Add line numbers to Markdown code snippets (#399)

* Add line numbers to Markdown code snippets

* Cosmetics

* Pass testing

diff_cover/snippets.py

@@ -116,18 +116,33 @@ def markdown(self):
         Return a Markdown representation of the snippet using Markdown fenced code blocks.
         See https://github.github.com/gfm/#fenced-code-blocks.
         """
+
+        line_number_length = len(str(self._last_line))
+
+        text = ""
+        for i, line in enumerate(self.text().splitlines(), start=self._start_line):
+            if i > self._start_line:
+                text += "\n"
+
+            notice = " "
+            if i in self._violation_lines:
+                notice = "!"
+
+            format_string = "{} {:>" + str(line_number_length) + "} {}"
+            text += format_string.format(notice, i, line)
+
         header = "Lines %d-%d\n\n" % (self._start_line, self._last_line)
         if self._lexer_name in self.LEXER_TO_MARKDOWN_CODE_HINT:
             return header + (
                 "```"
                 + self.LEXER_TO_MARKDOWN_CODE_HINT[self._lexer_name]
                 + "\n"
-                + self.text()
+                + text
                 + "\n```\n"
             )
 
         # unknown programming language, return a non-decorated fenced code block:
-        return "```\n" + self.text() + "\n```\n"
+        return "```\n" + text + "\n```\n"
 
     def terminal(self):
         """

tests/fixtures/snippet_list.md

@@ -1,38 +1,38 @@
 Lines 6-17
 
 ```python
-Line 6
-Line 7
-Line 8
-Line 9
-Line 10
-Line 11
-Line 12
-Line 13
-Line 14
-Line 15
-Line 16
-Line 17
+   6 Line 6
+   7 Line 7
+   8 Line 8
+   9 Line 9
+! 10 Line 10
+  11 Line 11
+! 12 Line 12
+! 13 Line 13
+  14 Line 14
+  15 Line 15
+  16 Line 16
+  17 Line 17
 ```
 
 
 Lines 46-61
 
 ```python
-Line 46
-Line 47
-Line 48
-Line 49
-Line 50
-Line 51
-Line 52
-Line 53
-Line 54
-Line 55
-Line 56
-Line 57
-Line 58
-Line 59
-Line 60
-Line 61
+  46 Line 46
+  47 Line 47
+  48 Line 48
+  49 Line 49
+! 50 Line 50
+! 51 Line 51
+  52 Line 52
+  53 Line 53
+! 54 Line 54
+! 55 Line 55
+  56 Line 56
+! 57 Line 57
+  58 Line 58
+  59 Line 59
+  60 Line 60
+  61 Line 61
 ```

tests/fixtures/snippet_list2.md

@@ -1,11 +1,10 @@
 Lines 1-7
 
 ```cpp
-#include <iostream>
-
-int main() {
-    std::cout << "Hello World!";
-    return 0;
-}
-
-```
+  1 #include <iostream>
+  2 
+  3 int main() {
+! 4     std::cout << "Hello World!";
+! 5     return 0;
+  6 }
+```

tests/fixtures/snippet_list3.md

@@ -1,13 +1,13 @@
 Lines 8-16
 
 ```cpp
-    // this is line 8
-    printf("Test2");
-
-    // this is line 11
-    printf("Test");
-}
-
-int main()
-{
+   8     // this is line 8
+   9     printf("Test2");
+  10 
+  11     // this is line 11
+! 12     printf("Test");
+  13 }
+  14 
+  15 int main()
+  16 {
 ```