templates: add head and tail functions

Add head and tail functions to get beginning or ends of strings.

E.g.:

{{"hello" | head 2}} will return "he"
{{"hello" | tail 2}} will return "lo"

Implements: https://todo.sr.ht/~rjarry/aerc/220
Changelog-added: New `head` and `tail` templates functions for strings.
Signed-off-by: Bence Ferdinandy <bence@ferdinandy.com>
Reviewed-by: Moritz Poldrack <moritz@poldrack.dev>
Acked-by: Robin Jarry <robin@jarry.cc>

2 files changed, 54 insertions, 0 deletions

diff --git a/lib/templates/functions.go b/lib/templates/functions.go
index 8b7af449..0d204b8f 100644
--- a/lib/templates/functions.go
+++ b/lib/templates/functions.go
@@ -334,6 +334,24 @@ func hasPrefix(prefix, s string) bool {
 	return strings.HasPrefix(s, prefix)
 }
 
+func head(n uint, s string) string {
+	r := []rune(s)
+	length := uint(len(r))
+	if length >= n {
+		return string(r[:n])
+	}
+	return s
+}
+
+func tail(n uint, s string) string {
+	r := []rune(s)
+	length := uint(len(r))
+	if length >= n {
+		return string(r[length-n:])
+	}
+	return s
+}
+
 var templateFuncs = template.FuncMap{
 	"quote":         quote,
 	"wrapText":      wrapText,
@@ -367,4 +385,6 @@ var templateFuncs = template.FuncMap{
 	"toLower":       strings.ToLower,
 	"toUpper":       strings.ToUpper,
 	"replace":       replace,
+	"head":          head,
+	"tail":          tail,
 }
diff --git a/lib/templates/functions_test.go b/lib/templates/functions_test.go
index 492aad01..3dac591e 100644
--- a/lib/templates/functions_test.go
+++ b/lib/templates/functions_test.go
@@ -121,3 +121,37 @@ func TestTemplates_DifferentInitialsFormats(t *testing.T) {
 		assert.Equal(t, c.initials, intls[0])
 	}
 }
+
+func TestTemplates_Head(t *testing.T) {
+	type testCase struct {
+		head   uint
+		input  string
+		output string
+	}
+	cases := []testCase{
+		{head: 3, input: "abcde", output: "abc"},
+		{head: 10, input: "abcde", output: "abcde"},
+	}
+
+	for _, c := range cases {
+		out := head(c.head, c.input)
+		assert.Equal(t, c.output, out)
+	}
+}
+
+func TestTemplates_Tail(t *testing.T) {
+	type testCase struct {
+		tail   uint
+		input  string
+		output string
+	}
+	cases := []testCase{
+		{tail: 2, input: "abcde", output: "de"},
+		{tail: 8, input: "abcde", output: "abcde"},
+	}
+
+	for _, c := range cases {
+		out := tail(c.tail, c.input)
+		assert.Equal(t, c.output, out)
+	}
+}