std::vec: implement a stable merge sort, deferring to insertion sort for

very small runs.

This uses a lot of unsafe code for speed, otherwise we would be having
to sort by sorting lists of indices and then do a pile of swaps to put
everything in the correct place.

Fixes #9819.

Author: huonw

src/libstd/vec.rs

@@ -1921,6 +1921,150 @@ impl<T:Eq> OwnedEqVector<T> for ~[T] {
     }
 }
 
+fn merge_sort<T>(v: &mut [T], less_eq: |&T, &T| -> bool) {
+    // warning: this wildly uses unsafe.
+    static INSERTION: uint = 8;
+
+    let len = v.len();
+
+    // allocate some memory to use as scratch memory, we keep the
+    // length 0 so we can keep shallow copies of the contents of `v`
+    // without risking the dtors running on an object twice if
+    // `less_eq` fails.
+    let mut working_space = with_capacity(2 * len);
+    // these both are buffers of length `len`.
+    let mut buf_dat = working_space.as_mut_ptr();
+    let mut buf_tmp = unsafe {buf_dat.offset(len as int)};
+
+    // length `len`.
+    let buf_v = v.as_ptr();
+
+    // step 1. sort short runs with insertion sort. This takes the
+    // values from `v` and sorts them into `buf_dat`, leaving that
+    // with sorted runs of length INSERTION.
+
+    // We could hardcode the sorting comparisons here, and we could
+    // manipulate/step the pointers themselves, rather than repeatedly
+    // .offset-ing.
+    for start in range_step(0, len, INSERTION) {
+        // start <= i <= len;
+        for i in range(start, cmp::min(start + INSERTION, len)) {
+            // j satisfies: start <= j <= i;
+            let mut j = i as int;
+            unsafe {
+                // `i` is in bounds.
+                let read_ptr = buf_v.offset(i as int);
+
+                // find where to insert, we need to do strict <,
+                // rather than <=, to maintain stability.
+
+                // start <= j - 1 < len, so .offset(j - 1) is in
+                // bounds.
+                while j > start as int && !less_eq(&*buf_dat.offset(j - 1), &*read_ptr) {
+                    j -= 1;
+                }
+
+                // shift everything to the right, to make space to
+                // insert this value.
+
+                // j + 1 could be `len` (for the last `i`), but in
+                // that case, `i == j` so we don't copy. The
+                // `.offset(j)` is always in bounds.
+                ptr::copy_memory(buf_dat.offset(j + 1),
+                                 buf_dat.offset(j),
+                                 i - j as uint);
+                ptr::copy_nonoverlapping_memory(buf_dat.offset(j), read_ptr, 1);
+            }
+        }
+    }
+
+    // step 2. merge the sorted runs.
+    let mut width = INSERTION;
+    while width < len {
+        // merge the sorted runs of length `width` in `buf_dat` two at
+        // a time, placing the result in `buf_tmp`.
+
+        // 0 <= start <= len.
+        for start in range_step(0, len, 2 * width) {
+            // manipulate pointers directly for speed (rather than
+            // using a `for` loop with `range` and `.offset` inside
+            // that loop).
+            unsafe {
+                // the end of the first run & start of the
+                // second. Offset of `len` is defined, since this is
+                // precisely one byte past the end of the object.
+                let right_start = buf_dat.offset(cmp::min(start + width, len) as int);
+                // end of the second. Similar reasoning to the above re safety.
+                let right_end_idx = cmp::min(start + 2 * width, len);
+                let right_end = buf_dat.offset(right_end_idx as int);
+
+                // the pointers to the elements under consideration
+                // from the two runs.
+
+                // both of these are in bounds.
+                let mut left = buf_dat.offset(start as int);
+                let mut right = right_start;
+
+                // where we're putting the results, it is a run of
+                // length `2*width`, so we step it once for each step
+                // of either `left` or `right`.  `buf_tmp` has length
+                // `len`, so these are in bounds.
+                let mut out = buf_tmp.offset(start as int);
+                let out_end = buf_tmp.offset(right_end_idx as int);
+
+                while out < out_end {
+                    // Either the left or the right run are exhausted,
+                    // so just copy the remainder from the other run
+                    // and move on; this gives a huge speed-up (order
+                    // of 25%) for mostly sorted vectors (the best
+                    // case).
+                    if left == right_start {
+                        // the number remaining in this run.
+                        let elems = (right_end as uint - right as uint) / mem::size_of::<T>();
+                        ptr::copy_nonoverlapping_memory(out, right, elems);
+                        break;
+                    } else if right == right_end {
+                        let elems = (right_start as uint - left as uint) / mem::size_of::<T>();
+                        ptr::copy_nonoverlapping_memory(out, left, elems);
+                        break;
+                    }
+
+                    // check which side is smaller, and that's the
+                    // next element for the new run.
+
+                    // `left < right_start` and `right < right_end`,
+                    // so these are valid.
+                    let to_copy = if less_eq(&*left, &*right) {
+                        step(&mut left)
+                    } else {
+                        step(&mut right)
+                    };
+                    ptr::copy_nonoverlapping_memory(out, to_copy, 1);
+                    step(&mut out);
+                }
+            }
+        }
+
+        util::swap(&mut buf_dat, &mut buf_tmp);
+
+        width *= 2;
+    }
+
+    // write the result to `v` in one go, so that there are never two copies
+    // of the same object in `v`.
+    unsafe {
+        ptr::copy_nonoverlapping_memory(v.as_mut_ptr(), buf_dat, len);
+    }
+
+    // increment the pointer, returning the old pointer.
+    #[inline(always)]
+    unsafe fn step<T>(ptr: &mut *mut T) -> *mut T {
+        let old = *ptr;
+        *ptr = ptr.offset(1);
+        old
+    }
+}
+
 /// Extension methods for vectors such that their elements are
 /// mutable.
 pub trait MutableVector<'a, T> {
@@ -2020,6 +2164,25 @@ pub trait MutableVector<'a, T> {
     /// Reverse the order of elements in a vector, in place
     fn reverse(self);
 
+    /// Sort the vector, in place, using `less_eq` to compare `a <=
+    /// b`.
+    ///
+    /// This sort is `O(n log n)` worst-case and stable, but allocates
+    /// approximately `2 * n`, where `n` is the length of `self`.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// let mut v = [5, 4, 1, 3, 2];
+    /// v.sort(|a, b| *a <= *b);
+    /// assert_eq!(v, [1, 2, 3, 4, 5]);
+    ///
+    /// // reverse sorting
+    /// v.sort(|a, b| *b <= *a);
+    /// assert_eq!(v, [5, 4, 3, 2, 1]);
+    /// ```
+    fn sort(self, less_eq: |&T, &T| -> bool);
+
     /**
      * Consumes `src` and moves as many elements as it can into `self`
      * from the range [start,end).
@@ -2164,6 +2327,11 @@ impl<'a,T> MutableVector<'a, T> for &'a mut [T] {
         }
     }
 
+    #[inline]
+    fn sort(self, less_eq: |&T, &T| -> bool) {
+        merge_sort(self, less_eq)
+    }
+
     #[inline]
     fn move_from(self, mut src: ~[T], start: uint, end: uint) -> uint {
         for (a, b) in self.mut_iter().zip(src.mut_slice(start, end).mut_iter()) {
@@ -2692,6 +2860,7 @@ mod tests {
     use vec::*;
     use cmp::*;
     use prelude::*;
+    use rand::{Rng, task_rng};
 
     fn square(n: uint) -> uint { n * n }
 
@@ -3298,6 +3467,57 @@ mod tests {
         assert!(v3.is_empty());
     }
 
+    #[test]
+    fn test_sort() {
+        for len in range(4u, 25) {
+            for _ in range(0, 100) {
+                let mut v = task_rng().gen_vec::<uint>(len);
+                v.sort(|a,b| a <= b);
+
+                assert!(v.windows(2).all(|w| w[0] <= w[1]));
+            }
+        }
+
+        // shouldn't fail/crash
+        let mut v: [uint, .. 0] = [];
+        v.sort(|a,b| a <= b);
+
+        let mut v = [0xDEADBEEF];
+        v.sort(|a,b| a <= b);
+        assert_eq!(v, [0xDEADBEEF]);
+    }
+
+    #[test]
+    fn test_sort_stability() {
+        for len in range(4, 25) {
+            for _ in range(0 , 10) {
+                let mut counts = [0, .. 10];
+
+                // create a vector like [(6, 1), (5, 1), (6, 2), ...],
+                // where the first item of each tuple is random, but
+                // the second item represents which occurrence of that
+                // number this element is, i.e. the second elements
+                // will occur in sorted order.
+                let mut v = range(0, len).map(|_| {
+                        let n = task_rng().gen::<uint>() % 10;
+                        counts[n] += 1;
+                        (n, counts[n])
+                    }).to_owned_vec();
+
+                // only sort on the first element, so an unstable sort
+                // may mix up the counts.
+                v.sort(|&(a,_), &(b,_)| a <= b);
+
+                // this comparison includes the count (the second item
+                // of the tuple), so elements with equal first items
+                // will need to be ordered with increasing
+                // counts... i.e. exactly asserting that this sort is
+                // stable.
+                assert!(v.windows(2).all(|w| w[0] <= w[1]));
+            }
+        }
+    }
+
     #[test]
     fn test_partition() {
         assert_eq!((~[]).partition(|x: &int| *x < 3), (~[], ~[]));
@@ -4124,7 +4344,8 @@ mod bench {
     use vec::VectorVector;
     use option::*;
     use ptr;
-    use rand::{weak_rng, Rng};
+    use rand::{weak_rng, task_rng, Rng};
+    use mem;
 
     #[bench]
     fn iterator(bh: &mut BenchHarness) {
@@ -4325,4 +4546,42 @@ mod bench {
                 }
             })
     }
+
+    fn sort_random_small(bh: &mut BenchHarness) {
+        let mut rng = weak_rng();
+        bh.iter(|| {
+            let mut v: ~[f64] = rng.gen_vec(5);
+            v.sort(|a,b| *a <= *b);
+        });
+        bh.bytes = 5 * mem::size_of::<f64>() as u64;
+    }
+
+    #[bench]
+    fn sort_random_medium(bh: &mut BenchHarness) {
+        let mut rng = weak_rng();
+        bh.iter(|| {
+            let mut v: ~[f64] = rng.gen_vec(100);
+            v.sort(|a,b| *a <= *b);
+        });
+        bh.bytes = 100 * mem::size_of::<f64>() as u64;
+    }
+
+    #[bench]
+    fn sort_random_large(bh: &mut BenchHarness) {
+        let mut rng = weak_rng();
+        bh.iter(|| {
+            let mut v: ~[f64] = rng.gen_vec(10000);
+            v.sort(|a,b| *a <= *b);
+        });
+        bh.bytes = 10000 * mem::size_of::<f64>() as u64;
+    }
+
+    #[bench]
+    fn sort_sorted(bh: &mut BenchHarness) {
+        let mut v = vec::from_fn(10000, |i| i);
+        bh.iter(|| {
+            v.sort(|a,b| *a <= *b);
+        });
+        bh.bytes = (v.len() * mem::size_of_val(&v[0])) as u64;
+    }
 }

src/test/run-pass/vector-sort-failure-safe.rs

@@ -0,0 +1,88 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::rand::{task_rng, Rng};
+
+static MAX_LEN: uint = 20;
+static mut drop_counts: [uint, .. MAX_LEN] = [0, .. MAX_LEN];
+static mut clone_count: uint = 0;
+
+#[deriving(Rand, Ord)]
+struct DropCounter { x: uint, clone_num: uint }
+
+impl Clone for DropCounter {
+    fn clone(&self) -> DropCounter {
+        let num = unsafe { clone_count };
+        unsafe { clone_count += 1; }
+        DropCounter {
+            x: self.x,
+            clone_num: num
+        }
+    }
+}
+
+impl Drop for DropCounter {
+    fn drop(&mut self) {
+        unsafe {
+            // Rand creates some with arbitrary clone_nums
+            if self.clone_num < MAX_LEN {
+                drop_counts[self.clone_num] += 1;
+            }
+        }
+    }
+}
+
+pub fn main() {
+    // len can't go above 64.
+    for len in range(2u, MAX_LEN) {
+        for _ in range(0, 10) {
+            let main = task_rng().gen_vec::<DropCounter>(len);
+
+            // work out the total number of comparisons required to sort
+            // this array...
+            let mut count = 0;
+            main.clone().sort(|a, b| { count += 1; a <= b });
+
+            // ... and then fail on each and every single one.
+            for fail_countdown in range(0, count) {
+                // refresh the counters.
+                unsafe {
+                    drop_counts = [0, .. MAX_LEN];
+                    clone_count = 0;
+                }
+
+                let v = main.clone();
+
+                std::task::try(proc() {
+                        let mut v = v;
+                        let mut fail_countdown = fail_countdown;
+                        v.sort(|a, b| {
+                                if fail_countdown == 0 {
+                                    fail!()
+                                }
+                                fail_countdown -= 1;
+                                a <= b
+                            })
+                    });
+
+                // check that the number of things dropped is exactly
+                // what we expect (i.e. the contents of `v`).
+                unsafe {
+                    for (i, &c) in drop_counts.iter().enumerate() {
+                        let expected = if i < len {1} else {0};
+                        assert!(c == expected,
+                                "found drop count == {} for i == {}, len == {}",
+                                c, i, len);
+                    }
+                }
+            }
+        }
+    }
+}