From de6fd1b1837a1a4bdfda33cf28e4c0df0976a39f Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Fri, 8 May 2026 12:28:31 +0000
Subject: [PATCH] =?UTF-8?q?mk:=20counto=20=E2=80=94=20count=20occurrences?=
 =?UTF-8?q?=20of=20x=20in=20l=20(intarith)?=
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Walks the list with a recursive count. On a head match, recurse and
add 1 via pluso-i; on no match (nafc), recurse forwarding the count.
Empty list yields 0.

6 new tests, 532/532 cumulative.
---
 lib/minikanren/intarith.sx     |  8 ++++++++
 lib/minikanren/tests/counto.sx | 35 ++++++++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+)
 create mode 100644 lib/minikanren/tests/counto.sx

diff --git a/lib/minikanren/intarith.sx b/lib/minikanren/intarith.sx
index 85b2cb69..51b2ee1e 100644
--- a/lib/minikanren/intarith.sx
+++ b/lib/minikanren/intarith.sx
@@ -127,3 +127,11 @@
       ((fresh (a d r-rest start-prime) (conso a d l) (conso (list start a) r-rest result) (pluso-i 1 start start-prime) (enumerate-from-i start-prime d r-rest))))))
 
 (define enumerate-i (fn (l result) (enumerate-from-i 0 l result)))
+
+(define
+  counto
+  (fn
+    (x l n)
+    (conde
+      ((nullo l) (== n 0))
+      ((fresh (a d n-rest) (conso a d l) (conde ((== a x) (counto x d n-rest) (pluso-i 1 n-rest n)) ((nafc (== a x)) (counto x d n))))))))
diff --git a/lib/minikanren/tests/counto.sx b/lib/minikanren/tests/counto.sx
new file mode 100644
index 00000000..0c9248fc
--- /dev/null
+++ b/lib/minikanren/tests/counto.sx
@@ -0,0 +1,35 @@
+;; lib/minikanren/tests/counto.sx — count occurrences of x in l (intarith).
+
+(mk-test
+  "counto-empty"
+  (run* q (counto 1 (list) q))
+  (list 0))
+(mk-test
+  "counto-not-found"
+  (run* q (counto 99 (list 1 2 3) q))
+  (list 0))
+(mk-test
+  "counto-once"
+  (run* q (counto 2 (list 1 2 3) q))
+  (list 1))
+(mk-test
+  "counto-thrice"
+  (run*
+    q
+    (counto
+      1
+      (list 1 2 1 3 1)
+      q))
+  (list 3))
+(mk-test
+  "counto-all-same"
+  (run*
+    q
+    (counto 7 (list 7 7 7 7) q))
+  (list 4))
+(mk-test
+  "counto-string"
+  (run* q (counto "x" (list "x" "y" "x") q))
+  (list 2))
+
+(mk-tests-run!)