//! This selects the curve25519_dalek_bits either by default from target_pointer_width or explicitly set

#![deny(clippy::unwrap_used, dead_code)]

#[allow(non_camel_case_types)]
#[derive(PartialEq, Debug, Clone, Copy)]
enum DalekBits {
    Dalek32,
    Dalek64,
}

use std::fmt::Formatter;

impl std::fmt::Display for DalekBits {
    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), std::fmt::Error> {
        let w_bits = match self {
            DalekBits::Dalek32 => "32",
            DalekBits::Dalek64 => "64",
        };
        write!(f, "{w_bits}")
    }
}

fn target_has_feature(feature: &str) -> bool {
    std::env::var("CARGO_CFG_TARGET_FEATURE")
        .map(|features| features.split(',').any(|f| f == feature))
        .unwrap_or(false)
}

fn main() {
    let target_arch = match std::env::var("CARGO_CFG_TARGET_ARCH") {
        Ok(arch) => arch,
        _ => "".to_string(),
    };

    let curve25519_dalek_bits = match std::env::var("CARGO_CFG_CURVE25519_DALEK_BITS").as_deref() {
        Ok("32") => DalekBits::Dalek32,
        Ok("64") => DalekBits::Dalek64,
        _ => deterministic::determine_curve25519_dalek_bits(&target_arch),
    };

    println!("cargo:rustc-cfg=curve25519_dalek_bits=\"{curve25519_dalek_bits}\"");

    let rustc_version = rustc_version::version().expect("failed to detect rustc version");
    if rustc_version.major == 1 && rustc_version.minor <= 64 {
        // Old versions of Rust complain when you have an `unsafe fn` and you use `unsafe {}` inside,
        // so for those we want to apply the `#[allow(unused_unsafe)]` attribute to get rid of that warning.
        println!("cargo:rustc-cfg=allow_unused_unsafe");
    }

    // Backend overrides / defaults
    let curve25519_dalek_backend =
        match std::env::var("CARGO_CFG_CURVE25519_DALEK_BACKEND").as_deref() {
            Ok("fiat") => "fiat",
            Ok("serial") => "serial",
            Ok("simd") => {
                // simd can only be enabled on x86_64 & 64bit target_pointer_width
                match is_capable_simd(&target_arch, curve25519_dalek_bits) {
                    true => "simd",
                    // If override is not possible this must result to compile error
                    // See: issues/532
                    false => panic!("Could not override curve25519_dalek_backend to simd"),
                }
            }
            Ok("avx512") => {
                // AVX-512 can only be enabled on x86_64 & 64bit target_pointer_width
                match is_capable_simd(&target_arch, curve25519_dalek_bits) {
                    true => {
                        // Enable SIMD as fallback through stable backend
                        // NOTE: Compiler permits duplicate / multi value on the same key
                        println!("cargo:rustc-cfg=curve25519_dalek_backend=\"simd\"");
                        "avx512"
                    }
                    // If override is not possible this must result to compile error
                    // See: issues/532
                    false => panic!("Could not override curve25519_dalek_backend to avx512"),
                }
            }
            // default between serial / simd / avx512 (if potentially capable)
            _ => match is_capable_avx512(&target_arch, curve25519_dalek_bits, rustc_version) {
                true => {
                    println!("cargo:rustc-cfg=curve25519_dalek_backend=\"simd\"");
                    "avx512"
                }
                false => match is_capable_simd(&target_arch, curve25519_dalek_bits) {
                    true => "simd",
                    false => "serial",
                },
            },
        };
    println!("cargo:rustc-cfg=curve25519_dalek_backend=\"{curve25519_dalek_backend}\"");
}

// Is the target arch & curve25519_dalek_bits potentially simd capable ?
fn is_capable_simd(arch: &str, bits: DalekBits) -> bool {
    arch == "x86_64" && bits == DalekBits::Dalek64
}

// Is the target arch & curve25519_dalek_bits potentially AVX-512 capable ?
fn is_capable_avx512(arch: &str, bits: DalekBits, rustc_version: rustc_version::Version) -> bool {
    // AVX-512 requires rustc >=1.89 or nightly
    if rustc_version.major == 1 && rustc_version.minor < 89 {
        let channel = rustc_version::version_meta()
            .expect("failed to detect rustc version")
            .channel;

        if channel != rustc_version::Channel::Nightly {
            return false;
        }
    }

    is_capable_simd(arch, bits)
        && target_has_feature("avx512ifma")
        && target_has_feature("avx512vl")
}

// Deterministic cfg(curve25519_dalek_bits) when this is not explicitly set.
mod deterministic {

    use super::*;

    // Custom Rust non-cargo build tooling needs to set CARGO_CFG_TARGET_POINTER_WIDTH
    static ERR_MSG_NO_POINTER_WIDTH: &str =
        "Standard Cargo TARGET_POINTER_WIDTH environment variable is not set.";

    // When either non-32 or 64 TARGET_POINTER_WIDTH detected
    static ERR_MSG_UNKNOWN_POINTER_WIDTH: &str = "Unknown TARGET_POINTER_WIDTH detected.";

    // Warning when the curve25519_dalek_bits cannot be determined
    fn determine_curve25519_dalek_bits_warning(cause: &str) {
        println!("cargo:warning=\"Defaulting to curve25519_dalek_bits=32: {cause}\"");
    }

    // Determine the curve25519_dalek_bits based on Rust standard TARGET triplet
    pub(super) fn determine_curve25519_dalek_bits(target_arch: &String) -> DalekBits {
        let target_pointer_width = match std::env::var("CARGO_CFG_TARGET_POINTER_WIDTH") {
            Ok(pw) => pw,
            Err(_) => {
                determine_curve25519_dalek_bits_warning(ERR_MSG_NO_POINTER_WIDTH);
                return DalekBits::Dalek32;
            }
        };

        #[allow(clippy::match_single_binding)]
        match &target_arch {
            //Issues: 449 and 456
            //TODO: When adding arch defaults use proper types not String match
            //TODO(Arm): Needs tests + benchmarks to back this up
            //TODO(Wasm32): Needs tests + benchmarks to back this up
            _ => match target_pointer_width.as_ref() {
                "64" => DalekBits::Dalek64,
                "32" => DalekBits::Dalek32,
                // Intended default solely for non-32/64 target pointer widths
                // Otherwise known target platforms only.
                _ => {
                    determine_curve25519_dalek_bits_warning(ERR_MSG_UNKNOWN_POINTER_WIDTH);
                    DalekBits::Dalek32
                }
            },
        }
    }
}