/**
 * Shallow-merges two ECharts option objects:
 *   - Top-level keys from both are combined.
 *   - Arrays are merged by index (element-wise shallow merge, primitive override).
 *   - Plain objects are shallow-merged.
 *   - Primitives are overridden by the override value.
 *
 * Designed for the two-stage memoization pattern: first build a base option
 * object from data + theme + formatters, then call `mergeOptions(base, override)`
 * inside a second `useMemo` keyed on `[base, override]`.
 */
export function mergeOptions<T extends Record<string, unknown>>(
  base: T,
  override?: Partial<T>,
): T {
  if (!override) return base
  const out: Record<string, unknown> = { ...base }
  for (const key of Object.keys(override) as (keyof T)[]) {
    const a = base[key]
    const b = override[key]
    if (b === undefined) continue
    out[key as string] = mergeValue(a, b)
  }
  return out as T
}

function mergeValue(a: unknown, b: unknown): unknown {
  if (Array.isArray(a) && Array.isArray(b)) {
    const len = Math.max(a.length, b.length)
    const result = new Array<unknown>(len)
    for (let i = 0; i < len; i++) {
      const av: unknown = a[i]
      const bv: unknown = b[i]
      if (bv === undefined) result[i] = av
      else if (av === undefined) result[i] = bv
      else result[i] = mergeValue(av, bv)
    }
    return result
  }
  if (isPlainObject(a) && isPlainObject(b)) {
    return { ...a, ...b }
  }
  return b
}

function isPlainObject(v: unknown): v is Record<string, unknown> {
  if (v === null || typeof v !== 'object') return false
  const proto = Object.getPrototypeOf(v) as unknown
  return proto === Object.prototype || proto === null
}