bux-lang/examples/ctfe_crc.bux

// Session 75 — CTFE tables for embedded / firmware-style use.
// Precomputes CRC-8 (poly 0x07) cells at compile time; runtime only indexes them.
import Std::Io::{PrintLine, PrintInt};

const POLY: int = 0x07;

// One shift step of CRC-8 (MSB-first).
const func CrcStep(crc: int) -> int {
    let c: int = crc & 0xFF;
    if (c & 0x80) != 0 {
        return ((c << 1) ^ POLY) & 0xFF;
    }
    return (c << 1) & 0xFF;
}

// Fold remaining shift steps (recursive — CTFE-friendly).
const func CrcFold(crc: int, bits: int) -> int {
    if bits <= 0 {
        return crc & 0xFF;
    }
    return CrcFold(CrcStep(crc), bits - 1);
}

const func Crc8Byte(byte: int) -> int {
    return CrcFold(byte & 0xFF, 8);
}

// Known table cells (full 256-entry array const init is future work).
const CRC_0: int = Crc8Byte(0);
const CRC_1: int = Crc8Byte(1);
const CRC_2: int = Crc8Byte(2);
const CRC_65: int = Crc8Byte(65);   // 'A'
const CRC_255: int = Crc8Byte(255);

// Table size as CTFE power-of-two (classic embedded pattern).
const func Pow2(n: int) -> int {
    if n <= 0 {
        return 1;
    }
    return 2 * Pow2(n - 1);
}
const TABLE_SIZE: int = Pow2(8); // 256

func Crc8Known(b: int) -> int {
    if b == 0 { return CRC_0; }
    if b == 1 { return CRC_1; }
    if b == 2 { return CRC_2; }
    if b == 65 { return CRC_65; }
    if b == 255 { return CRC_255; }
    return -1;
}

func Main() -> int {
    PrintInt(TABLE_SIZE);
    PrintLine("");
    PrintInt(CRC_0);
    PrintLine("");
    PrintInt(CRC_1);
    PrintLine("");
    PrintInt(CRC_65);
    PrintLine("");
    if TABLE_SIZE != 256 {
        PrintLine("FAIL ctfe_crc TABLE_SIZE");
        return 1;
    }
    if CRC_0 != 0 {
        PrintLine("FAIL ctfe_crc CRC_0");
        return 1;
    }
    // Reference: poly 0x07, byte 0x01 → 0x07 after 8 steps
    if CRC_1 != 7 {
        PrintLine("FAIL ctfe_crc CRC_1 expected 7");
        return 1;
    }
    let a: int = Crc8Known(1);
    if a != 7 {
        PrintLine("FAIL ctfe_crc runtime path");
        return 1;
    }
    PrintLine("PASS ctfe_crc");
    return 0;
}