LRANK: Add command (the command will be renamed LPOS).

The `LRANK` command returns the index (position) of a given element
within a list. Using the `direction` argument it is possible to specify
going from head to tail (acending, 1) or from tail to head (decending,
-1). Only the first found index is returend. The complexity is O(N).

When using lists as a queue it can be of interest at what position a
given element is, for instance to monitor a job processing through a
work queue. This came up within the Python `rq` project which is based
on Redis[0].

[0]: https://github.com/rq/rq/issues/1197

Signed-off-by: Paul Spooren <mail@aparcar.org>

src/help.h

@@ -668,6 +668,11 @@ struct commandHelp {
     "Remove elements from a list",
     2,
     "1.0.0" },
+    { "LRANK",
+    "key direction element",
+    "Return first index of element in list based on direction",
+    2,
+    "9.9.9" },
     { "LSET",
     "key index element",
     "Set the value of an element in a list by its index",

src/server.c

@@ -326,6 +326,10 @@ struct redisCommand redisCommandTable[] = {
      "write @list",
      0,NULL,1,1,1,0,0,0},
 
+    {"lrank",lrankCommand,4,
+     "read-only fast @list",
+     0,NULL,1,1,1,0,0,0},
+
     {"lrem",lremCommand,4,
      "write @list",
      0,NULL,1,1,1,0,0,0},

src/server.h

@@ -2269,6 +2269,7 @@ void flushdbCommand(client *c);
 void flushallCommand(client *c);
 void sortCommand(client *c);
 void lremCommand(client *c);
+void lrankCommand(client *c);
 void rpoplpushCommand(client *c);
 void infoCommand(client *c);
 void mgetCommand(client *c);

src/t_list.c

@@ -487,6 +487,40 @@ void ltrimCommand(client *c) {
     addReply(c,shared.ok);
 }
 
+void lrankCommand(client *c) {
+    robj *subject, *obj;
+    obj = c->argv[3];
+    long direction = 0;
+    long index = 0;
+
+    if ((getLongFromObjectOrReply(c, c->argv[2], &direction, NULL) != C_OK))
+        return;
+
+    subject = lookupKeyWriteOrReply(c,c->argv[1],shared.czero);
+    if (subject == NULL || checkType(c,subject,OBJ_LIST)) return;
+
+    listTypeIterator *li;
+    if (direction < 0) {
+        direction = -1;
+        li = listTypeInitIterator(subject,-1,LIST_HEAD);
+    } else {
+        direction = 1;
+        li = listTypeInitIterator(subject,0,LIST_TAIL);
+    }
+
+    listTypeEntry entry;
+    while (listTypeNext(li,&entry)) {
+        if (listTypeEqual(&entry,obj)) {
+            break;
+        }
+        index++;
+    }
+
+    listTypeReleaseIterator(li);
+
+    addReplyLongLong(c,index * direction);
+}
+
 void lremCommand(client *c) {
     robj *subject, *obj;
     obj = c->argv[3];