From 0757a921ab7db0f15ea703b5f5b185fa5a035ba6 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Wed, 13 Feb 2019 00:51:53 -0800
Subject: hash-symdiff: new function.

* hash.c (hash_symdiff): New function.
(hash_init): hash-symdiff intrinsic registered.

* hash.h (hash_symdiff): Declared.

* txr.1: Documented.
---
 hash.c | 35 +++++++++++++++++++++++++++++++++++
 hash.h |  1 +
 txr.1  | 22 +++++++++++++++++++++-
 3 files changed, 57 insertions(+), 1 deletion(-)

diff --git a/hash.c b/hash.c
index 4242fd58..272e1e2f 100644
--- a/hash.c
+++ b/hash.c
@@ -1418,6 +1418,40 @@ val hash_diff(val hash1, val hash2)
   }
 }
 
+val hash_symdiff(val hash1, val hash2)
+{
+  val self = lit("hash-symdiff");
+  struct hash *h1 = coerce(struct hash *, cobj_handle(self, hash1, hash_s));
+  struct hash *h2 = coerce(struct hash *, cobj_handle(self, hash2, hash_s));
+
+  if (h1->hops != h2->hops)
+    uw_throwf(error_s, lit("~a: ~s and ~a are incompatible hashes"),
+              self, hash1, hash2, nao);
+
+  {
+    val hout = make_similar_hash(hash1);
+    val hiter, entry;
+
+    for (hiter = hash_begin(hash1), entry = hash_next(hiter);
+         entry;
+         entry = hash_next(hiter))
+    {
+      if (!gethash_e(self, hash2, car(entry)))
+        sethash(hout, car(entry), cdr(entry));
+    }
+
+    for (hiter = hash_begin(hash2), entry = hash_next(hiter);
+         entry;
+         entry = hash_next(hiter))
+    {
+      if (!gethash_e(self, hash1, car(entry)))
+        sethash(hout, car(entry), cdr(entry));
+    }
+
+    return hout;
+  }
+}
+
 val hash_isec(val hash1, val hash2, val join_func)
 {
   val self = lit("hash-isec");
@@ -1586,6 +1620,7 @@ void hash_init(void)
   reg_fun(intern(lit("hash-alist"), user_package), func_n1(hash_alist));
   reg_fun(intern(lit("hash-uni"), user_package), func_n3o(hash_uni, 2));
   reg_fun(intern(lit("hash-diff"), user_package), func_n2(hash_diff));
+  reg_fun(intern(lit("hash-symdiff"), user_package), func_n2(hash_symdiff));
   reg_fun(intern(lit("hash-isec"), user_package), func_n3o(hash_isec, 2));
   reg_fun(intern(lit("hash-subset"), user_package), func_n2(hash_subset));
   reg_fun(intern(lit("hash-proper-subset"), user_package), func_n2(hash_proper_subset));
diff --git a/hash.h b/hash.h
index df2ca0b3..6b65daf0 100644
--- a/hash.h
+++ b/hash.h
@@ -65,6 +65,7 @@ val hash_pairs(val hash);
 val hash_alist(val hash);
 val hash_uni(val hash1, val hash2, val join_func);
 val hash_diff(val hash1, val hash2);
+val hash_symdiff(val hash1, val hash2);
 val hash_isec(val hash1, val hash2, val join_func);
 val hash_subset(val hash1, val hash2);
 val hash_proper_subset(val hash1, val hash2);
diff --git a/txr.1 b/txr.1
index f4242042..7118b588 100644
--- a/txr.1
+++ b/txr.1
@@ -41549,10 +41549,11 @@ dohash early using
 or
 .codn (return) .
 
-.coNP Functions @, hash-uni @ hash-diff and @ hash-isec
+.coNP Functions @, hash-uni @, hash-diff @ hash-symdiff and @ hash-isec
 .synb
 .mets (hash-uni < hash1 < hash2 <> [ join-func ])
 .mets (hash-diff < hash1 << hash2 )
+.mets (hash-symdiff < hash1 << hash2 )
 .mets (hash-isec < hash1 < hash2 <> [ join-func ])
 .syne
 .desc
@@ -41615,6 +41616,25 @@ in
 .code hash2
 are deleted.
 
+The
+.code hash-symdiff
+function performs a symmetric difference. A new hash is returned which
+contains all of the keys from
+.meta hash1
+that are not in
+.meta hash2
+and
+.IR "vice versa" :
+all of the keys from
+.meta hash2
+that are not in
+.metn hash1 .
+The keys carry their corresponding values from
+.meta hash1
+and
+.metn hash2 ,
+respectively.
+
 The
 .code hash-isec
 function performs a set intersection. The resulting hash contains
-- 
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