* hash.c (group_by): New function.

* hash.h (group_by): Declared.

* eval.c (eval_init): group_by registered as group-by intrinsic.

* txr.1: Documented.

1 files changed, 29 insertions, 0 deletions

diff --git a/hash.c b/hash.c
index 3721baf4..21168c0c 100644
--- a/hash.c
+++ b/hash.c
@@ -684,6 +684,35 @@ val hash_construct(val hashv_args, val pairs)
   return hash;
 }
 
+val group_by(val func, val seq, val hashv_args)
+{
+  val hash = hashv(hashv_args);
+
+  if (vectorp(seq)) {
+    cnum i, len;
+
+    for (i = 0, len = c_num(length(seq)); i < len; i++) {
+      val v = vecref(seq, num_fast(i));
+      pushhash(hash, funcall1(func, v), v);
+    }
+  } else {
+    for (; seq; seq = cdr(seq)) {
+      val v = car(seq);
+      pushhash(hash, funcall1(func, v), v);
+    }
+  }
+
+  {
+    val iter = hash_begin(hash);
+    val cell;
+
+    while ((cell = hash_next(iter)) != nil)
+      rplacd(cell, nreverse(cdr(cell)));
+
+    return hash;
+  }
+}
+
 static val hash_keys_lazy(val iter, val lcons)
 {
   val cell = hash_next(iter);