// Package issues derives deduplicated error "issues" from ingested telemetry,
// so the web UI can show a triaged list instead of raw traces and logs.
package issues

import (
	"fmt"
	"hash/fnv"
	"sort"
	"strings"
	"sync"
)

// maxIssues bounds memory use: once exceeded, the least-recently-seen issue
// is evicted to make room for a newly seen distinct one.
const maxIssues = 2000

// maxSamples bounds how many sample occurrences are kept per issue.
const maxSamples = 5

// Sample is one occurrence of an issue.
type Sample struct {
	Timestamp int64  `json:"timestamp"`
	TraceID   string `json:"traceId,omitempty"`
	SpanID    string `json:"spanId,omitempty"`
	Message   string `json:"message"`
	// URL is the page URL an occurrence happened on, populated only for
	// browser-originated (otel-rum) errors.
	URL string `json:"url,omitempty"`
}

// Issue is a deduplicated group of related errors, fingerprinted by kind,
// service and title.
type Issue struct {
	ID               string   `json:"id"`
	Title            string   `json:"title"`
	Kind             string   `json:"kind"` // "span_error" or "log_error"
	Service          string   `json:"service"`
	Count            int      `json:"count"`
	FirstSeen        int64    `json:"firstSeen"`
	LastSeen         int64    `json:"lastSeen"`
	AffectedServices []string `json:"affectedServices"`
	Samples          []Sample `json:"samples"`
}

// Tracker incrementally fingerprints and deduplicates errors as they're
// recorded. It is safe for concurrent use.
type Tracker struct {
	mu     sync.Mutex
	issues map[string]*Issue
}

// NewTracker creates an empty Tracker.
func NewTracker() *Tracker {
	return &Tracker{issues: make(map[string]*Issue)}
}

// Record adds an occurrence of an error, creating a new issue or merging into
// an existing one with a matching (kind, service, title) fingerprint.
func (t *Tracker) Record(kind, service, title string, sample Sample) {
	id := fingerprint(kind, service, title)

	t.mu.Lock()
	defer t.mu.Unlock()

	issue, ok := t.issues[id]
	if !ok {
		if len(t.issues) >= maxIssues {
			t.evictOldestLocked()
		}
		issue = &Issue{
			ID:        id,
			Title:     title,
			Kind:      kind,
			Service:   service,
			FirstSeen: sample.Timestamp,
		}
		t.issues[id] = issue
	}

	issue.Count++
	issue.LastSeen = sample.Timestamp
	issue.AffectedServices = addUnique(issue.AffectedServices, service)
	issue.Samples = append(issue.Samples, sample)
	if len(issue.Samples) > maxSamples {
		issue.Samples = issue.Samples[len(issue.Samples)-maxSamples:]
	}
}

// evictOldestLocked removes the issue with the smallest LastSeen. Callers
// must hold t.mu.
func (t *Tracker) evictOldestLocked() {
	var oldestID string
	var oldest int64
	first := true
	for id, issue := range t.issues {
		if first || issue.LastSeen < oldest {
			oldest = issue.LastSeen
			oldestID = id
			first = false
		}
	}
	if oldestID != "" {
		delete(t.issues, oldestID)
	}
}

// List returns issues sorted by LastSeen descending (most recently seen
// first). A non-positive limit returns all issues.
func (t *Tracker) List(limit int) []Issue {
	t.mu.Lock()
	defer t.mu.Unlock()

	result := make([]Issue, 0, len(t.issues))
	for _, issue := range t.issues {
		result = append(result, cloneIssue(issue))
	}

	sort.Slice(result, func(i, j int) bool {
		return result[i].LastSeen > result[j].LastSeen
	})

	if limit > 0 && len(result) > limit {
		result = result[:limit]
	}
	return result
}

// Get returns a single issue by id.
func (t *Tracker) Get(id string) (Issue, bool) {
	t.mu.Lock()
	defer t.mu.Unlock()

	issue, ok := t.issues[id]
	if !ok {
		return Issue{}, false
	}
	return cloneIssue(issue), true
}

// cloneIssue deep-copies the slice fields so returned issues don't share
// backing arrays with ones still being mutated by Record.
func cloneIssue(i *Issue) Issue {
	c := *i
	c.AffectedServices = append([]string(nil), i.AffectedServices...)
	c.Samples = append([]Sample(nil), i.Samples...)
	return c
}

func fingerprint(parts ...string) string {
	h := fnv.New64a()
	_, _ = h.Write([]byte(strings.Join(parts, "\x00")))
	return fmt.Sprintf("%016x", h.Sum64())
}

func addUnique(list []string, v string) []string {
	if v == "" {
		return list
	}
	for _, existing := range list {
		if existing == v {
			return list
		}
	}
	return append(list, v)
}