From 1bb127eb651ce8eda9f5e3b212345959f133c5e9 Mon Sep 17 00:00:00 2001
From: Scott Lystig Fritchie <slfritchie@snookles.com>
Date: Fri, 29 Aug 2014 18:20:05 +0900
Subject: [PATCH] Add scan_backward LPN limit + test

---
 prototype/tango-prototype/src/tango.erl       | 25 +++++++++++++------
 prototype/tango-prototype/test/tango_test.erl | 11 ++++++++
 2 files changed, 28 insertions(+), 8 deletions(-)

diff --git a/prototype/tango-prototype/src/tango.erl b/prototype/tango-prototype/src/tango.erl
index 8cf0c40..4513a93 100644
--- a/prototype/tango-prototype/src/tango.erl
+++ b/prototype/tango-prototype/src/tango.erl
@@ -25,6 +25,7 @@
 -export([pack_v1/3, unpack_v1/2,
          add_back_pointer/3,
          scan_backward/4,
+         scan_backward/5,
          pad_bin/2]).
 
 -define(MAGIC_NUMBER_V1, 16#88990011).
@@ -74,16 +75,21 @@ add_back_pointer([], New) ->
 add_back_pointer(BackPs, New) ->
     [New|BackPs].
 
-scan_backward(Proj, Stream, LastLPN, _WithPagesP) ->
-    lists:reverse(scan_backward2(Proj, Stream, LastLPN, _WithPagesP)).
+scan_backward(Proj, Stream, LastLPN, WithPagesP) ->
+    scan_backward(Proj, Stream, LastLPN, 0, WithPagesP).
 
-scan_backward2(Proj, Stream, LastLPN, WithPagesP) ->
+scan_backward(Proj, Stream, LastLPN, StopAtLPN, WithPagesP) ->
+    lists:reverse(scan_backward2(Proj, Stream, LastLPN, StopAtLPN, WithPagesP)).
+
+scan_backward2(_Proj, _Stream, LastLPN, StopAtLPN, _WithPagesP)
+  when LastLPN =< StopAtLPN ->
+    [];
+scan_backward2(Proj, Stream, LastLPN, StopAtLPN, WithPagesP) ->
     case corfurl:read_page(Proj, LastLPN) of
         {ok, FullPage} ->
             case proplists:get_value(Stream, unpack_v1(FullPage, stream_list)) of
                 undefined ->
-                    {gahh, lpn, LastLPN, unpack_v1(FullPage, stream_list)};
-                    %% [];
+                    {gah_fixme, lpn, LastLPN, unpack_v1(FullPage, stream_list)};
                 [] ->
                     if WithPagesP ->
                             [{LastLPN, unpack_v1(FullPage, page)}];
@@ -94,13 +100,16 @@ scan_backward2(Proj, Stream, LastLPN, WithPagesP) ->
                     if WithPagesP ->
                             [{LastLPN, unpack_v1(FullPage, page)}|
                              scan_backward2(Proj, Stream,
-                                            hd(BackPs),
+                                            hd(BackPs), StopAtLPN,
                                             WithPagesP)];
                        true ->
                             SkipLPN = lists:last(BackPs),
-                            [LastLPN] ++ (BackPs -- [SkipLPN]) ++
+                            AddLPNs = [LPN || LPN <- BackPs,
+                                              LPN /= SkipLPN,
+                                              LPN > StopAtLPN],
+                            [LastLPN] ++ AddLPNs ++
                                 scan_backward2(Proj, Stream,
-                                               SkipLPN,
+                                               SkipLPN, StopAtLPN,
                                                WithPagesP)
                     end
             end;
diff --git a/prototype/tango-prototype/test/tango_test.erl b/prototype/tango-prototype/test/tango_test.erl
index 29febf7..be7e7a9 100644
--- a/prototype/tango-prototype/test/tango_test.erl
+++ b/prototype/tango-prototype/test/tango_test.erl
@@ -136,6 +136,17 @@ scan_backward_test_int(PageSize, _Seq, P1) ->
          %% oldest -> newest (i.e. smallest LPN to largest LPN) order.
          ShouldBe = tango:scan_backward(P2, StreamNum, BackwardStartLPN,
                                         false),
+         StopAtLimit = NumPages div 2,
+         StopAtKicksInAt = StopAtLimit + 2,
+         {StopAtLPN, ShouldBeLPNS} =
+             if BackwardStartLPN < StopAtKicksInAt ->
+                     {0, ShouldBe};
+               true ->
+                     {StopAtLimit, [LPN || LPN <- ShouldBe, LPN > StopAtLimit]}
+            end,
+         ShouldBeLPNS =
+             tango:scan_backward(P2, StreamNum, BackwardStartLPN, StopAtLPN,
+                                 false),
 
          %% If we scan backward, we should get a list of LPNs in
          %% oldest -> newest (i.e. smallest LPN to largest LPN) order