Merge pull request #53 from dtolnay/sync

Reimplement pow5_factor using modular inverse

README.md

@@ -19,7 +19,7 @@ C, [https://github.com/ulfjack/ryu][upstream].
 uses nothing from the Rust standard library so is usable from no_std crates.*
 
 [paper]: https://dl.acm.org/citation.cfm?id=3192369
-[upstream]: https://github.com/ulfjack/ryu/tree/abf76d252bc97300354857e64e80d4a2bf664291
+[upstream]: https://github.com/ulfjack/ryu/tree/77e767f5e056bab96e895072fc21618ecff2f44b
 
 ```toml
 [dependencies]

src/d2s_intrinsics.rs

@@ -36,16 +36,16 @@ pub fn div100(x: u64) -> u64 {
 }
 
 #[cfg_attr(feature = "no-panic", inline)]
-fn pow5_factor(mut value: u64) -> u32 {
+pub(crate) fn pow5_factor(mut value: u64) -> u32 {
+    const M_INV_5: u64 = 14757395258967641293; // 5 * m_inv_5 = 1 (mod 2^64)
+    const N_DIV_5: u64 = 3689348814741910323; // #{ n | n = 0 (mod 2^64) } = 2^64 / 5
     let mut count = 0u32;
     loop {
         debug_assert!(value != 0);
-        let q = div5(value);
-        let r = (value as u32).wrapping_sub(5u32.wrapping_mul(q as u32));
-        if r != 0 {
+        value = value.wrapping_mul(M_INV_5);
+        if value > N_DIV_5 {
             break;
         }
-        value = q;
         count += 1;
     }
     count

tests/d2s_intrinsics_test.rs

@@ -0,0 +1,72 @@
+// Translated from C to Rust. The original C code can be found at
+// https://github.com/ulfjack/ryu and carries the following license:
+//
+// Copyright 2018 Ulf Adams
+//
+// The contents of this file may be used under the terms of the Apache License,
+// Version 2.0.
+//
+//    (See accompanying file LICENSE-Apache or copy at
+//     http://www.apache.org/licenses/LICENSE-2.0)
+//
+// Alternatively, the contents of this file may be used under the terms of
+// the Boost Software License, Version 1.0.
+//    (See accompanying file LICENSE-Boost or copy at
+//     https://www.boost.org/LICENSE_1_0.txt)
+//
+// Unless required by applicable law or agreed to in writing, this software
+// is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.
+
+#![allow(dead_code)]
+#![allow(
+    clippy::cast_lossless,
+    clippy::cast_possible_truncation,
+    clippy::unreadable_literal
+)]
+
+#[path = "../src/d2s_intrinsics.rs"]
+mod d2s_intrinsics;
+
+use d2s_intrinsics::pow5_factor;
+
+#[test]
+fn test_pow5_factor() {
+    assert_eq!(0, pow5_factor(1));
+    assert_eq!(0, pow5_factor(2));
+    assert_eq!(0, pow5_factor(3));
+    assert_eq!(0, pow5_factor(4));
+    assert_eq!(1, pow5_factor(5));
+    assert_eq!(0, pow5_factor(6));
+    assert_eq!(0, pow5_factor(7));
+    assert_eq!(0, pow5_factor(8));
+    assert_eq!(0, pow5_factor(9));
+    assert_eq!(1, pow5_factor(10));
+
+    assert_eq!(0, pow5_factor(12));
+    assert_eq!(0, pow5_factor(14));
+    assert_eq!(0, pow5_factor(16));
+    assert_eq!(0, pow5_factor(18));
+    assert_eq!(1, pow5_factor(20));
+
+    assert_eq!(2, pow5_factor(5 * 5));
+    assert_eq!(3, pow5_factor(5 * 5 * 5));
+    assert_eq!(4, pow5_factor(5 * 5 * 5 * 5));
+    assert_eq!(5, pow5_factor(5 * 5 * 5 * 5 * 5));
+    assert_eq!(6, pow5_factor(5 * 5 * 5 * 5 * 5 * 5));
+    assert_eq!(7, pow5_factor(5 * 5 * 5 * 5 * 5 * 5 * 5));
+    assert_eq!(8, pow5_factor(5 * 5 * 5 * 5 * 5 * 5 * 5 * 5));
+    assert_eq!(9, pow5_factor(5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5));
+    assert_eq!(10, pow5_factor(5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5));
+
+    assert_eq!(0, pow5_factor(42));
+    assert_eq!(1, pow5_factor(42 * 5));
+    assert_eq!(2, pow5_factor(42 * 5 * 5));
+    assert_eq!(3, pow5_factor(42 * 5 * 5 * 5));
+    assert_eq!(4, pow5_factor(42 * 5 * 5 * 5 * 5));
+    assert_eq!(5, pow5_factor(42 * 5 * 5 * 5 * 5 * 5));
+
+    assert_eq!(27, pow5_factor(7450580596923828125)); // 5^27, largest power of 5 < 2^64.
+    assert_eq!(1, pow5_factor(18446744073709551615)); // 2^64 - 1, largest multiple of 5 < 2^64.
+    assert_eq!(0, pow5_factor(18446744073709551614)); // 2^64 - 2, largest non-multiple of 5 < 2^64.
+}