package logging

import (
	"context"
	"fmt"
	"io"
	"log/slog"
	"strings"
	"sync"
	"time"
)

func Setup(format string, w io.Writer, level slog.Level) {
	var handler slog.Handler
	switch format {
	case "json":
		handler = slog.NewJSONHandler(w, &slog.HandlerOptions{Level: level})
	default:
		handler = newHumanHandler(w, level)
	}
	slog.SetDefault(slog.New(handler))
}

// humanHandler emits "<RFC3339> <LVL>: <msg> k=v k=v\n".
type humanHandler struct {
	mu    *sync.Mutex
	w     io.Writer
	level slog.Level
	attrs []slog.Attr
	group string
}

func newHumanHandler(w io.Writer, level slog.Level) *humanHandler {
	return &humanHandler{mu: &sync.Mutex{}, w: w, level: level}
}

func (h *humanHandler) Enabled(_ context.Context, l slog.Level) bool { return l >= h.level }

func (h *humanHandler) Handle(_ context.Context, r slog.Record) error {
	var b strings.Builder
	b.WriteString(r.Time.UTC().Format(time.RFC3339))
	b.WriteByte(' ')
	b.WriteString(shortLevel(r.Level))
	b.WriteString(": ")
	b.WriteString(r.Message)
	for _, a := range h.attrs {
		writeAttr(&b, h.group, a)
	}
	r.Attrs(func(a slog.Attr) bool {
		writeAttr(&b, h.group, a)
		return true
	})
	b.WriteByte('\n')
	h.mu.Lock()
	defer h.mu.Unlock()
	_, err := io.WriteString(h.w, b.String())
	return err
}

func (h *humanHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
	out := *h
	out.attrs = append(append([]slog.Attr{}, h.attrs...), attrs...)
	return &out
}

func (h *humanHandler) WithGroup(name string) slog.Handler {
	out := *h
	if h.group == "" {
		out.group = name
	} else {
		out.group = h.group + "." + name
	}
	return &out
}

func shortLevel(l slog.Level) string {
	switch {
	case l < slog.LevelInfo:
		return "DBG"
	case l < slog.LevelWarn:
		return "INF"
	case l < slog.LevelError:
		return "WRN"
	default:
		return "ERR"
	}
}

func writeAttr(b *strings.Builder, group string, a slog.Attr) {
	if a.Equal(slog.Attr{}) {
		return
	}
	b.WriteByte(' ')
	if group != "" {
		b.WriteString(group)
		b.WriteByte('.')
	}
	b.WriteString(a.Key)
	b.WriteByte('=')
	v := a.Value.String()
	if strings.ContainsAny(v, " \t\"") {
		fmt.Fprintf(b, "%q", v)
	} else {
		b.WriteString(v)
	}
}