//// Calculate CRC-32/ISO-HDLC checksums in pure Gleam.
////
//// This module is the supported public API for the `crc32` package. It provides
//// both one-shot checksum calculation and immutable incremental state for data
//// processed in chunks.
////
//// The implemented algorithm is CRC-32/ISO-HDLC, the conventional reflected
//// CRC-32 variant commonly used by ZIP, gzip, PNG, and Ethernet. The standard
//// check value for the ASCII bytes `"123456789"` is `0xCBF43926`.
////
//// The public API intentionally works with `BitArray` values and returns the
//// finalized checksum as a Gleam `Int`. Hexadecimal formatting is left to
//// callers that need textual output.
////
//// ```gleam
//// import crc32
////
//// pub fn main() {
////   let checksum = crc32.checksum(<<"123456789":utf8>>)
////   assert checksum == 0xCBF43926
//// }
//// ```
////
//// ```gleam
//// import crc32
////
//// pub fn main() {
////   let checksum =
////     crc32.new()
////     |> crc32.update(<<"hello ":utf8>>)
////     |> crc32.update(<<"world":utf8>>)
////     |> crc32.finalize()
////
////   assert checksum == crc32.checksum(<<"hello world":utf8>>)
//// }
//// ```
////
//// CRC-32 is useful for detecting accidental corruption. It is not a
//// cryptographic hash and must not be used for authentication or tamper
//// resistance.

import gleam/int
import gleam/list

// CRC-32/ISO-HDLC parameters:
// - Polynomial: 0x04C11DB7 (0xEDB88320 in reflected form)
// - Initial value: 0xFFFFFFFF
// - Input reflection: No
// - Output reflection: Yes
// - Final XOR: 0xFFFFFFFF
// - Check value for "123456789": 0xCBF43926

const initial_value: Int = 0xFFFFFFFF

const final_xor: Int = 0xFFFFFFFF

/// The lookup table for fast CRC-32 calculation.
/// Generated from the CRC-32/ISO-HDLC polynomial.
const crc_table: List(Int) = [
  0x00000000, 0x77073096, 0xEE0E612C, 0x990951BA, 0x076DC419, 0x706AF48F,
  0xE963A535, 0x9E6495A3, 0x0EDB8832, 0x79DCB8A4, 0xE0D5E91E, 0x97D2D988,
  0x09B64C2B, 0x7EB17CBD, 0xE7B82D07, 0x90BF1D91, 0x1DB71064, 0x6AB020F2,
  0xF3B97148, 0x84BE41DE, 0x1ADAD47D, 0x6DDDE4EB, 0xF4D4B551, 0x83D385C7,
  0x136C9856, 0x646BA8C0, 0xFD62F97A, 0x8A65C9EC, 0x14015C4F, 0x63066CD9,
  0xFA0F3D63, 0x8D080DF5, 0x3B6E20C8, 0x4C69105E, 0xD56041E4, 0xA2677172,
  0x3C03E4D1, 0x4B04D447, 0xD20D85FD, 0xA50AB56B, 0x35B5A8FA, 0x42B2986C,
  0xDBBBC9D6, 0xACBCF940, 0x32D86CE3, 0x45DF5C75, 0xDCD60DCF, 0xABD13D59,
  0x26D930AC, 0x51DE003A, 0xC8D75180, 0xBFD06116, 0x21B4F4B5, 0x56B3C423,
  0xCFBA9599, 0xB8BDA50F, 0x2802B89E, 0x5F058808, 0xC60CD9B2, 0xB10BE924,
  0x2F6F7C87, 0x58684C11, 0xC1611DAB, 0xB6662D3D, 0x76DC4190, 0x01DB7106,
  0x98D220BC, 0xEFD5102A, 0x71B18589, 0x06B6B51F, 0x9FBFE4A5, 0xE8B8D433,
  0x7807C9A2, 0x0F00F934, 0x9609A88E, 0xE10E9818, 0x7F6A0DBB, 0x086D3D2D,
  0x91646C97, 0xE6635C01, 0x6B6B51F4, 0x1C6C6162, 0x856530D8, 0xF262004E,
  0x6C0695ED, 0x1B01A57B, 0x8208F4C1, 0xF50FC457, 0x65B0D9C6, 0x12B7E950,
  0x8BBEB8EA, 0xFCB9887C, 0x62DD1DDF, 0x15DA2D49, 0x8CD37CF3, 0xFBD44C65,
  0x4DB26158, 0x3AB551CE, 0xA3BC0074, 0xD4BB30E2, 0x4ADFA541, 0x3DD895D7,
  0xA4D1C46D, 0xD3D6F4FB, 0x4369E96A, 0x346ED9FC, 0xAD678846, 0xDA60B8D0,
  0x44042D73, 0x33031DE5, 0xAA0A4C5F, 0xDD0D7CC9, 0x5005713C, 0x270241AA,
  0xBE0B1010, 0xC90C2086, 0x5768B525, 0x206F85B3, 0xB966D409, 0xCE61E49F,
  0x5EDEF90E, 0x29D9C998, 0xB0D09822, 0xC7D7A8B4, 0x59B33D17, 0x2EB40D81,
  0xB7BD5C3B, 0xC0BA6CAD, 0xEDB88320, 0x9ABFB3B6, 0x03B6E20C, 0x74B1D29A,
  0xEAD54739, 0x9DD277AF, 0x04DB2615, 0x73DC1683, 0xE3630B12, 0x94643B84,
  0x0D6D6A3E, 0x7A6A5AA8, 0xE40ECF0B, 0x9309FF9D, 0x0A00AE27, 0x7D079EB1,
  0xF00F9344, 0x8708A3D2, 0x1E01F268, 0x6906C2FE, 0xF762575D, 0x806567CB,
  0x196C3671, 0x6E6B06E7, 0xFED41B76, 0x89D32BE0, 0x10DA7A5A, 0x67DD4ACC,
  0xF9B9DF6F, 0x8EBEEFF9, 0x17B7BE43, 0x60B08ED5, 0xD6D6A3E8, 0xA1D1937E,
  0x38D8C2C4, 0x4FDFF252, 0xD1BB67F1, 0xA6BC5767, 0x3FB506DD, 0x48B2364B,
  0xD80D2BDA, 0xAF0A1B4C, 0x36034AF6, 0x41047A60, 0xDF60EFC3, 0xA867DF55,
  0x316E8EEF, 0x4669BE79, 0xCB61B38C, 0xBC66831A, 0x256FD2A0, 0x5268E236,
  0xCC0C7795, 0xBB0B4703, 0x220216B9, 0x5505262F, 0xC5BA3BBE, 0xB2BD0B28,
  0x2BB45A92, 0x5CB36A04, 0xC2D7FFA7, 0xB5D0CF31, 0x2CD99E8B, 0x5BDEAE1D,
  0x9B64C2B0, 0xEC63F226, 0x756AA39C, 0x026D930A, 0x9C0906A9, 0xEB0E363F,
  0x72076785, 0x05005713, 0x95BF4A82, 0xE2B87A14, 0x7BB12BAE, 0x0CB61B38,
  0x92D28E9B, 0xE5D5BE0D, 0x7CDCEFB7, 0x0BDBDF21, 0x86D3D2D4, 0xF1D4E242,
  0x68DDB3F8, 0x1FDA836E, 0x81BE16CD, 0xF6B9265B, 0x6FB077E1, 0x18B74777,
  0x88085AE6, 0xFF0F6A70, 0x66063BCA, 0x11010B5C, 0x8F659EFF, 0xF862AE69,
  0x616BFFD3, 0x166CCF45, 0xA00AE278, 0xD70DD2EE, 0x4E048354, 0x3903B3C2,
  0xA7672661, 0xD06016F7, 0x4969474D, 0x3E6E77DB, 0xAED16A4A, 0xD9D65ADC,
  0x40DF0B66, 0x37D83BF0, 0xA9BCAE53, 0xDEBB9EC5, 0x47B2CF7F, 0x30B5FFE9,
  0xBDBDF21C, 0xCABAC28A, 0x53B39330, 0x24B4A3A6, 0xBAD03605, 0xCDD70693,
  0x54DE5729, 0x23D967BF, 0xB3667A2E, 0xC4614AB8, 0x5D681B02, 0x2A6F2B94,
  0xB40BBE37, 0xC30C8EA1, 0x5A05DF1B, 0x2D02EF8D,
]

/// Opaque incremental state for a CRC-32/ISO-HDLC calculation.
///
/// Values of this type represent a checksum in progress. Construct them with
/// [`new`](#new), feed additional bytes with [`update`](#update), and then
/// obtain the finalized checksum with [`finalize`](#finalize) or compare it
/// with [`verify`](#verify).
pub opaque type Crc32 {
  Crc32(value: Int)
}

fn table_lookup(index: Int) -> Int {
  case list.drop(crc_table, index) {
    [head, ..] -> head
    [] -> 0
  }
}

fn update_bytes(crc: Int, data: BitArray) -> Int {
  case data {
    <<>> -> crc
    <<byte, rest:bits>> -> {
      let crc = int.bitwise_exclusive_or(crc, byte)
      let index = int.bitwise_and(crc, 0xFF)
      let crc = int.bitwise_shift_right(crc, 8)
      let crc = int.bitwise_exclusive_or(crc, table_lookup(index))
      update_bytes(crc, rest)
    }
    _ -> crc
  }
}

/// Calculate the CRC-32/ISO-HDLC checksum of the complete input.
///
/// This is the convenience entry point for callers that already have all bytes
/// available in one `BitArray`. It is equivalent to calling [`new`](#new),
/// [`update`](#update), and then [`finalize`](#finalize).
///
/// ## Examples
///
/// ```gleam
/// let value = crc32.checksum(<<"hello":utf8>>)
/// ```
///
pub fn checksum(data: BitArray) -> Int {
  new()
  |> update(data)
  |> finalize()
}

/// Start a new incremental CRC-32/ISO-HDLC calculation.
///
/// The returned state uses the standard initial value for CRC-32/ISO-HDLC.
/// Pass the state to [`update`](#update) for each chunk of input and then
/// finish with [`finalize`](#finalize).
///
/// ## Examples
///
/// ```gleam
/// let crc = crc32.new()
/// ```
///
pub fn new() -> Crc32 {
  Crc32(initial_value)
}

/// Feed another chunk of bytes into an incremental CRC-32 calculation.
///
/// This function is pipeline-friendly and returns a new state value. Updating
/// with multiple chunks produces the same finalized checksum as processing the
/// concatenated bytes in a single call to [`checksum`](#checksum).
///
/// ## Examples
///
/// ```gleam
/// crc32.new()
/// |> crc32.update(<<"hello ":utf8>>)
/// |> crc32.update(<<"world":utf8>>)
/// |> crc32.finalize()
/// // => same as crc32.checksum(<<"hello world":utf8>>)
/// ```
///
pub fn update(crc: Crc32, data: BitArray) -> Crc32 {
  Crc32(update_bytes(crc.value, data))
}

/// Finalize an incremental calculation and return its CRC-32 checksum.
///
/// The returned `Int` is the finalized CRC-32/ISO-HDLC value after the
/// algorithm's final XOR step.
///
/// ## Examples
///
/// ```gleam
/// let checksum =
///   crc32.new()
///   |> crc32.update(data)
///   |> crc32.finalize()
/// ```
///
pub fn finalize(crc: Crc32) -> Int {
  int.bitwise_exclusive_or(crc.value, final_xor)
}

/// Finalize an incremental calculation and compare it with an expected checksum.
///
/// This is a convenience helper for verification workflows. The `expected`
/// value should be the finalized CRC-32/ISO-HDLC checksum, typically written as
/// a hexadecimal integer literal in source code.
///
/// ## Examples
///
/// ```gleam
/// let valid =
///   crc32.new()
///   |> crc32.update(data)
///   |> crc32.verify(expected)
/// ```
///
pub fn verify(crc: Crc32, expected: Int) -> Bool {
  finalize(crc) == expected
}