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use solana_sdk::pubkey::Pubkey;

pub(crate) struct PubkeyBinCalculator16 {
    // how many bits from the first 2 bytes to shift away to ignore when calculating bin
    shift_bits: u32,
}

impl PubkeyBinCalculator16 {
    const fn num_bits<T>() -> usize {
        std::mem::size_of::<T>() * 8
    }

    fn log_2(x: u32) -> u32 {
        assert!(x > 0);
        Self::num_bits::<u32>() as u32 - x.leading_zeros() - 1
    }

    pub fn new(bins: usize) -> Self {
        const MAX_BITS: u32 = 16;
        assert!(bins > 0);
        let max_plus_1 = 1 << MAX_BITS;
        assert!(bins <= max_plus_1);
        assert!(bins.is_power_of_two());
        let bits = Self::log_2(bins as u32);
        Self {
            shift_bits: MAX_BITS - bits,
        }
    }

    pub fn bin_from_pubkey(&self, pubkey: &Pubkey) -> usize {
        let as_ref = pubkey.as_ref();
        ((as_ref[0] as usize * 256 + as_ref[1] as usize) as usize) >> self.shift_bits
    }
}

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

    #[test]
    fn test_pubkey_bins_log2() {
        assert_eq!(PubkeyBinCalculator16::num_bits::<u8>(), 8);
        assert_eq!(PubkeyBinCalculator16::num_bits::<u32>(), 32);
        for i in 0..32 {
            assert_eq!(PubkeyBinCalculator16::log_2(2u32.pow(i)), i);
        }
    }

    #[test]
    fn test_pubkey_bins() {
        for i in 0..=16 {
            let bins = 2u32.pow(i);
            let calc = PubkeyBinCalculator16::new(bins as usize);
            assert_eq!(calc.shift_bits, 16 - i, "i: {}", i);
        }
    }

    #[test]
    fn test_pubkey_bins_pubkeys() {
        let mut pk = Pubkey::new(&[0; 32]);
        for i in 0..=8 {
            let bins = 2usize.pow(i);
            let calc = PubkeyBinCalculator16::new(bins);

            let shift_bits = calc.shift_bits - 8; // we are only dealing with first byte

            pk.as_mut()[0] = 0;
            assert_eq!(0, calc.bin_from_pubkey(&pk));
            pk.as_mut()[0] = 0xff;
            assert_eq!(bins - 1, calc.bin_from_pubkey(&pk));

            for bin in 0..bins {
                pk.as_mut()[0] = (bin << shift_bits) as u8;
                assert_eq!(
                    bin,
                    calc.bin_from_pubkey(&pk),
                    "bin: {}/{}, bits: {}, val: {}",
                    bin,
                    bins,
                    shift_bits,
                    pk.as_ref()[0]
                );
                if bin > 0 {
                    pk.as_mut()[0] = ((bin << shift_bits) - 1) as u8;
                    assert_eq!(bin - 1, calc.bin_from_pubkey(&pk));
                }
            }
        }

        for i in 9..=16 {
            let mut pk = Pubkey::new(&[0; 32]);
            let bins = 2usize.pow(i);
            let calc = PubkeyBinCalculator16::new(bins);

            let shift_bits = calc.shift_bits;

            pk.as_mut()[1] = 0;
            assert_eq!(0, calc.bin_from_pubkey(&pk));
            pk.as_mut()[0] = 0xff;
            pk.as_mut()[1] = 0xff;
            assert_eq!(bins - 1, calc.bin_from_pubkey(&pk));

            let mut pk = Pubkey::new(&[0; 32]);
            for bin in 0..bins {
                let mut target = (bin << shift_bits) as u16;
                pk.as_mut()[0] = (target / 256) as u8;
                pk.as_mut()[1] = (target % 256) as u8;
                assert_eq!(
                    bin,
                    calc.bin_from_pubkey(&pk),
                    "bin: {}/{}, bits: {}, val: {}",
                    bin,
                    bins,
                    shift_bits,
                    pk.as_ref()[0]
                );
                if bin > 0 {
                    target -= 1;
                    pk.as_mut()[0] = (target / 256) as u8;
                    pk.as_mut()[1] = (target % 256) as u8;
                    assert_eq!(bin - 1, calc.bin_from_pubkey(&pk));
                }
            }
        }
    }

    #[test]
    #[should_panic(expected = "bins.is_power_of_two()")]
    fn test_pubkey_bins_illegal_bins3() {
        PubkeyBinCalculator16::new(3);
    }

    #[test]
    #[should_panic(expected = "bins <= max_plus_1")]
    fn test_pubkey_bins_illegal_bins2() {
        PubkeyBinCalculator16::new(65537);
    }
    #[test]
    #[should_panic(expected = "bins > 0")]
    fn test_pubkey_bins_illegal_bins() {
        PubkeyBinCalculator16::new(0);
    }
}