package preset

import (
	"fmt"
	"net/netip"
	"strconv"
	"strings"
)

// ParsePrefix accepts every spelling of a subnet mask the editor is required to
// take — "24", "/24" and "255.255.255.0" — and returns the prefix length.
//
// Only contiguous masks are accepted. 255.255.0.255 is a typo, not a netmask,
// and letting it through would produce a netsh call that fails confusingly.
func ParsePrefix(s string) (int, error) {
	s = strings.TrimSpace(s)
	if s == "" {
		return 0, fmt.Errorf("empty mask")
	}
	s = strings.TrimPrefix(s, "/")

	if strings.Contains(s, ".") {
		addr, err := netip.ParseAddr(s)
		if err != nil || !addr.Is4() {
			return 0, fmt.Errorf("%q is not a dotted-quad netmask", s)
		}
		b := addr.As4()
		mask := uint32(b[0])<<24 | uint32(b[1])<<16 | uint32(b[2])<<8 | uint32(b[3])

		ones := 0
		for ones < 32 && mask&(1<<uint(31-ones)) != 0 {
			ones++
		}
		// Everything below the leading run of ones must be zero.
		if ones < 32 && mask<<uint(ones) != 0 {
			return 0, fmt.Errorf("%q is not a contiguous netmask", s)
		}
		return ones, nil
	}

	n, err := strconv.Atoi(s)
	if err != nil {
		return 0, fmt.Errorf("%q is not a prefix length", s)
	}
	if n < 0 || n > 32 {
		return 0, fmt.Errorf("prefix /%d out of range", n)
	}
	return n, nil
}

// MaskString renders a prefix length as the dotted-quad netmask netsh expects
// on the command line. netsh will not take "/24".
func MaskString(prefix int) string {
	if prefix < 0 || prefix > 32 {
		prefix = 32
	}
	var mask uint32
	if prefix > 0 {
		mask = ^uint32(0) << uint(32-prefix)
	}
	return fmt.Sprintf("%d.%d.%d.%d", mask>>24&0xff, mask>>16&0xff, mask>>8&0xff, mask&0xff)
}

// FormatPrefix renders a prefix in whichever notation the user last chose.
// dotted controls the style; see config.Settings.MaskStyle.
func FormatPrefix(prefix int, dotted bool) string {
	if dotted {
		return MaskString(prefix)
	}
	return "/" + strconv.Itoa(prefix)
}