// Minimal JSON-schema subset validator (node built-ins only).
// Supports: type, required, enum, const, properties, items, minimum, oneOf, anyOf. A value that
// matches no oneOf branch is reported against the branch its const fields pick, if exactly one.
const typeOk = (t, v) =>
  t === "array" ? Array.isArray(v)
  : t === "null" ? v === null
  : t === "integer" ? Number.isInteger(v)
  : t === "object" ? v !== null && typeof v === "object" && !Array.isArray(v)
  : typeof v === t;

export function validate(schema, value, path = "$") {
  const errs = [];
  if (schema.type) {
    const types = [].concat(schema.type);
    if (!types.some((t) => typeOk(t, value))) return [`${path}: expected ${types.join("|")}`];
  }
  if (schema.enum && !schema.enum.includes(value)) errs.push(`${path}: not in enum ${JSON.stringify(schema.enum)}`);
  if ("const" in schema && value !== schema.const) errs.push(`${path}: expected ${JSON.stringify(schema.const)}`);
  if (schema.minimum !== undefined && typeof value === "number" && value < schema.minimum) errs.push(`${path}: below minimum ${schema.minimum}`);
  if (typeOk("object", value)) {
    for (const k of schema.required || []) if (!(k in value)) errs.push(`${path}: missing "${k}"`);
    for (const [k, s] of Object.entries(schema.properties || {})) if (k in value) errs.push(...validate(s, value[k], `${path}.${k}`));
  }
  if (Array.isArray(value) && schema.items) value.forEach((v, i) => errs.push(...validate(schema.items, v, `${path}[${i}]`)));
  if (schema.anyOf && !schema.anyOf.some((s) => validate(s, value, path).length === 0)) errs.push(`${path}: matches no anyOf branch`);
  if (schema.oneOf) {
    const n = schema.oneOf.filter((s) => validate(s, value, path).length === 0).length;
    // No branch matches: if the value's `const` fields pick out one branch (a clause's "kind",
    // an effect's "type"), say what is wrong with it; that is what the author meant.
    const meant = n === 0 ? schema.oneOf.filter((s) => discriminates(s, value)) : [];
    if (meant.length === 1) errs.push(...validate(meant[0], value, path));
    else if (n !== 1) errs.push(`${path}: matches ${n} oneOf branches (need 1)`);
  }
  return errs;
}

/** Does `value` have every `const` field of the object schema `s`, and does `s` have one? */
function discriminates(s, value) {
  const consts = Object.entries(s.properties || {}).filter(([, p]) => "const" in p);
  return typeOk("object", value) && consts.length > 0 && consts.every(([k, p]) => value[k] === p.const);
}