Index: src/sqlite.h.in
==================================================================
--- src/sqlite.h.in
+++ src/sqlite.h.in
@@ -8125,10 +8125,37 @@
 **
 ** The [sqlite3_snapshot_free()] interface is only available when the
 ** SQLITE_ENABLE_SNAPSHOT compile-time option is used.
 */
 SQLITE_EXPERIMENTAL void sqlite3_snapshot_free(sqlite3_snapshot*);
+
+/*
+** CAPI3REF: Compare the ages of two snapshot handles.
+** EXPERIMENTAL
+**
+** The sqlite3_snapshot_cmp(P1, P2) interface is used to compare the ages
+** of two valid snapshot handles. 
+**
+** If the two snapshot handles are not associated with the same database 
+** file, the result of the comparison is undefined. 
+**
+** Additionally, the result of the comparison is only valid if both of the
+** snapshot handles were obtained by calling sqlite3_snapshot_get() since the
+** last time the wal file was deleted. The wal file is deleted when the
+** database is changed back to rollback mode or when the number of database
+** clients drops to zero. If either snapshot handle was obtained before the 
+** wal file was last deleted, the value returned by this function 
+** is undefined.
+**
+** Otherwise, this API returns a negative value if P1 refers to an older
+** snapshot than P2, zero if the two handles refer to the same database
+** snapshot, and a positive value if P1 is a newer snapshot than P2.
+*/
+SQLITE_EXPERIMENTAL int sqlite3_snapshot_cmp(
+  sqlite3_snapshot *p1,
+  sqlite3_snapshot *p2
+);
 
 /*
 ** Undo the hack that converts floating point types to integer for
 ** builds on processors without floating point support.
 */

Index: src/test1.c
==================================================================
--- src/test1.c
+++ src/test1.c
@@ -2357,10 +2357,35 @@
   sqlite3_snapshot_free(pSnapshot);
   return TCL_OK;
 }
 #endif /* SQLITE_ENABLE_SNAPSHOT */
 
+#ifdef SQLITE_ENABLE_SNAPSHOT
+/*
+** Usage: sqlite3_snapshot_cmp SNAPSHOT1 SNAPSHOT2
+*/
+static int test_snapshot_cmp(
+  void * clientData,
+  Tcl_Interp *interp,
+  int objc,
+  Tcl_Obj *CONST objv[]
+){
+  int res;
+  sqlite3_snapshot *p1;
+  sqlite3_snapshot *p2;
+  if( objc!=3 ){
+    Tcl_WrongNumArgs(interp, 1, objv, "SNAPSHOT1 SNAPSHOT2");
+    return TCL_ERROR;
+  }
+  p1 = (sqlite3_snapshot*)sqlite3TestTextToPtr(Tcl_GetString(objv[1]));
+  p2 = (sqlite3_snapshot*)sqlite3TestTextToPtr(Tcl_GetString(objv[2]));
+  res = sqlite3_snapshot_cmp(p1, p2);
+  Tcl_SetObjResult(interp, Tcl_NewIntObj(res));
+  return TCL_OK;
+}
+#endif /* SQLITE_ENABLE_SNAPSHOT */
+
 /*
 ** Usage:  sqlite3_next_stmt  DB  STMT
 **
 ** Return the next statment in sequence after STMT.
 */
@@ -7247,10 +7272,11 @@
      { "vfs_current_time_int64",           vfsCurrentTimeInt64,   0 },
 #ifdef SQLITE_ENABLE_SNAPSHOT
      { "sqlite3_snapshot_get", test_snapshot_get, 0 },
      { "sqlite3_snapshot_open", test_snapshot_open, 0 },
      { "sqlite3_snapshot_free", test_snapshot_free, 0 },
+     { "sqlite3_snapshot_cmp", test_snapshot_cmp, 0 },
 #endif
   };
   static int bitmask_size = sizeof(Bitmask)*8;
   static int longdouble_size = sizeof(LONGDOUBLE_TYPE);
   int i;

Index: src/wal.c
==================================================================
--- src/wal.c
+++ src/wal.c
@@ -3397,10 +3397,27 @@
 /* Try to open on pSnapshot when the next read-transaction starts
 */
 void sqlite3WalSnapshotOpen(Wal *pWal, sqlite3_snapshot *pSnapshot){
   pWal->pSnapshot = (WalIndexHdr*)pSnapshot;
 }
+
+/* 
+** Return a +ve value if snapshot p1 is newer than p2. A -ve value if
+** p1 is older than p2 and zero if p1 and p2 are the same snapshot.
+*/
+int sqlite3_snapshot_cmp(sqlite3_snapshot *p1, sqlite3_snapshot *p2){
+  WalIndexHdr *pHdr1 = (WalIndexHdr*)p1;
+  WalIndexHdr *pHdr2 = (WalIndexHdr*)p2;
+
+  /* aSalt[0] is a copy of the value stored in the wal file header. It
+  ** is incremented each time the wal file is restarted.  */
+  if( pHdr1->aSalt[0]<pHdr2->aSalt[0] ) return -1;
+  if( pHdr1->aSalt[0]>pHdr2->aSalt[0] ) return +1;
+  if( pHdr1->mxFrame<pHdr2->mxFrame ) return -1;
+  if( pHdr1->mxFrame>pHdr2->mxFrame ) return +1;
+  return 0;
+}
 #endif /* SQLITE_ENABLE_SNAPSHOT */
 
 #ifdef SQLITE_ENABLE_ZIPVFS
 /*
 ** If the argument is not NULL, it points to a Wal object that holds a

Index: test/snapshot.test
==================================================================
--- test/snapshot.test
+++ test/snapshot.test
@@ -362,7 +362,81 @@
   db2 eval "BEGIN"
   list [catch {sqlite3_snapshot_open db2 main $::snapshot} msg] $msg
 } {1 SQLITE_ERROR}
 
 sqlite3_snapshot_free $snapshot
+
+#-------------------------------------------------------------------------
+# The following tests investigate the sqlite3_snapshot_cmp() API.
+#
+
+# Compare snapshots $p1 and $p2, checking that the result is $r.
+#
+proc do_snapshot_cmp_test {tn p1 p2 r} {
+  uplevel [list do_test $tn.1 [list sqlite3_snapshot_cmp $p1 $p2] $r]
+  uplevel [list do_test $tn.2 [list sqlite3_snapshot_cmp $p2 $p1] [expr $r*-1]]
+  uplevel [list do_test $tn.3 [list sqlite3_snapshot_cmp $p1 $p1] 0]
+  uplevel [list do_test $tn.4 [list sqlite3_snapshot_cmp $p2 $p2] 0]
+}
+
+catch { db2 close }
+reset_db
+
+do_execsql_test 7.1 {
+  PRAGMA journal_mode = wal;
+  CREATE TABLE t1(x);
+} wal
+
+do_test 7.1.2 {
+  execsql { BEGIN ; PRAGMA application_id }
+  set p1 [sqlite3_snapshot_get db main]
+  execsql {
+    INSERT INTO t1 VALUES(10);
+    COMMIT;
+  }
+  execsql { BEGIN ; PRAGMA application_id }
+  set p2 [sqlite3_snapshot_get db main]
+  execsql COMMIT
+} {}
+
+do_snapshot_cmp_test 7.1.3 $p1 $p2 -1
+sqlite3_snapshot_free $p1
+sqlite3_snapshot_free $p2
+
+do_execsql_test 7.2.1 {
+  INSERT INTO t1 VALUES(11);
+  INSERT INTO t1 VALUES(12);
+  INSERT INTO t1 VALUES(13);
+  BEGIN; 
+    PRAGMA application_id;
+} {0}
+do_test 7.2.2 {
+  set p1 [sqlite3_snapshot_get db main]
+  execsql {
+    COMMIT;
+    INSERT INTO t1 VALUES(14);
+    PRAGMA wal_checkpoint;
+    BEGIN;
+      PRAGMA application_id;
+  }
+  set p2 [sqlite3_snapshot_get db main]
+  execsql COMMIT
+} {}
+
+do_snapshot_cmp_test 7.2.3 $p1 $p2 -1
+sqlite3_snapshot_free $p2
+
+do_test 7.3.1 {
+  execsql {
+    INSERT INTO t1 VALUES(14);
+    BEGIN;
+      PRAGMA application_id;
+  }
+  set p2 [sqlite3_snapshot_get db main]
+  execsql COMMIT
+} {}
+
+do_snapshot_cmp_test 7.3.2 $p1 $p2 -1
+sqlite3_snapshot_free $p1
+sqlite3_snapshot_free $p2
 
 finish_test