From 0a2894f07a2c97390341b306618f86087c474677 Mon Sep 17 00:00:00 2001
From: Ilja <ilja@ilja.space>
Date: Mon, 25 Jul 2022 01:21:02 +0200
Subject: [PATCH] Fix bug when starting with $-sign

When a text token was running, it wouldn't stop on a $-sign if it wasn't followed with a "[".
This is good.

But when a token was finished and the next char was a $-sign, it would consider it an MFMOpen, even when not followed by a "[".
This is now fixed.
---
 lib/lexer.ex         | 23 ++++++++++++++++++-----
 lib/reader.ex        |  9 +++++++++
 test/lexer_test.exs  |  9 +++++++++
 test/reader_test.exs |  7 +++++++
 4 files changed, 43 insertions(+), 5 deletions(-)

diff --git a/lib/lexer.ex b/lib/lexer.ex
index d648f9f..54e86a9 100644
--- a/lib/lexer.ex
+++ b/lib/lexer.ex
@@ -31,11 +31,24 @@ defmodule MfmParser.Lexer do
 
   defp get_empty_token(input) do
     case Reader.peek(input) do
-      :eof -> :eof
-      "$" -> %MFM.Open{}
-      "]" -> %MFM.Close{}
-      "\n" -> %Newline{}
-      _ -> %Text{}
+      :eof ->
+        :eof
+
+      "$" ->
+        if Reader.peek(input, 2) == "[" do
+          %MFM.Open{}
+        else
+          %Text{}
+        end
+
+      "]" ->
+        %MFM.Close{}
+
+      "\n" ->
+        %Newline{}
+
+      _ ->
+        %Text{}
     end
   end
 
diff --git a/lib/reader.ex b/lib/reader.ex
index 89569b2..d614833 100644
--- a/lib/reader.ex
+++ b/lib/reader.ex
@@ -8,6 +8,15 @@ defmodule MfmParser.Reader do
     end
   end
 
+  def peek(input, steps) do
+    nth_char = input |> String.at(steps - 1)
+
+    case nth_char do
+      nil -> :eof
+      _ -> nth_char
+    end
+  end
+
   def next(input) do
     {next_char, rest} = String.split_at(input, 1)
 
diff --git a/test/lexer_test.exs b/test/lexer_test.exs
index f8eb0db..177f261 100644
--- a/test/lexer_test.exs
+++ b/test/lexer_test.exs
@@ -57,6 +57,15 @@ defmodule MfmParser.LexerTest do
 
       assert Lexer.peek("Eyes like $$") == %Text{content: "Eyes like $$"}
       assert Lexer.next("Eyes like $$") == {%Text{content: "Eyes like $$"}, ""}
+
+      assert Lexer.peek("$2 chocolatine") == %Text{content: "$2 chocolatine"}
+      assert Lexer.next("$2 chocolatine") == {%Text{content: "$2 chocolatine"}, ""}
+
+      assert Lexer.peek("$2") == %Text{content: "$2"}
+      assert Lexer.next("$2") == {%Text{content: "$2"}, ""}
+
+      assert Lexer.peek("$") == %Text{content: "$"}
+      assert Lexer.next("$") == {%Text{content: "$"}, ""}
     end
 
     test "it ends when a mfm token closes" do
diff --git a/test/reader_test.exs b/test/reader_test.exs
index 9de4a32..32aa6b3 100644
--- a/test/reader_test.exs
+++ b/test/reader_test.exs
@@ -6,12 +6,19 @@ defmodule MfmParser.ReaderTest do
     assert Reader.peek("chocolatine") == "c"
   end
 
+  test "it can peek at the nth character" do
+    assert Reader.peek("chocolatine", 7) == "a"
+  end
+
   test "it step to the next character" do
     assert Reader.next("chocolatine") == {"c", "hocolatine"}
   end
 
   test "it returns eof" do
     assert Reader.peek("") == :eof
+    assert Reader.peek("", 1) == :eof
+    assert Reader.peek("", 2) == :eof
+    assert Reader.peek("c", 3) == :eof
     assert Reader.next("") == :eof
   end
 end