feat: implements --tags

Implements `--tags`, which lists all tags in use.

internal/sheets/tags.go

@@ -0,0 +1,36 @@
+package sheets
+
+import (
+	"sort"
+
+	"github.com/cheat/cheat/internal/sheet"
+)
+
+// Tags returns a slice of all tags in use in any sheet
+func Tags(cheatpaths []map[string]sheet.Sheet) []string {
+
+	// create a map of all tags in use in any sheet
+	tags := make(map[string]bool)
+
+	// iterate over all tags on all sheets on all cheatpaths
+	for _, path := range cheatpaths {
+		for _, sheet := range path {
+			for _, tag := range sheet.Tags {
+				tags[tag] = true
+			}
+		}
+	}
+
+	// restructure the map into a slice
+	sorted := []string{}
+	for tag := range tags {
+		sorted = append(sorted, tag)
+	}
+
+	// sort the slice
+	sort.Slice(sorted, func(i, j int) bool {
+		return sorted[i] < sorted[j]
+	})
+
+	return sorted
+}

internal/sheets/tags_test.go

@@ -0,0 +1,51 @@
+package sheets
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/davecgh/go-spew/spew"
+
+	"github.com/cheat/cheat/internal/sheet"
+)
+
+// TestTags asserts that cheetsheet tags are properly returned
+func TestTags(t *testing.T) {
+
+	// mock cheatsheets available on multiple cheatpaths
+	cheatpaths := []map[string]sheet.Sheet{
+
+		// mock community cheatsheets
+		map[string]sheet.Sheet{
+			"foo": sheet.Sheet{Title: "foo", Tags: []string{"alpha"}},
+			"bar": sheet.Sheet{Title: "bar", Tags: []string{"alpha", "bravo"}},
+		},
+
+		// mock local cheatsheets
+		map[string]sheet.Sheet{
+			"bar": sheet.Sheet{Title: "bar", Tags: []string{"bravo", "charlie"}},
+			"baz": sheet.Sheet{Title: "baz", Tags: []string{"delta"}},
+		},
+	}
+
+	// consolidate the cheatsheets
+	tags := Tags(cheatpaths)
+
+	// specify the expected output
+	want := []string{
+		"alpha",
+		"bravo",
+		"charlie",
+		"delta",
+	}
+
+	// assert that the cheatsheets properly consolidated
+	if !reflect.DeepEqual(tags, want) {
+		t.Errorf(
+			"failed to return tags: want:\n%s, got:\n%s",
+			spew.Sdump(want),
+			spew.Sdump(tags),
+		)
+	}
+
+}