oxifft-codegen-impl 0.3.1

//! Dispatch-layer codelet generation for arbitrary user-specified sizes.
//!
//! Routes a user-specified FFT size `N` to the most appropriate code emitter:
//! - **Direct codelet set** {2, 4, 8, 16, 32, 64}:
//!   delegates to the existing hand-optimised emitters (`gen_notw`).
//! - **Winograd odd** {3, 5, 7}: delegates to `gen_odd`.
//! - **Rader hardcoded** {11, 13}: delegates to `gen_rader`.
//! - **Smooth-7 composites** (all prime factors in {2, 3, 5, 7}):
//!   emits a thin runtime wrapper that delegates to `Plan::dft_1d`.
//! - **Primes p <= 1021**: runtime wrapper (runtime Rader/Generic path).
//! - **Everything else** (large primes, non-smooth composites):
//!   emits a Bluestein runtime wrapper via `Plan::dft_1d`.
//!
//! # Codelet convention
//!
//! All generated functions follow the existing `OxiFFT` codelet convention:
//! ```ignore
//! pub fn codelet_any_{N}<T: crate::kernel::Float>(
//!     x: &mut [crate::kernel::Complex<T>],
//!     sign: i32,
//! )
//! ```
//! where `sign < 0` means forward (W = e^{-2*pi*i/N}) and `sign > 0` means inverse.
//!
//! See `Plan::dft_1d` in the `oxifft` crate for the primary entry point.

use proc_macro2::TokenStream;
use quote::{format_ident, quote};
use syn::LitInt;

// ============================================================================
// Error type
// ============================================================================

/// Error variants produced during any-size codelet generation.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum CodegenError {
    /// Size zero is not a valid FFT size.
    InvalidSize(usize),
    /// Size is valid but cannot be code-generated by any registered strategy.
    UnsupportedSize(usize),
    /// Code emission failed with the given message.
    EmitError(String),
}

impl core::fmt::Display for CodegenError {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        match self {
            Self::InvalidSize(n) => write!(f, "invalid codelet size: {n}"),
            Self::UnsupportedSize(n) => write!(f, "unsupported codelet size: {n}"),
            Self::EmitError(s) => write!(f, "codegen emit error: {s}"),
        }
    }
}

// ============================================================================
// Size classification
// ============================================================================

/// Classification of an FFT size for codelet routing.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum SizeClass {
    /// Handled by the non-twiddle emitter: {2, 4, 8, 16, 32, 64}.
    /// Also used for N=1 (identity, emitted directly).
    Notw(usize),
    /// Handled by the Winograd odd emitter: {3, 5, 7}.
    Odd(usize),
    /// Handled by the hardcoded Rader emitter (straight-line twiddles): {11, 13}.
    RaderHardcoded(usize),
    /// Smooth-7 composite (all prime factors in {2, 3, 5, 7}):
    /// routed to the runtime `Plan::dft_1d` mixed-radix path.
    MixedRadix(Vec<u16>),
    /// Small prime <= 1021 not in the hardcoded set: routed to `Plan::dft_1d`
    /// (which selects Winograd, Direct, or Generic internally).
    RaderPrime(usize),
    /// All other sizes: routed to `Plan::dft_1d` (Bluestein or Generic path).
    Bluestein(usize),
}

/// Classify `n` for codelet generation.
///
/// # Errors
///
/// Returns `CodegenError::InvalidSize(0)` when `n == 0`.
pub fn classify(n: usize) -> Result<SizeClass, CodegenError> {
    if n == 0 {
        return Err(CodegenError::InvalidSize(0));
    }
    if n == 1 {
        // N=1 is the identity transform; route to Notw so we emit the trivial codelet.
        return Ok(SizeClass::Notw(1));
    }

    // Hardcoded non-twiddle direct codelets {2, 4, 8, 16, 32, 64}
    if matches!(n, 2 | 4 | 8 | 16 | 32 | 64) {
        return Ok(SizeClass::Notw(n));
    }
    // Winograd odd codelets {3, 5, 7}
    if matches!(n, 3 | 5 | 7) {
        return Ok(SizeClass::Odd(n));
    }
    // Hardcoded Rader codelets (hand-optimised straight-line code) {11, 13}
    if matches!(n, 11 | 13) {
        return Ok(SizeClass::RaderHardcoded(n));
    }

    // Smooth-7: try to factor using only {2, 3, 5, 7}, greedy from largest supported radix
    if let Some(factors) = try_factor_smooth7(n) {
        return Ok(SizeClass::MixedRadix(factors));
    }

    // Prime p <= 1021 — known to have a primitive root; routed to runtime
    if is_prime(n) && n <= 1021 {
        return Ok(SizeClass::RaderPrime(n));
    }

    // Fallback: Bluestein (or Generic, as the runtime decides)
    Ok(SizeClass::Bluestein(n))
}

// ============================================================================
// Factoring helpers
// ============================================================================

fn try_factor_smooth7(mut n: usize) -> Option<Vec<u16>> {
    // Greedy peel: largest radix first so that the factor vec is roughly sorted.
    // All radices fit in u16, so the cast is safe.
    const RADICES: &[usize] = &[16, 8, 7, 5, 4, 3, 2];
    let mut factors = Vec::new();
    for &r in RADICES {
        while n % r == 0 {
            // SAFETY: every radix in RADICES is <= 16, which fits in u16.
            #[allow(clippy::cast_possible_truncation)]
            factors.push(r as u16);
            n /= r;
        }
    }
    if n == 1 && !factors.is_empty() {
        Some(factors)
    } else {
        None
    }
}

const fn is_prime(n: usize) -> bool {
    if n < 2 {
        return false;
    }
    if n == 2 {
        return true;
    }
    if n % 2 == 0 {
        return false;
    }
    let mut i = 3usize;
    // Use checked arithmetic to avoid overflow in i * i on 32-bit targets.
    while let Some(sq) = i.checked_mul(i) {
        if sq > n {
            break;
        }
        if n % i == 0 {
            return false;
        }
        i += 2;
    }
    true
}

// ============================================================================
// Code generation
// ============================================================================

/// Generate a codelet `TokenStream` for size `n`.
///
/// The emitted function is:
/// ```ignore
/// pub fn codelet_any_{n}<T: crate::kernel::Float>(
///     x: &mut [crate::kernel::Complex<T>],
///     sign: i32,
/// )
/// ```
///
/// # Errors
///
/// Returns `CodegenError` if `n == 0` or code emission fails.
pub fn generate(n: usize) -> Result<TokenStream, CodegenError> {
    match classify(n)? {
        SizeClass::Notw(sz) => generate_notw_any(sz),
        SizeClass::Odd(sz) => generate_odd_any(sz),
        SizeClass::RaderHardcoded(sz) => generate_rader_hardcoded(sz),
        SizeClass::MixedRadix(_) | SizeClass::RaderPrime(_) | SizeClass::Bluestein(_) => {
            Ok(generate_runtime_wrapper(n))
        }
    }
}

// ---- delegates to existing emitters ----------------------------------------

fn generate_notw_any(sz: usize) -> Result<TokenStream, CodegenError> {
    if sz == 1 {
        return Ok(generate_identity_codelet());
    }
    // Call gen_notw::generate with a synthetic literal token stream.
    let literal = proc_macro2::Literal::usize_unsuffixed(sz);
    let ts = quote! { #literal };
    crate::gen_notw::generate(ts).map_err(|e| CodegenError::EmitError(e.to_string()))
}

fn generate_odd_any(sz: usize) -> Result<TokenStream, CodegenError> {
    let literal = proc_macro2::Literal::usize_unsuffixed(sz);
    let ts = quote! { #literal };
    crate::gen_odd::generate_from_macro(ts).map_err(|e| CodegenError::EmitError(e.to_string()))
}

fn generate_rader_hardcoded(sz: usize) -> Result<TokenStream, CodegenError> {
    // gen_rader::generate_rader is #[must_use] and panics on unknown primes;
    // only call it for 11 and 13 (the hardcoded set).
    if matches!(sz, 11 | 13) {
        Ok(crate::gen_rader::generate_rader(sz))
    } else {
        Err(CodegenError::EmitError(format!(
            "generate_rader_hardcoded: size {sz} is not in the hardcoded set {{11, 13}}"
        )))
    }
}

// ---- identity (N=1) ---------------------------------------------------------

fn generate_identity_codelet() -> TokenStream {
    quote! {
        /// Size-1 DFT codelet (identity transform — single element is unchanged).
        #[inline(always)]
        #[allow(clippy::trivially_copy_pass_by_ref, unused_variables)]
        pub fn codelet_any_1<T: crate::kernel::Float>(
            x: &mut [crate::kernel::Complex<T>],
            sign: i32,
        ) {
            debug_assert!(x.len() >= 1, "codelet_any_1: input must have at least 1 element");
            // DFT-1 is the identity; nothing to do.
        }
    }
}

// ---- runtime-delegating wrapper --------------------------------------------

/// Emit a function that delegates to `Plan::dft_1d` at runtime.
///
/// Used for smooth-7 composites, runtime Rader primes, and Bluestein sizes —
/// i.e., any size handled by the `OxiFFT` runtime but not by a hardcoded codelet.
fn generate_runtime_wrapper(n: usize) -> TokenStream {
    let fn_name = format_ident!("codelet_any_{n}");
    let n_lit = proc_macro2::Literal::usize_unsuffixed(n);

    quote! {
        /// Runtime-delegating codelet generated for this size.
        ///
        /// Constructs an OxiFFT plan on each call and executes it.
        /// `sign < 0` selects the forward transform; `sign > 0` selects the inverse.
        pub fn #fn_name<T: crate::kernel::Float>(
            x: &mut [crate::kernel::Complex<T>],
            sign: i32,
        ) {
            use ::oxifft::api::{Direction, Flags, Plan};

            debug_assert_eq!(x.len(), #n_lit, "codelet input length mismatch");

            let direction = if sign < 0 {
                Direction::Forward
            } else {
                Direction::Backward
            };

            let plan = Plan::<T>::dft_1d(#n_lit, direction, Flags::ESTIMATE)
                .unwrap_or_else(|| {
                    panic!(
                        "OxiFFT: Plan::dft_1d failed for compile-time-verified size {}",
                        #n_lit
                    )
                });

            // Plan::execute is out-of-place; copy input to scratch, then execute in-place.
            let input_snapshot: ::std::vec::Vec<crate::kernel::Complex<T>> = x.to_vec();
            plan.execute(&input_snapshot, x);
        }
    }
}

// ============================================================================
// Proc-macro entry point
// ============================================================================

/// Parse `gen_any_codelet!(N)` input and dispatch.
///
/// # Syntax
/// ```ignore
/// gen_any_codelet!(8);    // size-8, generates codelet_any_8
/// gen_any_codelet!(15);   // size-15, runtime-delegating wrapper
/// gen_any_codelet!(2003); // size-2003, Bluestein runtime wrapper
/// ```
///
/// Returns a `compile_error!` token stream on parse or codegen failure.
#[must_use]
pub fn generate_from_macro(input: TokenStream) -> TokenStream {
    match parse_and_generate(input) {
        Ok(ts) => ts,
        Err(e) => {
            let msg = e.to_string();
            quote! { compile_error!(#msg); }
        }
    }
}

fn parse_and_generate(input: TokenStream) -> Result<TokenStream, CodegenError> {
    let size: LitInt = syn::parse2(input).map_err(|e| CodegenError::EmitError(e.to_string()))?;
    let n: usize = size
        .base10_parse()
        .map_err(|e| CodegenError::EmitError(e.to_string()))?;
    generate(n)
}

// ============================================================================
// Builder (programmatic interface)
// ============================================================================

/// Builder for programmatic codelet generation without proc-macros.
///
/// # Example
/// ```no_run
/// use oxifft_codegen_impl::CodeletBuilder;
///
/// let ts = CodeletBuilder::new(15).build().unwrap();
/// println!("{ts}");
/// ```
pub struct CodeletBuilder {
    n: usize,
    /// Reserved for future use: override the emitted function name.
    /// Not wired through `build()` yet; the generated name is always `codelet_any_{N}`.
    #[allow(dead_code)]
    name_override: Option<String>,
}

impl CodeletBuilder {
    /// Create a builder for the given FFT size.
    #[must_use]
    pub const fn new(n: usize) -> Self {
        Self {
            n,
            name_override: None,
        }
    }

    /// Override the generated function name (reserved for future use; currently has no effect).
    #[must_use]
    pub fn name(mut self, name: impl Into<String>) -> Self {
        self.name_override = Some(name.into());
        self
    }

    /// Generate the codelet `TokenStream`.
    ///
    /// # Errors
    ///
    /// Returns `CodegenError` if `n == 0` or code emission fails.
    pub fn build(self) -> Result<TokenStream, CodegenError> {
        generate(self.n)
    }
}

// ============================================================================
// Inline tests
// ============================================================================

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn classify_all_notw() {
        for &n in &[2usize, 4, 8, 16, 32, 64] {
            assert!(
                matches!(classify(n).unwrap(), SizeClass::Notw(_)),
                "n={n} should be Notw"
            );
        }
    }

    #[test]
    fn classify_all_odd() {
        for &n in &[3usize, 5, 7] {
            assert!(
                matches!(classify(n).unwrap(), SizeClass::Odd(_)),
                "n={n} should be Odd"
            );
        }
    }

    #[test]
    fn classify_rader_hardcoded() {
        assert!(matches!(
            classify(11).unwrap(),
            SizeClass::RaderHardcoded(11)
        ));
        assert!(matches!(
            classify(13).unwrap(),
            SizeClass::RaderHardcoded(13)
        ));
    }

    #[test]
    fn classify_rader_prime_runtime() {
        // Primes >13 and <=1021 not in the hardcoded set
        for &p in &[
            17usize, 19, 23, 29, 31, 37, 41, 43, 47, 53, 97, 101, 1013, 1019, 1021,
        ] {
            assert!(
                matches!(classify(p).unwrap(), SizeClass::RaderPrime(_)),
                "n={p} should be RaderPrime"
            );
        }
    }

    #[test]
    fn classify_bluestein_large_prime() {
        // 2003 is prime and > 1021
        assert!(matches!(
            classify(2003).unwrap(),
            SizeClass::Bluestein(2003)
        ));
    }

    #[test]
    fn classify_invalid_zero() {
        assert_eq!(classify(0).unwrap_err(), CodegenError::InvalidSize(0));
    }

    #[test]
    fn smooth7_factoring() {
        // These should all be MixedRadix
        for &n in &[
            6usize, 10, 12, 14, 15, 21, 24, 28, 30, 35, 40, 42, 48, 56, 60, 80, 84, 96, 112, 120,
            168, 240,
        ] {
            assert!(
                matches!(classify(n).unwrap(), SizeClass::MixedRadix(_)),
                "n={n} expected MixedRadix"
            );
        }
    }

    #[test]
    fn smooth7_factors_correct_for_15() {
        match classify(15).unwrap() {
            SizeClass::MixedRadix(factors) => {
                assert!(factors.contains(&5), "15 factors must include 5");
                assert!(factors.contains(&3), "15 factors must include 3");
            }
            other => panic!("expected MixedRadix, got {other:?}"),
        }
    }

    #[test]
    fn is_prime_helper() {
        assert!(is_prime(2));
        assert!(is_prime(3));
        assert!(is_prime(5));
        assert!(is_prime(7));
        assert!(is_prime(11));
        assert!(is_prime(97));
        assert!(!is_prime(0));
        assert!(!is_prime(1));
        assert!(!is_prime(4));
        assert!(!is_prime(100));
    }

    #[test]
    fn generate_emits_nonempty_for_direct_size() {
        let ts = generate(8).unwrap();
        assert!(!ts.to_string().is_empty());
    }

    #[test]
    fn generate_emits_nonempty_for_odd_size() {
        let ts = generate(3).unwrap();
        assert!(!ts.to_string().is_empty());
    }

    #[test]
    fn generate_emits_nonempty_for_rader_hardcoded() {
        let ts = generate(11).unwrap();
        assert!(!ts.to_string().is_empty());
    }

    #[test]
    fn generate_emits_nonempty_for_mixed_radix() {
        let ts = generate(15).unwrap();
        assert!(!ts.to_string().is_empty());
    }

    #[test]
    fn generate_emits_nonempty_for_bluestein() {
        let ts = generate(2003).unwrap();
        assert!(!ts.to_string().is_empty());
    }

    #[test]
    fn generate_emits_nonempty_for_identity() {
        let ts = generate(1).unwrap();
        assert!(!ts.to_string().is_empty());
    }

    #[test]
    fn generate_zero_returns_err() {
        assert!(generate(0).is_err());
    }

    #[test]
    fn codelet_builder_zero_returns_err() {
        assert!(CodeletBuilder::new(0).build().is_err());
    }

    #[test]
    fn codelet_builder_happy_path() {
        let ts = CodeletBuilder::new(8).build().unwrap();
        assert!(!ts.to_string().is_empty());
    }
}