Fix formatter escapes handling

Closes #49

Co-authored-by: JoeBew42 <joebew42@users.noreply.github.com>

Author: michalmuskala

formatter_test_suite/nested-maps.json

@@ -0,0 +1 @@
+[{"@id": "http://a/b","http://a/c": [{"@id": "http://a/d"}]}]

formatter_test_suite/nested-maps.min.json

@@ -0,0 +1 @@
+[{"@id":"http://a/b","http://a/c":[{"@id":"http://a/d"}]}]

formatter_test_suite/nested-maps.pretty.json

@@ -0,0 +1,10 @@
+[
+  {
+    "@id": "http://a/b",
+    "http://a/c": [
+      {
+        "@id": "http://a/d"
+      }
+    ]
+  }
+]

lib/formatter.ex

@@ -216,36 +216,27 @@ defmodule Jason.Formatter do
     {output_acc, &pp_string(&1, &2, in_bs, cont)}
   end
 
-  defp pp_string(<<?", rest::binary>>, output_acc, true = _in_bs, cont) do
-    pp_string(rest, [output_acc, ?"], false, cont)
+  defp pp_string(<<byte, rest::binary>>, output_acc, true = _in_bs, cont) do
+    pp_string(rest, [output_acc, byte], false, cont)
   end
 
   defp pp_string(<<?", rest::binary>>, output_acc, false = _in_bs, cont) do
     cont.(rest, [output_acc, ?"])
   end
 
-  defp pp_string(<<byte>>, output_acc, in_bs, cont) do
-    in_bs = not in_bs and byte == ?\\
-    {[output_acc, byte], &pp_string(&1, &2, in_bs, cont)}
+  defp pp_string(<<?\\, rest::binary>>, output_acc, false = _in_bs, cont) do
+    pp_string(rest, [output_acc, ?\\], true, cont)
   end
 
-  defp pp_string(binary, output_acc, _in_bs, cont) when is_binary(binary) do
-    size = byte_size(binary)
-
-    case :binary.match(binary, "\"") do
+  defp pp_string(binary, output_acc, false = _in_bs, cont) when is_binary(binary) do
+    case :binary.match(binary, ["\"", "\\"]) do
       :nomatch ->
-        skip = size - 2
-        <<_::binary-size(skip), prev, last>> = binary
-        in_bs = not (prev == ?\\ and last == ?\\) or last == ?\\
-        {[output_acc | binary], &pp_string(&1, &2, in_bs, cont)}
-
+        {[output_acc | binary], &pp_string(&1, &2, false, cont)}
       {pos, 1} ->
-        {leading, tail} = :erlang.split_binary(binary, pos + 1)
-        output = [output_acc | leading]
-
-        case :binary.at(binary, pos - 1) do
-          ?\\ -> pp_string(tail, output, false, cont)
-          _ -> cont.(tail, output)
+        {head, tail} = :erlang.split_binary(binary, pos + 1)
+        case :binary.at(binary, pos) do
+          ?\\ -> pp_string(tail, [output_acc | head], true, cont)
+          ?" -> cont.(tail, [output_acc | head])
         end
     end
   end

test/formatter_test.exs

@@ -10,7 +10,8 @@ defmodule Jason.FormatterTest do
     "simple-object",
     "multiple-objects",
     "backslash-string",
-    "empty-nest"
+    "empty-nest",
+    "nested-maps"
   ]
 
   for name <- @test_cases do
@@ -97,5 +98,9 @@ defmodule Jason.FormatterTest do
     input = ["\"abc\\", "\\", ?"]
     output = ~S|"abc\\"|
     assert(minimize(input) == output)
+
+    input = ~s|["a\\\\"]|
+    output = ~s|[\n  "a\\\\"\n]|
+    assert(pretty_print(input) == output)
   end
 end