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/// Rules that determine how a `MagicCustom` instance matches against input bytes.
///
/// This enum allows defining flexible matching strategies for custom file type detection.
/// It supports both byte signature matching and custom logic via function pointers,
/// making it suitable for integration in `static` contexts and `no_std` environments.
///
/// # Variants
///
/// - [`CustomMatchRules::Default`]: Matches by checking if any of the defined `signatures`
/// appears at any of the specified `offsets` within the input data.
/// This is the standard method used for most file types (e.g., PNG, ZIP, ELF).
///
/// - [`CustomMatchRules::WithFn(fn(&[u8]) -> bool)`]: Uses a **custom function** to determine
/// whether the input bytes match a specific format.
/// The function must:
/// - Be a `fn` pointer (not a closure or `Box<dyn Fn>`).
/// - Have the exact signature `fn(&[u8]) -> bool`.
/// - Be usable in `const` or `static` contexts.
///
/// # Why Function Pointers?
///
/// - Zero-cost: No heap allocation or vtable overhead.
/// - static compatible: Can be used in `static MAGIC_RULE: MagicCustom<T> = ...`.
/// - `no_std` friendly: No dependency on `std`, `alloc`, or runtime features.
/// - No closure: You cannot use inline closures like `|b| b.starts_with(...)` directly.
/// Instead, define a separate `const fn` and pass it.
///
/// # Example
///
/// ```rust
/// use magical_rs::magical::magic_custom::{MagicCustom, CustomMatchRules};
///
/// #[derive(Debug, Clone, Copy, PartialEq)]
/// enum ShoujuFile {
/// MahouShouju,
/// Unknown,
/// }
///
/// fn is_magical_girl(bytes: &[u8]) -> bool {
/// bytes.starts_with(b"MagicalGirl")
/// }
///
/// static SHOUJO_FILE: MagicCustom<ShoujuFile> = MagicCustom {
/// signatures: &[],
/// offsets: &[],
/// max_bytes_read: 32,
/// kind: ShoujuFile::MahouShouju,
/// rules: CustomMatchRules::WithFn(is_magical_girl),
/// };
///
/// ```
///
/// # Note
///
/// This enum is designed to work seamlessly with `MagicCustom<K>` for extensible,
/// compile-time file type detection without sacrificing performance or portability.
///
/// Detects the file type by matching against a custom list of `MagicCustom` rules.
///
/// This function iterates over the provided `rules` and returns the `kind` of the first rule
/// whose signature or custom logic matches the input `bytes`. If no rule matches,
/// it returns the provided `fallback` value.
///
/// # Type Parameters
/// - `K`: The type representing the file kind (e.g., an enum, `&'static str`, or custom struct).
/// Must implement `Clone` to be returned by value.
///
/// # Parameters
/// - `bytes`: The raw byte slice to analyze (e.g., file header or buffer).
/// - `rules`: A slice of `MagicCustom<K>` rules to check in order.
/// - `fallback`: The value to return if no rule matches.
///
/// # Returns
/// - `K`: The `kind` of the first matching rule.
/// - `fallback`: If no rule matches.
///
/// # Why This Matters
///
/// This function enables **fully customizable file type detection**:
/// - You can define your own `kind` type (e.g., `MyFileKind`, `&'static str`).
/// - You can create rules with custom signatures, offsets, or validation functions.
/// - It works in `no_std` environments — no heap allocation, no `std` dependency.
///
/// It is the core of the extensible detection system, allowing integration
/// into embedded systems, plugin architectures, or domain-specific formats.
///
/// # Example
///
/// ```rust
/// use magical_rs::magical::magic_custom::{MagicCustom, CustomMatchRules, match_types_custom};
///
/// #[derive(Debug, Clone, Copy, PartialEq)]
/// enum ShoujoKind {
/// Shoujo,
/// Unknown,
/// }
///
/// const SHOUJO: &[u8] = b"I_LOVE_MAGICAL_GIRL";
/// static SHOUJO_RULE: MagicCustom<ShoujoKind> = MagicCustom {
/// signatures: &[SHOUJO],
/// offsets: &[0],
/// max_bytes_read: 32,
/// kind: ShoujoKind::Shoujo,
/// rules: CustomMatchRules::Default,
/// };
///
/// let data = b"I_LOVE_MAGICAL_GIRL FOREVER";
/// let kind = match_types_custom(data, &[SHOUJO_RULE], ShoujoKind::Shoujo);
/// assert_eq!(kind, ShoujoKind::Shoujo);
/// ```
///
/// # Note
/// - Rules are checked in **order** — the first match wins.
/// - The function uses `.find()` and `.map_or()`, so it short-circuits on the first match.
/// - It does **not** enforce `max_bytes_read` — that is the caller's responsibility.
/// Use `bytes.len()` or external checks if needed.
///
/// # Performance
/// - Zero-cost abstraction: no allocation, no dynamic dispatch.
/// - `#[inline]`-friendly (you can add `#[inline]` if desired).
/// - Works in `no_std` and `const` contexts (with limitations).