From 1fc04a62217ab38efa2ce86654016f882c6763c1 Mon Sep 17 00:00:00 2001
From: antirez <antirez@gmail.com>
Date: Sat, 12 Apr 2014 10:55:28 +0200
Subject: [PATCH] HyperLogLog sparse to dense conversion function.

---
 src/hyperloglog.c | 47 ++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 44 insertions(+), 3 deletions(-)

diff --git a/src/hyperloglog.c b/src/hyperloglog.c
index a95a21c91..5e5064b1b 100644
--- a/src/hyperloglog.c
+++ b/src/hyperloglog.c
@@ -202,6 +202,8 @@ struct hllhdr {
 #define HLL_SPARSE 1 /* Sparse encoding */
 #define HLL_MAX_ENCODING 1
 
+#define HLL_SPARSE_MAX 3000
+
 /* =========================== Low level bit macros ========================= */
 
 /* Macros to access the dense representation.
@@ -555,8 +557,46 @@ double hllDenseSum(uint8_t *registers, double *PE, int *ezp) {
 
 /* ================== Sparse representation implementation  ================= */
 
-sds hllSparseToDense(sds *sparse) {
-    return sdsnew("TODO");
+/* Convert the HLL with sparse representation given as input in its dense
+ * representation. Both representations are represented by SDS strings, and
+ * the input representation is freed as a side effect. */
+sds hllSparseToDense(sds sparse) {
+    sds dense;
+    struct hllhdr *hdr, *oldhdr = (struct hllhdr*)sparse;
+    int idx = 0, runlen, regval;
+    uint8_t *p = (uint8_t*)sparse, *end = p+sdslen(sparse);
+
+    /* Create a string of the right size filled with zero bytes.
+     * Note that the cached cardinality is set to 0 as a side effect
+     * that is exactly the cardinality of an empty HLL. */
+    dense = sdsnewlen(NULL,HLL_DENSE_SIZE);
+    hdr = (struct hllhdr*) dense;
+    *hdr = *oldhdr; /* This will copy the magic and cached cardinality. */
+    hdr->encoding = HLL_DENSE;
+
+    /* Now read the sparse representation and set non-zero registers
+     * accordingly. */
+    p += HLL_HDR_SIZE;
+    while(p < end) {
+        if (HLL_SPARSE_IS_ZERO(p)) {
+            runlen = HLL_SPARSE_ZERO_LEN(p);
+            idx += runlen;
+        } else if (HLL_SPARSE_IS_XZERO(p)) {
+            runlen = HLL_SPARSE_XZERO_LEN(p);
+            idx += runlen;
+        } else {
+            runlen = HLL_SPARSE_VAL_LEN(p);
+            regval = HLL_SPARSE_VAL_VALUE(p);
+            while(runlen--) {
+                HLL_DENSE_SET_REGISTER(hdr->registers,idx,regval);
+                idx++;
+            }
+        }
+    }
+
+    /* Free the old representation and return the new one. */
+    sdsfree(sparse);
+    return dense;
 }
 
 /* "Add" the element in the sparse hyperloglog data structure.
@@ -739,6 +779,8 @@ int hllSparseAdd(robj *o, unsigned char *ele, size_t elesize) {
      int seqlen = seq-n;
      int oldlen = is_xzero ? 2 : 1;
      int deltalen = seqlen-oldlen;
+
+     if (deltalen > 0 && sdslen(o->ptr) > HLL_SPARSE_MAX) goto promote;
      if (deltalen && next) {
         memmove(next+deltalen,next,next-sparse);
         sdsIncrLen(o->ptr,deltalen);
@@ -774,7 +816,6 @@ double hllSparseSum(uint8_t *sparse, int sparselen, double *PE, int *ezp) {
     uint8_t *end = sparse+sparselen, *p = sparse;
 
     while(p < end) {
-        /* Set span to the number of registers covered by this opcode. */
         if (HLL_SPARSE_IS_ZERO(p)) {
             runlen = HLL_SPARSE_ZERO_LEN(p);
             idx += runlen;