Copy fast divmod100 from zmij

Author: dtolnay

src/lib.rs

@@ -220,6 +220,15 @@ static DECIMAL_PAIRS: DecimalPairs = DecimalPairs(
  8081828384858687888990919293949596979899",
 );
 
+// Returns {value / 100, value % 100} correct for values of up to 4 digits.
+fn divmod100(value: u32) -> (u32, u32) {
+ debug_assert!(value < 10_000);
+ const EXP: u32 = 19; // 19 is faster or equal to 12 even for 3 digits.
+ const SIG: u32 = (1 << EXP) / 100 + 1;
+ let div = (value * SIG) >> EXP; // value / 100
+ (div, value - div * 100)
+}
+
 /// This function converts a slice of ascii characters into a `&str` starting
 /// from `offset`.
 ///
@@ -266,25 +275,30 @@ macro_rules! impl_Unsigned {
  .expect("branch is not hit for types that cannot fit 1E4 (u8)");
  let quad = remain % scale;
  remain /= scale;
- let pair1 = (quad / 100) as usize;
- let pair2 = (quad % 100) as usize;
+ let (pair1, pair2) = divmod100(quad as u32);
  unsafe {
- buf[offset + 0].write(*DECIMAL_PAIRS.0.get_unchecked(pair1 * 2 + 0));
- buf[offset + 1].write(*DECIMAL_PAIRS.0.get_unchecked(pair1 * 2 + 1));
- buf[offset + 2].write(*DECIMAL_PAIRS.0.get_unchecked(pair2 * 2 + 0));
- buf[offset + 3].write(*DECIMAL_PAIRS.0.get_unchecked(pair2 * 2 + 1));
+ buf[offset + 0]
+ .write(*DECIMAL_PAIRS.0.get_unchecked(pair1 as usize * 2 + 0));
+ buf[offset + 1]
+ .write(*DECIMAL_PAIRS.0.get_unchecked(pair1 as usize * 2 + 1));
+ buf[offset + 2]
+ .write(*DECIMAL_PAIRS.0.get_unchecked(pair2 as usize * 2 + 0));
+ buf[offset + 3]
+ .write(*DECIMAL_PAIRS.0.get_unchecked(pair2 as usize * 2 + 1));
  }
  }
 
  // Format per two digits from the lookup table.
  if remain > 9 {
  offset -= 2;
 
- let pair = (remain % 100) as usize;
- remain /= 100;
+ let (last, pair) = divmod100(remain as u32);
+ remain = last as Self;
  unsafe {
- buf[offset + 0].write(*DECIMAL_PAIRS.0.get_unchecked(pair * 2 + 0));
- buf[offset + 1].write(*DECIMAL_PAIRS.0.get_unchecked(pair * 2 + 1));
+ buf[offset + 0]
+ .write(*DECIMAL_PAIRS.0.get_unchecked(pair as usize * 2 + 0));
+ buf[offset + 1]
+ .write(*DECIMAL_PAIRS.0.get_unchecked(pair as usize * 2 + 1));
  }
  }
 
@@ -349,25 +363,24 @@ impl Unsigned for u128 {
  // pull two pairs
  let quad = remain % 1_00_00;
  remain /= 1_00_00;
- let pair1 = (quad / 100) as usize;
- let pair2 = (quad % 100) as usize;
+ let (pair1, pair2) = divmod100(quad as u32);
  unsafe {
- buf[offset + 0].write(*DECIMAL_PAIRS.0.get_unchecked(pair1 * 2 + 0));
- buf[offset + 1].write(*DECIMAL_PAIRS.0.get_unchecked(pair1 * 2 + 1));
- buf[offset + 2].write(*DECIMAL_PAIRS.0.get_unchecked(pair2 * 2 + 0));
- buf[offset + 3].write(*DECIMAL_PAIRS.0.get_unchecked(pair2 * 2 + 1));
+ buf[offset + 0].write(*DECIMAL_PAIRS.0.get_unchecked(pair1 as usize * 2 + 0));
+ buf[offset + 1].write(*DECIMAL_PAIRS.0.get_unchecked(pair1 as usize * 2 + 1));
+ buf[offset + 2].write(*DECIMAL_PAIRS.0.get_unchecked(pair2 as usize * 2 + 0));
+ buf[offset + 3].write(*DECIMAL_PAIRS.0.get_unchecked(pair2 as usize * 2 + 1));
  }
  }
 
  // Format per two digits from the lookup table.
  if remain > 9 {
  offset -= 2;
 
- let pair = (remain % 100) as usize;
- remain /= 100;
+ let (last, pair) = divmod100(remain as u32);
+ remain = last as u64;
  unsafe {
- buf[offset + 0].write(*DECIMAL_PAIRS.0.get_unchecked(pair * 2 + 0));
- buf[offset + 1].write(*DECIMAL_PAIRS.0.get_unchecked(pair * 2 + 1));
+ buf[offset + 0].write(*DECIMAL_PAIRS.0.get_unchecked(pair as usize * 2 + 0));
+ buf[offset + 1].write(*DECIMAL_PAIRS.0.get_unchecked(pair as usize * 2 + 1));
  }
  }
 
@@ -398,13 +411,16 @@ fn enc_16lsd<const OFFSET: usize>(buf: &mut [MaybeUninit<u8>], n: u64) {
  // pull two pairs
  let quad = remain % 1_00_00;
  remain /= 1_00_00;
- let pair1 = (quad / 100) as usize;
- let pair2 = (quad % 100) as usize;
+ let (pair1, pair2) = divmod100(quad as u32);
  unsafe {
- buf[quad_index * 4 + OFFSET + 0].write(*DECIMAL_PAIRS.0.get_unchecked(pair1 * 2 + 0));
- buf[quad_index * 4 + OFFSET + 1].write(*DECIMAL_PAIRS.0.get_unchecked(pair1 * 2 + 1));
- buf[quad_index * 4 + OFFSET + 2].write(*DECIMAL_PAIRS.0.get_unchecked(pair2 * 2 + 0));
- buf[quad_index * 4 + OFFSET + 3].write(*DECIMAL_PAIRS.0.get_unchecked(pair2 * 2 + 1));
+ buf[quad_index * 4 + OFFSET + 0]
+ .write(*DECIMAL_PAIRS.0.get_unchecked(pair1 as usize * 2 + 0));
+ buf[quad_index * 4 + OFFSET + 1]
+ .write(*DECIMAL_PAIRS.0.get_unchecked(pair1 as usize * 2 + 1));
+ buf[quad_index * 4 + OFFSET + 2]
+ .write(*DECIMAL_PAIRS.0.get_unchecked(pair2 as usize * 2 + 0));
+ buf[quad_index * 4 + OFFSET + 3]
+ .write(*DECIMAL_PAIRS.0.get_unchecked(pair2 as usize * 2 + 1));
  }
  }
 }