From 6c1f9200881b859bcc6d7d5b0d9d37a5cc65eb60 Mon Sep 17 00:00:00 2001
From: Sam Anthony <sam@samanthony.xyz>
Date: Sat, 11 Jul 2026 13:09:00 -0230
Subject: encode

---
 go.mod          | 11 +++++++++
 go.sum          | 10 ++++++++
 htmlimg.go      | 23 ++++++++++++++++++
 htmlimg_test.go | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 117 insertions(+)
 create mode 100644 go.mod
 create mode 100644 go.sum
 create mode 100644 htmlimg.go
 create mode 100644 htmlimg_test.go

diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..9137d60
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,11 @@
+module git.samanthony.xyz/htmlimg
+
+go 1.23
+
+require github.com/stretchr/testify v1.11.1
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..c4c1710
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,10 @@
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu7U=
+github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
diff --git a/htmlimg.go b/htmlimg.go
new file mode 100644
index 0000000..332bc69
--- /dev/null
+++ b/htmlimg.go
@@ -0,0 +1,23 @@
+// Package htmlimg produces base64-encoded HTML <img> tags.
+//
+//	<img src="data:image/png;base64,iVBORw...">
+package htmlimg
+
+import (
+	"bytes"
+	"encoding/base64"
+	"html/template"
+	"image"
+	"image/png"
+)
+
+// Encode PNG- and then base64-encodes img and returns it embedded in an
+// HTML <img> tag.
+func Encode(m image.Image) (template.HTML, error) {
+	pngBuf := new(bytes.Buffer)
+	if err := png.Encode(pngBuf, m); err != nil {
+		return "", err
+	}
+	b64 := base64.StdEncoding.EncodeToString(pngBuf.Bytes())
+	return template.HTML(`<img src="data:image/png;base64,` + b64 + `">`), nil
+}
diff --git a/htmlimg_test.go b/htmlimg_test.go
new file mode 100644
index 0000000..a3fa857
--- /dev/null
+++ b/htmlimg_test.go
@@ -0,0 +1,73 @@
+package htmlimg_test
+
+import (
+	"bytes"
+	"encoding/base64"
+	"image"
+	"image/color"
+	"image/png"
+	"math/rand/v2"
+	"regexp"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"git.samanthony.xyz/htmlimg"
+)
+
+var tagExpr = regexp.MustCompile(`^<img src="data:image/png;base64,([A-Za-z0-9+/=]+)">$`)
+
+func TestOutputMatchRegexp(t *testing.T) {
+	t.Parallel()
+	img := newImg(16)
+	tag, err := htmlimg.Encode(img)
+	require.NoError(t, err)
+	require.Regexp(t, tagExpr, tag)
+}
+
+func TestDecodeImg(t *testing.T) {
+	t.Parallel()
+
+	img := newImg(16)
+
+	// Encode <img> tag
+	tag, err := htmlimg.Encode(img)
+	require.NoError(t, err)
+	t.Log(tag)
+
+	// Extract and decode base64 data from tag to get png data
+	groups := tagExpr.FindStringSubmatch(string(tag))
+	require.Len(t, groups, 2)
+	b64 := groups[1]
+	decPng, err := base64.StdEncoding.DecodeString(b64)
+	require.NoError(t, err)
+
+	// Decoded png should match original image's png
+	encPng := new(bytes.Buffer)
+	err = png.Encode(encPng, img)
+	require.NoError(t, err)
+	require.Equal(t, encPng.Bytes(), decPng)
+}
+
+func BenchmarkEncode(b *testing.B) {
+	img := newImg(1024)
+	for b.Loop() {
+		htmlimg.Encode(img)
+	}
+}
+
+func newImg(size int) image.Image {
+	w, h := size, size
+	img := image.NewNRGBA(image.Rect(0, 0, w, h))
+	for y := 0; y < h; y++ {
+		for x := 0; x < w; x++ {
+			img.Set(x, y, color.NRGBA{
+				R: uint8(rand.IntN(256)),
+				G: uint8(rand.IntN(256)),
+				B: uint8(rand.IntN(256)),
+				A: uint8(rand.IntN(256)),
+			})
+		}
+	}
+	return img
+}
-- 
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