// Copyright 2021 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 frontend

import (
	"fmt"
	"net/http"
	"sort"

	"golang.org/x/mod/semver"
	"golang.org/x/pkgsite/internal"
	"golang.org/x/pkgsite/internal/derrors"
	"golang.org/x/pkgsite/internal/experiment"
	"golang.org/x/sync/errgroup"
	vulnc "golang.org/x/vuln/client"
	"golang.org/x/vuln/osv"
)

// A Vuln contains information to display about a vulnerability.
type Vuln struct {
	// The vulndb ID.
	ID string
	// A description of the vulnerability, or the problem in obtaining it.
	Details string
	// The version is which the vulnerability has been fixed.
	FixedVersion string
}

type vulnEntriesFunc func(string) ([]*osv.Entry, error)

// VulnsForPackage obtains vulnerability information for the given package.
// If packagePath is empty, it returns all entries for the module at version.
// The getVulnEntries function should retrieve all entries for the given module path.
// It is passed to facilitate testing.
// If there is an error, VulnsForPackage returns a single Vuln that describes the error.
func VulnsForPackage(modulePath, version, packagePath string, getVulnEntries vulnEntriesFunc) []Vuln {
	vs, err := vulnsForPackage(modulePath, version, packagePath, getVulnEntries)
	if err != nil {
		return []Vuln{{Details: fmt.Sprintf("could not get vulnerability data: %v", err)}}
	}
	return vs
}

func vulnsForPackage(modulePath, version, packagePath string, getVulnEntries vulnEntriesFunc) (_ []Vuln, err error) {
	defer derrors.Wrap(&err, "vulns(%q, %q, %q)", modulePath, version, packagePath)

	// Get all the vulns for this module.
	entries, err := getVulnEntries(modulePath)
	if err != nil {
		return nil, err
	}
	// Each entry describes a single vuln. Select the ones that apply to this
	// package at this version.
	var vulns []Vuln
	for _, e := range entries {
		if vuln, ok := entryVuln(e, packagePath, version); ok {
			vulns = append(vulns, vuln)
		}
	}
	return vulns, nil
}

// VulnListPage holds the information for a page that lists all vuln entries.
type VulnListPage struct {
	basePage
	Entries []*osv.Entry
}

// VulnPage holds the information for a page that displays a single vuln entry.
type VulnPage struct {
	basePage
	Entry *osv.Entry
}

func entryVuln(e *osv.Entry, packagePath, version string) (Vuln, bool) {
	for _, a := range e.Affected {
		if (packagePath == "" || a.Package.Name == packagePath) && a.Ranges.AffectsSemver(version) {
			// Choose the latest fixed version, if any.
			var fixed string
			for _, r := range a.Ranges {
				if r.Type == osv.TypeGit {
					continue
				}
				for _, re := range r.Events {
					if re.Fixed != "" && (fixed == "" || semver.Compare(re.Fixed, fixed) > 0) {
						fixed = re.Fixed
					}
				}
			}
			if fixed != "" {
				fixed = "v" + fixed
			}
			return Vuln{
				ID:      e.ID,
				Details: e.Details,
				// TODO(golang/go#48223): handle stdlib versions
				FixedVersion: fixed,
			}, true
		}
	}
	return Vuln{}, false
}

func (s *Server) serveVuln(w http.ResponseWriter, r *http.Request, _ internal.DataSource) error {
	if !experiment.IsActive(r.Context(), internal.ExperimentVulns) {
		return &serverError{status: http.StatusNotFound}
	}
	switch r.URL.Path {
	case "/", "/list":
		// Serve a list of all entries.
		vulnListPage, err := newVulnListPage(s.vulnClient)
		if err != nil {
			return &serverError{status: derrors.ToStatus(err)}
		}
		vulnListPage.basePage = s.newBasePage(r, "Go Vulnerabilities List")
		s.servePage(r.Context(), w, "vuln/list", vulnListPage)
	default: // the path should be "/<ID>", e.g. "/GO-2021-0001".
		id := r.URL.Path[1:]
		vulnPage, err := newVulnPage(s.vulnClient, id)
		if err != nil {
			return &serverError{status: derrors.ToStatus(err)}
		}
		vulnPage.basePage = s.newBasePage(r, id)
		s.servePage(r.Context(), w, "vuln/entry", vulnPage)
	}
	return nil
}

func newVulnPage(client vulnc.Client, id string) (*VulnPage, error) {
	entry, err := client.GetByID(id)
	if err != nil {
		return nil, err
	}
	if entry == nil {
		return nil, derrors.NotFound
	}
	return &VulnPage{Entry: entry}, nil
}

func newVulnListPage(client vulnc.Client) (*VulnListPage, error) {
	const concurrency = 4

	ids, err := client.ListIDs()
	if err != nil {
		return nil, err
	}
	// Sort from most to least recent.
	sort.Slice(ids, func(i, j int) bool { return ids[i] > ids[j] })

	entries := make([]*osv.Entry, len(ids))
	sem := make(chan struct{}, concurrency)
	var g errgroup.Group
	for i, id := range ids {
		i := i
		id := id
		sem <- struct{}{}
		g.Go(func() error {
			defer func() { <-sem }()
			e, err := client.GetByID(id)
			if err != nil {
				return err
			}
			entries[i] = e
			return nil
		})
	}
	if err := g.Wait(); err != nil {
		return nil, err
	}
	return &VulnListPage{Entries: entries}, nil
}