From 2d6ebedb0f2217d81d26c8672d81d3d26a587e7a Mon Sep 17 00:00:00 2001
From: Richard Hillmann <Project0@users.noreply.github.com>
Date: Thu, 16 Jun 2022 17:36:08 +0200
Subject: [PATCH] feat: Replace data fillcontent with custom reader

---
 .github/workflows/main.yml |  2 +-
 .golangci.yml              |  1 +
 app.go                     | 19 +--------
 content.go                 | 66 ++++++++++++++++++++++++++++++
 content_test.go            | 82 ++++++++++++++++++++++++++++++++++++++
 5 files changed, 151 insertions(+), 19 deletions(-)
 create mode 100644 content.go
 create mode 100644 content_test.go

diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index e6afbeb..61e3f4c 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     env:
       GO_VERSION: 1.17
-      GOLANGCI_LINT_VERSION: v1.44.0
+      GOLANGCI_LINT_VERSION: v1.46.2
       CGO_ENABLED: 0
 
     steps:
diff --git a/.golangci.yml b/.golangci.yml
index 86523a5..1713c7b 100644
--- a/.golangci.yml
+++ b/.golangci.yml
@@ -39,6 +39,7 @@ linters:
     - varnamelen
     - nilnil
     - ifshort
+    - exhaustruct
 
 issues:
   exclude-use-default: false
diff --git a/app.go b/app.go
index 36665c3..965c213 100644
--- a/app.go
+++ b/app.go
@@ -1,7 +1,6 @@
 package main
 
 import (
-	"bytes"
 	"crypto/tls"
 	"crypto/x509"
 	"encoding/json"
@@ -181,7 +180,7 @@ func dataHandler(w http.ResponseWriter, r *http.Request) {
 		attachment = false
 	}
 
-	content := fillContent(size)
+	content := &contentReader{size: size}
 
 	if attachment {
 		w.Header().Set("Content-Disposition", "Attachment")
@@ -311,22 +310,6 @@ func healthHandler(w http.ResponseWriter, req *http.Request) {
 	}
 }
 
-func fillContent(length int64) io.ReadSeeker {
-	charset := "-ABCDEFGHIJKLMNOPQRSTUVWXYZ"
-	b := make([]byte, length)
-
-	for i := range b {
-		b[i] = charset[i%len(charset)]
-	}
-
-	if length > 0 {
-		b[0] = '|'
-		b[length-1] = '|'
-	}
-
-	return bytes.NewReader(b)
-}
-
 func getEnv(key, fallback string) string {
 	value := os.Getenv(key)
 	if len(value) == 0 {
diff --git a/content.go b/content.go
new file mode 100644
index 0000000..fce810e
--- /dev/null
+++ b/content.go
@@ -0,0 +1,66 @@
+package main
+
+import (
+	"errors"
+	"io"
+)
+
+const contentCharset = "-ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+
+type contentReader struct {
+	size int64
+	cur  int64
+}
+
+// Read implements the io.Read interface.
+func (r *contentReader) Read(p []byte) (int, error) {
+	length := r.size - 1
+
+	if r.cur >= length {
+		return 0, io.EOF
+	}
+	if len(p) == 0 {
+		return 0, nil
+	}
+
+	var n int
+	if r.cur == 0 {
+		p[n] = '|'
+		r.cur++
+		n++
+	}
+
+	for n < len(p) && r.cur <= length {
+		p[n] = contentCharset[int(r.cur)%len(contentCharset)]
+		r.cur++
+		n++
+	}
+
+	if r.cur >= length {
+		p[n-1] = '|'
+	}
+
+	return n, nil
+}
+
+// Seek implements the io.Seek interface.
+func (r *contentReader) Seek(offset int64, whence int) (int64, error) {
+	switch whence {
+	default:
+		return 0, errors.New("seek: invalid whence")
+	case io.SeekStart:
+		offset += 0
+	case io.SeekCurrent:
+		offset += r.cur
+	case io.SeekEnd:
+		offset += r.size - 1
+	}
+
+	if offset < 0 {
+		return 0, errors.New("seek: invalid offset")
+	}
+
+	r.cur = offset
+
+	return offset, nil
+}
diff --git a/content_test.go b/content_test.go
new file mode 100644
index 0000000..29eff84
--- /dev/null
+++ b/content_test.go
@@ -0,0 +1,82 @@
+package main
+
+import (
+	"io"
+	"testing"
+)
+
+func Test_contentReader_Read(t *testing.T) {
+	tests := []struct {
+		name    string
+		size    int64
+		content string
+	}{
+		{
+			name:    "simple",
+			size:    40,
+			content: "|ABCDEFGHIJKLMNOPQRSTUVWXYZ-ABCDEFGHIJK|",
+		},
+	}
+
+	for _, test := range tests {
+		test := test
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+
+			d := &contentReader{size: test.size}
+
+			b, err := io.ReadAll(d)
+			if err != nil {
+				t.Errorf("contentReader.Read() error = %v", err)
+				return
+			}
+
+			if string(b) != test.content {
+				t.Errorf("return content does not match expected value: got %s want %s", b, test.content)
+			}
+		})
+	}
+}
+
+func Test_contentReader_ReadSeek(t *testing.T) {
+	tests := []struct {
+		name       string
+		offset     int64
+		seekWhence int
+		size       int64
+		content    string
+	}{
+		{
+			name:       "simple",
+			offset:     10,
+			seekWhence: io.SeekCurrent,
+			size:       40,
+			content:    "JKLMNOPQRSTUVWXYZ-ABCDEFGHIJK|",
+		},
+	}
+
+	for _, test := range tests {
+		test := test
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+
+			d := &contentReader{size: test.size}
+
+			_, err := d.Seek(test.offset, test.seekWhence)
+			if err != nil {
+				t.Errorf("contentReader.Seek() error = %v", err)
+				return
+			}
+
+			b, err := io.ReadAll(d)
+			if err != nil {
+				t.Errorf("contentReader.Read() error = %v", err)
+				return
+			}
+
+			if string(b) != test.content {
+				t.Errorf("return content does not match expected value: got %s want %s", b, test.content)
+			}
+		})
+	}
+}