// Package bird renders BIRD2 configuration for flock-agent. The agent
// writes the rendered file to a shared volume; the bird sidecar reads it,
// and the agent calls birdc reload (over the shared birdc unix socket) on
// changes.
package bird

import (
	"bytes"
	"fmt"
	"net"
	"sort"
	"strings"
	"text/template"
)

// NodeBGP describes the inputs needed to render a node's BIRD config.
type NodeBGP struct {
	NodeName string
	RouterID string // IPv4 (any usable v4 on the node, typically the host's)
	LocalASN uint32
	Peers    []Peer
	// LocalV6 / LocalV4 are this node's local source addresses on the
	// same subnet as the v6 / v4 BGP peers. Used as `source address` in
	// each BGP protocol stanza (Cisco rejects v6 advertisements whose
	// next-hop is link-local-only — explicit source forces a global next-
	// hop self that crt001 accepts).
	LocalV6 string
	LocalV4 string
	// CIDR6 / CIDR4 are the per-node summary aggregates the agent wants
	// advertised. The agent installs blackhole kernel routes for each so
	// BIRD's protocol kernel imports them.
	CIDR6 []string
	CIDR4 []string
	// Anycast6/4 are the currently-Ready anycast /128 and /32 addresses.
	Anycast6 []string
	Anycast4 []string
}

type Peer struct {
	// Family is "v6" or "v4".
	Family  string
	Address string
	ASN     uint32
}

const tpl = `# Generated by flock-agent. DO NOT EDIT.
log syslog all;
router id {{.RouterID}};

protocol device { scan time 10; }
protocol direct { interface "lo"; }

protocol kernel kernel6 {
  learn;
  ipv6 {
    import all;
    # Do NOT push BIRD static routes back to the kernel; the agent owns
    # the kernel host routes for anycast. BIRD static is for advertise only.
    export filter {
      if source = RTS_STATIC then reject;
      accept;
    };
  };
}
protocol kernel kernel4 {
  learn;
  ipv4 {
    import all;
    export filter {
      if source = RTS_STATIC then reject;
      accept;
    };
  };
}


protocol static static6 {
  ipv6;
  {{range $cidr := .CIDR6}}route {{$cidr}} blackhole;
  {{end}}{{range $a := .Anycast6}}route {{$a}}/128 blackhole;
  {{end}}
}
protocol static static4 {
  ipv4;
  {{range $cidr := .CIDR4}}route {{$cidr}} blackhole;
  {{end}}{{range $a := .Anycast4}}route {{$a}}/32 blackhole;
  {{end}}
}
{{range $i, $p := .Peers}}{{if eq $p.Family "v6"}}
protocol bgp upstream6_{{$i}} {
  local{{if $.LocalV6}} {{$.LocalV6}}{{end}} as {{$.LocalASN}};
  neighbor {{$p.Address}} as {{$p.ASN}};
  graceful restart;
  ipv6 {
    import all;
    next hop self;
    export filter {
      {{range $cidr := $.CIDR6}}if net = {{$cidr}} then accept;
      {{end}}{{range $a := $.Anycast6}}if net = {{$a}}/128 then accept;
      {{end}}reject;
    };
  };
}
{{else if eq $p.Family "v4"}}
protocol bgp upstream4_{{$i}} {
  local{{if $.LocalV4}} {{$.LocalV4}}{{end}} as {{$.LocalASN}};
  neighbor {{$p.Address}} as {{$p.ASN}};
  graceful restart;
  ipv4 {
    import all;
    next hop self;
    export filter {
      {{range $cidr := $.CIDR4}}if net = {{$cidr}} then accept;
      {{end}}{{range $a := $.Anycast4}}if net = {{$a}}/32 then accept;
      {{end}}reject;
    };
  };
}
{{end}}{{end}}`

// Render produces the bird.conf text.
//
// The output is deterministic: the same NodeBGP input always produces the
// same string. CIDR lists, anycast lists, and peer lists are sorted before
// templating so that the only way the rendered config changes is when
// semantically meaningful inputs change. This stability matters because
// BirdManager compares Render output against the last-written config to
// avoid superfluous birdc reloads.
//
// Render validates every operator-supplied value that flows into the
// templated output (peer addresses, CIDRs, anycast IPs, source addresses)
// so a malformed NodeConfig or annotation cannot produce a malformed
// bird.conf — even one that BIRD would later reject.
func Render(in NodeBGP) (string, error) {
	if in.RouterID == "" {
		return "", fmt.Errorf("bird render: RouterID is required")
	}
	if net.ParseIP(in.RouterID) == nil {
		return "", fmt.Errorf("bird render: RouterID %q is not a valid IP", in.RouterID)
	}
	if in.LocalASN == 0 {
		return "", fmt.Errorf("bird render: LocalASN is required")
	}
	if err := validateLocalSource(in.LocalV6, "v6"); err != nil {
		return "", err
	}
	if err := validateLocalSource(in.LocalV4, "v4"); err != nil {
		return "", err
	}
	for i, p := range in.Peers {
		if err := validatePeer(p); err != nil {
			return "", fmt.Errorf("bird render: peer[%d]: %w", i, err)
		}
	}
	if err := validateCIDRs(in.CIDR6, "v6"); err != nil {
		return "", fmt.Errorf("bird render: cidr6: %w", err)
	}
	if err := validateCIDRs(in.CIDR4, "v4"); err != nil {
		return "", fmt.Errorf("bird render: cidr4: %w", err)
	}
	if err := validateAnycastIPs(in.Anycast6, "v6"); err != nil {
		return "", fmt.Errorf("bird render: anycast6: %w", err)
	}
	if err := validateAnycastIPs(in.Anycast4, "v4"); err != nil {
		return "", fmt.Errorf("bird render: anycast4: %w", err)
	}

	in = normalize(in)

	t, err := template.New("bird").Parse(tpl)
	if err != nil {
		return "", fmt.Errorf("bird template parse: %w", err)
	}
	var buf bytes.Buffer
	if err := t.Execute(&buf, in); err != nil {
		return "", fmt.Errorf("bird template execute: %w", err)
	}
	return buf.String(), nil
}

// validatePeer checks that a peer entry has a parseable IP whose family
// matches its declared Family field, and a non-zero ASN.
func validatePeer(p Peer) error {
	if p.ASN == 0 {
		return fmt.Errorf("ASN must be non-zero")
	}
	ip := net.ParseIP(p.Address)
	if ip == nil {
		return fmt.Errorf("address %q is not a valid IP", p.Address)
	}
	isV4 := ip.To4() != nil
	switch p.Family {
	case "v6":
		if isV4 {
			return fmt.Errorf("address %q is IPv4 but Family is v6", p.Address)
		}
	case "v4":
		if !isV4 {
			return fmt.Errorf("address %q is IPv6 but Family is v4", p.Address)
		}
	default:
		return fmt.Errorf("Family %q must be v6 or v4", p.Family)
	}
	return nil
}

// validateCIDRs parses each entry as a CIDR and rejects family mismatches.
// fam must be "v6" or "v4".
func validateCIDRs(cidrs []string, fam string) error {
	for _, c := range cidrs {
		_, n, err := net.ParseCIDR(c)
		if err != nil {
			return fmt.Errorf("invalid CIDR %q: %w", c, err)
		}
		isV4 := n.IP.To4() != nil
		if fam == "v6" && isV4 {
			return fmt.Errorf("CIDR %q is IPv4, expected IPv6", c)
		}
		if fam == "v4" && !isV4 {
			return fmt.Errorf("CIDR %q is IPv6, expected IPv4", c)
		}
	}
	return nil
}

// validateAnycastIPs parses each entry as a literal IP (no prefix) and rejects
// family mismatches.
func validateAnycastIPs(ips []string, fam string) error {
	for _, s := range ips {
		ip := net.ParseIP(s)
		if ip == nil {
			return fmt.Errorf("invalid IP %q", s)
		}
		isV4 := ip.To4() != nil
		if fam == "v6" && isV4 {
			return fmt.Errorf("IP %q is IPv4, expected IPv6", s)
		}
		if fam == "v4" && !isV4 {
			return fmt.Errorf("IP %q is IPv6, expected IPv4", s)
		}
	}
	return nil
}

// validateLocalSource validates an optional LocalV6/LocalV4 source address.
// Empty is allowed (BIRD picks its own); non-empty must be a parseable IP of
// the matching family.
func validateLocalSource(s, fam string) error {
	if s == "" {
		return nil
	}
	ip := net.ParseIP(s)
	if ip == nil {
		return fmt.Errorf("bird render: Local%s %q is not a valid IP", strings.ToUpper(fam), s)
	}
	isV4 := ip.To4() != nil
	if fam == "v6" && isV4 {
		return fmt.Errorf("bird render: LocalV6 %q is IPv4", s)
	}
	if fam == "v4" && !isV4 {
		return fmt.Errorf("bird render: LocalV4 %q is IPv6", s)
	}
	return nil
}

func normalize(in NodeBGP) NodeBGP {
	cp := in
	cp.CIDR6 = sortedUnique(in.CIDR6)
	cp.CIDR4 = sortedUnique(in.CIDR4)
	cp.Anycast6 = sortedUnique(in.Anycast6)
	cp.Anycast4 = sortedUnique(in.Anycast4)
	cp.Peers = append([]Peer(nil), in.Peers...)
	sort.SliceStable(cp.Peers, func(i, j int) bool {
		if cp.Peers[i].Family != cp.Peers[j].Family {
			return cp.Peers[i].Family < cp.Peers[j].Family
		}
		return cp.Peers[i].Address < cp.Peers[j].Address
	})
	return cp
}

func sortedUnique(s []string) []string {
	if len(s) == 0 {
		return nil
	}
	cp := append([]string(nil), s...)
	sort.Strings(cp)
	out := cp[:0]
	for i, v := range cp {
		if i == 0 || v != cp[i-1] {
			out = append(out, v)
		}
	}
	return out
}

// FamilyOf returns "v6" or "v4" for a peer address string.
func FamilyOf(addr string) string {
	ip := net.ParseIP(addr)
	if ip == nil {
		return ""
	}
	if ip.To4() != nil {
		return "v4"
	}
	return "v6"
}