//! Mask applied to a data source or destination

use crate::error::AsmError;

/// Bit mask to apply to a value
#[derive(Debug, Clone, Copy, Eq, PartialEq)]
pub struct Mask {
    /// Length of the selected bit slice
    len: u8,
    /// Offset of the selected bit slice from bit zero
    offset: u8,
}

impl Default for Mask {
    fn default() -> Self {
        Mask {
            len: 64,
            offset: 0,
        }
    }
}

impl Mask {
    pub const BYTE: Mask = Mask {
        len: 8,
        offset: 0,
    };
    pub const HALF_WORD: Mask = Mask {
        len: 16,
        offset: 0,
    };
    pub const WORD: Mask = Mask {
        len: 32,
        offset: 0,
    };
    pub const DOUBLE_WORD: Mask = Mask {
        len: 64,
        offset: 0,
    };
    pub const FULL: Mask = Self::DOUBLE_WORD;

    pub fn new(len: u8, offset: u8) -> Result<Self, AsmError> {
        if len == 0 || offset >= 64 {
            // create the invalid mask to display it in the error
            return Err(AsmError::BadMask(Mask {
                len,
                offset,
            }));
        }

        Ok(Self {
            len: len.min(64 - offset),
            offset,
        })
    }

    /// Get a binary mask representing the span
    pub fn as_bitmask(self) -> u64 {
        ((1 << self.len) - 1) << self.offset
    }

    pub fn is_default(self) -> bool {
        self == Self::default()
    }
}