import { deflateSync, inflateSync } from 'fflate'

// ── Loadout codec ─────────────────────────────────────────────────────────────
//
// A generic, WoW-talent-loadout-style binary share codec: pack *bits and
// indices* (not JSON), DEFLATE the body, frame it with a version + checksum, and
// emit a short, opaque, clipboard-safe base64url string under a namespacing
// prefix. Domain code supplies only the body schema (`write`/`read` over the
// BitWriter/BitReader); everything else — compression, integrity, framing,
// whitespace tolerance, typed errors — lives here so a new shareable thing
// (e.g. an enabled-skills set) is just a new `createLoadout({ … })`.

// ── Little-endian bit writer (WoW's WriteBits, low bit first) ────────────────
export class BitWriter {
  private bits: number[] = []

  bit(v: 0 | 1 | boolean): void {
    this.bits.push(v ? 1 : 0)
  }

  uint(value: number, width: number): void {
    let v = value >>> 0

    for (let i = 0; i < width; i += 1) {
      this.bits.push(v & 1)
      v >>>= 1
    }
  }

  // LEB128-style varint: 7 payload bits per group, high "continue" bit set while
  // more groups follow.
  varint(value: number): void {
    let v = Math.max(0, Math.floor(value))

    do {
      const group = v & 0x7f
      v = Math.floor(v / 128)
      this.bit(v > 0 ? 1 : 0)
      this.uint(group, 7)
    } while (v > 0)
  }

  str(s: string): void {
    const bytes = new TextEncoder().encode(s)
    this.varint(bytes.length)

    for (const b of bytes) {
      this.uint(b, 8)
    }
  }

  bytes(): Uint8Array {
    const out = new Uint8Array(Math.ceil(this.bits.length / 8))

    for (let i = 0; i < this.bits.length; i += 1) {
      if (this.bits[i]) {
        out[i >> 3]! |= 1 << (i & 7)
      }
    }

    return out
  }
}

export class BitReader {
  private pos = 0

  constructor(private readonly buf: Uint8Array) {}

  bit(): number {
    if (this.pos >= this.buf.length * 8) {
      throw new RangeError('loadout truncated')
    }

    const i = this.pos++

    return (this.buf[i >> 3]! >> (i & 7)) & 1
  }

  uint(width: number): number {
    let v = 0

    for (let i = 0; i < width; i += 1) {
      v |= this.bit() << i
    }

    return v >>> 0
  }

  varint(): number {
    let v = 0
    let shift = 0

    for (;;) {
      const cont = this.bit()
      v += this.uint(7) * 2 ** shift
      shift += 7

      if (!cont) {
        return v
      }
    }
  }

  str(): string {
    const len = this.varint()
    const bytes = new Uint8Array(len)

    for (let i = 0; i < len; i += 1) {
      bytes[i] = this.uint(8)
    }

    return new TextDecoder().decode(bytes)
  }
}

// Interns repeated strings (labels, categories, …) so each record spends one
// varint id instead of the full string; DEFLATE then squeezes the dictionary.
export class Dict {
  private readonly index = new Map<string, number>()
  readonly list: string[] = []

  id(s: string): number {
    const hit = this.index.get(s)

    if (hit !== undefined) {
      return hit
    }

    const id = this.list.length
    this.index.set(s, id)
    this.list.push(s)

    return id
  }
}

// Index of `value` in a fixed enum table, clamped to 0 so an unknown value
// decodes to the table's first (default) member instead of throwing.
export const idxOf = <T extends readonly string[]>(table: T, value: string): number => {
  const i = table.indexOf(value as T[number])

  return i < 0 ? 0 : i
}

// Bits needed to address `n` items positionally (fixed-width back-references).
export const indexBits = (n: number): number => (n <= 1 ? 1 : Math.ceil(Math.log2(n)))

// ── base64url over the raw bytes (URL- and clipboard-safe, no padding) ────────
function toBase64Url(buf: Uint8Array): string {
  let bin = ''

  for (const b of buf) {
    bin += String.fromCharCode(b)
  }

  return btoa(bin).replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '')
}

function fromBase64Url(s: string): Uint8Array {
  const b64 = s.replace(/-/g, '+').replace(/_/g, '/')
  const bin = atob(b64 + '='.repeat((4 - (b64.length % 4)) % 4))
  const out = new Uint8Array(bin.length)

  for (let i = 0; i < bin.length; i += 1) {
    out[i] = bin.charCodeAt(i)
  }

  return out
}

// FNV-1a over the body bytes, low 16 bits — a tamper/corruption gate, not crypto.
function checksum16(buf: Uint8Array): number {
  let h = 0x811c9dc5

  for (const b of buf) {
    h ^= b
    h = Math.imul(h, 0x01000193)
  }

  return (h >>> 0) & 0xffff
}

export class LoadoutError extends Error {}

export interface Loadout<T> {
  decode(code: string): T
  encode(value: T): string
}

export interface LoadoutSpec<T> {
  /** Namespacing prefix (like WoW's leading bytes), e.g. 'HML'. */
  prefix: string
  /** Bumped whenever the body schema changes incompatibly. */
  version: number
  /** Write the domain body; framing/compression/checksum are added around it. */
  write: (w: BitWriter, value: T) => void
  /** Read the domain body back. May throw — it's wrapped as a LoadoutError. */
  read: (r: BitReader) => T
  /** Noun for user-facing error messages, e.g. 'map code'. Default: 'code'. */
  noun?: string
  /** Error subclass to throw, so callers can `instanceof` their own type. */
  error?: new (message: string) => LoadoutError
}

const HEAD_BYTES = 3 // 8-bit version + 16-bit checksum

// Build an encode/decode pair for a domain value. The body schema is the only
// thing a caller writes; everything else (deflate, version+checksum frame,
// base64url, whitespace tolerance, typed errors) is shared.
export function createLoadout<T>(spec: LoadoutSpec<T>): Loadout<T> {
  const Err = spec.error ?? LoadoutError
  const noun = spec.noun ?? 'code'
  const Noun = noun.charAt(0).toUpperCase() + noun.slice(1)

  const encode = (value: T): string => {
    const body = new BitWriter()
    spec.write(body, value)
    const payload = deflateSync(body.bytes(), { level: 9 })

    const head = new BitWriter()
    head.uint(spec.version, 8)
    head.uint(checksum16(payload), 16)
    const headBytes = head.bytes()

    const framed = new Uint8Array(headBytes.length + payload.length)
    framed.set(headBytes, 0)
    framed.set(payload, headBytes.length)

    return spec.prefix + toBase64Url(framed)
  }

  const decode = (code: string): T => {
    // Strip ALL whitespace, not just the ends — a pasted code often picks up soft
    // wraps / newlines, and base64 decoding chokes on any of it.
    const cleaned = code.replace(/\s+/g, '')
    const raw = cleaned.startsWith(spec.prefix) ? cleaned.slice(spec.prefix.length) : cleaned

    if (!raw) {
      throw new Err(`That doesn't look like a ${noun}.`)
    }

    let framed: Uint8Array

    try {
      framed = fromBase64Url(raw)
    } catch {
      throw new Err(`That doesn't look like a ${noun}.`)
    }

    if (framed.length <= HEAD_BYTES) {
      throw new Err(`${Noun} is too short to be valid.`)
    }

    const head = new BitReader(framed.subarray(0, HEAD_BYTES))
    const version = head.uint(8)
    const storedSum = head.uint(16)

    if (version !== spec.version) {
      throw new Err(`${Noun} is version ${version}; this build reads version ${spec.version}.`)
    }

    const payload = framed.subarray(HEAD_BYTES)

    if (checksum16(payload) !== storedSum) {
      throw new Err(`${Noun} looks corrupted (checksum mismatch).`)
    }

    try {
      return spec.read(new BitReader(inflateSync(payload)))
    } catch (err) {
      throw new Err(err instanceof Error ? `${Noun} is malformed: ${err.message}` : `${Noun} is malformed.`)
    }
  }

  return { decode, encode }
}