package agent

import (
	"errors"
	"fmt"
	"strings"
	"time"
)

// errPRGone marks a 404 from the PR lookup itself. A 404 from any other endpoint
// can be a proxy or ingress blip and is left to the ordinary failure cap.
var errPRGone = errors.New("PR no longer visible")

// IsPRGone reports whether err is a 404 from the PR lookup, meaning the PR is no
// longer visible rather than one endpoint being briefly unreachable.
func IsPRGone(err error) bool {
	return errors.Is(err, errPRGone)
}

// Mergeability is Gitea's mergeable flag. Gitea 1.26 always sends a plain bool,
// and sends false both for a real conflict and while it recomputes the merge
// base after a push, so false on its own decides nothing (prWatch resolves it).
// Unknown covers what the bool cannot carry: an absent or null flag from another
// Gitea build, and a snapshot no successful poll ever filled in.
type Mergeability int

const (
	MergeUnknown Mergeability = iota
	MergeYes
	MergeNo
)

func (m Mergeability) String() string {
	switch m {
	case MergeYes:
		return "true"
	case MergeNo:
		return "false"
	}
	return "unknown"
}

func (m Mergeability) MarshalJSON() ([]byte, error) {
	switch m {
	case MergeYes:
		return []byte("true"), nil
	case MergeNo:
		return []byte("false"), nil
	}
	return []byte("null"), nil
}

func (m *Mergeability) UnmarshalJSON(b []byte) error {
	switch strings.TrimSpace(string(b)) {
	case "true":
		*m = MergeYes
	case "false":
		*m = MergeNo
	case "null":
		*m = MergeUnknown
	default:
		return fmt.Errorf("mergeable: unexpected value %s", b)
	}
	return nil
}

// PRState is a point-in-time snapshot of the PR attributes watchpr tracks.
type PRState struct {
	Ref              PRRef        `json:"ref"`
	State            string       `json:"state"` // open / closed
	Merged           bool         `json:"merged"`
	HeadSHA          string       `json:"head_sha"`
	BaseSHA          string       `json:"base_sha"`
	Mergeable        Mergeability `json:"mergeable"`
	CIStatus         string       `json:"ci_status"` // success / pending / failure / error / ""
	NonAgentComments int          `json:"non_agent_comments"`
	Title            string       `json:"title"`
	URL              string       `json:"url"`
}

// FetchState builds a PRState for the given ref. agentLogin's comments are
// excluded from the non-agent comment count.
func FetchState(c *GiteaClient, ref PRRef, agentLogin string) (PRState, error) {
	pr, err := c.GetPR(ref.RepoPath(), ref.Number)
	if err != nil {
		if IsNotFound(err) {
			return PRState{}, fmt.Errorf("%w: %w", errPRGone, err)
		}
		return PRState{}, err
	}
	// A 404 here means the head commit is gone (branch deleted after a squash/
	// rebase merge); the PR object is still authoritative, so treat CI as absent
	// rather than discarding the merge signal and hanging the watch loop.
	ci, err := c.CommitStatus(ref.RepoPath(), pr.Head.Sha)
	if err != nil && !IsNotFound(err) {
		return PRState{}, err
	}
	comments, err := c.ListComments(ref.RepoPath(), ref.Number)
	if err != nil {
		return PRState{}, err
	}
	return PRState{
		Ref:              ref,
		State:            pr.State,
		Merged:           pr.Merged,
		HeadSHA:          pr.Head.Sha,
		BaseSHA:          pr.Base.Sha,
		Mergeable:        pr.Mergeable,
		CIStatus:         ci,
		NonAgentComments: countNonAgentComments(comments, agentLogin),
		Title:            pr.Title,
		URL:              pr.HTMLURL,
	}, nil
}

// StateFetcher fetches the current PRState for a ref. *GiteaClient satisfies it
// via its FetchState method; tests inject fakes.
type StateFetcher interface {
	FetchState(ref PRRef, agentLogin string) (PRState, error)
}

// FetchState makes *GiteaClient a StateFetcher.
func (c *GiteaClient) FetchState(ref PRRef, agentLogin string) (PRState, error) {
	return FetchState(c, ref, agentLogin)
}

// WatchResult is the change that ended a watch.
type WatchResult struct {
	Ref    PRRef
	Reason string
	State  PRState
}

// terminalState reports whether a PR has reached a final state from which no
// further meaningful change is possible, with a human-readable reason. Unlike a
// transition (see MeaningfulChange) this holds for a single snapshot, so it also
// catches a PR that is already merged/closed the moment watchpr starts.
func terminalState(st PRState) (bool, string) {
	if st.Merged {
		return true, "PR merged"
	}
	if st.State == "closed" {
		return true, "PR closed without merging"
	}
	return false, ""
}

// conflictWindow is how long an unbroken run of non-mergeable polls must span
// before it is reported as a conflict. What the run has to outlast is Gitea's
// merge recompute, which is a duration, so counting polls cannot express it:
// --interval varies from seconds to hours between callers. Recomputes have been
// observed finishing in ~5-20s with no bound on the tail, so this is set well
// clear of that; the cost of it being too long is only a later conflict alert,
// while too short is a false one.
const conflictWindow = 2 * time.Minute

// prWatch tracks one PR across polls, because mergeability needs more memory
// than the previous snapshot. Gitea reports mergeable=false while it recomputes
// the merge base after a push, so a false is only trusted once this watch has
// seen a merge computation start: the PR was mergeable at some point, or its
// head or base SHA moved. A bare false inherited from the baseline says nothing
// -- it is equally a conflict the operator is already waiting on and a recompute
// in flight -- so it arms nothing. Arming and the run of falses are independent:
// movement only arms and never resets the run, because a base branch that moves
// under the PR on every push to main would otherwise restart it forever. The
// window is instead measured from the later of the run's start and the arm, so
// an arming poll confirms nothing it has not itself outlasted.
type prWatch struct {
	prev  PRState
	armed bool
	// armedAt is when the rule armed mid-watch; nil while disarmed, and nil when
	// the baseline armed it, since then there is no transition to measure from.
	armedAt *time.Time
	// conflictSince is when the current unbroken run of non-mergeable polls
	// began; nil when no run is in progress.
	conflictSince *time.Time
}

func newPRWatch(baseline PRState) *prWatch {
	return &prWatch{prev: baseline, armed: baseline.Mergeable == MergeYes}
}

// arm records the disarmed->armed transition and when it happened. Re-arming is
// a no-op, so a base branch moving under every poll advances nothing.
func (w *prWatch) arm(now time.Time) {
	if w.armed {
		return
	}
	w.armed = true
	at := now
	w.armedAt = &at
}

// mergeInputsChanged reports whether the commits Gitea merges have moved, which
// starts a fresh merge computation whose result is attributable to this watch.
func mergeInputsChanged(prev, cur PRState) bool {
	return cur.HeadSHA != prev.HeadSHA || cur.BaseSHA != prev.BaseSHA
}

// track folds one snapshot's mergeability into the run of observations. The
// observations that confirm a conflict must be adjacent, so anything but
// another non-mergeable one breaks the run.
func (w *prWatch) track(st PRState, now time.Time) {
	switch st.Mergeable {
	case MergeNo:
		if w.conflictSince == nil {
			at := now
			w.conflictSince = &at
		}
	case MergeYes:
		w.arm(now)
		w.conflictSince = nil
	default:
		w.conflictSince = nil
	}
}

// missed records a poll that never produced a snapshot; the run of adjacent
// non-mergeable observations does not survive the gap.
func (w *prWatch) missed() {
	w.conflictSince = nil
}

// windowStart is the later of the run's first observation and the arm, so the
// window always covers observations this watch can attribute a merge
// computation to. A baseline arm records no time and leaves the run governing.
func (w *prWatch) windowStart() *time.Time {
	if w.conflictSince == nil {
		return nil
	}
	if w.armedAt != nil && w.armedAt.After(*w.conflictSince) {
		return w.armedAt
	}
	return w.conflictSince
}

// confirmed reports whether the run of non-mergeable observations has spanned
// the recompute window since it started counting, so neither a lone
// non-mergeable poll nor the poll that armed the rule confirms anything.
func (w *prWatch) confirmed(now time.Time) bool {
	start := w.windowStart()
	return start != nil && now.Sub(*start) >= conflictWindow
}

// observe folds in the newest snapshot, taken at now, and reports whether the
// watch should end.
func (w *prWatch) observe(cur PRState, now time.Time) (bool, string) {
	changed, reason := MeaningfulChange(w.prev, cur)
	if mergeInputsChanged(w.prev, cur) {
		w.arm(now)
	}
	w.prev = cur
	w.track(cur, now)
	if changed {
		return true, reason
	}
	if w.armed && w.confirmed(now) && cur.State == "open" {
		return true, "PR lost mergeability (conflict)"
	}
	return false, ""
}

// MaxPollFailures is how many consecutive failed polls of the same PR are
// tolerated before Watch gives up. The abort fires on the 20th failed tick, so
// at watchpr's default 60s interval a watch rides out ~19 minutes of failure.
const MaxPollFailures = 20

// Watch establishes a baseline for each ref, then polls on every tick until a
// tracked PR changes meaningfully, returning the first such change. A PR that is
// already terminal (merged/closed) at baseline is reported immediately rather
// than polled forever. Transient poll errors are handed to onError and the loop
// continues, but never blindly: a baseline fetch error, an authentication
// failure surviving a token re-mint, a 404 from the PR lookup itself (the repo
// is gone, renamed, or no longer visible), and MaxPollFailures consecutive
// failures of one PR all abort, because a watcher that sees nothing must not
// look healthy.
// onBaseline, if set, receives every captured baseline once, before the first
// tick, so a caller can show what state the watch started from.
func Watch(f StateFetcher, refs []PRRef, agentLogin string, ticks <-chan time.Time, onBaseline func([]PRState), onError func(PRRef, error)) (WatchResult, error) {
	watches := make(map[string]*prWatch, len(refs))
	baselines := make([]PRState, 0, len(refs))
	for _, ref := range refs {
		st, err := f.FetchState(ref, agentLogin)
		if err != nil {
			return WatchResult{}, err
		}
		if terminal, reason := terminalState(st); terminal {
			return WatchResult{Ref: ref, Reason: reason, State: st}, nil
		}
		watches[ref.String()] = newPRWatch(st)
		baselines = append(baselines, st)
	}
	if onBaseline != nil {
		onBaseline(baselines)
	}
	fails := make(map[string]int, len(refs))
	// The tick carries the time it fired, which is the clock the conflict
	// window is measured on.
	for now := range ticks {
		for _, ref := range refs {
			key := ref.String()
			cur, err := f.FetchState(ref, agentLogin)
			if err != nil {
				if IsAuthError(err) || IsPRGone(err) {
					return WatchResult{}, fmt.Errorf("polling %s: %w", key, err)
				}
				fails[key]++
				watches[key].missed()
				if onError != nil {
					onError(ref, err)
				}
				if fails[key] >= MaxPollFailures {
					return WatchResult{}, fmt.Errorf("polling %s: giving up after %d consecutive failures: %w", key, fails[key], err)
				}
				continue
			}
			fails[key] = 0
			if changed, reason := watches[key].observe(cur, now); changed {
				return WatchResult{Ref: ref, Reason: reason, State: cur}, nil
			}
		}
	}
	return WatchResult{}, nil
}

// countNonAgentComments counts comments authored by anyone other than agentLogin.
func countNonAgentComments(comments []Comment, agentLogin string) int {
	n := 0
	for _, cm := range comments {
		if cm.User.Login != agentLogin {
			n++
		}
	}
	return n
}

// isFailedCI reports whether a combined CI state is a terminal failure.
func isFailedCI(state string) bool {
	return state == "failure" || state == "error"
}

// MeaningfulChange compares a previous state to the current one and reports
// whether a change warrants alerting the operator, with a human-readable
// reason. Benign transitions (CI pending→success, the agent's own comments, a
// new head or base commit, an unchanged snapshot) return false. Mergeability is
// not decided here: it takes a whole run of observations, which prWatch keeps.
//
// Alerting conditions:
//   - the PR merged
//   - the PR closed without merging
//   - a new comment from someone other than the agent
//   - CI transitioned into failure/error
func MeaningfulChange(prev, cur PRState) (bool, string) {
	if !prev.Merged && cur.Merged {
		return true, "PR merged"
	}
	// Closed (not merged): only alert on the open→closed edge.
	if prev.State == "open" && cur.State == "closed" && !cur.Merged {
		return true, "PR closed without merging"
	}
	if cur.NonAgentComments > prev.NonAgentComments {
		return true, "new comment from a non-agent user"
	}
	if isFailedCI(cur.CIStatus) && !isFailedCI(prev.CIStatus) {
		return true, "CI failed (" + cur.CIStatus + ")"
	}
	return false, ""
}