From 0a6ac81276b2380685020fcb660712cd2b693b7f Mon Sep 17 00:00:00 2001
From: Simon Cruanes <simon.cruanes.2007@m4x.org>
Date: Tue, 31 Aug 2021 16:33:47 -0400
Subject: [PATCH] wip: slice sort

---
 src/core/CCSort.ml | 192 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 192 insertions(+)
 create mode 100644 src/core/CCSort.ml

diff --git a/src/core/CCSort.ml b/src/core/CCSort.ml
new file mode 100644
index 00000000..cf8f4410
--- /dev/null
+++ b/src/core/CCSort.ml
@@ -0,0 +1,192 @@
+
+(** Reusable sort algorithm *)
+
+(** Array-like structure.
+
+    We only require constant-time indexing inside the array. *)
+module type ARRAY = sig
+  type elt
+  type t
+
+  val compare : elt -> elt -> int
+
+  val length : t -> int
+
+  val get : t -> int -> elt
+
+  val set : t -> int -> elt -> unit
+
+  val swap : t -> int -> int -> unit
+end
+
+type 'a compare = ('a -> 'a -> int)
+(** Type for a comparison function *)
+
+module type S = sig
+  module A : ARRAY
+
+  val sort_slice : A.t -> i:int -> len:int -> unit
+
+  val sort : A.t -> unit
+
+  val sort_uniq_slice : A.t -> i:int -> len:int -> int
+
+  val sort_uniq : A.t -> int
+end
+
+module Make(A : ARRAY)
+  : S with module A = A
+= struct
+  module A = A
+  open A
+
+  let rec insert_ a i k =
+    if k<i then ()
+    else if compare (A.get a k) (A.get a (k+1)) > 0 then (
+      swap a k (k+1);
+      insert_ a i (k-1)
+    )
+
+  let insertion_sort_ a ~i:first ~j:last =
+    if first+1<last then (
+      for i=start+1 to last-1 do
+        let j= ref i in
+        (* insert a[i] into slice [start… i-1] *)
+        while !j > start && A.compare (get a (!j - 1)) (get a !j) > 0 do
+          swap a (!j - 1) !j;
+          decr j;
+        done;
+      done
+    )
+
+  (* sort between [i] and [j], both included *)
+  let rec sort_slice_ a ~i ~j : unit =
+    if j - i < 2 then ()
+    else if j - i < 16 then (
+      (* base case *)
+      insertion_sort_ a ~i ~j
+    ) else (
+      (* quicksort *)
+
+    )
+
+  let sort_slice a ~i ~len =
+    if len > 1 then sort_slice_ a ~i ~j:(i+len-1)
+
+  let[@inline] sort a = sort_slice a ~i:0 ~len:(A.length a)
+
+  (* assume [a[i:i+len]] is sorted, return new length *)
+  let uniq_ a ~i:i0 ~len : int =
+    let cur = ref i0 in
+    let i = ref i0 in
+    while !i < i0 + len do
+      let x = get a !i in
+      if A.compare x (get a !cur) = 0 then (
+        incr i; (* dup, skip it *)
+      ) else (
+        (* move it to [!cur] *)
+        if !i <> !cur then set a !cur x;
+        incr i;
+        incr cur;
+      );
+    done;
+    !cur - i0
+
+  let sort_uniq_slice a ~i ~len : int =
+    sort_slice a ~i ~len;
+    uniq_ a ~i ~len
+
+  let[@inline] sort_uniq a = sort_uniq_slice a ~i:0 ~len:(A.length a)
+end[@inline]
+
+(*
+(* Dual Pivot Quicksort (Yaroslavskiy)
+   from "average case analysis of Java 7's Dual Pivot Quicksort" *)
+module SortGeneric(A : MONO_ARRAY) = struct
+  module Rand = Random.State
+
+  let seed_ = [|123456|]
+
+  type state = {
+    mutable l: int; (* left pointer *)
+    mutable g: int; (* right pointer *)
+    mutable k: int;
+  }
+
+  let rand_idx_ rand i j = i + Rand.int rand (j-i)
+
+  let swap_ a i j =
+    if i=j then ()
+    else (
+      let tmp = A.get a i in
+      A.set a i (A.get a j);
+      A.set a j tmp
+    )
+
+  let sort ~cmp a =
+    let rec insert_ a i k =
+      if k<i then ()
+      else if cmp (A.get a k) (A.get a (k+1)) > 0 then (
+        swap_ a k (k+1);
+        insert_ a i (k-1)
+      )
+    in
+    (* recursive part of insertion sort *)
+    let rec sort_insertion_rec a i j k =
+      if k<j then (
+        insert_ a i (k-1);
+        sort_insertion_rec a i j (k+1)
+      )
+    in
+    (* insertion sort, for small slices *)
+    let sort_insertion a i j =
+      if j-i > 1 then sort_insertion_rec a i j (i+1)
+    in
+    let rand = Rand.make seed_ in
+    (* sort slice.
+       There is a chance that the two pivots are equal, but it's unlikely. *)
+    let rec sort_slice_ ~st a i j =
+      if j-i>10 then (
+        st.l <- i;
+        st.g <- j-1;
+        st.k <- i;
+        (* choose pivots *)
+        let p = A.get a (rand_idx_ rand i j) in
+        let q = A.get a (rand_idx_ rand i j) in
+        (* invariant: st.p <= st.q, swap them otherwise *)
+        let p, q = if cmp p q > 0 then q, p else p, q in
+        while st.k <= st.g do
+          let cur = A.get a st.k in
+          if cmp cur p < 0 then (
+            (* insert in leftmost band *)
+            if st.k <> st.l then swap_ a st.k st.l;
+            st.l <- st.l + 1
+          ) else if cmp cur q > 0 then (
+            (* insert in rightmost band *)
+            while st.k < st.g && cmp (A.get a st.g) q > 0 do
+              st.g <- st.g - 1
+            done;
+            swap_ a st.k st.g;
+            st.g <- st.g - 1;
+            (* the element swapped from the right might be in the first situation.
+               that is, < p  (we know it's <= q already) *)
+            if cmp (A.get a st.k) p < 0 then (
+              if st.k <> st.l then swap_ a st.k st.l;
+              st.l <- st.l + 1
+            )
+          );
+          st.k <- st.k + 1
+        done;
+        (* save values before recursing *)
+        let l = st.l and g = st.g and sort_middle = cmp p q < 0 in
+        sort_slice_ ~st a i l;
+        if sort_middle then sort_slice_ ~st a l (g+1);
+        sort_slice_ ~st a (g+1) j;
+      ) else sort_insertion a i j
+    in
+    if A.length a > 0 then (
+      let st = { l=0; g=A.length a; k=0; } in
+      sort_slice_ ~st a 0 (A.length a)
+    )
+end
+   *)