dag: implement binary_search_by for VecDeque

Summary:
This makes it easier to replace Vec<Span> with VecDeque<Span> in SpanSet for
efficient push_front and deprecates SpanSetAsc (which uses Id in a bit hacky
way - they are not real Ids).

Reviewed By: sfilipco

Differential Revision: D23385245

fbshipit-source-id: b612cd816223a301e2705084057bd24865beccf0

eden/scm/lib/dag/src/bsearch.rs

@@ -0,0 +1,65 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This software may be used and distributed according to the terms of the
+ * GNU General Public License version 2.
+ */
+
+use std::cmp::Ordering;
+use std::collections::VecDeque;
+
+use Ordering::Equal;
+use Ordering::Greater;
+use Ordering::Less;
+
+// Based on rust stdlib slice::binary_search_by:
+// https://github.com/rust-lang/rust/blob/dbad8c93680710e80c54cbaf8416821f7a5750c8/library/core/src/slice/mod.rs#L1721
+
+pub trait BinarySearchBy<T> {
+    /// Binary searches this sorted slice with a comparator function.
+    ///
+    /// The comparator function should implement an order consistent
+    /// with the sort order of the underlying slice, returning an
+    /// order code that indicates whether its argument is `Less`,
+    /// `Equal` or `Greater` the desired target.
+    ///
+    /// If the value is found then [`Result::Ok`] is returned, containing the
+    /// index of the matching element. If there are multiple matches, then any
+    /// one of the matches could be returned. If the value is not found then
+    /// [`Result::Err`] is returned, containing the index where a matching
+    /// element could be inserted while maintaining sorted order.
+    fn binary_search_by<F>(&self, f: F) -> Result<usize, usize>
+    where
+        F: FnMut(&T) -> Ordering;
+}
+
+impl<T> BinarySearchBy<T> for VecDeque<T> {
+    fn binary_search_by<F>(&self, mut f: F) -> Result<usize, usize>
+    where
+        F: FnMut(&T) -> Ordering,
+    {
+        let s = self;
+        let mut size = s.len();
+        if size == 0 {
+            return Err(0);
+        }
+        let mut base = 0usize;
+        while size > 1 {
+            let half = size / 2;
+            let mid = base + half;
+            // mid is always in [0, size), that means mid is >= 0 and < size.
+            // mid >= 0: by definition
+            // mid < size: mid = size / 2 + size / 4 + size / 8 ...
+            let cmp = f(s.get(mid).unwrap());
+            base = if cmp == Greater { base } else { mid };
+            size -= half;
+        }
+        // base is always in [0, size) because base <= mid.
+        let cmp = f(s.get(base).unwrap());
+        if cmp == Equal {
+            Ok(base)
+        } else {
+            Err(base + (cmp == Less) as usize)
+        }
+    }
+}

eden/scm/lib/dag/src/lib.rs

@@ -11,6 +11,7 @@
 //!
 //! Building blocks for the commit graph used by source control.
 
+mod bsearch;
 mod default_impl;
 mod delegate;
 pub mod errors;