// Copyright 2023 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package client

import (
	"context"
	"encoding/json"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"path"
	"time"

	"golang.org/x/vuln/internal"
	"golang.org/x/vuln/internal/derrors"
	"golang.org/x/vuln/osv"
)

type httpSource struct {
	url    string // the base URI of the source (without trailing "/"). e.g. https://vuln.golang.org
	c      *http.Client
	cache  Cache
	dbName string

	// indexCalls counts the number of times Index()
	// method has been called. httpCalls counts the
	// number of times GetByModule makes an http request
	// to the vuln db for a module path. Used for testing
	// privacy properties of httpSource.
	indexCalls int
	httpCalls  int
}

func newHTTPClient(uri *url.URL, opts Options) (_ *httpSource, err error) {
	hs := &httpSource{url: uri.String()}
	hs.dbName = uri.Hostname()
	if opts.HTTPCache != nil {
		hs.cache = opts.HTTPCache
	}
	if opts.HTTPClient != nil {
		hs.c = opts.HTTPClient
	} else {
		hs.c = new(http.Client)
	}
	return hs, nil
}

func (hs *httpSource) Index(ctx context.Context) (_ DBIndex, err error) {
	hs.indexCalls++ // for testing privacy properties
	defer derrors.Wrap(&err, "Index()")

	var cachedIndex DBIndex
	var cachedIndexRetrieved *time.Time

	if hs.cache != nil {
		index, retrieved, err := hs.cache.ReadIndex(hs.dbName)
		if err != nil {
			return nil, err
		}

		cachedIndex = index
		if cachedIndex != nil {
			if time.Since(retrieved) < time.Hour*2 {
				return cachedIndex, nil
			}

			cachedIndexRetrieved = &retrieved
		}
	}

	req, err := http.NewRequestWithContext(ctx, "GET", fmt.Sprintf("%s/index.json", hs.url), nil)
	if err != nil {
		return nil, err
	}
	if cachedIndexRetrieved != nil {
		req.Header.Add("If-Modified-Since", cachedIndexRetrieved.Format(http.TimeFormat))
	}
	resp, err := hs.c.Do(req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()
	if cachedIndexRetrieved != nil && resp.StatusCode == http.StatusNotModified {
		// If status has not been modified, this is equivalent to returning the
		// same index. We update the timestamp so the next cache index read does
		// not require a roundtrip to the server.
		if err = hs.cache.WriteIndex(hs.dbName, cachedIndex, time.Now()); err != nil {
			return nil, err
		}
		return cachedIndex, nil
	}
	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("unexpected status code: %d", resp.StatusCode)
	}
	b, err := io.ReadAll(resp.Body)
	if err != nil {
		return nil, err
	}
	var index DBIndex
	if err = json.Unmarshal(b, &index); err != nil {
		return nil, err
	}

	if hs.cache != nil {
		if err = hs.cache.WriteIndex(hs.dbName, index, time.Now()); err != nil {
			return nil, err
		}
	}

	return index, nil
}

func (*httpSource) unexported() {}

func (hs *httpSource) GetByModule(ctx context.Context, modulePath string) (_ []*osv.Entry, err error) {
	defer derrors.Wrap(&err, "httpSource.GetByModule(%q)", modulePath)

	index, err := hs.Index(ctx)
	if err != nil {
		return nil, err
	}

	lastModified, present := index[modulePath]
	if !present {
		return nil, nil
	}

	if hs.cache != nil {
		cached, err := hs.cache.ReadEntries(hs.dbName, modulePath)
		if err != nil {
			return nil, err
		}
		if len(cached) > 0 && !latestModifiedTime(cached).Before(lastModified) {
			return cached, nil
		}
	}

	epath, err := EscapeModulePath(modulePath)
	if err != nil {
		return nil, err
	}
	hs.httpCalls++ // for testing privacy properties
	entries, err := httpReadJSON[[]*osv.Entry](ctx, hs, epath+".json")
	if err != nil || entries == nil {
		return nil, err
	}
	// TODO: we may want to check that the returned entries actually match
	// the module we asked about, so that the cache cannot be poisoned
	if hs.cache != nil {
		if err := hs.cache.WriteEntries(hs.dbName, modulePath, entries); err != nil {
			return nil, err
		}
	}
	return entries, nil
}

func (hs *httpSource) GetByID(ctx context.Context, id string) (_ *osv.Entry, err error) {
	defer derrors.Wrap(&err, "GetByID(%q)", id)

	return httpReadJSON[*osv.Entry](ctx, hs, fmt.Sprintf("%s/%s.json", internal.IDDirectory, id))
}

func (hs *httpSource) GetByAlias(ctx context.Context, alias string) (entries []*osv.Entry, err error) {
	defer derrors.Wrap(&err, "httpSource.GetByAlias(%q)", alias)

	aliasToIDs, err := httpReadJSON[map[string][]string](ctx, hs, "aliases.json")
	if err != nil {
		return nil, err
	}
	ids := aliasToIDs[alias]
	if len(ids) == 0 {
		return nil, nil
	}
	return getByIDs(ctx, hs, ids)
}

func (hs *httpSource) ListIDs(ctx context.Context) (_ []string, err error) {
	defer derrors.Wrap(&err, "ListIDs()")

	return httpReadJSON[[]string](ctx, hs, path.Join(internal.IDDirectory, "index.json"))
}

func httpReadJSON[T any](ctx context.Context, hs *httpSource, relativePath string) (T, error) {
	var zero T
	content, err := hs.readBody(ctx, fmt.Sprintf("%s/%s", hs.url, relativePath))
	if err != nil {
		return zero, err
	}
	if len(content) == 0 {
		return zero, nil
	}
	var t T
	if err := json.Unmarshal(content, &t); err != nil {
		return zero, err
	}
	return t, nil
}

// This is the format for the last-modified header, as described at
// https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Last-Modified.
var lastModifiedFormat = "Mon, 2 Jan 2006 15:04:05 GMT"

func (hs *httpSource) LastModifiedTime(ctx context.Context) (_ time.Time, err error) {
	defer derrors.Wrap(&err, "LastModifiedTime()")

	// Assume that if anything changes, the index does.
	url := fmt.Sprintf("%s/index.json", hs.url)
	req, err := http.NewRequestWithContext(ctx, http.MethodHead, url, nil)
	if err != nil {
		return time.Time{}, err
	}
	resp, err := hs.c.Do(req)
	if err != nil {
		return time.Time{}, err
	}
	if resp.StatusCode != 200 {
		return time.Time{}, fmt.Errorf("got status code %d", resp.StatusCode)
	}
	h := resp.Header.Get("Last-Modified")
	return time.Parse(lastModifiedFormat, h)
}

func (hs *httpSource) readBody(ctx context.Context, url string) ([]byte, error) {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
	if err != nil {
		return nil, err
	}
	resp, err := hs.c.Do(req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()
	if resp.StatusCode == http.StatusNotFound {
		return nil, nil
	}
	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("got HTTP status %s", resp.Status)
	}
	// might want this to be a LimitedReader
	return io.ReadAll(resp.Body)
}

type Options struct {
	HTTPClient *http.Client
	HTTPCache  Cache
}