type LogLevel = "debug" | "info" | "warn" | "error";
type LogFields = Record<string, unknown>;

const LEVEL_ORDER: Record<LogLevel, number> = { debug: 0, info: 1, warn: 2, error: 3 };

function resolveMinLevel(): LogLevel {
  const configured = process.env.LOG_LEVEL?.toLowerCase();
  if (configured === "debug" || configured === "info" || configured === "warn" || configured === "error") {
    return configured;
  }
  return process.env.NODE_ENV === "production" ? "info" : "debug";
}

const MIN_LEVEL = resolveMinLevel();

function formatFields(fields?: LogFields): string {
  if (!fields) return "";
  const parts = Object.entries(fields)
    .filter(([, value]) => value !== undefined)
    .map(([key, value]) => `${key}=${typeof value === "string" ? value : JSON.stringify(value)}`);
  return parts.length > 0 ? ` ${parts.join(" ")}` : "";
}

function write(level: LogLevel, scope: string, message: string, fields?: LogFields): void {
  if (LEVEL_ORDER[level] < LEVEL_ORDER[MIN_LEVEL]) return;
  const line = `${new Date().toISOString()} ${level.toUpperCase().padEnd(5)} [${scope}] ${message}${formatFields(fields)}`;
  if (level === "error") console.error(line);
  else if (level === "warn") console.warn(line);
  else console.log(line);
}

export interface Logger {
  debug(message: string, fields?: LogFields): void;
  info(message: string, fields?: LogFields): void;
  warn(message: string, fields?: LogFields): void;
  error(message: string, fields?: LogFields): void;
  /** Returns a logger that merges `bindings` into every call's fields - for attaching
   * per-request context (reqId, route) without threading it through every log call. */
  child(bindings: LogFields): Logger;
}

/** Scoped logger. `scope` is the bracketed tag, e.g. createLogger("api") -> "[api]". */
export function createLogger(scope: string, bindings?: LogFields): Logger {
  const log = (level: LogLevel, message: string, fields?: LogFields) =>
    write(level, scope, message, bindings || fields ? { ...bindings, ...fields } : undefined);

  return {
    debug: (message, fields) => log("debug", message, fields),
    info: (message, fields) => log("info", message, fields),
    warn: (message, fields) => log("warn", message, fields),
    error: (message, fields) => log("error", message, fields),
    child: (extra) => createLogger(scope, { ...bindings, ...extra }),
  };
}