PFDEBUG DECODE added.

Provides a human readable description of the opcodes composing a
run-length encoded HLL (sparse encoding).
The command is only useful for debugging / development tasks.

src/hyperloglog.c

@@ -1243,6 +1243,41 @@ void pfdebugCommand(redisClient *c) {
             HLL_DENSE_GET_REGISTER(val,hdr->registers,j);
             addReplyLongLong(c,val);
         }
+    }
+    /* PFDEBUG DECODE <key> */
+    else if (!strcasecmp(cmd,"decode")) {
+        if (c->argc != 3) goto arityerr;
+
+        uint8_t *p = o->ptr, *end = p+sdslen(o->ptr);
+        sds decoded = sdsempty();
+
+        if (hdr->encoding != HLL_SPARSE) {
+            addReplyError(c,"HLL encoding is not sparse");
+            return;
+        }
+
+        p += HLL_HDR_SIZE;
+        while(p < end) {
+            int runlen, regval;
+
+            if (HLL_SPARSE_IS_ZERO(p)) {
+                runlen = HLL_SPARSE_ZERO_LEN(p);
+                p++;
+                decoded = sdscatprintf(decoded,"z:%d ",runlen);
+            } else if (HLL_SPARSE_IS_XZERO(p)) {
+                runlen = HLL_SPARSE_XZERO_LEN(p);
+                p += 2;
+                decoded = sdscatprintf(decoded,"Z:%d ",runlen);
+            } else {
+                runlen = HLL_SPARSE_VAL_LEN(p);
+                regval = HLL_SPARSE_VAL_VALUE(p);
+                p++;
+                decoded = sdscatprintf(decoded,"v:%d,%d ",regval,runlen);
+            }
+        }
+        decoded = sdstrim(decoded," ");
+        addReplyBulkCBuffer(c,decoded,sdslen(decoded));
+        sdsfree(decoded);
     } else {
         addReplyErrorFormat(c,"Unknown PFDEBUG subcommand '%s'", cmd);
     }