From 923421fd2ddef5ecabfe812d15adda2590cc0b79 Mon Sep 17 00:00:00 2001
From: Conrad Ludgate <conrad.ludgate@truelayer.com>
Date: Mon, 17 Oct 2022 11:38:28 +0100
Subject: [PATCH] avoid int formatting as much as possible (net -70%/-68% on
 2822/3339 respectively)

---
 src/format/mod.rs     | 39 +++++++++++++++++++++++++++------------
 src/naive/date.rs     | 12 ++++++++++--
 src/naive/time/mod.rs |  8 +++++++-
 3 files changed, 44 insertions(+), 15 deletions(-)

diff --git a/src/format/mod.rs b/src/format/mod.rs
index ab020c5cf8..ed63e4eb7a 100644
--- a/src/format/mod.rs
+++ b/src/format/mod.rs
@@ -40,7 +40,6 @@ use alloc::string::{String, ToString};
 #[cfg(any(feature = "alloc", feature = "std", test))]
 use core::borrow::Borrow;
 use core::fmt;
-#[cfg(any(feature = "alloc", feature = "std", test))]
 use core::fmt::Write;
 use core::str::FromStr;
 #[cfg(any(feature = "std", test))]
@@ -798,21 +797,37 @@ fn write_rfc2822_inner(
         return Err(fmt::Error);
     }
 
+    result.push_str(locale.short_weekdays[d.weekday().num_days_from_sunday() as usize]);
+    result.push_str(", ");
+    write_hundreds(result, d.day() as u8)?;
+    result.push(' ');
+    result.push_str(locale.short_months[d.month0() as usize]);
+    result.push(' ');
+    write_hundreds(result, (year / 100) as u8)?;
+    write_hundreds(result, (year % 100) as u8)?;
+    result.push(' ');
+    write_hundreds(result, t.hour() as u8)?;
+    result.push(':');
+    write_hundreds(result, t.minute() as u8)?;
+    result.push(':');
     let sec = t.second() + t.nanosecond() / 1_000_000_000;
-    write!(
-        result,
-        "{}, {:02} {} {:04} {:02}:{:02}:{:02} ",
-        locale.short_weekdays[d.weekday().num_days_from_sunday() as usize],
-        d.day(),
-        locale.short_months[d.month0() as usize],
-        year,
-        t.hour(),
-        t.minute(),
-        sec
-    )?;
+    write_hundreds(result, sec as u8)?;
+    result.push(' ');
     write_local_minus_utc(result, off, false, Colons::None)
 }
 
+/// Equivalent to `{:02}` formatting for n < 100.
+pub(crate) fn write_hundreds(w: &mut impl Write, n: u8) -> fmt::Result {
+    if n >= 100 {
+        return Err(fmt::Error);
+    }
+
+    let tens = b'0' + n / 10;
+    let ones = b'0' + n % 10;
+    w.write_char(tens as char)?;
+    w.write_char(ones as char)
+}
+
 /// Tries to format given arguments with given formatting items.
 /// Internally used by `DelayedFormat`.
 #[cfg(any(feature = "alloc", feature = "std", test))]
diff --git a/src/naive/date.rs b/src/naive/date.rs
index c5fb7b1725..4e282bd1f9 100644
--- a/src/naive/date.rs
+++ b/src/naive/date.rs
@@ -1830,14 +1830,22 @@ impl DoubleEndedIterator for NaiveDateWeeksIterator {
 /// ```
 impl fmt::Debug for NaiveDate {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        use core::fmt::Write;
+
         let year = self.year();
         let mdf = self.mdf();
         if (0..=9999).contains(&year) {
-            write!(f, "{:04}-{:02}-{:02}", year, mdf.month(), mdf.day())
+            crate::format::write_hundreds(f, (year / 100) as u8)?;
+            crate::format::write_hundreds(f, (year % 100) as u8)?;
         } else {
             // ISO 8601 requires the explicit sign for out-of-range years
-            write!(f, "{:+05}-{:02}-{:02}", year, mdf.month(), mdf.day())
+            write!(f, "{:+05}", year)?;
         }
+
+        f.write_char('-')?;
+        crate::format::write_hundreds(f, mdf.month() as u8)?;
+        f.write_char('-')?;
+        crate::format::write_hundreds(f, mdf.day() as u8)
     }
 }
 
diff --git a/src/naive/time/mod.rs b/src/naive/time/mod.rs
index c79fe6b3c6..4e273373d6 100644
--- a/src/naive/time/mod.rs
+++ b/src/naive/time/mod.rs
@@ -1188,7 +1188,13 @@ impl fmt::Debug for NaiveTime {
             (sec, self.frac)
         };
 
-        write!(f, "{:02}:{:02}:{:02}", hour, min, sec)?;
+        use core::fmt::Write;
+        crate::format::write_hundreds(f, hour as u8)?;
+        f.write_char(':')?;
+        crate::format::write_hundreds(f, min as u8)?;
+        f.write_char(':')?;
+        crate::format::write_hundreds(f, sec as u8)?;
+
         if nano == 0 {
             Ok(())
         } else if nano % 1_000_000 == 0 {