Add frontmatter extractor

frontmatter.go

@@ -0,0 +1,23 @@
+package main
+
+import "bytes"
+
+var (
+	frontmatterOpen  = []byte("---")
+	frontmatterClose = []byte("\n---")
+)
+
+func extractFrontmatter(b []byte) (frontmatter, content []byte) {
+	if !bytes.HasPrefix(b, frontmatterOpen) {
+		return nil, b
+	}
+	fm := b[len(frontmatterOpen):]
+	if len(fm) != 0 && fm[0] != '\n' {
+		return nil, b
+	}
+	i := bytes.Index(fm, frontmatterClose)
+	if i == -1 {
+		i = len(fm)
+	}
+	return fm[:i], b[len(frontmatterOpen)+len(fm):]
+}

frontmatter_test.go

@@ -0,0 +1,69 @@
+package main
+
+import (
+	"testing"
+)
+
+func TestExtractFrontmatter(t *testing.T) {
+	tests := []struct {
+		Raw         string
+		Frontmatter string
+		Content     string
+	}{
+		{
+			Raw:         "",
+			Frontmatter: "",
+			Content:     "",
+		},
+		{
+			Raw:         "---\na: b\nc: d\n---",
+			Frontmatter: "\na: b\nc: d",
+			Content:     "",
+		},
+		{
+			Raw:         "# Hello, world!",
+			Frontmatter: "",
+			Content:     "# Hello, world!",
+		},
+		{
+			Raw:         "---\ne: f\ng: h",
+			Frontmatter: "\ne: f\ng: h",
+			Content:     "",
+		},
+		{
+			Raw:         "----\na: b\nc: d",
+			Frontmatter: "",
+			Content:     "----\na: b\nc: d",
+		},
+		{
+			Raw:         "---\n---",
+			Frontmatter: "",
+			Content:     "",
+		},
+		{
+			Raw:         "\n---\n---\nHello, world!",
+			Frontmatter: "",
+			Content:     "\n---\n---\nHello, world!",
+		},
+		{
+			Raw:         "---",
+			Frontmatter: "",
+			Content:     "",
+		},
+		{
+			Raw:         "----",
+			Frontmatter: "",
+			Content:     "----",
+		},
+	}
+
+	for _, test := range tests {
+		frontmatter, content := extractFrontmatter([]byte(test.Raw))
+		if string(frontmatter) != test.Frontmatter {
+			t.Fatalf("expected frontmatter %q, got %q", test.Frontmatter, string(frontmatter))
+		}
+		if string(content) != test.Content {
+			t.Fatalf("expected content %q, got %q", test.Content, string(content))
+		}
+	}
+}