/** Structured logger: one JSON line per record on stderr, human line on stdout when SRC_LOG_PRETTY=1. */
export type LogLevel = "debug" | "info" | "warn" | "error";

const LEVELS: Record<LogLevel, number> = { debug: 10, info: 20, warn: 30, error: 40 };
const minLevel = LEVELS[(process.env.SRC_LOG_LEVEL as LogLevel) ?? "info"] ?? 20;
const pretty = process.env.SRC_LOG_PRETTY !== "0";

export interface Logger {
  debug(msg: string, data?: Record<string, unknown>): void;
  info(msg: string, data?: Record<string, unknown>): void;
  warn(msg: string, data?: Record<string, unknown>): void;
  error(msg: string, data?: Record<string, unknown>): void;
  child(bindings: Record<string, unknown>): Logger;
}

function emit(level: LogLevel, scope: string, bindings: Record<string, unknown>, msg: string, data?: Record<string, unknown>) {
  if (LEVELS[level] < minLevel) return;
  const rec = { t: new Date().toISOString(), level, scope, msg, ...bindings, ...(data ?? {}) };
  if (pretty) {
    const extra = { ...bindings, ...(data ?? {}) };
    const tail = Object.keys(extra).length ? " " + JSON.stringify(extra) : "";
    const line = `${rec.t.slice(11, 19)} ${level.toUpperCase().padEnd(5)} [${scope}] ${msg}${tail}`;
    (level === "error" || level === "warn" ? process.stderr : process.stdout).write(line + "\n");
  } else {
    process.stderr.write(JSON.stringify(rec) + "\n");
  }
}

export function createLogger(scope: string, bindings: Record<string, unknown> = {}): Logger {
  return {
    debug: (m, d) => emit("debug", scope, bindings, m, d),
    info: (m, d) => emit("info", scope, bindings, m, d),
    warn: (m, d) => emit("warn", scope, bindings, m, d),
    error: (m, d) => emit("error", scope, bindings, m, d),
    child: (b) => createLogger(scope, { ...bindings, ...b }),
  };
}