// Command watchpr polls one or more Gitea pull requests and exits when a
// tracked PR changes in a way worth alerting on: it merges or closes, gets a
// new comment from someone other than the agent, its CI fails, or it loses
// mergeability after the baseline. Benign transitions (CI pending→success, the
// agent's own pushes and comments) are ignored, and so is any condition that was
// already true at the baseline -- which watchpr prints on stderr (as a JSON
// record under --json), so a run started against a conflicted or CI-red PR says
// so. One case is never reported at all: a conflict introduced by the push
// immediately before the watch started, on a PR whose head and base then never
// move, stays silent for the whole run, because nothing in Gitea's payload
// separates it from a merge check still in flight. The baseline line is the
// only notice of it.
//
//	watchpr owner/repo#12 owner/repo:15
//	watchpr --once --json owner/repo#12
//	watchpr --interval 30s owner/repo#12
//	watchpr --interval 30 owner/repo#12
package main

import (
	"encoding/json"
	"fmt"
	"io"
	"os"
	"strings"
	"time"

	"git.unkin.net/unkin/agent-tools/internal/agent"

	"github.com/spf13/cobra"
)

var version = "dev"

func main() {
	// cobra prints the error itself (SilenceErrors stays off); we only need to
	// turn any command error into a non-zero exit.
	if err := newRootCmd().Execute(); err != nil {
		os.Exit(1)
	}
}

// newRootCmd builds the watchpr command tree. It is separated from main so
// tests can invoke Execute and assert the exit behaviour without spawning a
// process.
func newRootCmd() *cobra.Command {
	var intervalFlag string
	var once, jsonMode bool

	root := &cobra.Command{
		Use:   "watchpr [flags] owner/repo#N [owner/repo#N ...]",
		Short: "Poll Gitea PRs and exit when one changes meaningfully.",
		Long: "watchpr polls each PR every --interval and exits (reporting what changed)\n" +
			"when a PR merges/closes, gets a new non-agent comment, its CI fails, or it\n" +
			"loses mergeability after the baseline. Conditions already true at the\n" +
			"baseline are printed, not alerted on. Refs take owner/repo#N or owner/repo:N.",
		Version:      version,
		Args:         cobra.ArbitraryArgs,
		SilenceUsage: true,
		RunE: func(cmd *cobra.Command, args []string) error {
			if len(args) == 0 {
				return fmt.Errorf("no PR references given (e.g. owner/repo#12)")
			}
			interval, err := agent.ParseDurationFlag("interval", intervalFlag)
			if err != nil {
				return err
			}
			refs := make([]agent.PRRef, 0, len(args))
			for _, a := range args {
				ref, err := agent.ParsePRRef(a)
				if err != nil {
					return err
				}
				refs = append(refs, ref)
			}
			c := clientFor()
			if once {
				return runOnce(c, refs, jsonMode)
			}
			return runWatch(c, refs, interval, jsonMode)
		},
	}
	root.SetVersionTemplate("{{.Version}}\n")

	f := root.Flags()
	f.StringVar(&intervalFlag, "interval", "60s", "Polling interval: a duration (30s, 2m, 1h30m) or a bare number of seconds")
	f.BoolVar(&once, "once", false, "Check once, print current state, and exit")
	f.BoolVar(&jsonMode, "json", false, "Emit JSON")

	root.AddCommand(&cobra.Command{
		Use:          "version",
		Short:        "Print the version",
		Run:          func(cmd *cobra.Command, args []string) { fmt.Println(version) },
		SilenceUsage: true,
	})
	return root
}

// clientFor builds the Gitea client. Watching public repos works anonymously,
// so an unavailable token is a warning, not a failure; a poll that is actually
// rejected re-mints then.
func clientFor() *agent.GiteaClient {
	token, err := agent.GiteaToken()
	if err != nil {
		fmt.Fprintf(os.Stderr, "warning: no Gitea token (%v); polling anonymously\n", err)
		token = ""
	}
	return agent.NewGiteaClient(token)
}

// runOnce fetches and prints the current state of each PR, then exits 0.
func runOnce(c *agent.GiteaClient, refs []agent.PRRef, jsonMode bool) error {
	login := agent.AgentLogin()
	states := make([]agent.PRState, 0, len(refs))
	for _, ref := range refs {
		st, err := agent.FetchState(c, ref, login)
		if err != nil {
			return describeFailure(err)
		}
		states = append(states, st)
	}
	if jsonMode {
		return json.NewEncoder(os.Stdout).Encode(states)
	}
	for _, st := range states {
		printState(st)
	}
	return nil
}

// runWatch establishes a baseline then polls until a tracked PR changes
// meaningfully, at which point it reports the change and returns.
func runWatch(c *agent.GiteaClient, refs []agent.PRRef, interval time.Duration, jsonMode bool) error {
	login := agent.AgentLogin()

	ticker := time.NewTicker(interval)
	defer ticker.Stop()

	onBaseline := func(states []agent.PRState) {
		emitBaselines(os.Stderr, states, interval, jsonMode)
	}
	onError := func(ref agent.PRRef, err error) {
		fmt.Fprintf(os.Stderr, "warning: polling %s: %v\n", ref.String(), err)
	}

	res, err := agent.Watch(c, refs, login, ticker.C, onBaseline, onError)
	if err != nil {
		return describeFailure(err)
	}
	report(res.Ref.String(), res.Reason, res.State, jsonMode)
	return nil
}

// baselineRecord is the --json form of baselineLine. Both go to stderr, leaving
// the stdout contract a single result record: a caller automating watchpr is
// precisely the one who needs to be told the watch started against a PR that is
// already conflicted, since no alert will ever follow for it.
type baselineRecord struct {
	Baseline   bool          `json:"baseline"`
	Suppressed string        `json:"suppressed,omitempty"`
	State      agent.PRState `json:"state"`
}

// emitBaselines writes the state each watch started from, in whichever form the
// caller asked for.
func emitBaselines(w io.Writer, states []agent.PRState, interval time.Duration, jsonMode bool) {
	if jsonMode {
		enc := json.NewEncoder(w)
		for _, st := range states {
			_ = enc.Encode(baselineRecord{Baseline: true, Suppressed: suppressedAtBaseline(st), State: st})
		}
		return
	}
	_, _ = fmt.Fprintf(w, "watching %d PR(s) every %s; baseline established\n", len(states), interval)
	for _, st := range states {
		_, _ = fmt.Fprintln(w, baselineLine(st))
	}
}

// describeFailure names the cause of a terminal failure so a watcher that stops
// says why. An anonymous rejection, a permission boundary and a token that
// outlived its Vault lease are three different problems and only the last is
// fixed by a fresh token.
func describeFailure(err error) error {
	switch {
	case agent.IsNoCredential(err):
		return fmt.Errorf("gitea requires authentication and no token could be minted, aborted: %w", err)
	case agent.IsPermissionDenied(err):
		return fmt.Errorf("gitea denied access to %s (a fresh token will not help), aborted: %w", agent.AgentLogin(), err)
	case agent.IsAuthError(err):
		return fmt.Errorf("gitea rejected the token and re-minting did not recover it, aborted: %w", err)
	case agent.IsPRGone(err):
		return fmt.Errorf("PR no longer visible (repo deleted, renamed, or made private), aborted: %w", err)
	}
	return err
}

// report emits the change that ended the watch.
func report(key, reason string, st agent.PRState, jsonMode bool) {
	if jsonMode {
		_ = json.NewEncoder(os.Stdout).Encode(struct {
			Changed bool          `json:"changed"`
			Reason  string        `json:"reason"`
			State   agent.PRState `json:"state"`
		}{true, reason, st})
		return
	}
	fmt.Printf("%s changed: %s\n", key, reason)
	printState(st)
}

func printState(st agent.PRState) {
	fmt.Println(stateLine(st))
}

func stateLine(st agent.PRState) string {
	return fmt.Sprintf("%s state=%s merged=%t mergeable=%s ci=%s head=%s base=%s comments(non-agent)=%d",
		st.Ref.String(), st.State, st.Merged, st.Mergeable, ciOrNone(st.CIStatus),
		shortSHA(st.HeadSHA), shortSHA(st.BaseSHA), st.NonAgentComments)
}

// baselineLine describes the state a watch started from, naming the conditions
// it will deliberately stay silent about: watchpr alerts on changes, so a PR
// that is already conflicted or already CI-red produces no alert for either, and
// that suppression has to be visible to whoever started the watch.
func baselineLine(st agent.PRState) string {
	line := "  baseline " + stateLine(st)
	if s := suppressedAtBaseline(st); s != "" {
		return line + " -- " + s + "; not alerting on a pre-existing condition"
	}
	return line
}

func suppressedAtBaseline(st agent.PRState) string {
	var conds []string
	if st.Mergeable == agent.MergeNo {
		conds = append(conds, "already non-mergeable (a real conflict, or Gitea still recomputing)")
	}
	if st.CIStatus == "failure" || st.CIStatus == "error" {
		conds = append(conds, "CI already "+st.CIStatus)
	}
	return strings.Join(conds, ", ")
}

func ciOrNone(s string) string {
	if s == "" {
		return "none"
	}
	return s
}

func shortSHA(s string) string {
	if len(s) > 8 {
		return s[:8]
	}
	if s == "" {
		return "-"
	}
	return s
}