From a05a726dc5febb1a688b16221b84a21edd4de5eb Mon Sep 17 00:00:00 2001
From: bejaratommy <tommy@bejara.net>
Date: Thu, 19 Mar 2026 12:47:09 +0100
Subject: [PATCH] test(math): add missing mod function tests and improve docs

The mod function was missing test coverage in numeric_test.go, even
though it was documented in docs/math.md. Add tests covering basic
modulo, even divisors, and equality cases.

Also expand the mod documentation to show a concrete example and
clarify argument ordering, which is non-obvious in piped template
syntax.

Fixes #458

Signed-off-by: Tommy <tommy@bejarano.io>
---
 docs/math.md    |  9 ++++++++-
 numeric_test.go | 22 ++++++++++++++++++++++
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/docs/math.md b/docs/math.md
index b08d0a2f..0d19e1e0 100644
--- a/docs/math.md
+++ b/docs/math.md
@@ -24,7 +24,14 @@ Perform integer division with `div`
 
 ## mod
 
-Modulo with `mod`
+Perform modulo with `mod`. The result is the remainder of dividing the first
+argument by the second.
+
+```
+mod 10 3
+```
+
+The above will return `1`, because `10 % 3 = 1`.
 
 ## mul
 
diff --git a/numeric_test.go b/numeric_test.go
index 1252cd73..44915c7f 100644
--- a/numeric_test.go
+++ b/numeric_test.go
@@ -249,6 +249,28 @@ func TestDivf(t *testing.T) {
 	}
 }
 
+func TestMod(t *testing.T) {
+	tpl := `{{ mod 10 3 }}`
+	if err := runt(tpl, `1`); err != nil {
+		t.Error(err)
+	}
+
+	tpl = `{{ mod 10 2 }}`
+	if err := runt(tpl, `0`); err != nil {
+		t.Error(err)
+	}
+
+	tpl = `{{ mod 10 10 }}`
+	if err := runt(tpl, `0`); err != nil {
+		t.Error(err)
+	}
+
+	tpl = `{{ mod 7 3 }}`
+	if err := runt(tpl, `1`); err != nil {
+		t.Error(err)
+	}
+}
+
 func TestMul(t *testing.T) {
 	tpl := `{{ 1 | mul "2" 3 "4"}}`
 	if err := runt(tpl, `24`); err != nil {