// Package applog provides geniusrun's structured JSON logging: a
// log/slog-based logger writing to stdout, plus the App helper that tags
// records with the schema fields every geniusrun log line carries --
// "type" ("app" | "http" | "wrapper"), "package" (Go package; absent on
// Python-emitted lines), "file" (Go/Python source file basename), "class"
// (Go receiver type or Python class, omitted when there is none), and
// "method" (the emitting Go/Python function). "msg" is optional:
// NewLogger's handler drops it when empty.
package applog

import (
	"io"
	"log/slog"
	"path/filepath"
	"runtime"
	"strings"
)

// NewLogger builds a JSON-handler *slog.Logger writing to w at the given
// level ("debug"|"info"|"warn"|"error", case-insensitive; anything else
// defaults to info). Empty msg values are omitted from the output --
// records whose meaning is fully carried by type/class/method and attrs
// (e.g. the HTTP request line) don't need one.
func NewLogger(level string, w io.Writer) *slog.Logger {
	return slog.New(slog.NewJSONHandler(w, &slog.HandlerOptions{
		Level: parseLevel(level),
		ReplaceAttr: func(groups []string, a slog.Attr) slog.Attr {
			if len(groups) == 0 && a.Key == slog.MessageKey && a.Value.String() == "" {
				return slog.Attr{}
			}
			return a
		},
	}))
}

// App returns the default logger tagged with the schema fields for an
// ordinary application record (type=app), deriving the emitting location
// from the caller via the runtime: package, file, class (the receiver
// type, omitted for free functions), method. Derivation instead of
// hand-typed strings means the labels can never drift from the code. The
// http and wrapper emitters (internal/api's logging middleware,
// internal/garmin's execute/forwardWrapperStderr) tag their own type and
// location instead.
func App() *slog.Logger {
	pkg, file, class, method := location(1)
	logger := slog.Default().With("type", "app", "package", pkg, "file", file)
	if class != "" {
		logger = logger.With("class", class)
	}
	return logger.With("method", method)
}

// location resolves the caller (skip frames above this function's caller)
// into the log schema's package/file/class/method fields. A method's
// runtime name looks like "geniusrun/backend/internal/api.(*Server).X";
// a free function's like "geniusrun/backend/internal/api.X"; a closure
// gets its parent's name plus ".funcN", kept verbatim in method.
func location(skip int) (pkg, file, class, method string) {
	pc, path, _, ok := runtime.Caller(skip + 1)
	if !ok {
		return "unknown", "unknown", "", "unknown"
	}
	file = filepath.Base(path)
	full := runtime.FuncForPC(pc).Name()
	base := full[strings.LastIndex(full, "/")+1:]
	pkg, rest, _ := strings.Cut(base, ".")
	if strings.HasPrefix(rest, "(*") {
		if end := strings.Index(rest, ")."); end != -1 {
			class = rest[2:end]
			rest = rest[end+2:]
		}
	}
	return pkg, file, class, rest
}

func parseLevel(level string) slog.Level {
	switch strings.ToLower(level) {
	case "debug":
		return slog.LevelDebug
	case "warn":
		return slog.LevelWarn
	case "error":
		return slog.LevelError
	default:
		return slog.LevelInfo
	}
}