// ─── I/O instrumentation (opt-in, for mcp/bench) ───
//
// A process-wide counter of shard reads and parsed bytes, so the benchmark can
// report the STRUCTURAL cost of a query (how many pages, how many bytes) next
// to its wall time. That matters on a shared box specifically: wall time is only
// meaningful when the machine is idle, but read counts and byte counts are
// properties of the query plan and hold under any load.
//
// Off unless MCP_IO_STATS=1, and even then it is two integer adds per read.
//
// Moved here from mcp/src/source.ts with one change: the enable flag is guarded
// so this module can be imported from a browser bundle. `process.env?.X` behind
// a `typeof process` check is a dynamic read Next does NOT inline, which is
// exactly right for a server-only diagnostic — the browser gets `undefined` and
// the counters stay off, rather than the bundle failing on a missing global.

export type IoStats = { reads: number; bytes: number };

const IO_STATS_ON =
  typeof process !== "undefined" && process.env?.MCP_IO_STATS === "1";

const ioTotals: Record<string, IoStats> = {};

export function recordRead(kind: string, bytes: number): void {
  if (!IO_STATS_ON) return;
  const slot = (ioTotals[kind] ??= { reads: 0, bytes: 0 });
  slot.reads++;
  slot.bytes += bytes;
}

// A snapshot of every counter so far, for diffing across one tool call.
export function ioStatsSnapshot(): Record<string, IoStats> {
  const out: Record<string, IoStats> = {};
  for (const [k, v] of Object.entries(ioTotals)) out[k] = { ...v };
  return out;
}

export function ioStatsEnabled(): boolean {
  return IO_STATS_ON;
}