Build the Tentacle SDK without cgo

The SDK reaches internal/datetime through settings validation, and that
package had only a cgo strftime implementation, so statically linked
Tentacle packages could not build. Split the locale-aware cgo formatter
into its own file and add a C-locale fallback for builds without cgo.
The reader itself still uses the cgo implementation.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/datetime/datetime.go

@@ -4,17 +4,7 @@
 
 package datetime
 
-/*
-#include <locale.h>
-#include <stdlib.h>
-#include <time.h>
-*/
-import "C"
-
-import (
-	"time"
-	"unsafe"
-)
+import "time"
 
 type Option struct {
 	Value string
@@ -56,12 +46,6 @@ var dateFormats = []struct {
 	{"abbreviated-ymd", "%Y. %b %-d, %a"},
 }
 
-func init() {
-	locale := C.CString("")
-	defer C.free(unsafe.Pointer(locale))
-	C.setlocale(C.LC_TIME, locale)
-}
-
 func TimeOptions(now time.Time) []Option {
 	return options(timeFormats, now)
 }
@@ -133,22 +117,3 @@ func valid(formats []struct {
 	}
 	return false
 }
-
-func format(now time.Time, spec string) string {
-	cSpec := C.CString(spec)
-	defer C.free(unsafe.Pointer(cSpec))
-	tm := C.struct_tm{
-		tm_year:  C.int(now.Year() - 1900),
-		tm_mon:   C.int(int(now.Month()) - 1),
-		tm_mday:  C.int(now.Day()),
-		tm_hour:  C.int(now.Hour()),
-		tm_min:   C.int(now.Minute()),
-		tm_sec:   C.int(now.Second()),
-		tm_wday:  C.int(int(now.Weekday())),
-		tm_yday:  C.int(now.YearDay() - 1),
-		tm_isdst: -1,
-	}
-	buffer := make([]C.char, 512)
-	length := C.strftime(&buffer[0], C.size_t(len(buffer)), cSpec, &tm)
-	return C.GoStringN(&buffer[0], C.int(length))
-}

internal/datetime/format_cgo.go

@@ -0,0 +1,44 @@
+// Copyright (c) 2026 Storm Dragon
+// Authors: Storm Dragon, Angus Kola
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+//go:build cgo
+
+package datetime
+
+/*
+#include <locale.h>
+#include <stdlib.h>
+#include <time.h>
+*/
+import "C"
+
+import (
+	"time"
+	"unsafe"
+)
+
+func init() {
+	locale := C.CString("")
+	defer C.free(unsafe.Pointer(locale))
+	C.setlocale(C.LC_TIME, locale)
+}
+
+func format(now time.Time, spec string) string {
+	cSpec := C.CString(spec)
+	defer C.free(unsafe.Pointer(cSpec))
+	tm := C.struct_tm{
+		tm_year:  C.int(now.Year() - 1900),
+		tm_mon:   C.int(int(now.Month()) - 1),
+		tm_mday:  C.int(now.Day()),
+		tm_hour:  C.int(now.Hour()),
+		tm_min:   C.int(now.Minute()),
+		tm_sec:   C.int(now.Second()),
+		tm_wday:  C.int(int(now.Weekday())),
+		tm_yday:  C.int(now.YearDay() - 1),
+		tm_isdst: -1,
+	}
+	buffer := make([]C.char, 512)
+	length := C.strftime(&buffer[0], C.size_t(len(buffer)), cSpec, &tm)
+	return C.GoStringN(&buffer[0], C.int(length))
+}

internal/datetime/format_nocgo.go

@@ -0,0 +1,80 @@
+// Copyright (c) 2026 Storm Dragon
+// Authors: Storm Dragon, Angus Kola
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+//go:build !cgo
+
+package datetime
+
+import (
+	"strconv"
+	"strings"
+	"time"
+)
+
+// format is a C-locale strftime subset for builds without cgo, such as the
+// statically linked Tentacle packages that only validate settings. The reader
+// itself is built with cgo and uses the locale-aware implementation.
+func format(now time.Time, spec string) string {
+	var out strings.Builder
+	for index := 0; index < len(spec); index++ {
+		if spec[index] != '%' || index+1 == len(spec) {
+			out.WriteByte(spec[index])
+			continue
+		}
+		index++
+		pad := true
+		if spec[index] == '-' && index+1 < len(spec) {
+			pad = false
+			index++
+		}
+		two := func(value int) string {
+			if pad && value < 10 {
+				return "0" + strconv.Itoa(value)
+			}
+			return strconv.Itoa(value)
+		}
+		switch spec[index] {
+		case 'H':
+			out.WriteString(two(now.Hour()))
+		case 'I':
+			hour := now.Hour() % 12
+			if hour == 0 {
+				hour = 12
+			}
+			out.WriteString(two(hour))
+		case 'M':
+			out.WriteString(two(now.Minute()))
+		case 'S':
+			out.WriteString(two(now.Second()))
+		case 'p':
+			out.WriteString(now.Format("PM"))
+		case 'X':
+			out.WriteString(now.Format("15:04:05"))
+		case 'x':
+			out.WriteString(now.Format("01/02/06"))
+		case 'd':
+			out.WriteString(two(now.Day()))
+		case 'm':
+			out.WriteString(two(int(now.Month())))
+		case 'Y':
+			out.WriteString(strconv.Itoa(now.Year()))
+		case 'y':
+			out.WriteString(now.Format("06"))
+		case 'A':
+			out.WriteString(now.Weekday().String())
+		case 'a':
+			out.WriteString(now.Weekday().String()[:3])
+		case 'B':
+			out.WriteString(now.Month().String())
+		case 'b':
+			out.WriteString(now.Month().String()[:3])
+		case '%':
+			out.WriteByte('%')
+		default:
+			out.WriteByte('%')
+			out.WriteByte(spec[index])
+		}
+	}
+	return out.String()
+}

internal/datetime/format_nocgo_test.go

@@ -0,0 +1,28 @@
+// Copyright (c) 2026 Storm Dragon
+// Authors: Storm Dragon, Angus Kola
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+//go:build !cgo
+
+package datetime
+
+import (
+	"testing"
+	"time"
+)
+
+func TestFormatFallbackMatchesCLocale(t *testing.T) {
+	now := time.Date(2026, time.March, 5, 7, 4, 9, 0, time.UTC)
+	for spec, want := range map[string]string{
+		"%I:%M %p": "07:04 AM", "%H:%M:%S": "07:04:09", "%X": "07:04:09", "%x": "03/05/26",
+		"%A, %-d %B, %Y": "Thursday, 5 March, 2026", "%a, %b %-d": "Thu, Mar 5", "%d/%m/%Y": "05/03/2026",
+		"%H hours and %M minutes": "07 hours and 04 minutes", "100%%": "100%",
+	} {
+		if got := format(now, spec); got != want {
+			t.Fatalf("%q = %q, want %q", spec, got, want)
+		}
+	}
+	if !ValidTime("24-hour") || !ValidDate("full-mdy") {
+		t.Fatal("validation changed without cgo")
+	}
+}