// Package blocklist downloads public IP blocklists on a schedule, keeps the
// last good copy of each on disk and in memory, and answers which lists
// contain an address.
package blocklist

import (
	"context"
	"log/slog"
	"net/netip"
	"sync"
	"time"

	"go4.org/netipx"
)

// Format is how a list's file is written.
type Format int

const (
	// FormatLines has one address or network per line, such as 192.0.2.1 or
	// 192.0.2.0/24. Lines that start with # are comments.
	FormatLines Format = iota

	// FormatSpamhausJSON has one JSON object per line, with a network in its
	// cidr field. The last line holds the file's metadata and no network.
	FormatSpamhausJSON

	// FormatDShield has one network per line, as tab-separated fields that
	// start with the network's first and last address. Lines that start
	// with # are comments.
	FormatDShield
)

// List is one public blocklist.
type List struct {
	// Name names the list in lookup answers, and names its copy on disk.
	Name string

	// Description says in one line what being on the list means.
	Description string

	// URL is where the publisher serves the whole list.
	URL string

	// Interval is how long a copy is used before the list is downloaded
	// again.
	Interval time.Duration

	// Format is how the list's file is written.
	Format Format
}

// Listing says that a list contains an address.
type Listing struct {
	Name        string `json:"name"`
	Description string `json:"description"`

	// Updated is when routewatch last downloaded the list, or last heard from
	// its publisher that its copy was still current.
	Updated time.Time `json:"updated"`
}

// listCopy is a list's last good copy in memory.
type listCopy struct {
	addresses *netipx.IPSet
	updated   time.Time

	// validators identify the version of the list this copy holds.
	validators validators
}

// Params is what New needs.
type Params struct {
	// Dir is the directory that holds the copies on disk.
	Dir string

	// Lists are the lists to download and look addresses up in.
	Lists []List

	Logger *slog.Logger
}

// Blocklists keeps a copy of each list and answers which lists contain an
// address. Start loads the copies from disk and starts downloading the lists.
type Blocklists struct {
	dir    string
	lists  []List
	logger *slog.Logger

	// copies holds each list's last good copy, by list name. A list with no
	// good copy yet has none.
	mu     sync.RWMutex
	copies map[string]listCopy

	// stop ends the goroutine Start runs, and done is closed when it returns.
	stop context.CancelFunc
	done chan struct{}
}

// New returns a Blocklists with no copies. Start loads them.
func New(params Params) *Blocklists {
	return &Blocklists{
		dir:    params.Dir,
		lists:  params.Lists,
		logger: params.Logger.With("component", "blocklists"),
		copies: make(map[string]listCopy),
		done:   make(chan struct{}),
	}
}

// Lookup returns a Listing for each list whose copy contains addr, in the
// order of the lists given to New. It makes no outside request. When no list
// contains addr, it returns an empty list, not nil.
func (b *Blocklists) Lookup(addr netip.Addr) []Listing {
	b.mu.RLock()
	defer b.mu.RUnlock()

	listings := []Listing{}

	for _, list := range b.lists {
		c, ok := b.copies[list.Name]
		if ok && c.addresses.Contains(addr.Unmap()) {
			listings = append(listings, Listing{
				Name:        list.Name,
				Description: list.Description,
				Updated:     c.updated,
			})
		}
	}

	return listings
}

// ShortestInterval returns the shortest Interval of the lists. That is the
// soonest a copy can change, and so how long a lookup answer stays current.
func (b *Blocklists) ShortestInterval() time.Duration {
	var shortest time.Duration

	for i, list := range b.lists {
		if i == 0 || list.Interval < shortest {
			shortest = list.Interval
		}
	}

	return shortest
}

// updated returns when the named list's copy was last downloaded or found
// current, or the zero time when the list has no copy.
func (b *Blocklists) updated(name string) time.Time {
	b.mu.RLock()
	defer b.mu.RUnlock()

	return b.copies[name].updated
}

// copyValidators returns the validators of the named list's copy. They are
// empty when the list has no copy.
func (b *Blocklists) copyValidators(name string) validators {
	b.mu.RLock()
	defer b.mu.RUnlock()

	return b.copies[name].validators
}

// setCopy makes c the named list's copy.
func (b *Blocklists) setCopy(name string, c listCopy) {
	b.mu.Lock()
	defer b.mu.Unlock()

	b.copies[name] = c
}

// setUpdated records that the named list's copy was found current at t.
func (b *Blocklists) setUpdated(name string, t time.Time) {
	b.mu.Lock()
	defer b.mu.Unlock()

	c, ok := b.copies[name]
	if ok {
		c.updated = t
		b.copies[name] = c
	}
}