import { createHash } from "node:crypto";

export function sha256(input: string | Buffer): string {
  return createHash("sha256").update(input).digest("hex");
}

export function shortHash(input: string, len = 16): string {
  return sha256(input).slice(0, len);
}

/**
 * Structural fingerprint of a payload: the sorted set of key paths with primitive types,
 * ignoring values and array indexes. Used to detect schema drift on public endpoints.
 */
export function schemaFingerprint(payload: unknown, maxDepth = 6): string {
  const paths = new Set<string>();
  const walk = (v: unknown, path: string, depth: number) => {
    if (depth > maxDepth) return;
    if (Array.isArray(v)) {
      if (v.length === 0) paths.add(`${path}[]`);
      // Sample first 3 elements — enough to capture the shape.
      for (const item of v.slice(0, 3)) walk(item, `${path}[]`, depth + 1);
      return;
    }
    if (v && typeof v === "object") {
      for (const [k, val] of Object.entries(v as Record<string, unknown>)) walk(val, path ? `${path}.${k}` : k, depth + 1);
      return;
    }
    paths.add(`${path}:${v === null ? "null" : typeof v}`);
  };
  walk(payload, "", 0);
  return shortHash([...paths].sort().join("|"), 20);
}

/** Deterministic fingerprint for deduplicating observations replayed after a reconnect. */
export function observationFingerprint(parts: {
  sourceId: string;
  instrumentId: string;
  field: string;
  value: number;
  sourceTimestamp: number | null;
  sequence?: number | string | null;
}): string {
  return shortHash(
    `${parts.sourceId}|${parts.instrumentId}|${parts.field}|${parts.value}|${parts.sourceTimestamp ?? ""}|${parts.sequence ?? ""}`,
    24,
  );
}