package inventory

import (
	"context"
	"fmt"
	"net/netip"
)

// The mesh assigns overlay addresses. A node computes nothing about the network it is joining —
// it generates a keypair, publishes the public half, and receives the rest
// (novox/hq 08-connectivity).

// AssignAddress gives a node its place on the private network, if it has none.
//
// Idempotent: a node that already has an address keeps it. Reassigning would change every other
// node's peer list to chase it, and an address that moves is the thing declaring the hub was
// meant to stop.
//
// **The adopted tunnel's addresses come first** (novox/hq ADR 0105). The hub that took over a
// tunnel is at the tunnel's own address. A node enrolling with a key the tunnel already routed
// to keeps the address the tunnel had for it — nothing a peer knows changes. And an address the
// tunnel holds for a peer that has not enrolled is never handed to anyone else: that peer is
// still reaching the hub at it.
//
// The rest is allocated in order from the range, taking the lowest free one. Not random: a person
// reading a peer list should be able to guess which node an address belongs to, and reuse of a
// released address is a smaller problem than a list nobody can hold in their head.
func (i *Inventory) AssignAddress(ctx context.Context, node, cidr string) (string, error) {
	prefix, err := netip.ParsePrefix(cidr)
	if err != nil {
		return "", fmt.Errorf("%q is not a network the mesh can allocate from: %w", cidr, err)
	}

	var existing, key *string
	var name string
	var hub bool
	if err := i.store.Pool().QueryRow(ctx,
		`select name, host(overlay_address), overlay_key, is_hub from node where id = $1`, node).
		Scan(&name, &existing, &key, &hub); err != nil {
		return "", err
	}
	if existing != nil && *existing != "" {
		return *existing, nil
	}

	tunnel, hubName, adopted, err := i.AdoptedTunnel(ctx)
	if err != nil {
		return "", err
	}
	if adopted && hub && hubName == name {
		address, err := netip.ParsePrefix(tunnel.Address)
		if err != nil {
			return "", fmt.Errorf("the adopted tunnel's address %q: %w", tunnel.Address, err)
		}
		return i.place(ctx, node, address.Addr().String())
	}
	carried, err := i.CarriedPeers(ctx)
	if err != nil {
		return "", err
	}
	taken := map[string]bool{}
	if adopted {
		// The tunnel's own address is the hub's whether or not the hub has been placed yet.
		if address, err := netip.ParsePrefix(tunnel.Address); err == nil {
			taken[address.Addr().String()] = true
		}
	}
	for _, p := range carried {
		if key != nil && p.PublicKey == *key {
			// The tunnel already routes to this key: the node keeps that address, and the peer
			// notices nothing when its machine enrols.
			//
			// **Unless the operator named it something else** (novox/hq issue 112). The name is
			// what the mesh has been answering for this address in the meantime; a machine
			// enrolling under a different one would silently split the two — the name resolving
			// here, the node known as that — so it is refused where the operator can read it.
			if p.Named != "" && p.Named != name {
				return "", fmt.Errorf(
					"the carried peer at %s was named %q, and %q is enrolling under its key — "+
						"enrol it as %q, or rename the peer first (`overlay name`)",
					p.Address, p.Named, name, p.Named)
			}
			return i.place(ctx, node, p.Address)
		}
		taken[p.Address] = true
	}

	rows, err := i.store.Pool().Query(ctx,
		`select host(overlay_address) from node where overlay_address is not null`)
	if err != nil {
		return "", err
	}
	for rows.Next() {
		var a string
		if err := rows.Scan(&a); err != nil {
			rows.Close()
			return "", err
		}
		taken[a] = true
	}
	rows.Close()
	if err := rows.Err(); err != nil {
		return "", err
	}

	// The first address in a range is conventionally the network itself and is skipped, so
	// allocation starts one past it.
	candidate := prefix.Masked().Addr().Next()
	for prefix.Contains(candidate) {
		if !taken[candidate.String()] {
			return i.place(ctx, node, candidate.String())
		}
		candidate = candidate.Next()
	}
	// Said plainly rather than returning an empty address that fails later on a machine. A mesh
	// that has outgrown its range needs a person, and renumbering is not something to attempt
	// halfway through assigning one node.
	return "", fmt.Errorf(
		"every address in %s is taken, so %s cannot be given one. The mesh has outgrown its "+
			"range and renumbering it is a deliberate act", cidr, name)
}

func (i *Inventory) place(ctx context.Context, node, address string) (string, error) {
	if _, err := i.store.Pool().Exec(ctx,
		`update node set overlay_address = $2::inet where id = $1`, node, address); err != nil {
		return "", err
	}
	return address, nil
}