//! A crate for generating arrays of prime numbers at compile time.
//!
//! The crate currently uses trial division to compute primes and generation of very large arrays
//! will therefore take a long time.
//!
//! `#![no_std]` compatible.
//!
//! # Example
//!
//! ```
//! # use const_primes::Primes;
//! const PRIMES: Primes<10> = Primes::new();
//! assert_eq!(PRIMES[5], 13);
//! assert_eq!(PRIMES, [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]);
//! ```

#![forbid(unsafe_code)]
#![no_std]

/// [`Primes`] consists of an array of integers of this type. Currently `u32`.
// Just change this to whatever unsigned primitive integer type you want and it should work.
type Underlying = u32;

/// Returns the largest integer smaller than or equal to sqrt(x).
const fn isqrt(x: Underlying) -> Underlying {
    let mut left = 0;
    let mut right = x + 1;

    while left != right - 1 {
        let mid = left + (right - left) / 2;
        if mid * mid <= x {
            left = mid;
        } else {
            right = mid;
        }
    }

    left
}

/// A wrapper around an array that consists of the first `N` primes and can be created in const contexts.
///
/// # Examples
/// Basic usage
/// ```
/// # use const_primes::Primes;
/// const PRIMES: Primes<3> = Primes::new();
/// assert_eq!(PRIMES[2], 5);
/// assert_eq!(PRIMES, [2, 3, 5]);
/// ```
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct Primes<const N: usize> {
    primes: [Underlying; N],
}

impl<const N: usize> Primes<N> {
    /// Generates a new instance of `Primes` which contains the first `N` primes.
    /// # Example
    /// Basic usage
    /// ```
    /// # use const_primes::Primes;
    /// const PRIMES: Primes<3> = Primes::new();
    /// assert_eq!(PRIMES, [2, 3, 5]);
    /// ```
    /// Determine N through type inference
    /// ```
    /// # use const_primes::Primes;
    /// assert_eq!(Primes::new(), [2, 3, 5, 7, 11]);
    /// ```
    /// Specify N manually
    /// ```
    /// # use const_primes::Primes;
    /// let primes = Primes::<5>::new();
    /// assert_eq!(primes, [2, 3, 5, 7, 11]);
    /// ```
    /// # Panic
    /// Panics if `N` is zero. In const contexts this will fail to compile
    /// ```compile_fail
    /// # use const_primes::Primes;
    /// const NO_PRIMES: Primes<0> = Primes::new();
    /// ```
    /// In other contexts it will panic at runtime
    /// ```should_panic
    /// # use const_primes::Primes;
    /// let no_primes = Primes::<0>::new();
    /// ```
    pub const fn new() -> Self {
        assert!(N >= 1, "`N` must be at least 1");

        let mut primes = [2; N];
        let mut number = 3;
        let mut i = 1;

        while i < N {
            let mut j = 0;
            let mut is_prime = true;
            while primes[j] < isqrt(number) + 1 {
                if number % primes[j] == 0 {
                    is_prime = false;
                    break;
                }
                j += 1;
            }
            if is_prime {
                primes[i] = number;
                i += 1;
            }
            number += 1;
        }

        Self { primes }
    }

    /// Converts `self` into an array of size `N`.
    #[inline]
    #[must_use]
    pub const fn into_array(self) -> [Underlying; N] {
        self.primes
    }

    /// Returns a slice of the underlying array.
    #[inline]
    #[must_use]
    pub const fn as_slice(&self) -> &[Underlying; N] {
        &self.primes
    }

    /// Returns a reference to the element at the given index if it is within bounds.
    ///
    /// # Example
    /// Basic usage
    /// ```
    /// # use const_primes::Primes;
    /// const PRIMES: Primes<5> = Primes::new();
    /// assert_eq!(PRIMES.get(2), Some(&5));
    /// ```
    #[inline]
    #[must_use]
    pub const fn get(&self, index: usize) -> Option<&Underlying> {
        if index < N {
            Some(&self.primes[index])
        } else {
            None
        }
    }

    /// Returns the last prime in `self`. This is also the largest prime in `self`.
    ///
    /// # Example
    /// Basic usage
    /// ```
    /// # use const_primes::Primes;
    /// const PRIMES: Primes<5> = Primes::new();
    /// assert_eq!(PRIMES.last(), &11);
    /// ```
    #[inline]
    #[must_use]
    pub const fn last(&self) -> &Underlying {
        match self.primes.last() {
            Some(l) => l,
            None => panic!("this should panic during creation"),
        }
    }

    /// Returns the number of primes in `self`.
    ///
    /// # Example
    /// ```
    /// # use const_primes::Primes;
    /// const PRIMES: Primes<5> = Primes::new();
    /// assert_eq!(PRIMES.len(), 5);
    /// ```
    // Can never be empty since we panic if the user tries to create an empty `Primes`.
    #[allow(clippy::len_without_is_empty)]
    pub const fn len(&self) -> usize {
        N
    }
}

impl<const N: usize, I> core::ops::Index<I> for Primes<N>
where
    I: core::slice::SliceIndex<[Underlying]>,
{
    type Output = I::Output;
    #[inline]
    fn index(&self, index: I) -> &Self::Output {
        &self.primes[index]
    }
}

impl<const N: usize> PartialEq<[Underlying; N]> for Primes<N> {
    fn eq(&self, other: &[Underlying; N]) -> bool {
        &self.primes == other
    }
}

impl<const N: usize> PartialEq<Primes<N>> for [Underlying; N] {
    fn eq(&self, other: &Primes<N>) -> bool {
        self == &other.primes
    }
}

impl<const N: usize> From<Primes<N>> for [Underlying; N] {
    #[inline]
    fn from(const_primes: Primes<N>) -> Self {
        const_primes.primes
    }
}

impl<const N: usize> AsRef<[Underlying]> for Primes<N> {
    #[inline]
    fn as_ref(&self) -> &[Underlying] {
        &self.primes
    }
}

impl<const N: usize> IntoIterator for Primes<N> {
    type Item = <[Underlying; N] as IntoIterator>::Item;
    type IntoIter = <[Underlying; N] as IntoIterator>::IntoIter;
    fn into_iter(self) -> Self::IntoIter {
        self.primes.into_iter()
    }
}

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

    #[test]
    fn verify_5() {
        const PRIMES: Primes<5> = Primes::new();
        assert_eq!([2, 3, 5, 7, 11], PRIMES);
    }

    #[test]
    fn verify_10() {
        const PRIMES: Primes<10> = Primes::new();
        assert_eq!([2, 3, 5, 7, 11, 13, 17, 19, 23, 29], PRIMES);
    }

    #[test]
    fn verify_1000() {
        const PRIMES: Primes<1000> = Primes::new();
        #[rustfmt::skip]
        const CORRECT: [Underlying; 1000] = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337, 347, 349, 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433, 439, 443, 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523, 541, 547, 557, 563, 569, 571, 577, 587, 593, 599, 601, 607, 613, 617, 619, 631, 641, 643, 647, 653, 659, 661, 673, 677, 683, 691, 701, 709, 719, 727, 733, 739, 743, 751, 757, 761, 769, 773, 787, 797, 809, 811, 821, 823, 827, 829, 839, 853, 857, 859, 863, 877, 881, 883, 887, 907, 911, 919, 929, 937, 941, 947, 953, 967, 971, 977, 983, 991, 997, 1009, 1013, 1019, 1021, 1031, 1033, 1039, 1049, 1051, 1061, 1063, 1069, 1087, 1091, 1093, 1097, 1103, 1109, 1117, 1123, 1129, 1151, 1153, 1163, 1171, 1181, 1187, 1193, 1201, 1213, 1217, 1223, 1229, 1231, 1237, 1249, 1259, 1277, 1279, 1283, 1289, 1291, 1297, 1301, 1303, 1307, 1319, 1321, 1327, 1361, 1367, 1373, 1381, 1399, 1409, 1423, 1427, 1429, 1433, 1439, 1447, 1451, 1453, 1459, 1471, 1481, 1483, 1487, 1489, 1493, 1499, 1511, 1523, 1531, 1543, 1549, 1553, 1559, 1567, 1571, 1579, 1583, 1597, 1601, 1607, 1609, 1613, 1619, 1621, 1627, 1637, 1657, 1663, 1667, 1669, 1693, 1697, 1699, 1709, 1721, 1723, 1733, 1741, 1747, 1753, 1759, 1777, 1783, 1787, 1789, 1801, 1811, 1823, 1831, 1847, 1861, 1867, 1871, 1873, 1877, 1879, 1889, 1901, 1907, 1913, 1931, 1933, 1949, 1951, 1973, 1979, 1987, 1993, 1997, 1999, 2003, 2011, 2017, 2027, 2029, 2039, 2053, 2063, 2069, 2081, 2083, 2087, 2089, 2099, 2111, 2113, 2129, 2131, 2137, 2141, 2143, 2153, 2161, 2179, 2203, 2207, 2213, 2221, 2237, 2239, 2243, 2251, 2267, 2269, 2273, 2281, 2287, 2293, 2297, 2309, 2311, 2333, 2339, 2341, 2347, 2351, 2357, 2371, 2377, 2381, 2383, 2389, 2393, 2399, 2411, 2417, 2423, 2437, 2441, 2447, 2459, 2467, 2473, 2477, 2503, 2521, 2531, 2539, 2543, 2549, 2551, 2557, 2579, 2591, 2593, 2609, 2617, 2621, 2633, 2647, 2657, 2659, 2663, 2671, 2677, 2683, 2687, 2689, 2693, 2699, 2707, 2711, 2713, 2719, 2729, 2731, 2741, 2749, 2753, 2767, 2777, 2789, 2791, 2797, 2801, 2803, 2819, 2833, 2837, 2843, 2851, 2857, 2861, 2879, 2887, 2897, 2903, 2909, 2917, 2927, 2939, 2953, 2957, 2963, 2969, 2971, 2999, 3001, 3011, 3019, 3023, 3037, 3041, 3049, 3061, 3067, 3079, 3083, 3089, 3109, 3119, 3121, 3137, 3163, 3167, 3169, 3181, 3187, 3191, 3203, 3209, 3217, 3221, 3229, 3251, 3253, 3257, 3259, 3271, 3299, 3301, 3307, 3313, 3319, 3323, 3329, 3331, 3343, 3347, 3359, 3361, 3371, 3373, 3389, 3391, 3407, 3413, 3433, 3449, 3457, 3461, 3463, 3467, 3469, 3491, 3499, 3511, 3517, 3527, 3529, 3533, 3539, 3541, 3547, 3557, 3559, 3571, 3581, 3583, 3593, 3607, 3613, 3617, 3623, 3631, 3637, 3643, 3659, 3671, 3673, 3677, 3691, 3697, 3701, 3709, 3719, 3727, 3733, 3739, 3761, 3767, 3769, 3779, 3793, 3797, 3803, 3821, 3823, 3833, 3847, 3851, 3853, 3863, 3877, 3881, 3889, 3907, 3911, 3917, 3919, 3923, 3929, 3931, 3943, 3947, 3967, 3989, 4001, 4003, 4007, 4013, 4019, 4021, 4027, 4049, 4051, 4057, 4073, 4079, 4091, 4093, 4099, 4111, 4127, 4129, 4133, 4139, 4153, 4157, 4159, 4177, 4201, 4211, 4217, 4219, 4229, 4231, 4241, 4243, 4253, 4259, 4261, 4271, 4273, 4283, 4289, 4297, 4327, 4337, 4339, 4349, 4357, 4363, 4373, 4391, 4397, 4409, 4421, 4423, 4441, 4447, 4451, 4457, 4463, 4481, 4483, 4493, 4507, 4513, 4517, 4519, 4523, 4547, 4549, 4561, 4567, 4583, 4591, 4597, 4603, 4621, 4637, 4639, 4643, 4649, 4651, 4657, 4663, 4673, 4679, 4691, 4703, 4721, 4723, 4729, 4733, 4751, 4759, 4783, 4787, 4789, 4793, 4799, 4801, 4813, 4817, 4831, 4861, 4871, 4877, 4889, 4903, 4909, 4919, 4931, 4933, 4937, 4943, 4951, 4957, 4967, 4969, 4973, 4987, 4993, 4999, 5003, 5009, 5011, 5021, 5023, 5039, 5051, 5059, 5077, 5081, 5087, 5099, 5101, 5107, 5113, 5119, 5147, 5153, 5167, 5171, 5179, 5189, 5197, 5209, 5227, 5231, 5233, 5237, 5261, 5273, 5279, 5281, 5297, 5303, 5309, 5323, 5333, 5347, 5351, 5381, 5387, 5393, 5399, 5407, 5413, 5417, 5419, 5431, 5437, 5441, 5443, 5449, 5471, 5477, 5479, 5483, 5501, 5503, 5507, 5519, 5521, 5527, 5531, 5557, 5563, 5569, 5573, 5581, 5591, 5623, 5639, 5641, 5647, 5651, 5653, 5657, 5659, 5669, 5683, 5689, 5693, 5701, 5711, 5717, 5737, 5741, 5743, 5749, 5779, 5783, 5791, 5801, 5807, 5813, 5821, 5827, 5839, 5843, 5849, 5851, 5857, 5861, 5867, 5869, 5879, 5881, 5897, 5903, 5923, 5927, 5939, 5953, 5981, 5987, 6007, 6011, 6029, 6037, 6043, 6047, 6053, 6067, 6073, 6079, 6089, 6091, 6101, 6113, 6121, 6131, 6133, 6143, 6151, 6163, 6173, 6197, 6199, 6203, 6211, 6217, 6221, 6229, 6247, 6257, 6263, 6269, 6271, 6277, 6287, 6299, 6301, 6311, 6317, 6323, 6329, 6337, 6343, 6353, 6359, 6361, 6367, 6373, 6379, 6389, 6397, 6421, 6427, 6449, 6451, 6469, 6473, 6481, 6491, 6521, 6529, 6547, 6551, 6553, 6563, 6569, 6571, 6577, 6581, 6599, 6607, 6619, 6637, 6653, 6659, 6661, 6673, 6679, 6689, 6691, 6701, 6703, 6709, 6719, 6733, 6737, 6761, 6763, 6779, 6781, 6791, 6793, 6803, 6823, 6827, 6829, 6833, 6841, 6857, 6863, 6869, 6871, 6883, 6899, 6907, 6911, 6917, 6947, 6949, 6959, 6961, 6967, 6971, 6977, 6983, 6991, 6997, 7001, 7013, 7019, 7027, 7039, 7043, 7057, 7069, 7079, 7103, 7109, 7121, 7127, 7129, 7151, 7159, 7177, 7187, 7193, 7207, 7211, 7213, 7219, 7229, 7237, 7243, 7247, 7253, 7283, 7297, 7307, 7309, 7321, 7331, 7333, 7349, 7351, 7369, 7393, 7411, 7417, 7433, 7451, 7457, 7459, 7477, 7481, 7487, 7489, 7499, 7507, 7517, 7523, 7529, 7537, 7541, 7547, 7549, 7559, 7561, 7573, 7577, 7583, 7589, 7591, 7603, 7607, 7621, 7639, 7643, 7649, 7669, 7673, 7681, 7687, 7691, 7699, 7703, 7717, 7723, 7727, 7741, 7753, 7757, 7759, 7789, 7793, 7817, 7823, 7829, 7841, 7853, 7867, 7873, 7877, 7879, 7883, 7901, 7907, 7919];
        assert_eq!(CORRECT, PRIMES);
    }

    #[test]
    fn check_isqrt() {
        #[rustfmt::skip]
        const TEST_CASES: [(Underlying, Underlying); 100] = [(0, 0), (1, 1), (2, 1), (3, 1), (4, 2), (5, 2), (6, 2), (7, 2), (8, 2), (9, 3), (10, 3), (11, 3), (12, 3), (13, 3), (14, 3), (15, 3), (16, 4), (17, 4), (18, 4), (19, 4), (20, 4), (21, 4), (22, 4), (23, 4), (24, 4), (25, 5), (26, 5), (27, 5), (28, 5), (29, 5), (30, 5), (31, 5), (32, 5), (33, 5), (34, 5), (35, 5), (36, 6), (37, 6), (38, 6), (39, 6), (40, 6), (41, 6), (42, 6), (43, 6), (44, 6), (45, 6), (46, 6), (47, 6), (48, 6), (49, 7), (50, 7), (51, 7), (52, 7), (53, 7), (54, 7), (55, 7), (56, 7), (57, 7), (58, 7), (59, 7), (60, 7), (61, 7), (62, 7), (63, 7), (64, 8), (65, 8), (66, 8), (67, 8), (68, 8), (69, 8), (70, 8), (71, 8), (72, 8), (73, 8), (74, 8), (75, 8), (76, 8), (77, 8), (78, 8), (79, 8), (80, 8), (81, 9), (82, 9), (83, 9), (84, 9), (85, 9), (86, 9), (87, 9), (88, 9), (89, 9), (90, 9), (91, 9), (92, 9), (93, 9), (94, 9), (95, 9), (96, 9), (97, 9), (98, 9), (99, 9)];
        for (x, ans) in TEST_CASES {
            assert_eq!(isqrt(x), ans);
        }
    }
}