rewrite html generator with html/template

internal/disk_usage/disk_usage_windows.go

@@ -0,0 +1,37 @@
+package disk_usage
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+// Loaded on first call. kernel32.dll is a KnownDLL,
+// Windows always loads it from the system directory.
+var procGetDiskFreeSpaceExW = syscall.NewLazyDLL("kernel32.dll").NewProc("GetDiskFreeSpaceExW")
+
+func diskUsage(path string) (Usage, error) {
+	pathPtr, err := syscall.UTF16PtrFromString(path)
+	if err != nil {
+		return Usage{}, err
+	}
+	// BOOL GetDiskFreeSpaceExW(LPCWSTR lpDirectoryName,
+	//     PULARGE_INTEGER lpFreeBytesAvailableToCaller,  // respects user disk quotas
+	//     PULARGE_INTEGER lpTotalNumberOfBytes,          // respects user disk quotas
+	//     PULARGE_INTEGER lpTotalNumberOfFreeBytes);     // whole disk
+	var freeToCaller, total, totalFree uint64
+	ok, _, callErr := procGetDiskFreeSpaceExW.Call(
+		uintptr(unsafe.Pointer(pathPtr)),
+		uintptr(unsafe.Pointer(&freeToCaller)),
+		uintptr(unsafe.Pointer(&total)),
+		uintptr(unsafe.Pointer(&totalFree)))
+	if ok == 0 {
+		// Call always returns non-nil error, it is meaningful only if the function failed
+		return Usage{}, callErr
+	}
+	return Usage{
+		Total: int64(total),
+		// with quotas totalFree can be > total
+		Used: max(int64(total)-int64(totalFree), 0),
+		Free: int64(freeToCaller),
+	}, nil
+}