Add "hash" template function

Signed-off-by: Knut Ahlers <knut@ahlers.me>

README.md

@@ -35,10 +35,16 @@
   $ echo '{{ file "hello" }}' | korvike
   Hello World
   ```
+- `{{ hash <algo> <string> }}`  
+  Hash string with given algorithm (supported algorithms: md5, sha1, sha256, sha512)
+  ```console
+  $ echo '{{ hash "sha256" "this is a test" }}' | korvike
+  2e99758548972a8e8822ad47fa1017ff72f06f3ff6a016851f45c398732bc50c
+  ```
 - `{{ markdown <source> }}`  
   Format the source using a markdown parser
   ```console
-  $ echo '{{ markdown "# headline" | korvike }}'
+  $ echo '{{ markdown "# headline" }}' | korvike
   <h1>headline</h1>
   ```
 - `{{ now <format string> }}`  

functions/func_hash.go

@@ -0,0 +1,40 @@
+package functions
+
+import (
+	"crypto/md5"
+	"crypto/sha1"
+	"crypto/sha256"
+	"crypto/sha512"
+	"errors"
+	"fmt"
+	"hash"
+)
+
+func init() {
+	registerFunction("hash", func(name string, v ...string) (string, error) {
+		if len(v) < 1 {
+			return "", errors.New("no string to hash")
+		}
+
+		var hash hash.Hash
+		switch name {
+		case "md5":
+			hash = md5.New()
+		case "sha1":
+			hash = sha1.New()
+		case "sha256":
+			hash = sha256.New()
+		case "sha512":
+			hash = sha512.New()
+
+		default:
+			return "", fmt.Errorf("hash algo %q not supported", name)
+		}
+
+		if _, err := hash.Write([]byte(v[0])); err != nil {
+			return "", fmt.Errorf("writing to hash: %w", err)
+		}
+
+		return fmt.Sprintf("%x", hash.Sum(nil)), nil
+	})
+}

functions/func_test.go

@@ -73,6 +73,20 @@ func Test_file(t *testing.T) {
 	}
 }
 
+func Test_hash(t *testing.T) {
+	input := "I'm a string to hash"
+	for algo, exp := range map[string]string{
+		"md5":    "d5adddaa0fd9f924b85e7874dc85f814",
+		"sha1":   "bd41599338445f401b8d3751fbe718e8a0b52004",
+		"sha256": "ba32f090baf28862816a10da05509b31393704184ae49c68f9eb2933afa9e4d1",
+		"sha512": "3288edcff4f28526fe8ecfb6d5182f2a446ab0572550c9591d1f5cacd377397af25c0274c9c2428c35422d215ccdc304d0353c093c76e750f9d7c4d54e64eed8",
+	} {
+		if res := renderHelper(fmt.Sprintf("{{ hash %q %q }}", algo, input), nil); res != exp {
+			t.Errorf("Hash algo %q yield unexpected result: exp=%q res=%q", algo, exp, res)
+		}
+	}
+}
+
 func Test_now(t *testing.T) {
 	if _, err := time.Parse(time.RFC3339Nano, renderHelper(fmt.Sprintf("{{now %q}}", time.RFC3339Nano), nil)); err != nil {
 		t.Errorf("[now] did not produce expected time format")