fastalp 0.1.33

World's fastest and highest-ratio lossless floating-point compression / 全球最快、压缩比最高的通用时序浮点无损压缩
Documentation 
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use crate::constants::BITS_U64;

/// Packed parameters bitfield masks and shift constants.
/// 参数位域掩码与位移常量
pub const EXP_MASK: u16 = 0x001F;
pub const FAC_SHIFT: u16 = 5;
pub const FAC_MASK: u16 = 0x000F;
pub const BIT_WIDTH_SHIFT: u16 = 9;
pub const BIT_WIDTH_MASK: u16 = 0x007F;

/// Packs exponent (5b), factor (4b), and bit width (7b) into a 2-byte unsigned integer.
/// 将 exp (5b), fac (4b), bit_width (7b) 打包进 2 字节 u16
#[inline(always)]
pub const fn pack_params(exp: u8, fac: u8, bit_width: u8) -> u16 {
  ((exp as u16) & EXP_MASK)
    | (((fac as u16) & FAC_MASK) << FAC_SHIFT)
    | (((bit_width as u16) & BIT_WIDTH_MASK) << BIT_WIDTH_SHIFT)
}

/// Unpacks (exponent, factor, bit width) from a 2-byte unsigned integer.
/// 从 2 字节 u16 解包 (exp, fac, bit_width)
#[inline(always)]
pub const fn unpack_params(params: u16) -> (u8, u8, u8) {
  let exp = (params & EXP_MASK) as u8;
  let fac = ((params >> FAC_SHIFT) & FAC_MASK) as u8;
  let bit_width = ((params >> BIT_WIDTH_SHIFT) & BIT_WIDTH_MASK) as u8;
  (exp, fac, bit_width)
}

/// Calculates minimum bit width required to represent an unsigned integer (0..=64).
/// 快速计算表示数值所需的最少比特位数 (0..=64，无分支实现)
#[inline(always)]
pub const fn bits_needed(max_val: u64) -> u8 {
  (u64::BITS - max_val.leading_zeros()) as u8
}

/// Computes lower bitmask for a given bit width (0..=64).
/// 快速计算 0..=64 比特宽度的低位掩码
#[inline(always)]
pub const fn bit_mask(bit_width: u8) -> u64 {
  if bit_width >= BITS_U64 as u8 {
    u64::MAX
  } else {
    (1u64 << bit_width).wrapping_sub(1)
  }
}