package backend

import (
	_ "embed"
	"strings"
	"sync"
)

// IEEE OUI registry as a compact text file: each line is 6 hex characters
// (uppercase, no separators) followed immediately by the organization name.
// Sourced from https://standards-oui.ieee.org/oui/oui.csv and pre-processed
// to strip the address column and common corporate suffixes (Inc, Ltd, ...).
//
//go:embed oui_data.txt
var ouiData string

var (
	ouiOnce  sync.Once
	ouiTable map[string]string
)

func loadOUITable() {
	ouiTable = make(map[string]string, 40000)
	for line := range strings.SplitSeq(ouiData, "\n") {
		if len(line) < 7 {
			continue
		}
		ouiTable[line[:6]] = line[6:]
	}
}

// LookupVendor returns the organization assigned to the OUI prefix of mac,
// or an empty string when the prefix is unknown or mac is malformed. Locally
// administered MACs (the U/L bit is set) are explicitly skipped — they are
// randomized by the device, not assigned to a vendor, so any match would be
// a false positive.
func LookupVendor(mac string) string {
	ouiOnce.Do(loadOUITable)

	prefix := normalizeOUI(mac)
	if prefix == "" {
		return ""
	}
	return ouiTable[prefix]
}

// normalizeOUI extracts the 6-hex-char OUI from a MAC address (any of
// "aa:bb:cc:...", "aa-bb-cc-...", "aabb.ccdd.eeff", "aabbcc...") and returns
// it uppercased. Returns "" for randomized MACs (locally administered bit
// set) or anything that doesn't parse.
func normalizeOUI(mac string) string {
	var hex strings.Builder
	hex.Grow(12)
	for i := 0; i < len(mac) && hex.Len() < 6; i++ {
		c := mac[i]
		switch {
		case c >= '0' && c <= '9', c >= 'A' && c <= 'F':
			hex.WriteByte(c)
		case c >= 'a' && c <= 'f':
			hex.WriteByte(c - 32)
		}
	}
	if hex.Len() < 6 {
		return ""
	}
	out := hex.String()

	// Skip locally administered MACs. The U/L bit is bit 1 of the first
	// octet — when set, the MAC was generated by the device (private
	// address randomization on iOS/Android, virtual interfaces, ...), not
	// assigned by IEEE, so OUI lookup is meaningless.
	first, ok := hexByte(out[0], out[1])
	if !ok || first&0x02 != 0 {
		return ""
	}
	return out
}

func hexByte(hi, lo byte) (byte, bool) {
	h, ok1 := hexNibble(hi)
	l, ok2 := hexNibble(lo)
	if !ok1 || !ok2 {
		return 0, false
	}
	return h<<4 | l, true
}

func hexNibble(c byte) (byte, bool) {
	switch {
	case c >= '0' && c <= '9':
		return c - '0', true
	case c >= 'A' && c <= 'F':
		return c - 'A' + 10, true
	case c >= 'a' && c <= 'f':
		return c - 'a' + 10, true
	}
	return 0, false
}