// Package deadlinks implements a liveness checker for hyperlinks in HTML and
// gemtext documents.
//
// # Storage
//
// By default DeadLinks uses an in-memory SQLite database for tracking the
// status of resources and the links between them. If memory usage becomes a
// problem it is also possible to use a SQLite database file:
//
//	store := deadlinks.NewSQLiteStore(&deadlinks.SQLiteStoreOpts{
//		Path: "/path/to/db/file.sqlite",
//	})
//
//	// TODO initialize DeadLinks
package deadlinks

import (
	"context"
	"errors"
	"fmt"
	"regexp"
	"runtime"
	"sync"
	"time"

	"code.betamike.com/mediocregopher/mediocre-go-lib/miter"
)

// Opts are optional fields which can be provided to New. A nil Opts is
// equivalent to an empty one.
type Opts struct {
	Client Client // Defaults to `NewClient(nil)`
	Parser Parser // Defaults to `NewParser()`

	// Concurrency determines the maximum number of URLs which can be checked
	// simultaneously.
	//
	// Default: `runtime.NumCPU()`
	Concurrency int

	// OnError, if set, will be called whenever DeadLinks encounters an error
	// internally that it would otherwise skip over.
	OnError func(error)
}

func (o *Opts) withDefaults() *Opts {
	if o == nil {
		o = new(Opts)
	}

	if o.Client == nil {
		o.Client = NewClient(nil)
	}

	if o.Parser == nil {
		o.Parser = NewParser()
	}

	if o.Concurrency == 0 {
		o.Concurrency = runtime.NumCPU()
	}

	return o
}

// DeadLinks crawls a configured space of URLs and keeps track of any broken
// links which it finds.
//
// DeadLinks supports multiple web protocols and document formats
// out-of-the-box, and will traverse between them as necessary based on URL
// schemas. See the `NewClient` and `NewParser` functions for more details.
type DeadLinks struct {
	opts     Opts
	store    Store
	patterns []*regexp.Regexp
}

// New initializes and returns a DeadLinks instance which will track the
// liveness of the given set of pinned URLs, as well as all URLs linked to from
// those. If a linked URL matches one of the given regexp patterns then any
// URLs linked to from it will be tracked as well.
//
// If a non-empty Store is passed to New then whatever set of previously pinned
// URLs were present will be overwritten with the given ones.
func New(
	ctx context.Context,
	store Store,
	pinnedURLStrs,
	patternStrs []string,
	opts *Opts,
) (
	*DeadLinks, error,
) {
	var (
		err        error
		pinnedURLs = make([]URL, len(pinnedURLStrs))
		patterns   = make([]*regexp.Regexp, len(patternStrs))
	)

	for i, u := range pinnedURLStrs {
		if pinnedURLs[i], err = ParseURL(u); err != nil {
			return nil, fmt.Errorf("parsing url %q: %w", u, err)
		}
	}

	for i, p := range patternStrs {
		if patterns[i], err = regexp.Compile(p); err != nil {
			return nil, fmt.Errorf("compiling regexp %q: %w", p, err)
		}
	}

	d := &DeadLinks{
		opts:     *opts.withDefaults(),
		store:    store,
		patterns: patterns,
	}

	if err := d.store.SetPinned(ctx, pinnedURLs); err != nil {
		return nil, fmt.Errorf("pinning URLs: %w", err)
	}

	return d, nil
}

func (d *DeadLinks) onError(ctx context.Context, err error) {
	if d.opts.OnError == nil ||
		(ctx.Err() != nil && errors.Is(err, ctx.Err())) {
		return
	}

	d.opts.OnError(err)
}

func (d *DeadLinks) shouldFollowURL(url URL) bool {
	urlStr := string(url)
	for _, pattern := range d.patterns {
		if pattern.MatchString(urlStr) {
			return true
		}
	}
	return false
}

func (d *DeadLinks) getURL(ctx context.Context, url URL) ([]URL, error) {
	mimeType, body, err := d.opts.Client.Get(ctx, url)
	if err != nil {
		return nil, err
	}
	defer body.Close()

	if !d.shouldFollowURL(url) {
		return nil, nil
	}

	return d.opts.Parser.Parse(mimeType, body)
}

// checkURL only returns an error if storing the results of the check fails.
func (d *DeadLinks) checkURL(ctx context.Context, url URL) error {
	var (
		now      = time.Now()
		status   = ResourceStatusOK
		errorStr string
	)

	outgoingURLs, err := d.getURL(ctx, url)
	if err != nil {
		status = ResourceStatusError
		errorStr = err.Error()
	}

	err = d.store.Update(ctx, now, url, status, errorStr, outgoingURLs)
	if err != nil {
		return fmt.Errorf(
			"failed storing status:%v (errorStr:%q) and %d outgoingURLs: %w",
			status, errorStr, len(outgoingURLs), err,
		)
	}

	return nil
}

// Update runs through all pinned or previously discovered URLs which were
// last checked prior to the given time (or which have never been checked) and
// updates the internal storage with any new URLs and links to dead URLs which
// it finds.
func (d *DeadLinks) Update(
	ctx context.Context, lastCheckedBefore time.Time,
) error {
	var (
		wg = new(sync.WaitGroup)
		ch = make(chan URL, d.opts.Concurrency)
	)

	wg.Add(d.opts.Concurrency)
	for i := 0; i < d.opts.Concurrency; i++ {
		go func() {
			defer wg.Done()
			for url := range ch {
				if err := d.checkURL(ctx, url); err != nil {
					d.onError(ctx, fmt.Errorf("checking url %q: %w", url, err))
				}
			}
		}()
	}

	var err error

	// Because checking URLs can result in new URLs being inserted into the
	// Store, we query the Store in a loop until it stops producing
	// unvisited/stale URLs.
	for {
		var (
			n        int
			urlsIter = d.store.GetURLsByLastChecked(lastCheckedBefore)
		)

		err = miter.ForEach(ctx, urlsIter, func(url URL) error {
			ch <- url
			n++
			return nil
		})

		if err != nil || n == 0 {
			break
		}
	}

	close(ch)
	wg.Wait()

	if err != nil {
		return fmt.Errorf("iterating urls needing checked: %w", err)
	}

	if err := d.store.GC(ctx); err != nil {
		return fmt.Errorf("garbage collecting: %w", err)
	}

	return nil
}

// TODO expose GetByStatus