import * as Bytes from './Bytes.js'
import * as Errors from './Errors.js'
import * as Hex from './Hex.js'

/**
 * Encodes an integer using Bitcoin's CompactSize variable-length encoding.
 *
 * | Range | Encoding | Bytes |
 * |---|---|---|
 * | 0–252 | Direct value | 1 |
 * | 253–65,535 | `0xFD` + 2 bytes LE | 3 |
 * | 65,536–4,294,967,295 | `0xFE` + 4 bytes LE | 5 |
 * | \> 4,294,967,295 | `0xFF` + 8 bytes LE | 9 |
 *
 * @example
 * ```ts twoslash
 * import { CompactSize } from 'ox'
 *
 * const bytes = CompactSize.toBytes(252)
 * // Uint8Array [252]
 *
 * const bytes2 = CompactSize.toBytes(253)
 * // Uint8Array [253, 253, 0]
 * ```
 *
 * @param value - The integer to encode.
 * @returns The CompactSize-encoded bytes.
 */
export function toBytes(value: bigint | number): Bytes.Bytes {
  const n = BigInt(value)
  if (n < 0n) throw new NegativeValueError({ value: n })
  if (n <= 252n) return new Uint8Array([Number(n)])
  if (n <= 0xffffn) {
    const buf = new Uint8Array(3)
    buf[0] = 0xfd
    const view = new DataView(buf.buffer)
    view.setUint16(1, Number(n), true)
    return buf
  }
  if (n <= 0xffffffffn) {
    const buf = new Uint8Array(5)
    buf[0] = 0xfe
    const view = new DataView(buf.buffer)
    view.setUint32(1, Number(n), true)
    return buf
  }
  const buf = new Uint8Array(9)
  buf[0] = 0xff
  const view = new DataView(buf.buffer)
  view.setBigUint64(1, n, true)
  return buf
}

export declare namespace toBytes {
  type ErrorType = NegativeValueError | Errors.GlobalErrorType
}

/**
 * Encodes an integer using Bitcoin's CompactSize variable-length encoding and returns it as {@link ox#Hex.Hex}.
 *
 * @example
 * ```ts twoslash
 * import { CompactSize } from 'ox'
 *
 * const hex = CompactSize.toHex(252)
 * // '0xfc'
 * ```
 *
 * @param value - The integer to encode.
 * @returns The CompactSize-encoded hex string.
 */
export function toHex(value: bigint | number): Hex.Hex {
  return Hex.fromBytes(toBytes(value))
}

export declare namespace toHex {
  type ErrorType = toBytes.ErrorType | Errors.GlobalErrorType
}

/**
 * Decodes a CompactSize-encoded value from {@link ox#Bytes.Bytes}.
 *
 * @example
 * ```ts twoslash
 * import { CompactSize } from 'ox'
 *
 * const result = CompactSize.fromBytes(new Uint8Array([0xfd, 0x00, 0x01]))
 * // { value: 256, size: 3 }
 * ```
 *
 * @param data - The bytes to decode from.
 * @returns The decoded value and number of bytes consumed.
 */
export function fromBytes(data: Bytes.Bytes): fromBytes.ReturnType {
  if (data.length === 0)
    throw new InsufficientBytesError({ expected: 1, actual: 0 })
  const first = data[0]!
  if (first < 0xfd) return { value: BigInt(first), size: 1 }
  const view = new DataView(data.buffer, data.byteOffset)
  if (first === 0xfd) {
    if (data.length < 3)
      throw new InsufficientBytesError({ expected: 3, actual: data.length })
    return { value: BigInt(view.getUint16(1, true)), size: 3 }
  }
  if (first === 0xfe) {
    if (data.length < 5)
      throw new InsufficientBytesError({ expected: 5, actual: data.length })
    return { value: BigInt(view.getUint32(1, true)), size: 5 }
  }
  if (data.length < 9)
    throw new InsufficientBytesError({ expected: 9, actual: data.length })
  return {
    value: view.getBigUint64(1, true),
    size: 9,
  }
}

export declare namespace fromBytes {
  type ReturnType = {
    /** The decoded integer value. */
    value: bigint
    /** The number of bytes consumed. */
    size: number
  }

  type ErrorType = InsufficientBytesError | Errors.GlobalErrorType
}

/**
 * Decodes a CompactSize-encoded value from {@link ox#Hex.Hex}.
 *
 * @example
 * ```ts twoslash
 * import { CompactSize } from 'ox'
 *
 * const result = CompactSize.fromHex('0xfd0001')
 * // { value: 256, size: 3 }
 * ```
 *
 * @param data - The hex string to decode from.
 * @returns The decoded value and number of bytes consumed.
 */
export function fromHex(data: Hex.Hex): fromBytes.ReturnType {
  return fromBytes(Bytes.fromHex(data))
}

export declare namespace fromHex {
  type ErrorType = fromBytes.ErrorType | Errors.GlobalErrorType
}

/** Thrown when a CompactSize value is negative. */
export class NegativeValueError extends Errors.BaseError {
  override readonly name = 'CompactSize.NegativeValueError'

  constructor({ value }: { value: bigint }) {
    super(`CompactSize value must be non-negative, got ${value}.`)
  }
}

/** Thrown when there are insufficient bytes to decode a CompactSize value. */
export class InsufficientBytesError extends Errors.BaseError {
  override readonly name = 'CompactSize.InsufficientBytesError'

  constructor({ expected, actual }: { expected: number; actual: number }) {
    super(
      `Insufficient bytes for CompactSize decoding. Expected at least ${expected}, got ${actual}.`,
    )
  }
}