package main

import (
	"bufio"
	"encoding/json"
	"net/http"
	"os"
	"sync"
	"sync/atomic"
	"time"

	"github.com/meshcore-analyzer/perfio"
)

// PerfIOResponse holds per-process disk I/O metrics derived from /proc/self/io.
//
// `Ingestor` is the same shape as the top-level fields, sourced from the
// ingestor's own /proc/self/io snapshot (published via the ingestor stats file).
// Issue #1120 calls for "Both ingestor and server" — this is the ingestor half.
//
// `CancelledWriteBytesPerSec` surfaces `cancelled_write_bytes` from
// /proc/self/io — bytes the kernel discarded before they hit disk (e.g. file
// truncated/unlinked while dirty). Useful signal when chasing
// write-amplification anomalies (cf. the BackfillPathJSON loop in #1119).
type PerfIOResponse struct {
	ReadBytesPerSec           float64       `json:"readBytesPerSec"`
	WriteBytesPerSec          float64       `json:"writeBytesPerSec"`
	CancelledWriteBytesPerSec float64       `json:"cancelledWriteBytesPerSec"`
	SyscallsRead              float64       `json:"syscallsRead"`
	SyscallsWrite             float64       `json:"syscallsWrite"`
	Ingestor                  *PerfIOSample `json:"ingestor,omitempty"`
}

// PerfIOSample is the canonical per-process I/O rate sample, shared with the
// ingestor via internal/perfio. Sharing the type prevents silent JSON contract
// drift between the publisher (ingestor) and the consumer (server) (#1167).
type PerfIOSample = perfio.Sample

// PerfSqliteResponse holds SQLite-specific perf metrics.
type PerfSqliteResponse struct {
	WalSizeMB    float64 `json:"walSizeMB"`
	WalSize      int64   `json:"walSize"`
	PageCount    int64   `json:"pageCount"`
	PageSize     int64   `json:"pageSize"`
	CacheSize    int64   `json:"cacheSize"`
	CacheHitRate float64 `json:"cacheHitRate"`
}

// procIOSample is a snapshot of /proc/self/io counters.
type procIOSample struct {
	at             time.Time
	readBytes      int64
	writeBytes     int64
	cancelledWrite int64
	syscR          int64
	syscW          int64
}

// perfIOTracker keeps the previous sample so handlePerfIO can compute deltas.
var (
	perfIOMu       sync.Mutex
	perfIOLastSample procIOSample
)

// readIngestorStatsParseCalls counts full json.Unmarshal calls performed by
// readIngestorIOSample (cache miss path). Exported (lowercase + same-package
// access) for tests asserting the cache eliminates redundant decodes.
// Carmack must-fix #2.
var readIngestorStatsParseCalls atomic.Int64

// resetIngestorIOCache wipes the cached snapshot. Test-only helper.
func resetIngestorIOCache() {
	ingestorIOCache.Lock()
	ingestorIOCache.mtimeUnixNano = 0
	ingestorIOCache.size = 0
	ingestorIOCache.sample = nil
	ingestorIOCache.Unlock()
}

// ingestorIOCache is the byte-stable snapshot cache for readIngestorIOSample
// (Carmack must-fix #2). Keyed by (file mtime nanoseconds, size); on hit we
// return the previously decoded sample without re-opening the file.
var ingestorIOCache struct {
	sync.Mutex
	mtimeUnixNano int64
	size          int64
	sample        *PerfIOSample
}

// readProcIO parses /proc/self/io. Returns a zero-time sample (at.IsZero())
// on non-Linux, read failure, or when no recognised keys were parsed
// (Carmack must-fix #6 — never publish a phantom-zero counter set, the
// next tick would treat the real counters as a giant delta).
func readProcIO() procIOSample {
	s := procIOSample{at: time.Now()}
	f, err := os.Open("/proc/self/io")
	if err != nil {
		return procIOSample{}
	}
	defer f.Close()
	if !parseProcIOInto(bufio.NewScanner(f), &s) {
		return procIOSample{}
	}
	return s
}

// parseProcIOInto reads /proc/self/io-shaped key:value lines from sc and
// populates the byte/syscall fields on s. Returns true iff at least one
// recognised key was successfully parsed (Carmack must-fix #6).
//
// Implementation delegates to perfio.ParseProcIO — single source of truth
// shared with the ingestor (Carmack must-fix #7; previously two divergent
// copies, which is how the empty-key gate was missing on this side).
func parseProcIOInto(sc *bufio.Scanner, s *procIOSample) bool {
	var c perfio.Counters
	ok := perfio.ParseProcIO(sc, &c)
	s.readBytes = c.ReadBytes
	s.writeBytes = c.WriteBytes
	s.cancelledWrite = c.CancelledWriteBytes
	s.syscR = c.SyscR
	s.syscW = c.SyscW
	return ok
}

// handlePerfIO returns delta-rate disk I/O for the server process (per-second).
// On the first call (no prior sample), rates are zero; subsequent calls
// report the delta divided by elapsed seconds.
func (s *Server) handlePerfIO(w http.ResponseWriter, r *http.Request) {
	cur := readProcIO()
	resp := PerfIOResponse{}

	perfIOMu.Lock()
	prev := perfIOLastSample
	perfIOLastSample = cur
	perfIOMu.Unlock()

	if !prev.at.IsZero() {
		dt := cur.at.Sub(prev.at).Seconds()
		if dt < 0.001 {
			dt = 0.001
		}
		resp.ReadBytesPerSec = float64(cur.readBytes-prev.readBytes) / dt
		resp.WriteBytesPerSec = float64(cur.writeBytes-prev.writeBytes) / dt
		resp.CancelledWriteBytesPerSec = float64(cur.cancelledWrite-prev.cancelledWrite) / dt
		resp.SyscallsRead = float64(cur.syscR-prev.syscR) / dt
		resp.SyscallsWrite = float64(cur.syscW-prev.syscW) / dt
	}
	// Ingestor block: GREEN commit replaces stub readIngestorIOSample with
	// real parsing of the ingestor stats file's procIO section (#1120
	// follow-up — "Both ingestor and server").
	if ing := readIngestorIOSample(); ing != nil {
		resp.Ingestor = ing
	}
	writeJSON(w, resp)
}

// IngestorStatsStaleThreshold is the maximum age (sampledAt → now) of an
// ingestor stats snapshot before it is treated as dead and dropped from the
// /api/perf/io response. Default writer interval is ~1s; 5× that catches a
// wedged writer goroutine without flapping on a brief tick miss.
//
// #1167 must-fix #1: serving stale procIO as live disguises a dead ingestor.
const IngestorStatsStaleThreshold = 5 * time.Second

// ingestorIOPeek is the minimal subset of IngestorStats that
// readIngestorIOSample actually needs. Decoding into this instead of the
// full IngestorStats avoids allocating BackfillUpdates (a map) and the
// ~10 unused counter fields on every /api/perf/io request (Carmack
// must-fix #1).
type ingestorIOPeek struct {
	SampledAt string        `json:"sampledAt"`
	ProcIO    *PerfIOSample `json:"procIO,omitempty"`
}

// readIngestorIOSample reads the per-process I/O block from the ingestor stats
// file. Returns nil if the file is missing, malformed, carries no proc-IO
// block (older ingestor builds), OR the snapshot is older than
// IngestorStatsStaleThreshold (#1167 must-fix #1 — operators must not see
// stale numbers under .ingestor when the ingestor is down). Never errors —
// diagnostics only.
//
// Cached by (file mtime nanoseconds, size): the underlying file is byte-stable
// between 1Hz writer ticks, so polling the endpoint at 1Hz from N tabs MUST
// NOT cause N file-opens + N json.Unmarshal per second on identical bytes
// (Carmack must-fix #2). The cache invalidates as soon as either mtime or
// size differs from the cached entry.
func readIngestorIOSample() *PerfIOSample {
	path := IngestorStatsPath()
	info, statErr := os.Stat(path)
	if statErr != nil {
		return nil
	}
	mtimeNs := info.ModTime().UnixNano()
	size := info.Size()

	ingestorIOCache.Lock()
	if ingestorIOCache.mtimeUnixNano == mtimeNs && ingestorIOCache.size == size && ingestorIOCache.sample != nil {
		s := ingestorIOCache.sample
		ingestorIOCache.Unlock()
		// Re-validate freshness on cache hit too: a stale-but-byte-stable
		// file (writer wedged) MUST still drop after the threshold.
		if s.SampledAt != "" {
			if ts, err := time.Parse(time.RFC3339, s.SampledAt); err == nil {
				if time.Since(ts) > IngestorStatsStaleThreshold {
					return nil
				}
			}
		}
		return s
	}
	ingestorIOCache.Unlock()

	data, err := os.ReadFile(path)
	if err != nil {
		return nil
	}
	readIngestorStatsParseCalls.Add(1)
	var st ingestorIOPeek
	if err := json.Unmarshal(data, &st); err != nil {
		return nil
	}
	if st.ProcIO == nil {
		return nil
	}
	stamp := st.SampledAt
	if stamp == "" {
		stamp = st.ProcIO.SampledAt
	}
	if stamp == "" {
		return nil
	}
	ts, err := time.Parse(time.RFC3339, stamp)
	if err != nil {
		return nil
	}
	if time.Since(ts) > IngestorStatsStaleThreshold {
		return nil
	}

	ingestorIOCache.Lock()
	ingestorIOCache.mtimeUnixNano = mtimeNs
	ingestorIOCache.size = size
	ingestorIOCache.sample = st.ProcIO
	ingestorIOCache.Unlock()

	return st.ProcIO
}

// handlePerfSqlite returns SQLite WAL size + cache hit-rate stats.
func (s *Server) handlePerfSqlite(w http.ResponseWriter, r *http.Request) {
	resp := PerfSqliteResponse{}
	if s.db != nil && s.db.conn != nil {
		var pageCount, pageSize int64
		_ = s.db.conn.QueryRow("PRAGMA page_count").Scan(&pageCount)
		_ = s.db.conn.QueryRow("PRAGMA page_size").Scan(&pageSize)
		var cacheSize int64
		_ = s.db.conn.QueryRow("PRAGMA cache_size").Scan(&cacheSize)
		resp.PageCount = pageCount
		resp.PageSize = pageSize
		resp.CacheSize = cacheSize

		// Cache hit rate: derived from PacketStore cache (rw_cache). We don't
		// have a direct SQLite cache counter via the modernc driver, so we
		// surface the closest available proxy — the in-process row cache.
		if s.store != nil {
			cs := s.store.GetCacheStatsTyped()
			total := cs.Hits + cs.Misses
			if total > 0 {
				resp.CacheHitRate = float64(cs.Hits) / float64(total)
			}
		}

		if s.db.path != "" && s.db.path != ":memory:" {
			if info, err := os.Stat(s.db.path + "-wal"); err == nil {
				resp.WalSize = info.Size()
				resp.WalSizeMB = float64(info.Size()) / 1048576
			}
		}
	}
	writeJSON(w, resp)
}

// IngestorStats is the on-disk JSON shape the ingestor writes periodically
// for the server to expose via /api/perf/write-sources.
type IngestorStats struct {
	SampledAt           string           `json:"sampledAt"`
	TxInserted          int64            `json:"tx_inserted"`
	ObsInserted         int64            `json:"obs_inserted"`
	DuplicateTx         int64            `json:"tx_dupes"`
	NodeUpserts         int64            `json:"node_upserts"`
	ObserverUpserts     int64            `json:"observer_upserts"`
	WriteErrors         int64            `json:"write_errors"`
	SignatureDrops      int64            `json:"sig_drops"`
	WALCommits          int64            `json:"walCommits"`
	GroupCommitFlushes  int64            `json:"groupCommitFlushes"`
	BackfillUpdates     map[string]int64 `json:"backfillUpdates"`
	// ProcIO is the ingestor's own /proc/self/io rates (since its previous
	// sample). Optional — older ingestor builds don't publish this. See #1120.
	ProcIO *PerfIOSample `json:"procIO,omitempty"`
}

// IngestorStatsPath is the well-known location where the ingestor writes its
// rolling stats snapshot. Overridable by env CORESCOPE_INGESTOR_STATS for tests.
func IngestorStatsPath() string {
	if p := os.Getenv("CORESCOPE_INGESTOR_STATS"); p != "" {
		return p
	}
	return "/tmp/corescope-ingestor-stats.json"
}

// handlePerfWriteSources reads the ingestor's stats file and returns a flat
// map of source-name -> counter, plus the sample timestamp.
func (s *Server) handlePerfWriteSources(w http.ResponseWriter, r *http.Request) {
	out := map[string]interface{}{
		"sources":  map[string]int64{},
		"sampleAt": "",
	}

	data, err := os.ReadFile(IngestorStatsPath())
	if err != nil {
		writeJSON(w, out)
		return
	}
	var st IngestorStats
	if err := json.Unmarshal(data, &st); err != nil {
		writeJSON(w, out)
		return
	}
	sources := map[string]int64{
		"tx_inserted":      st.TxInserted,
		"tx_dupes":         st.DuplicateTx,
		"obs_inserted":     st.ObsInserted,
		"node_upserts":     st.NodeUpserts,
		"observer_upserts": st.ObserverUpserts,
		"write_errors":     st.WriteErrors,
		"sig_drops":        st.SignatureDrops,
		"walCommits":       st.WALCommits,
		"groupCommitFlushes": st.GroupCommitFlushes,
	}
	for name, v := range st.BackfillUpdates {
		sources["backfill_"+name] = v
	}
	out["sources"] = sources
	out["sampleAt"] = st.SampledAt
	writeJSON(w, out)
}