/*
A small structural diff of two JSON values: objects by key, arrays by index. A leaf that
differs, a key present on one side only, and a change of type each give one entry.
*/
export interface JsonDifference {
  path: string
  a: unknown
  b: unknown
}

const kind = (v: unknown): string => (v === null ? 'null' : Array.isArray(v) ? 'array' : typeof v)

export function jsonDiff(a: unknown, b: unknown, path = ''): JsonDifference[] {
  const ka = kind(a)
  if (ka !== kind(b)) return [{ path, a, b }]
  if (ka === 'array') {
    const [xs, ys] = [a as unknown[], b as unknown[]]
    const out: JsonDifference[] = []
    for (let i = 0; i < Math.max(xs.length, ys.length); i++) {
      const here = `${path}[${i}]`
      if (i >= xs.length || i >= ys.length) out.push({ path: here, a: xs[i], b: ys[i] })
      else out.push(...jsonDiff(xs[i], ys[i], here))
    }
    return out
  }
  if (ka === 'object') {
    const [x, y] = [a as Record<string, unknown>, b as Record<string, unknown>]
    const out: JsonDifference[] = []
    for (const key of new Set([...Object.keys(x), ...Object.keys(y)])) {
      const here = path === '' ? key : `${path}.${key}`
      const [inX, inY] = [Object.hasOwn(x, key), Object.hasOwn(y, key)]
      if (inX && inY) out.push(...jsonDiff(x[key], y[key], here))
      else out.push({ path: here, a: x[key], b: y[key] })
    }
    return out
  }
  return a === b || (Number.isNaN(a) && Number.isNaN(b)) ? [] : [{ path, a, b }]
}