From 76cc8e97a2cf3265b39cb6c1b444c7926871f6a0 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date: Tue, 28 Jul 2015 18:39:00 +0200
Subject: [PATCH] Add sort primop

---
 doc/manual/expressions/builtins.xml | 23 ++++++++++++++++++
 src/libexpr/primops.cc              | 37 ++++++++++++++++++++++++++++-
 tests/lang/eval-okay-sort.exp       |  1 +
 tests/lang/eval-okay-sort.nix       |  8 +++++++
 4 files changed, 68 insertions(+), 1 deletion(-)
 create mode 100644 tests/lang/eval-okay-sort.exp
 create mode 100644 tests/lang/eval-okay-sort.nix

diff --git a/doc/manual/expressions/builtins.xml b/doc/manual/expressions/builtins.xml
index 446157056..6642513f6 100644
--- a/doc/manual/expressions/builtins.xml
+++ b/doc/manual/expressions/builtins.xml
@@ -779,6 +779,29 @@ builtins.replaceStrings ["oo" "a"] ["a" "i"] "foobar"
   </varlistentry>
 
 
+  <varlistentry><term><function>builtins.sort</function>
+  <replaceable>comparator</replaceable> <replaceable>list</replaceable></term>
+
+    <listitem><para>Return <replaceable>list</replaceable> in sorted
+    order. It repeatedly calls the function
+    <replaceable>comparator</replaceable> with two elements. The
+    comparator should return <literal>true</literal> if the first
+    element is less than the second, and <literal>false</literal>
+    otherwise. For example,
+
+<programlisting>
+builtins.sort builtins.lessThan [ 483 249 526 147 42 77 ]
+</programlisting>
+
+    produces the list <literal>[ 42 77 147 249 483 526
+    ]</literal>.</para>
+
+    <para>This is a stable sort: it preserves the relative order of
+    elements deemed equal by the comparator.</para></listitem>
+
+  </varlistentry>
+
+
   <varlistentry><term><function>builtins.stringLength</function>
   <replaceable>e</replaceable></term>
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 5da2f3463..5b9ecc018 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -1364,7 +1364,6 @@ static void prim_all(EvalState & state, const Pos & pos, Value * * args, Value &
 }
 
 
-/* Apply a function to every element of a list. */
 static void prim_genList(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     state.forceFunction(*args[0], pos);
@@ -1383,6 +1382,41 @@ static void prim_genList(EvalState & state, const Pos & pos, Value * * args, Val
 }
 
 
+static void prim_lessThan(EvalState & state, const Pos & pos, Value * * args, Value & v);
+
+
+static void prim_sort(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    state.forceFunction(*args[0], pos);
+    state.forceList(*args[1], pos);
+
+    auto len = args[1]->listSize();
+    state.mkList(v, len);
+    for (unsigned int n = 0; n < len; ++n) {
+        state.forceValue(*args[1]->listElems()[n]);
+        v.listElems()[n] = args[1]->listElems()[n];
+    }
+
+
+    auto comparator = [&](Value * a, Value * b) {
+        /* Optimization: if the comparator is lessThan, bypass
+           callFunction. */
+        if (args[0]->type == tPrimOp && args[0]->primOp->fun == prim_lessThan)
+            return CompareValues()(a, b);
+
+        Value vTmp1, vTmp2;
+        state.callFunction(*args[0], *a, vTmp1, pos);
+        state.callFunction(vTmp1, *b, vTmp2, pos);
+        return state.forceBool(vTmp2);
+    };
+
+    /* FIXME: std::sort can segfault if the comparator is not a strict
+       weak ordering. What to do? std::stable_sort() seems more
+       resilient, but no guarantees... */
+    std::stable_sort(v.listElems(), v.listElems() + len, comparator);
+}
+
+
 /*************************************************************
  * Integer arithmetic
  *************************************************************/
@@ -1779,6 +1813,7 @@ void EvalState::createBaseEnv()
     addPrimOp("__any", 2, prim_any);
     addPrimOp("__all", 2, prim_all);
     addPrimOp("__genList", 2, prim_genList);
+    addPrimOp("__sort", 2, prim_sort);
 
     // Integer arithmetic
     addPrimOp("__add", 2, prim_add);
diff --git a/tests/lang/eval-okay-sort.exp b/tests/lang/eval-okay-sort.exp
new file mode 100644
index 000000000..148b93516
--- /dev/null
+++ b/tests/lang/eval-okay-sort.exp
@@ -0,0 +1 @@
+[ [ 42 77 147 249 483 526 ] [ 526 483 249 147 77 42 ] [ "bar" "fnord" "foo" "xyzzy" ] [ { key = 1; value = "foo"; } { key = 1; value = "fnord"; } { key = 2; value = "bar"; } ] ]
diff --git a/tests/lang/eval-okay-sort.nix b/tests/lang/eval-okay-sort.nix
new file mode 100644
index 000000000..8299c3a4a
--- /dev/null
+++ b/tests/lang/eval-okay-sort.nix
@@ -0,0 +1,8 @@
+with builtins;
+
+[ (sort lessThan [ 483 249 526 147 42 77 ])
+  (sort (x: y: y < x) [ 483 249 526 147 42 77 ])
+  (sort lessThan [ "foo" "bar" "xyzzy" "fnord" ])
+  (sort (x: y: x.key < y.key)
+    [ { key = 1; value = "foo"; } { key = 2; value = "bar"; } { key = 1; value = "fnord"; } ]) 
+]