From 9a2f9df335393284f6af2d95dbd65cb606792ad8 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Wed, 20 Jan 2021 01:03:51 -0800
Subject: New function: hash-keys-of.

* hash.c (hash_keys_of): New function.
(hash_init): Register hash-keys-of intrinsic

* hash.h (hash_keys_of): Declared.

* txr.1: Documented.
---
 hash.c | 21 +++++++++++++++++++++
 hash.h |  1 +
 txr.1  | 11 ++++++++++-
 3 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/hash.c b/hash.c
index a6772c29..8040fa68 100644
--- a/hash.c
+++ b/hash.c
@@ -1808,6 +1808,26 @@ val hash_revget(val hash, val value, val test, val keyfun)
   return nil;
 }
 
+val hash_keys_of(val hash, val value, val test, val keyfun)
+{
+  val self = lit("hash-keys-of");
+  val cell;
+  struct hash_iter hi;
+  list_collect_decl(out, ptail);
+
+  hash_iter_init(&hi, hash, self);
+
+  test = default_arg(test, eql_f);
+  keyfun = default_arg(keyfun, identity_f);
+
+  while ((cell = hash_iter_next(&hi)) != nil) {
+    if (funcall2(test, value, funcall1(keyfun, us_cdr(cell))))
+      ptail = list_collect(ptail, us_car(cell));
+  }
+
+  return out;
+}
+
 val hash_invert(val hash, val joinfun, val unitfun, struct args *hashv_args)
 {
   val self = lit("hash-invert");
@@ -1916,6 +1936,7 @@ void hash_init(void)
   reg_fun(intern(lit("hash-update-1"), user_package),
           func_n4o(hash_update_1, 3));
   reg_fun(intern(lit("hash-revget"), user_package), func_n4o(hash_revget, 2));
+  reg_fun(intern(lit("hash-keys-of"), user_package), func_n4o(hash_keys_of, 2));
   reg_fun(intern(lit("hash-invert"), user_package), func_n3ov(hash_invert, 1));
   reg_fun(intern(lit("hash-begin"), user_package), func_n1(hash_begin));
   reg_fun(intern(lit("hash-next"), user_package), func_n1(hash_next));
diff --git a/hash.h b/hash.h
index f9fcb75b..542b3e1a 100644
--- a/hash.h
+++ b/hash.h
@@ -89,6 +89,7 @@ val hash_proper_subset(val hash1, val hash2);
 val hash_update(val hash, val fun);
 val hash_update_1(val hash, val key, val fun, val init);
 val hash_revget(val hash, val value, val test, val keyfun);
+val hash_keys_of(val hash, val value, val test, val keyfun);
 val hash_invert(val hash, val joinfun, val unitfun, struct args *hashv_args);
 
 void hash_process_weak(void);
diff --git a/txr.1 b/txr.1
index 54550b31..bf1233e2 100644
--- a/txr.1
+++ b/txr.1
@@ -48244,9 +48244,10 @@ to
 The function returns
 .codn nil .
 
-.coNP Function @ hash-revget
+.coNP Functions @ hash-revget and @ hash-keys-of
 .synb
 .mets (hash-revget < hash < value >> [ testfun <> [ keyfun ]])
+.mets (hash-keys-of < hash < value >> [ testfun <> [ keyfun ]])
 .syne
 .desc
 The
@@ -48267,6 +48268,14 @@ is returned.
 If multiple matching entries exist, it is not specified which entry's
 key is returned.
 
+The
+.code hash-keys-of
+function has exactly the same argument conventions, and likewise
+searches the
+.metn hash .
+However, it returns a list of all keys whose values match
+.metn value .
+
 The
 .meta keyfun
 function is applied to each value in
-- 
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