import { InvalidKeyError } from '../errors.js'

export const DEFAULT_SEPARATOR = '/'

/** Reserved segments. User segments can never start with `~` (a leading `~` is escaped). */
export const RESERVED = {
  index: '~i',
  unique: '~u',
  events: '~e',
  projections: '~p',
} as const

export type Key = string | number

/** The character immediately after `c` (by UTF-16 code unit). */
export function succ(c: string): string {
  return String.fromCharCode(c.charCodeAt(0) + 1)
}

export function validateSeparator(sep: string): void {
  if (sep.length !== 1) throw new TypeError(`Separator must be a single character, got ${JSON.stringify(sep)}`)
  const code = sep.charCodeAt(0)
  if (code <= 0x20 || code >= 0x7f || sep === '%' || sep === '~') {
    throw new TypeError(`Separator must be printable ASCII other than "%" and "~", got ${JSON.stringify(sep)}`)
  }
}

function hex2(code: number): string {
  return '%' + code.toString(16).toUpperCase().padStart(2, '0')
}

/** Converts a user key to a segment string, rejecting `undefined`/`null`/empty. */
export function toSegment(key: unknown): string {
  if (key === undefined || key === null) throw new InvalidKeyError(`Key must not be ${key}`)
  if (typeof key === 'number') {
    if (!Number.isFinite(key)) throw new InvalidKeyError(`Numeric key must be finite, got ${key}`)
    return String(key)
  }
  if (typeof key !== 'string') throw new InvalidKeyError(`Key must be a string or number, got ${typeof key}`)
  if (key === '') throw new InvalidKeyError('Key must not be empty')
  return key
}

/**
 * Escapes one path segment: percent-encodes `%`, the separator, and a leading `~`.
 * The result never contains a raw separator and never starts with `~`.
 */
export function escapeSegment(segment: string, sep: string = DEFAULT_SEPARATOR): string {
  // Fast path: nothing to escape (the overwhelmingly common case).
  if (segment.indexOf('%') === -1 && segment.indexOf(sep) === -1 && segment.charCodeAt(0) !== 0x7e) return segment
  let out = ''
  for (let i = 0; i < segment.length; i++) {
    const c = segment[i]!
    if (c === '%' || c === sep || (i === 0 && c === '~')) out += hex2(c.charCodeAt(0))
    else out += c
  }
  return out
}

export function unescapeSegment(escaped: string): string {
  if (escaped.indexOf('%') === -1) return escaped
  return escaped.replace(/%([0-9A-Fa-f]{2})/g, (_, h: string) => String.fromCharCode(parseInt(h, 16)))
}

export function isReservedSegment(escaped: string): boolean {
  return escaped.charCodeAt(0) === 0x7e // '~'
}

/** Joins unescaped segments into an escaped key. */
export function joinPath(path: readonly string[], sep: string = DEFAULT_SEPARATOR): string {
  let out = ''
  for (let i = 0; i < path.length; i++) {
    if (i > 0) out += sep
    out += escapeSegment(path[i]!, sep)
  }
  return out
}

/** Splits an escaped key into unescaped segments. */
export function splitPath(key: string, sep: string = DEFAULT_SEPARATOR): string[] {
  if (key === '') return []
  return key.split(sep).map(unescapeSegment)
}

/**
 * Precomputed key strings for a node path.
 * - `selfKey`: escaped segments joined by the separator (`''` for the root)
 * - `prefix`: `selfKey + sep` (`''` for the root) — every descendant key starts with it
 * - `end`: exclusive upper bound of the subtree range (`selfKey + succ(sep)`; `undefined` for the root)
 */
export interface PathKeys {
  readonly path: readonly string[]
  readonly sep: string
  readonly selfKey: string
  readonly prefix: string
  readonly end: string | undefined
}

export function pathKeys(path: readonly string[], sep: string): PathKeys {
  const selfKey = joinPath(path, sep)
  const root = path.length === 0
  return {
    path,
    sep,
    selfKey,
    prefix: root ? '' : selfKey + sep,
    end: root ? undefined : selfKey + succ(sep),
  }
}