package main

import (
	"encoding/json"
	"fmt"
	"net/http"
	"net/url"
	"os"
	"path/filepath"
	"regexp"
	"slices"
	"strings"
)

// The packages page: upgrading nixpkgs (the kernel and every package) without
// a new release of nixos-router. The work is done by router-upgrade (root, the
// systemd unit router-upgrade.service); the page asks for a step in
// upgrade/request.json and shows what the steps left in upgrade/: check.json,
// preview.json and the unit's log. The channel is the setting
// router.update.channel.

func (s System) upgradeDir() string { return filepath.Join(s.StateDir, "upgrade") }

func (s System) versionPath() string {
	if s.VersionFile != "" {
		return s.VersionFile
	}
	return "/run/current-system/nixos-version"
}

// runningVersion is the NixOS version of the running system (26.11.20261027.31ecbac).
func (s System) runningVersion() string {
	data, _ := os.ReadFile(s.versionPath())
	return strings.TrimSpace(string(data))
}

var (
	releaseRe = regexp.MustCompile(`^[0-9]+\.[0-9]+`)
	channelRe = regexp.MustCompile(`^(unstable|[0-9]{2}\.[0-9]{2})$`)
)

// releaseOf is the release (26.11) of a version like 26.11.20261027.31ecbac.
func releaseOf(version string) string { return releaseRe.FindString(version) }

// UpgradeCheck is check.json: the channel's newest revision against the running one.
type UpgradeCheck struct {
	Channel string `json:"channel"`
	Branch  string `json:"branch"`
	Time    string `json:"time"`
	Running struct {
		Rev     string `json:"rev"`
		Version string `json:"version"`
	} `json:"running"`
	Newest struct {
		Rev string `json:"rev"`
	} `json:"newest"`
	UpToDate bool `json:"upToDate"`
}

// UpgradeChange is a package that has another version in the new system.
type UpgradeChange struct {
	Name string `json:"name"`
	From string `json:"from"`
	To   string `json:"to"`
}

// UpgradePreview is preview.json.
type UpgradePreview struct {
	Channel      string          `json:"channel"`
	Rev          string          `json:"rev"`
	Version      string          `json:"version"`
	Time         string          `json:"time"`
	Download     string          `json:"download"`
	Builds       int             `json:"builds"`
	RebootNeeded bool            `json:"rebootNeeded"`
	Changes      []UpgradeChange `json:"changes"`
}

func readUpgradeFile(path string, v any) bool {
	data, err := os.ReadFile(path)
	return err == nil && json.Unmarshal(data, v) == nil
}

// channel is the saved setting router.update.channel ("" for none).
func (s Settings) channel() string { return str(s, "router", "update", "channel") }

// setChannel changes router.update.channel in settings.json and, since it takes
// effect when upgrading rather than when applying, in the confirmed settings
// too, so it does not show up as an unapplied change.
func (sys System) setChannel(channel string) error {
	set := func(path string, required bool) error {
		s, err := loadSettings(path)
		if err != nil {
			if !required && os.IsNotExist(err) {
				return nil
			}
			return err
		}
		router := obj(s, "router")
		if channel == "" {
			if u, ok := router["update"].(map[string]any); ok {
				delete(u, "channel")
				if len(u) == 0 {
					delete(router, "update")
				}
			}
		} else {
			obj(router, "update")["channel"] = channel
		}
		return s.save(path)
	}
	if err := set(sys.settingsPath(), true); err != nil {
		return err
	}
	return set(sys.confirmedPath(), false)
}

// UpgradeRun is the state of the router-upgrade unit.
type UpgradeRun = UpdateStatus

func (s System) UpgradeStatus() UpgradeRun { return unitRun("router-upgrade.service") }

// requestUpgrade asks the router-upgrade unit for one step.
func (s System) requestUpgrade(action string, force bool, timeout string) error {
	data, _ := json.Marshal(map[string]any{"action": action, "force": force, "timeout": timeout})
	if err := os.MkdirAll(s.upgradeDir(), 0o700); err != nil {
		return err
	}
	if err := writeFileAtomic(filepath.Join(s.upgradeDir(), "request.json"), data, 0o600); err != nil {
		return err
	}
	return startUnit("router-upgrade.service")
}

type channelChoice struct{ Value, Label string }

type upgradeData struct {
	Version  string // the running NixOS version
	Channel  string
	Choices  []channelChoice
	Check    *UpgradeCheck
	Preview  *UpgradePreview
	Run      UpgradeRun
	Updating bool // router-update is running
	Pending  bool // a system waits for confirmation
}

// stableChannels are the stable NixOS releases the page offers (the branch
// nixos-26.05 and so on). They are listed here until the router can ask the
// channel server which exist; the release a router runs says nothing about it:
// nixos-unstable already calls itself the release that comes next.
var stableChannels = []string{"26.05"}

// channelChoices are the branches worth offering: none (the packages that came
// with the release), unstable and the stable releases. A configured channel
// that is none of these stays listed.
func channelChoices(current string) []channelChoice {
	choices := []channelChoice{{"", "None: the packages that came with this release of nixos-router"}, {"unstable", "unstable (newest, may break)"}}
	for _, r := range stableChannels {
		choices = append(choices, channelChoice{r, r + " (stable)"})
	}
	if current != "" && !slices.ContainsFunc(choices, func(c channelChoice) bool { return c.Value == current }) {
		choices = append(choices, channelChoice{current, current})
	}
	return choices
}

func (a *app) upgradeData() upgradeData {
	d := upgradeData{Version: a.sys.runningVersion(), Run: a.sys.UpgradeStatus()}
	d.Updating = a.sys.UpdateStatus().Running
	d.Pending = a.sys.Status().RollbackIn > 0
	if s, err := loadSettings(a.sys.settingsPath()); err == nil {
		d.Channel = s.channel()
	}
	d.Choices = channelChoices(d.Channel)
	// What router-upgrade printed: a finished one-shot unit has no journal to find.
	if data, err := os.ReadFile(filepath.Join(a.sys.upgradeDir(), "last.log")); err == nil {
		d.Run.Log = tail(string(data), 64<<10)
	}
	var c UpgradeCheck
	// What was checked before another channel was chosen or before the last
	// upgrade says nothing about now.
	if readUpgradeFile(filepath.Join(a.sys.upgradeDir(), "check.json"), &c) && c.Channel == d.Channel && c.Running.Version == d.Version {
		d.Check = &c
	}
	var p UpgradePreview
	if readUpgradeFile(filepath.Join(a.sys.upgradeDir(), "preview.json"), &p) && p.Channel == d.Channel {
		d.Preview = &p
	}
	return d
}

func (a *app) upgradePage(w http.ResponseWriter, r *http.Request, sess session) {
	d := a.upgradeData()
	p := page{Title: "Packages", CSRF: sess.csrf, LoggedIn: true, Data: d, Flash: r.URL.Query().Get("msg")}
	if d.Run.Running {
		p.Refresh = 3
	}
	a.render(w, "upgrade", p)
}

func (a *app) upgradeBack(w http.ResponseWriter, r *http.Request, msg string) {
	http.Redirect(w, r, "/upgrade?msg="+url.QueryEscape(msg), http.StatusSeeOther)
}

func (a *app) upgradeChannel(w http.ResponseWriter, r *http.Request, _ session) {
	v := strings.TrimSpace(r.PostFormValue("channel"))
	if !r.PostForm.Has("channel") {
		a.upgradeBack(w, r, "Not saved: no channel was sent.")
		return
	}
	if v != "" && !channelRe.MatchString(v) {
		a.upgradeBack(w, r, "Not saved: the channel is unstable or a release like 26.05.")
		return
	}
	if err := a.sys.setChannel(v); err != nil {
		a.upgradeBack(w, r, "Not saved: "+err.Error())
		return
	}
	a.upgradeBack(w, r, "Saved.")
}

var upgradeSteps = map[string]string{
	"check":    "Checking.",
	"preview":  "Working out what would change.",
	"download": "Downloading.",
	"apply":    "Upgrading.",
}

// upgradeStep starts one step of router-upgrade.
func (a *app) upgradeStep(w http.ResponseWriter, r *http.Request, _ session) {
	action := r.PathValue("step")
	msg, ok := upgradeSteps[action]
	if !ok {
		http.NotFound(w, r)
		return
	}
	// Looking is harmless beside a change that waits for confirmation;
	// changing the system or filling the disk is not.
	if a.sys.UpgradeStatus().Running || a.sys.UpdateStatus().Running {
		a.upgradeBack(w, r, "Another upgrade or update is running.")
		return
	}
	if (action == "download" || action == "apply") && a.busy() {
		a.upgradeBack(w, r, "Another change is in progress or waiting for confirmation.")
		return
	}
	d := a.upgradeData()
	if d.Channel == "" {
		a.upgradeBack(w, r, "Choose a channel first.")
		return
	}
	if (action == "download" || action == "apply") && d.Preview == nil {
		a.upgradeBack(w, r, "Preview first: it says what would change.")
		return
	}
	if err := a.sys.requestUpgrade(action, r.PostFormValue("force") == "on", ""); err != nil {
		a.upgradeBack(w, r, fmt.Sprintf("Could not start it: %v", err))
		return
	}
	a.upgradeBack(w, r, msg)
}

// tail is the last n bytes of s, from a line start.
func tail(s string, n int) string {
	if len(s) <= n {
		return s
	}
	s = s[len(s)-n:]
	if i := strings.IndexByte(s, '\n'); i >= 0 {
		s = s[i+1:]
	}
	return s
}