * where integers are encoded as actual integers instead of a series of
+ * characters. It allows push and pop operations on either side of the list
+ * in O(1) time. However, because every operation requires a reallocation of
+ * the memory used by the ziplist, the actual complexity is related to the
+ * amount of memory used by the ziplist.

  *
  *

- * <zlbytes> is an unsigned integer to hold the number of bytes that
- * the ziplist occupies. This is stored to not have to traverse the ziplist
- * to know the new length when pushing.
+ * ----------------------------------------------------------------------------

  *
  *

- * <zllen> is the number of items in the ziplist. When this value is
- * greater than 254, we need to traverse the entire list to know
- * how many items it holds.
+ * ZIPLIST OVERALL LAYOUT:
+ * The general layout of the ziplist is as follows:
+ * <zlbytes><zltail><zllen><entry><entry><zlend>

  *
  *

- * <len> is the number of bytes occupied by a single entry. When this
- * number is greater than 253, the length will occupy 5 bytes, where
- * the extra bytes contain an unsigned integer to hold the length.
+ * <zlbytes> is an unsigned integer to hold the number of bytes that the
+ * ziplist occupies. This value needs to be stored to be able to resize the
+ * entire structure without the need to traverse it first.
+ *
+ * <zltail> is the offset to the last entry in the list. This allows a pop
+ * operation on the far side of the list without the need for full traversal.
+ *
+ * <zllen> is the number of entries.When this value is larger than 2**16-2,
+ * we need to traverse the entire list to know how many items it holds.
+ *
+ * <zlend> is a single byte special value, equal to 255, which indicates the
+ * end of the list.
+ *
+ * ZIPLIST ENTRIES:
+ * Every entry in the ziplist is prefixed by a header that contains two pieces
+ * of information. First, the length of the previous entry is stored to be
+ * able to traverse the list from back to front. Second, the encoding with an
+ * optional string length of the entry itself is stored.
+ *
+ * The length of the previous entry is encoded in the following way:
+ * If this length is smaller than 254 bytes, it will only consume a single
+ * byte that takes the length as value. When the length is greater than or
+ * equal to 254, it will consume 5 bytes. The first byte is set to 254 to
+ * indicate a larger value is following. The remaining 4 bytes take the
+ * length of the previous entry as value.
+ *
+ * The other header field of the entry itself depends on the contents of the
+ * entry. When the entry is a string, the first 2 bits of this header will hold
+ * the type of encoding used to store the length of the string, followed by the
+ * actual length of the string. When the entry is an integer the first 2 bits
+ * are both set to 1. The following 2 bits are used to specify what kind of
+ * integer will be stored after this header. An overview of the different
+ * types and encodings is as follows:
+ *
+ * |00pppppp| - 1 byte
+ *      String value with length less than or equal to 63 bytes (6 bits).
+ * |01pppppp|qqqqqqqq| - 2 bytes
+ *      String value with length less than or equal to 16383 bytes (14 bits).
+ * |10______|qqqqqqqq|rrrrrrrr|ssssssss|tttttttt| - 5 bytes
+ *      String value with length greater than or equal to 16384 bytes.
+ * |1100____| - 1 byte
+ *      Integer encoded as int16_t (2 bytes).
+ * |1101____| - 1 byte
+ *      Integer encoded as int32_t (4 bytes).
+ * |1110____| - 1 byte
+ *      Integer encoded as int64_t (8 bytes).

  */
 
 #include <stdio.h>
  */
 
 #include <stdio.h>


@@ -25,25 +71,20 @@
 
 int ll2string(char *s, size_t len, long long value);
 
 
 int ll2string(char *s, size_t len, long long value);
 

-/* Important note: the ZIP_END value is used to depict the end of the
- * ziplist structure. When a pointer contains an entry, the first couple
- * of bytes contain the encoded length of the previous entry. This length
- * is encoded as ZIP_ENC_RAW length, so the first two bits will contain 00
- * and the byte will therefore never have a value of 255. */

 #define ZIP_END 255
 #define ZIP_BIGLEN 254
 
 #define ZIP_END 255
 #define ZIP_BIGLEN 254
 

-/* Entry encoding */
-#define ZIP_ENC_RAW     0
-#define ZIP_ENC_INT16   1
-#define ZIP_ENC_INT32   2
-#define ZIP_ENC_INT64   3
-#define ZIP_ENCODING(p) ((p)[0] >> 6)
+/* Different encoding/length possibilities */
+#define ZIP_STR_06B (0 << 6)
+#define ZIP_STR_14B (1 << 6)
+#define ZIP_STR_32B (2 << 6)
+#define ZIP_INT_16B (0xc0 | 0<<4)
+#define ZIP_INT_32B (0xc0 | 1<<4)
+#define ZIP_INT_64B (0xc0 | 2<<4)

 
 

-/* Length encoding for raw entries */
-#define ZIP_LEN_INLINE  0
-#define ZIP_LEN_UINT16  1
-#define ZIP_LEN_UINT32  2
+/* Macro's to determine type */
+#define ZIP_IS_STR(enc) (((enc) & 0xc0) < 0xc0)
+#define ZIP_IS_INT(enc) (!ZIP_IS_STR(enc) && ((enc) & 0x30) < 0x30)

 
 /* Utility macros */
 #define ZIPLIST_BYTES(zl)       (*((uint32_t*)(zl)))
 
 /* Utility macros */
 #define ZIPLIST_BYTES(zl)       (*((uint32_t*)(zl)))


@@ -67,14 +108,25 @@ typedef struct zlentry {
     unsigned char *p;
 } zlentry;
 
     unsigned char *p;
 } zlentry;
 

+/* Return the encoding pointer to by 'p'. */
+static unsigned int zipEntryEncoding(unsigned char *p) {
+    /* String encoding: 2 MSBs */
+    unsigned char b = p[0] & 0xc0;
+    if (b < 0xc0) {
+        return b;
+    } else {
+        /* Integer encoding: 4 MSBs */
+        return p[0] & 0xf0;
+    }
+    assert(NULL);
+}
+

 /* Return bytes needed to store integer encoded by 'encoding' */
 /* Return bytes needed to store integer encoded by 'encoding' */

-static unsigned int zipEncodingSize(unsigned char encoding) {
-    if (encoding == ZIP_ENC_INT16) {
-        return sizeof(int16_t);
-    } else if (encoding == ZIP_ENC_INT32) {
-        return sizeof(int32_t);
-    } else if (encoding == ZIP_ENC_INT64) {
-        return sizeof(int64_t);
+static unsigned int zipIntSize(unsigned char encoding) {
+    switch(encoding) {
+    case ZIP_INT_16B: return sizeof(int16_t);
+    case ZIP_INT_32B: return sizeof(int32_t);
+    case ZIP_INT_64B: return sizeof(int64_t);

     }
     assert(NULL);
 }
     }
     assert(NULL);
 }


@@ -82,23 +134,28 @@ static unsigned int zipEncodingSize(unsigned char encoding) {
 /* Decode the encoded length pointed by 'p'. If a pointer to 'lensize' is
  * provided, it is set to the number of bytes required to encode the length. */
 static unsigned int zipDecodeLength(unsigned char *p, unsigned int *lensize) {
 /* Decode the encoded length pointed by 'p'. If a pointer to 'lensize' is
  * provided, it is set to the number of bytes required to encode the length. */
 static unsigned int zipDecodeLength(unsigned char *p, unsigned int *lensize) {

-    unsigned char encoding = ZIP_ENCODING(p), lenenc;
+    unsigned char encoding = zipEntryEncoding(p);

     unsigned int len;
 
     unsigned int len;
 

-    if (encoding == ZIP_ENC_RAW) {
-        lenenc = (p[0] >> 4) & 0x3;
-        if (lenenc == ZIP_LEN_INLINE) {
-            len = p[0] & 0xf;
+    if (ZIP_IS_STR(encoding)) {
+        switch(encoding) {
+        case ZIP_STR_06B:
+            len = p[0] & 0x3f;

             if (lensize) *lensize = 1;
             if (lensize) *lensize = 1;

-        } else if (lenenc == ZIP_LEN_UINT16) {
-            len = p[1] | (p[2] << 8);
-            if (lensize) *lensize = 3;
-        } else {
-            len = p[1] | (p[2] << 8) | (p[3] << 16) | (p[4] << 24);
+            break;
+        case ZIP_STR_14B:
+            len = ((p[0] & 0x3f) << 6) | p[1];
+            if (lensize) *lensize = 2;
+            break;
+        case ZIP_STR_32B:
+            len = (p[1] << 24) | (p[2] << 16) | (p[3] << 8) | p[4];

             if (lensize) *lensize = 5;
             if (lensize) *lensize = 5;

+            break;
+        default:
+            assert(NULL);

         }
     } else {
         }
     } else {

-        len = zipEncodingSize(encoding);
+        len = zipIntSize(encoding);

         if (lensize) *lensize = 1;
     }
     return len;
         if (lensize) *lensize = 1;
     }
     return len;


@@ -106,34 +163,36 @@ static unsigned int zipDecodeLength(unsigned char *p, unsigned int *lensize) {
 
 /* Encode the length 'l' writing it in 'p'. If p is NULL it just returns
  * the amount of bytes required to encode such a length. */
 
 /* Encode the length 'l' writing it in 'p'. If p is NULL it just returns
  * the amount of bytes required to encode such a length. */

-static unsigned int zipEncodeLength(unsigned char *p, char encoding, unsigned int rawlen) {
-    unsigned char len = 1, lenenc, buf[5];
-    if (encoding == ZIP_ENC_RAW) {
-        if (rawlen <= 0xf) {
+static unsigned int zipEncodeLength(unsigned char *p, unsigned char encoding, unsigned int rawlen) {
+    unsigned char len = 1, buf[5];
+
+    if (ZIP_IS_STR(encoding)) {
+        /* Although encoding is given it may not be set for strings,
+         * so we determine it here using the raw length. */
+        if (rawlen <= 0x3f) {

             if (!p) return len;
             if (!p) return len;

-            lenenc = ZIP_LEN_INLINE;
-            buf[0] = rawlen;
-        } else if (rawlen <= 0xffff) {
-            len += 2;
+            buf[0] = ZIP_STR_06B | rawlen;
+        } else if (rawlen <= 0x3fff) {
+            len += 1;

             if (!p) return len;
             if (!p) return len;

-            lenenc = ZIP_LEN_UINT16;
-            buf[1] = (rawlen     ) & 0xff;
-            buf[2] = (rawlen >> 8) & 0xff;
+            buf[0] = ZIP_STR_14B | ((rawlen >> 8) & 0x3f);
+            buf[1] = rawlen & 0xff;

         } else {
             len += 4;
             if (!p) return len;
         } else {
             len += 4;
             if (!p) return len;

-            lenenc = ZIP_LEN_UINT32;
-            buf[1] = (rawlen      ) & 0xff;
-            buf[2] = (rawlen >>  8) & 0xff;
-            buf[3] = (rawlen >> 16) & 0xff;
-            buf[4] = (rawlen >> 24) & 0xff;
+            buf[0] = ZIP_STR_32B;
+            buf[1] = (rawlen >> 24) & 0xff;
+            buf[2] = (rawlen >> 16) & 0xff;
+            buf[3] = (rawlen >> 8) & 0xff;
+            buf[4] = rawlen & 0xff;

         }
         }

-        buf[0] = (lenenc << 4) | (buf[0] & 0xf);
+    } else {
+        /* Implies integer encoding, so length is always 1. */
+        if (!p) return len;
+        buf[0] = encoding;

     }
     }

-    if (!p) return len;

 
 

-    /* Apparently we need to store the length in 'p' */
-    buf[0] = (encoding << 6) | (buf[0] & 0x3f);
+    /* Store this length at p */

     memcpy(p,buf,len);
     return len;
 }
     memcpy(p,buf,len);
     return len;
 }


@@ -198,11 +257,11 @@ static int zipTryEncoding(unsigned char *entry, unsigned int entrylen, long long
         /* Great, the string can be encoded. Check what's the smallest
          * of our encoding types that can hold this value. */
         if (value >= INT16_MIN && value <= INT16_MAX) {
         /* Great, the string can be encoded. Check what's the smallest
          * of our encoding types that can hold this value. */
         if (value >= INT16_MIN && value <= INT16_MAX) {

-            *encoding = ZIP_ENC_INT16;
+            *encoding = ZIP_INT_16B;

         } else if (value >= INT32_MIN && value <= INT32_MAX) {
         } else if (value >= INT32_MIN && value <= INT32_MAX) {

-            *encoding = ZIP_ENC_INT32;
+            *encoding = ZIP_INT_32B;

         } else {
         } else {

-            *encoding = ZIP_ENC_INT64;
+            *encoding = ZIP_INT_64B;

         }
         *v = value;
         return 1;
         }
         *v = value;
         return 1;


@@ -215,13 +274,13 @@ static void zipSaveInteger(unsigned char *p, int64_t value, unsigned char encodi
     int16_t i16;
     int32_t i32;
     int64_t i64;
     int16_t i16;
     int32_t i32;
     int64_t i64;

-    if (encoding == ZIP_ENC_INT16) {
+    if (encoding == ZIP_INT_16B) {

         i16 = value;
         memcpy(p,&i16,sizeof(i16));
         i16 = value;
         memcpy(p,&i16,sizeof(i16));

-    } else if (encoding == ZIP_ENC_INT32) {
+    } else if (encoding == ZIP_INT_32B) {

         i32 = value;
         memcpy(p,&i32,sizeof(i32));
         i32 = value;
         memcpy(p,&i32,sizeof(i32));

-    } else if (encoding == ZIP_ENC_INT64) {
+    } else if (encoding == ZIP_INT_64B) {

         i64 = value;
         memcpy(p,&i64,sizeof(i64));
     } else {
         i64 = value;
         memcpy(p,&i64,sizeof(i64));
     } else {


@@ -234,13 +293,13 @@ static int64_t zipLoadInteger(unsigned char *p, unsigned char encoding) {
     int16_t i16;
     int32_t i32;
     int64_t i64, ret;
     int16_t i16;
     int32_t i32;
     int64_t i64, ret;

-    if (encoding == ZIP_ENC_INT16) {
+    if (encoding == ZIP_INT_16B) {

         memcpy(&i16,p,sizeof(i16));
         ret = i16;
         memcpy(&i16,p,sizeof(i16));
         ret = i16;

-    } else if (encoding == ZIP_ENC_INT32) {
+    } else if (encoding == ZIP_INT_32B) {

         memcpy(&i32,p,sizeof(i32));
         ret = i32;
         memcpy(&i32,p,sizeof(i32));
         ret = i32;

-    } else if (encoding == ZIP_ENC_INT64) {
+    } else if (encoding == ZIP_INT_64B) {

         memcpy(&i64,p,sizeof(i64));
         ret = i64;
     } else {
         memcpy(&i64,p,sizeof(i64));
         ret = i64;
     } else {


@@ -255,7 +314,7 @@ static zlentry zipEntry(unsigned char *p) {
     e.prevrawlen = zipPrevDecodeLength(p,&e.prevrawlensize);
     e.len = zipDecodeLength(p+e.prevrawlensize,&e.lensize);
     e.headersize = e.prevrawlensize+e.lensize;
     e.prevrawlen = zipPrevDecodeLength(p,&e.prevrawlensize);
     e.len = zipDecodeLength(p+e.prevrawlensize,&e.lensize);
     e.headersize = e.prevrawlensize+e.lensize;

-    e.encoding = ZIP_ENCODING(p+e.prevrawlensize);
+    e.encoding = zipEntryEncoding(p+e.prevrawlensize);

     e.p = p;
     return e;
 }
     e.p = p;
     return e;
 }


@@ -327,7 +386,7 @@ static unsigned char *__ziplistInsert(unsigned char *zl, unsigned char *p, unsig
     unsigned int curlen = ZIPLIST_BYTES(zl), reqlen, prevlen = 0;
     unsigned int offset, nextdiff = 0;
     unsigned char *tail;
     unsigned int curlen = ZIPLIST_BYTES(zl), reqlen, prevlen = 0;
     unsigned int offset, nextdiff = 0;
     unsigned char *tail;

-    unsigned char encoding = ZIP_ENC_RAW;
+    unsigned char encoding = 0;

     long long value;
     zlentry entry;
 
     long long value;
     zlentry entry;
 


@@ -344,11 +403,13 @@ static unsigned char *__ziplistInsert(unsigned char *zl, unsigned char *p, unsig
 
     /* See if the entry can be encoded */
     if (zipTryEncoding(s,slen,&value,&encoding)) {
 
     /* See if the entry can be encoded */
     if (zipTryEncoding(s,slen,&value,&encoding)) {

-        reqlen = zipEncodingSize(encoding);
+        /* 'encoding' is set to the appropriate integer encoding */
+        reqlen = zipIntSize(encoding);

     } else {
     } else {

+        /* 'encoding' is untouched, however zipEncodeLength will use the
+         * string length to figure out how to encode it. */

         reqlen = slen;
     }
         reqlen = slen;
     }

-

     /* We need space for both the length of the previous entry and
      * the length of the payload. */
     reqlen += zipPrevEncodeLength(NULL,prevlen);
     /* We need space for both the length of the previous entry and
      * the length of the payload. */
     reqlen += zipPrevEncodeLength(NULL,prevlen);


@@ -380,10 +441,10 @@ static unsigned char *__ziplistInsert(unsigned char *zl, unsigned char *p, unsig
     /* Write the entry */
     p += zipPrevEncodeLength(p,prevlen);
     p += zipEncodeLength(p,encoding,slen);
     /* Write the entry */
     p += zipPrevEncodeLength(p,prevlen);
     p += zipEncodeLength(p,encoding,slen);

-    if (encoding != ZIP_ENC_RAW) {
-        zipSaveInteger(p,value,encoding);
-    } else {
+    if (ZIP_IS_STR(encoding)) {

         memcpy(p,s,slen);
         memcpy(p,s,slen);

+    } else {
+        zipSaveInteger(p,value,encoding);

     }
     ZIPLIST_INCR_LENGTH(zl,1);
     return zl;
     }
     ZIPLIST_INCR_LENGTH(zl,1);
     return zl;


@@ -463,7 +524,7 @@ unsigned int ziplistGet(unsigned char *p, unsigned char **sstr, unsigned int *sl
     if (sstr) *sstr = NULL;
 
     entry = zipEntry(p);
     if (sstr) *sstr = NULL;
 
     entry = zipEntry(p);

-    if (entry.encoding == ZIP_ENC_RAW) {
+    if (ZIP_IS_STR(entry.encoding)) {

         if (sstr) {
             *slen = entry.len;
             *sstr = p+entry.headersize;
         if (sstr) {
             *slen = entry.len;
             *sstr = p+entry.headersize;


@@ -510,7 +571,7 @@ unsigned int ziplistCompare(unsigned char *p, unsigned char *sstr, unsigned int
     if (p[0] == ZIP_END) return 0;
 
     entry = zipEntry(p);
     if (p[0] == ZIP_END) return 0;
 
     entry = zipEntry(p);

-    if (entry.encoding == ZIP_ENC_RAW) {
+    if (ZIP_IS_STR(entry.encoding)) {

         /* Raw compare */
         if (entry.len == slen) {
             return memcmp(p+entry.headersize,sstr,slen) == 0;
         /* Raw compare */
         if (entry.len == slen) {
             return memcmp(p+entry.headersize,sstr,slen) == 0;


@@ -562,7 +623,7 @@ void ziplistRepr(unsigned char *zl) {
         entry = zipEntry(p);
         printf("{offset %ld, header %u, payload %u} ",p-zl,entry.headersize,entry.len);
         p += entry.headersize;
         entry = zipEntry(p);
         printf("{offset %ld, header %u, payload %u} ",p-zl,entry.headersize,entry.len);
         p += entry.headersize;

-        if (entry.encoding == ZIP_ENC_RAW) {
+        if (ZIP_IS_STR(entry.encoding)) {

             fwrite(p,entry.len,1,stdout);
         } else {
             printf("%lld", (long long) zipLoadInteger(p,entry.encoding));
             fwrite(p,entry.len,1,stdout);
         } else {
             printf("%lld", (long long) zipLoadInteger(p,entry.encoding));