package logger

import (
	"context"
	"log/slog"
	"sync"
	"time"
)

// DefaultRingCapacity is how many recent log records the observability ring
// buffer retains before the oldest is evicted. Compile-time constant.
const DefaultRingCapacity = 2000

// Entry is one normalized log record as the admin observability log tail serves
// it: {time, level, source, msg}. Time is the record's own timestamp; Source is
// the provenance tag ("module/subfeature") when the record carried one.
type Entry struct {
	Time   time.Time
	Level  string
	Source string
	Msg    string
}

// RingBuffer is a concurrency-safe bounded buffer of recent log entries. When
// full the oldest entry is overwritten, so it never grows past its capacity and
// Append never blocks or fails. It is the storage behind
// GET /api/v1/admin/observability/logs.
type RingBuffer struct {
	mu   sync.RWMutex
	buf  []Entry
	next int
	full bool
	cap  int
}

// NewRingBuffer builds a ring buffer holding up to capacity entries, falling
// back to DefaultRingCapacity when capacity is not positive.
func NewRingBuffer(capacity int) *RingBuffer {
	if capacity <= 0 {
		capacity = DefaultRingCapacity
	}
	return &RingBuffer{buf: make([]Entry, capacity), cap: capacity}
}

// Append stores e, overwriting the oldest entry when the buffer is full.
func (r *RingBuffer) Append(e Entry) {
	if r == nil {
		return
	}
	r.mu.Lock()
	defer r.mu.Unlock()
	r.buf[r.next] = e
	r.next = (r.next + 1) % r.cap
	if r.next == 0 {
		r.full = true
	}
}

// Len returns how many entries are currently retained (at most the capacity).
func (r *RingBuffer) Len() int {
	if r == nil {
		return 0
	}
	r.mu.RLock()
	defer r.mu.RUnlock()
	if r.full {
		return r.cap
	}
	return r.next
}

// Snapshot returns up to limit most-recent entries, newest first. A non-positive
// limit returns every retained entry.
func (r *RingBuffer) Snapshot(limit int) []Entry {
	if r == nil {
		return nil
	}
	r.mu.RLock()
	defer r.mu.RUnlock()
	count := r.next
	if r.full {
		count = r.cap
	}
	if limit > 0 && limit < count {
		count = limit
	}
	out := make([]Entry, 0, count)
	for i := 0; i < count; i++ {
		idx := (r.next - 1 - i + r.cap*2) % r.cap
		out = append(out, r.buf[idx])
	}
	return out
}

// ringHandler wraps another slog.Handler: every record the inner handler emits
// is also normalized into buf. The inner handler's own output (console/file) is
// untouched, so installing the ring changes nothing downstream.
type ringHandler struct {
	inner slog.Handler
	buf   *RingBuffer
	attrs []slog.Attr
}

// newRingHandler wraps inner so emitted records are captured into buf. A nil
// buffer returns inner unchanged.
func newRingHandler(inner slog.Handler, buf *RingBuffer) slog.Handler {
	if buf == nil {
		return inner
	}
	return &ringHandler{inner: inner, buf: buf}
}

// Enabled delegates to the inner handler: the ring captures exactly the records
// the configured level lets through, never more.
func (h *ringHandler) Enabled(ctx context.Context, level slog.Level) bool {
	return h.inner.Enabled(ctx, level)
}

// Handle captures rec into the ring buffer and forwards it to the inner handler.
func (h *ringHandler) Handle(ctx context.Context, rec slog.Record) error {
	h.buf.Append(Entry{
		Time:   rec.Time,
		Level:  levelName(rec.Level),
		Source: h.sourceOf(rec),
		Msg:    rec.Message,
	})
	return h.inner.Handle(ctx, rec)
}

// WithAttrs keeps the captured attributes in sync with the child logger, so a
// source tagged with logger.WithSource is visible to Handle.
func (h *ringHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
	if len(attrs) == 0 {
		return h
	}
	return &ringHandler{
		inner: h.inner.WithAttrs(attrs),
		buf:   h.buf,
		attrs: append(append([]slog.Attr{}, h.attrs...), attrs...),
	}
}

// WithGroup keeps the group nesting in sync with the child logger.
func (h *ringHandler) WithGroup(name string) slog.Handler {
	if name == "" {
		return h
	}
	return &ringHandler{inner: h.inner.WithGroup(name), buf: h.buf, attrs: h.attrs}
}

// sourceOf resolves a record's provenance: the "source" attribute tagged by
// logger.WithSource when present, else the "module" (+"/"+"sub") pair tagged by
// logger.Scoped, else "".
func (h *ringHandler) sourceOf(rec slog.Record) string {
	attrs := make([]slog.Attr, 0, len(h.attrs)+rec.NumAttrs())
	attrs = append(attrs, h.attrs...)
	rec.Attrs(func(a slog.Attr) bool {
		attrs = append(attrs, a)
		return true
	})
	if v, ok := attrValue(attrs, "source"); ok {
		return v
	}
	module, ok := attrValue(attrs, "module")
	if !ok {
		return ""
	}
	if sub, ok := attrValue(attrs, "sub"); ok && sub != "" {
		return module + "/" + sub
	}
	return module
}

// attrValue returns the value of key in attrs, recursing into groups.
func attrValue(attrs []slog.Attr, key string) (string, bool) {
	for _, a := range attrs {
		if a.Value.Kind() == slog.KindGroup {
			if v, ok := attrValue(a.Value.Group(), key); ok {
				return v, true
			}
			continue
		}
		if a.Key == key {
			return a.Value.String(), true
		}
	}
	return "", false
}

// levelName maps a slog level to the wire name used by the log tail.
func levelName(l slog.Level) string {
	switch {
	case l >= slog.LevelError:
		return "error"
	case l >= slog.LevelWarn:
		return "warn"
	case l >= slog.LevelInfo:
		return "info"
	default:
		return "debug"
	}
}