seq: Add a print_seq fast path function for integer and positive increments

A lot of custom logic, we basically do arithmetic on character
arrays, but this comes at with huge performance gains.

Unlike coreutils `seq`, we do this for all positive increments
(because why not), and we do not fall back to slow path if
the last parameter is in scientific notation.

Also, add some tests for empty separator, as that may catch
some corner cases.

src/uu/seq/BENCHMARKING.md

@@ -76,5 +76,19 @@ write!(stdout, "{separator}")?
 
 The change above resulted in a ~10% speedup.
 
+### Fast increment path
+
+When dealing with positive integer values (first/increment/last), and
+the default format is used, we use a custom fast path that does arithmetic
+on u8 arrays (i.e. strings), instead of repeatedly calling into
+formatting format.
+
+This provides _massive_ performance gains, in the order of 10-20x compared
+with the default implementation, at the expense of some added code complexity.
+
+Just from performance numbers, it is clear that GNU `seq` uses similar
+tricks, but we are more liberal on when we use our fast path (e.g. large
+increments are supported). Our fast path implementation gets within ~10%
+of `seq` performance.
 
 [0]: https://github.com/sharkdp/hyperfine

src/uu/seq/src/seq.rs

@@ -2,11 +2,13 @@
 //
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
-// spell-checker:ignore (ToDO) bigdecimal extendedbigdecimal numberparse hexadecimalfloat
+// spell-checker:ignore (ToDO) bigdecimal extendedbigdecimal numberparse hexadecimalfloat biguint
 use std::ffi::OsString;
 use std::io::{BufWriter, ErrorKind, Write, stdout};
 
 use clap::{Arg, ArgAction, Command};
+use num_bigint::BigUint;
+use num_traits::ToPrimitive;
 use num_traits::Zero;
 
 use uucore::error::{FromIo, UResult};
@@ -190,12 +192,17 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
         }
     };
 
+    // Allow fast printing, `print_seq` will do further checks.
+    let fast_allowed = options.format.is_none() && !options.equal_width && precision == Some(0);
+
     let result = print_seq(
         (first.number, increment.number, last.number),
         &options.separator,
         &options.terminator,
         &format,
+        fast_allowed,
     );
+
     match result {
         Ok(()) => Ok(()),
         Err(err) if err.kind() == ErrorKind::BrokenPipe => Ok(()),
@@ -245,6 +252,118 @@ pub fn uu_app() -> Command {
         )
 }
 
+// Fast code path increment function.
+// Add inc to the string val[start..end]. This operates on ASCII digits, assuming
+// val and inc are well formed.
+// Returns the new value for start.
+fn fast_inc(val: &mut [u8], start: usize, end: usize, inc: &[u8]) -> usize {
+    // To avoid a lot of casts to signed integers, we make sure to decrement pos
+    // as late as possible, so that it does not ever go negative.
+    let mut pos = end;
+    let mut carry = 0u8;
+
+    // First loop, add all digits of inc into val.
+    for inc_pos in (0..inc.len()).rev() {
+        pos -= 1;
+
+        let mut new_val = inc[inc_pos] + carry;
+        // Be careful here, only add existing digit of val.
+        if pos >= start {
+            new_val += val[pos] - b'0';
+        }
+        if new_val > b'9' {
+            carry = 1;
+            new_val -= 10;
+        } else {
+            carry = 0;
+        }
+        val[pos] = new_val;
+    }
+
+    // Done, now, if we have a carry, add that to the upper digits of val.
+    if carry == 0 {
+        return start.min(pos);
+    }
+    while pos > start {
+        pos -= 1;
+
+        if val[pos] == b'9' {
+            // 9+1 = 10. Carry propagating, keep going.
+            val[pos] = b'0';
+        } else {
+            // Carry stopped propagating, return unchanged start.
+            val[pos] += 1;
+            return start;
+        }
+    }
+
+    // The carry propagated so far that a new digit was added.
+    val[start - 1] = b'1';
+    start - 1
+}
+
+/// Integer print, default format, positive increment: fast code path
+/// that avoids reformating digit at all iterations.
+/// TODO: We could easily support equal_width (we do quite a bit of work
+/// _not_ supporting that and aligning the number to the left).
+fn fast_print_seq(
+    mut stdout: impl Write,
+    first: &BigUint,
+    increment: u64,
+    last: &BigUint,
+    separator: &str,
+    terminator: &str,
+) -> std::io::Result<()> {
+    // Nothing to do, just return.
+    if last < first {
+        return Ok(());
+    }
+
+    // Do at most u64::MAX loops. We can print in the order of 1e8 digits per second,
+    // u64::MAX is 1e19, so it'd take hundreds of years for this to complete anyway.
+    // TODO: we can move this test to `print_seq` if we care about this case.
+    let loop_cnt = ((last - first) / increment).to_u64().unwrap_or(u64::MAX);
+
+    // Format the first number.
+    let first_str = first.to_string();
+
+    // Makeshift log10.ceil
+    let last_length = last.to_string().len();
+
+    // Allocate a large u8 buffer, that contains a preformatted string
+    // of the number followed by the `separator`.
+    //
+    // | ... head space ... | number | separator |
+    // ^0                   ^ start  ^ num_end   ^ size (==buf.len())
+    //
+    // We keep track of start in this buffer, as the number grows.
+    // When printing, we take a slice between start and end.
+    let size = separator.len() + last_length;
+    let mut buf = vec![0u8; size];
+    let buf = buf.as_mut_slice();
+
+    let num_end = buf.len() - separator.len();
+    let mut start = num_end - first_str.len();
+
+    // Initialize buf with first and separator.
+    buf[start..num_end].copy_from_slice(first_str.as_bytes());
+    buf[num_end..].copy_from_slice(separator.as_bytes());
+
+    // Prepare the number to increment with as a string
+    let inc_str = increment.to_string();
+    let inc_str = inc_str.as_bytes();
+
+    for _ in 0..loop_cnt {
+        stdout.write_all(&buf[start..])?;
+        start = fast_inc(buf, start, num_end, inc_str);
+    }
+    // Write the last number without separator, but with terminator.
+    stdout.write_all(&buf[start..num_end])?;
+    write!(stdout, "{terminator}")?;
+    stdout.flush()?;
+    Ok(())
+}
+
 fn done_printing<T: Zero + PartialOrd>(next: &T, increment: &T, last: &T) -> bool {
     if increment >= &T::zero() {
         next > last
@@ -253,16 +372,40 @@ fn done_printing<T: Zero + PartialOrd>(next: &T, increment: &T, last: &T) -> boo
     }
 }
 
-/// Floating point based code path
+/// Arbitrary precision decimal number code path ("slow" path)
 fn print_seq(
     range: RangeFloat,
     separator: &str,
     terminator: &str,
     format: &Format<num_format::Float, &ExtendedBigDecimal>,
+    fast_allowed: bool,
 ) -> std::io::Result<()> {
     let stdout = stdout().lock();
     let mut stdout = BufWriter::new(stdout);
     let (first, increment, last) = range;
+
+    if fast_allowed {
+        // Test if we can use fast code path.
+        // First try to convert the range to BigUint (u64 for the increment).
+        let (first_bui, increment_u64, last_bui) = (
+            first.to_biguint(),
+            increment.to_biguint().and_then(|x| x.to_u64()),
+            last.to_biguint(),
+        );
+        if let (Some(first_bui), Some(increment_u64), Some(last_bui)) =
+            (first_bui, increment_u64, last_bui)
+        {
+            return fast_print_seq(
+                stdout,
+                &first_bui,
+                increment_u64,
+                &last_bui,
+                separator,
+                terminator,
+            );
+        }
+    }
+
     let mut value = first;
 
     let mut is_first_iteration = true;
@@ -281,3 +424,58 @@ fn print_seq(
     stdout.flush()?;
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test_fast_inc_simple() {
+        use crate::fast_inc;
+
+        let mut val = [b'.', b'.', b'.', b'0', b'_'];
+        let inc = [b'4'].as_ref();
+        assert_eq!(fast_inc(val.as_mut(), 3, 4, inc), 3);
+        assert_eq!(val, "...4_".as_bytes());
+        assert_eq!(fast_inc(val.as_mut(), 3, 4, inc), 3);
+        assert_eq!(val, "...8_".as_bytes());
+        assert_eq!(fast_inc(val.as_mut(), 3, 4, inc), 2); // carried 1 more digit
+        assert_eq!(val, "..12_".as_bytes());
+
+        let mut val = [b'0', b'_'];
+        let inc = [b'2'].as_ref();
+        assert_eq!(fast_inc(val.as_mut(), 0, 1, inc), 0);
+        assert_eq!(val, "2_".as_bytes());
+        assert_eq!(fast_inc(val.as_mut(), 0, 1, inc), 0);
+        assert_eq!(val, "4_".as_bytes());
+        assert_eq!(fast_inc(val.as_mut(), 0, 1, inc), 0);
+        assert_eq!(val, "6_".as_bytes());
+    }
+
+    // Check that we handle increment > val correctly.
+    #[test]
+    fn test_fast_inc_large_inc() {
+        use crate::fast_inc;
+
+        let mut val = [b'.', b'.', b'.', b'7', b'_'];
+        let inc = "543".as_bytes();
+        assert_eq!(fast_inc(val.as_mut(), 3, 4, inc), 1); // carried 2 more digits
+        assert_eq!(val, ".550_".as_bytes());
+        assert_eq!(fast_inc(val.as_mut(), 1, 4, inc), 0); // carried 1 more digit
+        assert_eq!(val, "1093_".as_bytes());
+    }
+
+    // Check that we handle longer carries
+    #[test]
+    fn test_fast_inc_carry() {
+        use crate::fast_inc;
+
+        let mut val = [b'.', b'9', b'9', b'9', b'_'];
+        let inc = "1".as_bytes();
+        assert_eq!(fast_inc(val.as_mut(), 1, 4, inc), 0);
+        assert_eq!(val, "1000_".as_bytes());
+
+        let mut val = [b'.', b'9', b'9', b'9', b'_'];
+        let inc = "11".as_bytes();
+        assert_eq!(fast_inc(val.as_mut(), 1, 4, inc), 0);
+        assert_eq!(val, "1010_".as_bytes());
+    }
+}

src/uucore/src/lib/features/extendedbigdecimal.rs

@@ -2,7 +2,7 @@
 //
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
-// spell-checker:ignore bigdecimal extendedbigdecimal
+// spell-checker:ignore bigdecimal extendedbigdecimal biguint
 //! An arbitrary precision float that can also represent infinity, NaN, etc.
 //!
 //! The finite values are stored as [`BigDecimal`] instances. Because
@@ -25,7 +25,9 @@ use std::ops::Add;
 use std::ops::Neg;
 
 use bigdecimal::BigDecimal;
+use bigdecimal::num_bigint::BigUint;
 use num_traits::FromPrimitive;
+use num_traits::Signed;
 use num_traits::Zero;
 
 #[derive(Debug, Clone)]
@@ -107,6 +109,20 @@ impl ExtendedBigDecimal {
     pub fn one() -> Self {
         Self::BigDecimal(1.into())
     }
+
+    pub fn to_biguint(&self) -> Option<BigUint> {
+        match self {
+            ExtendedBigDecimal::BigDecimal(big_decimal) => {
+                let (bi, scale) = big_decimal.as_bigint_and_scale();
+                if bi.is_negative() || scale > 0 || scale < -(u32::MAX as i64) {
+                    return None;
+                }
+                bi.to_biguint()
+                    .map(|bi| bi * BigUint::from(10u32).pow(-scale as u32))
+            }
+            _ => None,
+        }
+    }
 }
 
 impl Zero for ExtendedBigDecimal {

tests/by-util/test_seq.rs

@@ -216,6 +216,10 @@ fn test_separator_and_terminator() {
         .args(&["-s", ",", "2", "6"])
         .succeeds()
         .stdout_is("2,3,4,5,6\n");
+    new_ucmd!()
+        .args(&["-s", "", "2", "6"])
+        .succeeds()
+        .stdout_is("23456\n");
     new_ucmd!()
         .args(&["-s", "\n", "2", "6"])
         .succeeds()
@@ -286,6 +290,10 @@ fn test_separator_and_terminator_floats() {
         .args(&["-s", ",", "-t", "!", "2.0", "6"])
         .succeeds()
         .stdout_is("2.0,3.0,4.0,5.0,6.0!");
+    new_ucmd!()
+        .args(&["-s", "", "-t", "!", "2.0", "6"])
+        .succeeds()
+        .stdout_is("2.03.04.05.06.0!");
 }
 
 #[test]