//! Bit twiddling.

use super::IndexMode;
use crate::{Direction, Edge, Resolution, Vertex};
use core::{cmp, num::NonZeroU8};

/// Offset (in bits) of the mode in an H3 index.
const MODE_OFFSET: usize = 59;
const MODE_MASK: u64 = 0b1111 << MODE_OFFSET;

/// Offset (in bits) of the cell edge in an H3 index.
const EDGE_OFFSET: usize = 56;
const EDGE_MASK: u64 = 0b111 << EDGE_OFFSET;

/// Offset (in bits) of the cell vertex in an H3 index.
const VERTEX_OFFSET: usize = 56;
const VERTEX_MASK: u64 = 0b111 << VERTEX_OFFSET;

// Bitmask to select the directions bits in an H3 index.
pub const DIRECTIONS_MASK: u64 = 0x0000_1fff_ffff_ffff;

/// Returns the H3 index mode  bits.
#[must_use]
pub const fn get_mode(bits: u64) -> u8 {
    ((bits & MODE_MASK) >> MODE_OFFSET) as u8
}

/// Clears the H3 index mode bits.
#[must_use]
pub const fn clr_mode(bits: u64) -> u64 {
    bits & !MODE_MASK
}

/// Sets the H3 index mode bits.
#[must_use]
pub const fn set_mode(bits: u64, mode: IndexMode) -> u64 {
    clr_mode(bits) | ((mode as u64) << MODE_OFFSET)
}

/// Returns the H3 index cell edge bits.
#[must_use]
pub const fn get_edge(bits: u64) -> u8 {
    ((bits & EDGE_MASK) >> EDGE_OFFSET) as u8
}

/// Sets the H3 index mode bits.
#[must_use]
pub fn set_edge(bits: u64, edge: Edge) -> u64 {
    clr_edge(bits) | (u64::from(edge) << EDGE_OFFSET)
}

/// Clears the H3 index cell edge bits.
#[must_use]
pub const fn clr_edge(bits: u64) -> u64 {
    bits & !EDGE_MASK
}

/// Returns the H3 index cell vertex bits.
#[must_use]
pub const fn get_vertex(bits: u64) -> u8 {
    ((bits & VERTEX_MASK) >> VERTEX_OFFSET) as u8
}

/// Sets the H3 index mode bits.
#[must_use]
pub fn set_vertex(bits: u64, vertex: Vertex) -> u64 {
    clr_vertex(bits) | (u64::from(vertex) << VERTEX_OFFSET)
}

/// Clears the H3 index cell vertex bits.
#[must_use]
pub const fn clr_vertex(bits: u64) -> u64 {
    bits & !VERTEX_MASK
}

/// Returns the H3 index resolution.
#[must_use]
pub const fn get_resolution(bits: u64) -> Resolution {
    // SAFETY: the masking restricts the value on 4 bits (thus 0-15).
    Resolution::new_unchecked(h3o_bit::get_resolution(bits))
}

/// Sets the H3 index resolution bits.
#[must_use]
pub fn set_resolution(bits: u64, resolution: Resolution) -> u64 {
    h3o_bit::set_resolution(bits, resolution.into())
}

/// Returns the H3 index direction bits at the given resolution.
#[must_use]
pub fn get_direction(bits: u64, resolution: Resolution) -> u8 {
    debug_assert!(resolution != Resolution::Zero, "res 0 means no directions");
    h3o_bit::get_direction(bits, resolution.into())
}

/// Clears the H3 index direction at the given resolution.
#[must_use]
pub fn clr_direction(bits: u64, resolution: Resolution) -> u64 {
    debug_assert!(resolution != Resolution::Zero, "res 0 means no directions");
    h3o_bit::clr_direction(bits, resolution.into())
}

/// Set the H3 index direction bits at the given resolution.
pub fn set_direction(bits: u64, cell: u8, resolution: Resolution) -> u64 {
    debug_assert!(resolution != Resolution::Zero, "res 0 means no directions");
    h3o_bit::set_direction(bits, resolution.into(), cell)
}

/// Sets unused directions in an H3 index at the given resolution.
#[must_use]
pub fn set_unused(bits: u64, resolution: Resolution) -> u64 {
    let unused_end_offset = resolution.direction_offset();
    let unused_bits = (1 << unused_end_offset) - 1;

    bits | unused_bits
}

/// Returns the direction bits of the first non-center direction from an H3
/// index.
#[must_use]
pub fn first_axe(bits: u64) -> Option<NonZeroU8> {
    const PREFIX_LENGTH: u32 = DIRECTIONS_MASK.leading_zeros();

    let resolution = get_resolution(bits);
    // No direction at res 0 (only base cell), thus no non-zero direction.
    if resolution == Resolution::Zero {
        return None;
    }

    // Ignore the `0` from the hidden bits (hence `- PREFIX_LENGTH`).
    let bit_index = (bits & DIRECTIONS_MASK).leading_zeros() - PREFIX_LENGTH;

    // +1 because the first direction is at resolution 1, not 0.
    #[expect(clippy::cast_possible_truncation, reason = "values are in range")]
    let resolution = cmp::min(
        ((bit_index / h3o_bit::DIRECTION_BITSIZE as u32) + 1) as u8,
        resolution.into(),
    );

    NonZeroU8::new(get_direction(bits, Resolution::new_unchecked(resolution)))
}

/// Returns a cell rotated `count` time, by 60 degrees step.
pub fn rotate60<const CCW: bool>(mut bits: u64, count: usize) -> u64 {
    // Using specialization (through `rotate60_once`) actually pays off.
    if count == 1 {
        for resolution in
            Resolution::range(Resolution::One, get_resolution(bits))
        {
            let direction =
                Direction::new_unchecked(get_direction(bits, resolution));

            bits = set_direction(
                bits,
                direction.rotate60_once::<CCW>().into(),
                resolution,
            );
        }
    } else {
        for resolution in
            Resolution::range(Resolution::One, get_resolution(bits))
        {
            let direction =
                Direction::new_unchecked(get_direction(bits, resolution));

            bits = set_direction(
                bits,
                direction.rotate60::<CCW>(count).into(),
                resolution,
            );
        }
    }

    bits
}

/// Returns a pentagonal cell rotated by 60 degrees.
pub fn pentagon_rotate60<const CCW: bool>(mut bits: u64) -> u64 {
    let res = get_resolution(bits);
    if res == Resolution::Zero {
        return bits;
    }

    // Direction that would be rotated to the K axe (and thus require an
    // adjustment).
    let direction = if CCW { Direction::JK } else { Direction::IK }.axe();

    // Using specialization (through `rotate60_once`) actually pays off.
    if first_axe(bits) == direction {
        for resolution in Resolution::range(Resolution::One, res) {
            let direction =
                Direction::new_unchecked(get_direction(bits, resolution));

            bits = set_direction(
                bits,
                direction.rotate60::<CCW>(2).into(),
                resolution,
            );
        }
    } else {
        for resolution in Resolution::range(Resolution::One, res) {
            let direction =
                Direction::new_unchecked(get_direction(bits, resolution));

            bits = set_direction(
                bits,
                direction.rotate60_once::<CCW>().into(),
                resolution,
            );
        }
    }

    bits
}

#[cfg(test)]
#[path = "./bits_tests.rs"]
mod tests;
