SDS: New sds type 5 implemented.

This is an attempt to use the refcount feature of the sds.c fork
provided in the Pull Request #2509. A new type, SDS_TYPE_5 is introduced
having a one byte header with just the string length, without
information about the available additional length at the end of the
string (this means that sdsMakeRoomFor() will be required each time
we want to append something, since the string will always report to have
0 bytes available).

More work needed in order to avoid common SDS functions will pay the
cost of this type. For example both sdscatprintf() and sdscatfmt()
should try to upgrade to SDS_TYPE_8 ASAP when appending chars.

1 files changed, 47 insertions, 21 deletions

diff --git a/src/sds.h b/src/sds.h
index 9201a751c..1fcbe1155 100644
--- a/src/sds.h
+++ b/src/sds.h
@@ -39,43 +39,53 @@
 
 typedef char *sds;
 
+/* Note: sdshdr5 is never used, we just access the flags byte directly.
+ * However is here to document the layout of type 5 SDS strings. */
+struct __attribute__ ((__packed__)) sdshdr5 {
+    unsigned char flags; /* 3 lsb of type, and 5 msb of string length */
+    char buf[];
+};
 struct __attribute__ ((__packed__)) sdshdr8 {
     uint8_t len; /* used */
     uint8_t alloc; /* excluding the header and null terminator */
-    char flags; /* 2 lsb of type, and 6 msb of refcount */
+    unsigned char flags; /* 3 lsb of type, 5 unused bits */
     char buf[];
 };
 struct __attribute__ ((__packed__)) sdshdr16 {
     uint16_t len; /* used */
     uint16_t alloc; /* excluding the header and null terminator */
-    char flags; /* 2 lsb of type, and 6 msb of refcount */
+    unsigned char flags; /* 3 lsb of type, 5 unused bits */
     char buf[];
 };
 struct __attribute__ ((__packed__)) sdshdr32 {
     uint32_t len; /* used */
     uint32_t alloc; /* excluding the header and null terminator */
-    char flags; /* 2 lsb of type, and 6 msb of refcount */
+    unsigned char flags; /* 3 lsb of type, 5 unused bits */
     char buf[];
 };
 struct __attribute__ ((__packed__)) sdshdr64 {
     uint64_t len; /* used */
     uint64_t alloc; /* excluding the header and null terminator */
-    char flags; /* 2 lsb of type, and 6 msb of refcount */
+    unsigned char flags; /* 3 lsb of type, 5 unused bits */
     char buf[];
 };
 
-#define SDS_TYPE_8  0
-#define SDS_TYPE_16 1
-#define SDS_TYPE_32 2
-#define SDS_TYPE_64 3
-#define SDS_TYPE_MASK 3
-#define SDS_TYPE_BITS 2
+#define SDS_TYPE_5  0
+#define SDS_TYPE_8  1
+#define SDS_TYPE_16 2
+#define SDS_TYPE_32 3
+#define SDS_TYPE_64 4
+#define SDS_TYPE_MASK 7
+#define SDS_TYPE_BITS 3
 #define SDS_HDR_VAR(T,s) struct sdshdr##T *sh = (void*)((s)-(sizeof(struct sdshdr##T)));
 #define SDS_HDR(T,s) ((struct sdshdr##T *)((s)-(sizeof(struct sdshdr##T))))
+#define SDS_TYPE_5_LEN(f) ((f)>>SDS_TYPE_BITS)
 
 static inline size_t sdslen(const sds s) {
-    char flags = s[-1];
+    unsigned char flags = s[-1];
     switch(flags&SDS_TYPE_MASK) {
+        case SDS_TYPE_5:
+            return SDS_TYPE_5_LEN(flags);
         case SDS_TYPE_8:
             return SDS_HDR(8,s)->len;
         case SDS_TYPE_16:
@@ -89,8 +99,11 @@ static inline size_t sdslen(const sds s) {
 }
 
 static inline size_t sdsavail(const sds s) {
-    char flags = s[-1];
+    unsigned char flags = s[-1];
     switch(flags&SDS_TYPE_MASK) {
+        case SDS_TYPE_5: {
+            return 0;
+        }
         case SDS_TYPE_8: {
             SDS_HDR_VAR(8,s);
             return sh->alloc - sh->len;
@@ -112,8 +125,14 @@ static inline size_t sdsavail(const sds s) {
 }
 
 static inline void sdssetlen(sds s, size_t newlen) {
-    char flags = s[-1];
+    unsigned char flags = s[-1];
     switch(flags&SDS_TYPE_MASK) {
+        case SDS_TYPE_5:
+            {
+                unsigned char *fp = ((unsigned char*)s)-1;
+                *fp = SDS_TYPE_5 | (newlen << SDS_TYPE_BITS);
+            }
+            break;
         case SDS_TYPE_8:
             SDS_HDR(8,s)->len = newlen;
             break;
@@ -130,8 +149,15 @@ static inline void sdssetlen(sds s, size_t newlen) {
 }
 
 static inline void sdsinclen(sds s, size_t inc) {
-    char flags = s[-1];
+    unsigned char flags = s[-1];
     switch(flags&SDS_TYPE_MASK) {
+        case SDS_TYPE_5:
+            {
+                unsigned char *fp = ((unsigned char*)s)-1;
+                unsigned char newlen = SDS_TYPE_5_LEN(flags)+inc;
+                *fp = SDS_TYPE_5 | (newlen << SDS_TYPE_BITS);
+            }
+            break;
         case SDS_TYPE_8:
             SDS_HDR(8,s)->len += inc;
             break;
@@ -149,8 +175,10 @@ static inline void sdsinclen(sds s, size_t inc) {
 
 /* sdsalloc() = sdsavail() + sdslen() */
 static inline size_t sdsalloc(const sds s) {
-    char flags = s[-1];
+    unsigned char flags = s[-1];
     switch(flags&SDS_TYPE_MASK) {
+        case SDS_TYPE_5:
+            return SDS_TYPE_5_LEN(flags);
         case SDS_TYPE_8:
             return SDS_HDR(8,s)->alloc;
         case SDS_TYPE_16:
@@ -164,8 +192,11 @@ static inline size_t sdsalloc(const sds s) {
 }
 
 static inline void sdssetalloc(sds s, size_t newlen) {
-    char flags = s[-1];
+    unsigned char flags = s[-1];
     switch(flags&SDS_TYPE_MASK) {
+        case SDS_TYPE_5:
+            /* Nothing to do, this type has no total allocation info. */
+            break;
         case SDS_TYPE_8:
             SDS_HDR(8,s)->alloc = newlen;
             break;
@@ -193,11 +224,6 @@ sds sdscatsds(sds s, const sds t);
 sds sdscpylen(sds s, const char *t, size_t len);
 sds sdscpy(sds s, const char *t);
 
-/* we can add a reference count on top of any 
- * existing sds. (max up to 63 references) */
-void sdsIncRefcount(sds s);
-void sdsDecRefcount(sds s);
-
 sds sdscatvprintf(sds s, const char *fmt, va_list ap);
 #ifdef __GNUC__
 sds sdscatprintf(sds s, const char *fmt, ...)