stdlib: Correct and optimize pretty printing of strings

Avoid traversing all of the list/string when only part of it will be
used. An explicit check that the list is flat is needed since
string:slice() accepts deep lists and more.

2 files changed, 72 insertions, 13 deletions

diff --git a/lib/stdlib/src/io_lib_pretty.erl b/lib/stdlib/src/io_lib_pretty.erl
index 5483ea87b5..8f2fd7ea8f 100644
--- a/lib/stdlib/src/io_lib_pretty.erl
+++ b/lib/stdlib/src/io_lib_pretty.erl
@@ -721,7 +721,7 @@ printable_list(_L, 1, _T, _Enc) ->
 printable_list(L, _D, T, latin1) when T < 0 ->
     io_lib:printable_latin1_list(L);
 printable_list(L, _D, T, Enc) when T >= 0 ->
-    case slice(L, tsub(T, 2)) of
+    case slice(L, tsub(T, 2), Enc) of
         false ->
             false;
         {prefix, Prefix} when Enc =:= latin1 ->
@@ -737,20 +737,46 @@ printable_list(L, _D, T, Enc) when T >= 0 ->
 printable_list(L, _D, T, _Uni) when T < 0->
     io_lib:printable_list(L).
 
-slice(L, N) ->
-    try io_lib:chars_length(L) =< N of
-        true ->
+slice(L, N, latin1) ->
+    try lists:split(N, L) of
+        {_, []} ->
             all;
-        false ->
-            case string:slice(L, 0, N) of
-                "" ->
-                    false;
-                Prefix ->
-                    {prefix, Prefix}
+        {[], _} ->
+            false;
+        {L1, _} ->
+            {prefix, L1}
+    catch
+        _:_ ->
+            all
+    end;
+slice(L, N, _Uni) ->
+    %% Be careful not to traverse more of L than necessary.
+    try string:slice(L, 0, N) of
+        "" ->
+            false;
+        Prefix ->
+            %% Assume no binaries are introduced by string:slice().
+            case is_flat(L, lists:flatlength(Prefix)) of
+                true ->
+                    case string:equal(Prefix, L) of
+                        true ->
+                            all;
+                        false ->
+                            {prefix, Prefix}
+                    end;
+                false ->
+                    false
             end
     catch _:_ -> false
     end.
 
+is_flat(_L, 0) ->
+    true;
+is_flat([C|Cs], N) when is_integer(C) ->
+    is_flat(Cs, N - 1);
+is_flat(_, _N) ->
+    false.
+
 printable_bin0(Bin, D, T, Enc) ->
     Len = case D >= 0 of
               true ->
diff --git a/lib/stdlib/test/io_SUITE.erl b/lib/stdlib/test/io_SUITE.erl
index f097552e8c..824f5d19f2 100644
--- a/lib/stdlib/test/io_SUITE.erl
+++ b/lib/stdlib/test/io_SUITE.erl
@@ -1,7 +1,7 @@
 %%
 %% %CopyrightBegin%
 %% 
-%% Copyright Ericsson AB 1999-2018. All Rights Reserved.
+%% Copyright Ericsson AB 1999-2019. All Rights Reserved.
 %% 
 %% Licensed under the Apache License, Version 2.0 (the "License");
 %% you may not use this file except in compliance with the License.
@@ -32,7 +32,7 @@
          io_with_huge_message_queue/1, format_string/1,
 	 maps/1, coverage/1, otp_14178_unicode_atoms/1, otp_14175/1,
          otp_14285/1, limit_term/1, otp_14983/1, otp_15103/1, otp_15076/1,
-         otp_15159/1]).
+         otp_15159/1, otp_15639/1]).
 
 -export([pretty/2, trf/3]).
 
@@ -64,7 +64,8 @@ all() ->
      io_lib_print_binary_depth_one, otp_10302, otp_10755, otp_10836,
      io_lib_width_too_small, io_with_huge_message_queue,
      format_string, maps, coverage, otp_14178_unicode_atoms, otp_14175,
-     otp_14285, limit_term, otp_14983, otp_15103, otp_15076, otp_15159].
+     otp_14285, limit_term, otp_14983, otp_15103, otp_15076, otp_15159,
+     otp_15639].
 
 %% Error cases for output.
 error_1(Config) when is_list(Config) ->
@@ -2647,3 +2648,35 @@ otp_15076(_Config) ->
     {'EXIT', {badarg, _}} = (catch io_lib:build_text(L)),
     {'EXIT', {badarg, _}} = (catch io_lib:build_text(L, [])),
     ok.
+
+otp_15639(_Config) ->
+    L = lists:duplicate(10, "a"),
+    LOpts = [{encoding, latin1},  {chars_limit, 10}],
+    UOpts = [{encoding, unicode}, {chars_limit, 10}],
+    "[[...]|...]" = pretty(L, LOpts),
+    "[[...]|...]" = pretty(L, UOpts),
+    "[\"a\",[...]|...]" =
+        pretty(L, [{chars_limit, 12}, {encoding, latin1}]),
+    "[\"a\",[...]|...]" =
+        pretty(L, [{chars_limit, 12}, {encoding, unicode}]),
+
+    %% Latin-1
+    "\"12345678\"" = pretty("12345678", LOpts),
+    "\"12345678\"..." = pretty("123456789", LOpts),
+    "\"\\r\\n123456\"..." = pretty("\r\n1234567", LOpts),
+    "\"\\r1234567\"..." = pretty("\r12345678", LOpts),
+    "\"\\r\\n123456\"..." = pretty("\r\n12345678", LOpts),
+    "\"12345678\"..." = pretty("12345678"++[x], LOpts),
+    "[49,50|...]" = pretty("1234567"++[x], LOpts),
+    "[49,x]" = pretty("1"++[x], LOpts),
+    "[[...]|...]" = pretty(["1","2","3","4","5","6","7","8"], LOpts),
+    %% Unicode
+    "\"12345678\"" = pretty("12345678", UOpts),
+    "\"12345678\"..." = pretty("123456789", UOpts),
+    "\"\\r\\n1234567\"" = pretty("\r\n1234567", UOpts),
+    "\"\\r1234567\"..." = pretty("\r12345678", UOpts),
+    "\"\\r\\n1234567\"..." = pretty("\r\n12345678", UOpts),
+    "[49,50|...]" = pretty("12345678"++[x], UOpts),
+    "\"12345678\"..." = pretty("123456789"++[x], UOpts),
+    "[[...]|...]" = pretty(["1","2","3","4","5","6","7","8"], UOpts),
+    ok.