Use different buffer size for every integer type

src/lib.rs

@@ -55,8 +55,8 @@ pub trait Integer: private::Sealed {
     fn fmt<W: fmt::Write>(self, W) -> fmt::Result;
 }
 
-trait IntegerPrivate {
-    fn write_to(self, buf: &mut [u8; MAX_LEN]) -> &[u8];
+trait IntegerPrivate<B> {
+    fn write_to(self, buf: &mut B) -> &[u8];
 }
 
 const DEC_DIGITS_LUT: &'static[u8] =
@@ -66,23 +66,23 @@ const DEC_DIGITS_LUT: &'static[u8] =
       6061626364656667686970717273747576777879\
       8081828384858687888990919293949596979899";
 
-const MAX_LEN: usize = 40; // i128::MIN (including minus sign)
-
 // Adaptation of the original implementation at
 // https://github.com/rust-lang/rust/blob/b8214dc6c6fc20d0a660fb5700dca9ebf51ebe89/src/libcore/fmt/num.rs#L188-L266
 macro_rules! impl_IntegerCommon {
-    ($t:ident) => {
+    ($max_len:expr, $t:ident) => {
         impl Integer for $t {
             #[cfg(feature = "std")]
+            #[inline]
             fn write<W: io::Write>(self, mut wr: W) -> io::Result<usize> {
-                let mut buf = unsafe { mem::uninitialized() };
+                let mut buf: [u8; $max_len] = unsafe { mem::uninitialized() };
                 let bytes = self.write_to(&mut buf);
                 try!(wr.write_all(bytes));
                 Ok(bytes.len())
             }
 
+            #[inline]
             fn fmt<W: fmt::Write>(self, mut wr: W) -> fmt::Result {
-                let mut buf = unsafe { mem::uninitialized() };
+                let mut buf: [u8; $max_len] = unsafe { mem::uninitialized() };
                 let bytes = self.write_to(&mut buf);
                 wr.write_str(unsafe { str::from_utf8_unchecked(bytes) })
             }
@@ -93,12 +93,13 @@ macro_rules! impl_IntegerCommon {
 }
 
 macro_rules! impl_Integer {
-    ($($t:ident),* as $conv_fn:ident) => {$(
-        impl_IntegerCommon!($t);
+    ($($max_len:expr => $t:ident),* as $conv_fn:ident) => {$(
+        impl_IntegerCommon!($max_len, $t);
 
-        impl IntegerPrivate for $t {
+        impl IntegerPrivate<[u8; $max_len]> for $t {
             #[allow(unused_comparisons)]
-            fn write_to(self, buf: &mut [u8; MAX_LEN]) -> &[u8] {
+            #[inline]
+            fn write_to(self, buf: &mut [u8; $max_len]) -> &[u8] {
                 let is_nonnegative = self >= 0;
                 let mut n = if is_nonnegative {
                     self as $conv_fn
@@ -160,23 +161,44 @@ macro_rules! impl_Integer {
     )*};
 }
 
-impl_Integer!(i8, u8, i16, u16, i32, u32 as u32);
-impl_Integer!(i64, u64 as u64);
+const I8_MAX_LEN: usize = 4;
+const U8_MAX_LEN: usize = 3;
+const I16_MAX_LEN: usize = 6;
+const U16_MAX_LEN: usize = 5;
+const I32_MAX_LEN: usize = 11;
+const U32_MAX_LEN: usize = 10;
+const I64_MAX_LEN: usize = 20;
+const U64_MAX_LEN: usize = 20;
+
+impl_Integer!(
+    I8_MAX_LEN => i8,
+    U8_MAX_LEN => u8,
+    I16_MAX_LEN => i16,
+    U16_MAX_LEN => u16,
+    I32_MAX_LEN => i32,
+    U32_MAX_LEN => u32 
+    as u32);
+
+impl_Integer!(I64_MAX_LEN => i64, U64_MAX_LEN => u64 as u64);
+
 #[cfg(target_pointer_width = "16")]
-impl_Integer!(isize, usize as u16);
+impl_Integer!(I16_MAX_LEN => isize, U16_MAX_LEN => usize as u16);
+
 #[cfg(target_pointer_width = "32")]
-impl_Integer!(isize, usize as u32);
+impl_Integer!(I32_MAX_LEN => isize, U32_MAX_LEN => usize as u32);
+
 #[cfg(target_pointer_width = "64")]
-impl_Integer!(isize, usize as u64);
+impl_Integer!(I64_MAX_LEN => isize, U64_MAX_LEN => usize as u64);
 
 #[cfg(all(feature = "i128"))]
 macro_rules! impl_Integer128 {
-    ($($t:ident),*) => {$(
-        impl_IntegerCommon!($t);
+    ($($max_len:expr => $t:ident),*) => {$(
+        impl_IntegerCommon!($max_len, $t);
 
-        impl IntegerPrivate for $t {
+        impl IntegerPrivate<[u8; $max_len]> for $t {
             #[allow(unused_comparisons)]
-            fn write_to(self, buf: &mut [u8; MAX_LEN]) -> &[u8] {
+            #[inline]
+            fn write_to(self, buf: &mut [u8; $max_len]) -> &[u8] {
                 let is_nonnegative = self >= 0;
                 let n = if is_nonnegative {
                     self as u128
@@ -190,7 +212,8 @@ macro_rules! impl_Integer128 {
                 unsafe {
                     // Divide by 10^19 which is the highest power less than 2^64.
                     let (n, rem) = udiv128::udivmod_1e19(n);
-                    curr -= rem.write_to(buf).len() as isize;
+                    let buf1 = buf_ptr.offset(curr - U64_MAX_LEN as isize) as *mut [u8; U64_MAX_LEN];
+                    curr -= rem.write_to(&mut *buf1).len() as isize;
 
                     if n != 0 {
                         // Memset the base10 leading zeros of rem.
@@ -200,7 +223,7 @@ macro_rules! impl_Integer128 {
 
                         // Divide by 10^19 again.
                         let (n, rem) = udiv128::udivmod_1e19(n);
-                        let buf2 = buf_ptr.offset(curr - buf.len() as isize) as *mut _;
+                        let buf2 = buf_ptr.offset(curr - U64_MAX_LEN as isize) as *mut [u8; U64_MAX_LEN];
                         curr -= rem.write_to(&mut *buf2).len() as isize;
 
                         if n != 0 {
@@ -230,4 +253,9 @@ macro_rules! impl_Integer128 {
 }
 
 #[cfg(all(feature = "i128"))]
-impl_Integer128!(i128, u128);
+const U128_MAX_LEN: usize = 39;
+#[cfg(all(feature = "i128"))]
+const I128_MAX_LEN: usize = 40;
+
+#[cfg(all(feature = "i128"))]
+impl_Integer128!(I128_MAX_LEN => i128, U128_MAX_LEN => u128);

src/udiv128.rs

@@ -21,6 +21,7 @@
 // (https://github.com/rust-lang/rust/issues/44545) and to allow function
 // inlining which doesn’t happen with the intrinsic.
 
+#[inline]
 pub fn udivmod_1e19(n: u128) -> (u128, u64) {
     let d = 10_000_000_000_000_000_000_u64; // 10^19