lint: Check for README presence

After exercism/meta#15, READMEs are to be
generated and placed into each track's exercise implementation
directory.

My (unsubstantiated) assumption is that they will be required to be
present after Nextercism, as we don't want to keep generating READMEs on
the fly.

If this assumption is correct, it seems necessary to check that READMEs
are present on all exercises.

With the attached fixture change and attached test changes:

* The example test would fail without the attached code change.
* The added TestMissingReadme would fail if the attached code change
  were incorrect.
* The TestLintTrack against `fixtures/numbers` actually does not fail,
  because there are other reasons for the track to be invalid, so the
  attached code change was not necessary. This points to the unit test
  being too coarse, but this is a discussion for another day. For now,
  the other two tests suffice.

Closes exercism/discussions#200
Closes exercism/configlet#86

cmd/lint.go

@@ -89,6 +89,10 @@ func lintTrack(path string) bool {
 			check: missingMetadata,
 			msg:   "An implementation for '%v' was found, but config.json does not reference this exercise.",
 		},
+		{
+			check: missingReadme,
+			msg:   "The implementation for '%v' is missing a README.",
+		},
 		{
 			check: missingSolution,
 			msg:   "The implementation for '%v' is missing an example solution.",
@@ -201,6 +205,25 @@ func missingSolution(t track.Track) []string {
 	return slugs
 }
 
+func missingReadme(t track.Track) []string {
+	readmes := map[string]bool{}
+	for _, exercise := range t.Exercises {
+		readmes[exercise.Slug] = exercise.HasReadme()
+	}
+	// Don't complain about missing readmes in foregone exercises.
+	for _, slug := range t.Config.ForegoneSlugs {
+		readmes[slug] = true
+	}
+
+	slugs := []string{}
+	for slug, ok := range readmes {
+		if !ok {
+			slugs = append(slugs, slug)
+		}
+	}
+	return slugs
+}
+
 func missingTestSuite(t track.Track) []string {
 	tests := map[string]bool{}
 	for _, exercise := range t.Exercises {

cmd/lint_example_test.go

@@ -16,6 +16,7 @@ func ExampleLint() {
 	lintTrack(filepath.FromSlash("../fixtures/numbers"))
 	// Output:
 	// -> An exercise with slug 'bajillion' is referenced in config.json, but no implementation was found.
+	// -> The implementation for 'four' is missing a README.
 	// -> The implementation for 'three' is missing an example solution.
 	// -> The implementation for 'two' is missing a test suite.
 	// -> The exercise 'one' was found in config.json, but does not have a UUID.

cmd/lint_test.go

@@ -110,6 +110,27 @@ func TestMissingMetadata(t *testing.T) {
 	assert.Equal(t, "cherry", slugs[1])
 }
 
+func TestMissingReadme(t *testing.T) {
+	track := track.Track{
+		Exercises: []track.Exercise{
+			{Slug: "apple"},
+			{Slug: "banana", ReadmePath: "README.md"},
+			{Slug: "cherry"},
+		},
+	}
+
+	slugs := missingReadme(track)
+
+	if len(slugs) != 2 {
+		t.Fatalf("Expected missing solutions in 2 exercises, missing in %d", len(slugs))
+	}
+
+	sort.Strings(slugs)
+
+	assert.Equal(t, "apple", slugs[0])
+	assert.Equal(t, "cherry", slugs[1])
+}
+
 func TestMissingSolution(t *testing.T) {
 	track := track.Track{
 		Exercises: []track.Exercise{

fixtures/numbers/config.json

@@ -23,6 +23,12 @@
       "topics": [],
       "difficulty": 1
     },
+    {
+      "uuid": "444",
+      "slug": "four",
+      "topics": [],
+      "difficulty": 1
+    },
     {
       "uuid": "ddd",
       "slug": "bajillion",

track/track_test.go

@@ -19,6 +19,7 @@ func TestNewTrack(t *testing.T) {
 		"one":   false,
 		"two":   false,
 		"three": false,
+		"four":  false,
 	}
 
 	assert.Equal(t, len(slugs), len(track.Exercises))