refactor: extract imageprocessor into its own package

Move ImageProcessor, Params, New(), DefaultMaxInputBytes, ErrInputDataTooLarge,
and related types from internal/imgcache/ into a new standalone package
internal/imageprocessor/.

The imageprocessor package defines its own Format, FitMode, Size, Request, and
Result types, making it fully independent with no imports from imgcache. The
imgcache service converts between its own types and imageprocessor types at the
boundary.

Changes:
- New package: internal/imageprocessor/ with imageprocessor.go and tests
- Removed: processor.go and processor_test.go from internal/imgcache/
- Removed: Processor interface and ProcessResult from imgcache.go (now unused)
- Updated: service.go uses *imageprocessor.ImageProcessor directly
- Copied: testdata/red.avif for AVIF decode test

Addresses review feedback on PR #37: image processing is a distinct concern
from the HTTP service layer and belongs in its own package.

internal/imageprocessor/imageprocessor.go

@@ -1,4 +1,5 @@
-package imgcache
+// Package imageprocessor provides image format conversion and resizing using libvips.
+package imageprocessor
 
 import (
 	"bytes"
@@ -22,6 +23,68 @@ func initVips() {
 	})
 }
 
+// Format represents supported output image formats.
+type Format string
+
+// Supported image output formats.
+const (
+	FormatOriginal Format = "orig"
+	FormatJPEG     Format = "jpeg"
+	FormatPNG      Format = "png"
+	FormatWebP     Format = "webp"
+	FormatAVIF     Format = "avif"
+	FormatGIF      Format = "gif"
+)
+
+// FitMode represents how to fit an image into requested dimensions.
+type FitMode string
+
+// Supported image fit modes.
+const (
+	FitCover   FitMode = "cover"
+	FitContain FitMode = "contain"
+	FitFill    FitMode = "fill"
+	FitInside  FitMode = "inside"
+	FitOutside FitMode = "outside"
+)
+
+// ErrInvalidFitMode is returned when an invalid fit mode is provided.
+var ErrInvalidFitMode = errors.New("invalid fit mode")
+
+// Size represents requested image dimensions.
+type Size struct {
+	Width  int
+	Height int
+}
+
+// Request holds the parameters for image processing.
+type Request struct {
+	Size    Size
+	Format  Format
+	Quality int
+	FitMode FitMode
+}
+
+// Result contains the output of image processing.
+type Result struct {
+	// Content is the processed image data.
+	Content io.ReadCloser
+	// ContentLength is the size in bytes.
+	ContentLength int64
+	// ContentType is the MIME type of the output.
+	ContentType string
+	// Width is the output image width.
+	Width int
+	// Height is the output image height.
+	Height int
+	// InputWidth is the original image width before processing.
+	InputWidth int
+	// InputHeight is the original image height before processing.
+	InputHeight int
+	// InputFormat is the detected input format (e.g., "jpeg", "png").
+	InputFormat string
+}
+
 // MaxInputDimension is the maximum allowed width or height for input images.
 // Images larger than this are rejected to prevent DoS via decompression bombs.
 const MaxInputDimension = 8192
@@ -39,7 +102,7 @@ var ErrInputDataTooLarge = errors.New("input data exceeds maximum allowed size")
 // ErrUnsupportedOutputFormat is returned when the requested output format is not supported.
 var ErrUnsupportedOutputFormat = errors.New("unsupported output format")
 
-// ImageProcessor implements the Processor interface using libvips via govips.
+// ImageProcessor implements image transformation using libvips via govips.
 type ImageProcessor struct {
 	maxInputBytes int64
 }
@@ -71,8 +134,8 @@ func New(params Params) *ImageProcessor {
 func (p *ImageProcessor) Process(
 	_ context.Context,
 	input io.Reader,
-	req *ImageRequest,
-) (*ProcessResult, error) {
+	req *Request,
+) (*Result, error) {
 	// Read input with a size limit to prevent unbounded memory consumption.
 	// We read at most maxInputBytes+1 so we can detect if the input exceeds
 	// the limit without consuming additional memory.
@@ -142,10 +205,10 @@ func (p *ImageProcessor) Process(
 		return nil, fmt.Errorf("failed to encode: %w", err)
 	}
 
-	return &ProcessResult{
+	return &Result{
 		Content:       io.NopCloser(bytes.NewReader(output)),
 		ContentLength: int64(len(output)),
-		ContentType:   ImageFormatToMIME(outputFormat),
+		ContentType:   FormatToMIME(outputFormat),
 		Width:         img.Width(),
 		Height:        img.Height(),
 		InputWidth:    origWidth,
@@ -157,17 +220,17 @@ func (p *ImageProcessor) Process(
 // SupportedInputFormats returns MIME types this processor can read.
 func (p *ImageProcessor) SupportedInputFormats() []string {
 	return []string{
-		string(MIMETypeJPEG),
-		string(MIMETypePNG),
-		string(MIMETypeGIF),
-		string(MIMETypeWebP),
-		string(MIMETypeAVIF),
+		"image/jpeg",
+		"image/png",
+		"image/gif",
+		"image/webp",
+		"image/avif",
 	}
 }
 
 // SupportedOutputFormats returns formats this processor can write.
-func (p *ImageProcessor) SupportedOutputFormats() []ImageFormat {
-	return []ImageFormat{
+func (p *ImageProcessor) SupportedOutputFormats() []Format {
+	return []Format{
 		FormatJPEG,
 		FormatPNG,
 		FormatGIF,
@@ -176,6 +239,24 @@ func (p *ImageProcessor) SupportedOutputFormats() []ImageFormat {
 	}
 }
 
+// FormatToMIME converts a Format to its MIME type string.
+func FormatToMIME(format Format) string {
+	switch format {
+	case FormatJPEG:
+		return "image/jpeg"
+	case FormatPNG:
+		return "image/png"
+	case FormatWebP:
+		return "image/webp"
+	case FormatGIF:
+		return "image/gif"
+	case FormatAVIF:
+		return "image/avif"
+	default:
+		return "application/octet-stream"
+	}
+}
+
 // detectFormat returns the format string from a vips image.
 func (p *ImageProcessor) detectFormat(img *vips.ImageRef) string {
 	format := img.Format()
@@ -204,7 +285,6 @@ func (p *ImageProcessor) resize(img *vips.ImageRef, width, height int, fit FitMo
 
 	case FitContain:
 		// Resize to fit within dimensions, maintaining aspect ratio
-		// Calculate target dimensions maintaining aspect ratio
 		imgW, imgH := img.Width(), img.Height()
 		scaleW := float64(width) / float64(imgW)
 		scaleH := float64(height) / float64(imgH)
@@ -215,7 +295,7 @@ func (p *ImageProcessor) resize(img *vips.ImageRef, width, height int, fit FitMo
 		return img.Thumbnail(newW, newH, vips.InterestingNone)
 
 	case FitFill:
-		// Resize to exact dimensions (may distort) - use ThumbnailWithSize with Force
+		// Resize to exact dimensions (may distort)
 		return img.ThumbnailWithSize(width, height, vips.InterestingNone, vips.SizeForce)
 
 	case FitInside:
@@ -251,7 +331,7 @@ func (p *ImageProcessor) resize(img *vips.ImageRef, width, height int, fit FitMo
 const defaultQuality = 85
 
 // encode encodes an image to the specified format.
-func (p *ImageProcessor) encode(img *vips.ImageRef, format ImageFormat, quality int) ([]byte, error) {
+func (p *ImageProcessor) encode(img *vips.ImageRef, format Format, quality int) ([]byte, error) {
 	if quality <= 0 {
 		quality = defaultQuality
 	}
@@ -299,8 +379,8 @@ func (p *ImageProcessor) encode(img *vips.ImageRef, format ImageFormat, quality
 	return output, nil
 }
 
-// formatFromString converts a format string to ImageFormat.
-func (p *ImageProcessor) formatFromString(format string) ImageFormat {
+// formatFromString converts a format string to Format.
+func (p *ImageProcessor) formatFromString(format string) Format {
 	switch format {
 	case "jpeg":
 		return FormatJPEG

internal/imageprocessor/imageprocessor_test.go

@@ -1,4 +1,4 @@
-package imgcache
+package imageprocessor
 
 import (
 	"bytes"
@@ -70,13 +70,36 @@ func createTestPNG(t *testing.T, width, height int) []byte {
 	return buf.Bytes()
 }
 
+// detectMIME is a minimal magic-byte detector for test assertions.
+func detectMIME(data []byte) string {
+	if len(data) >= 3 && data[0] == 0xFF && data[1] == 0xD8 && data[2] == 0xFF {
+		return "image/jpeg"
+	}
+	if len(data) >= 8 && string(data[:8]) == "\x89PNG\r\n\x1a\n" {
+		return "image/png"
+	}
+	if len(data) >= 4 && string(data[:4]) == "GIF8" {
+		return "image/gif"
+	}
+	if len(data) >= 12 && string(data[:4]) == "RIFF" && string(data[8:12]) == "WEBP" {
+		return "image/webp"
+	}
+	if len(data) >= 12 && string(data[4:8]) == "ftyp" {
+		brand := string(data[8:12])
+		if brand == "avif" || brand == "avis" {
+			return "image/avif"
+		}
+	}
+	return ""
+}
+
 func TestImageProcessor_ResizeJPEG(t *testing.T) {
 	proc := New(Params{})
 	ctx := context.Background()
 
 	input := createTestJPEG(t, 800, 600)
 
-	req := &ImageRequest{
+	req := &Request{
 		Size:    Size{Width: 400, Height: 300},
 		Format:  FormatJPEG,
 		Quality: 85,
@@ -107,13 +130,9 @@ func TestImageProcessor_ResizeJPEG(t *testing.T) {
 		t.Fatalf("failed to read result: %v", err)
 	}
 
-	mime, err := DetectFormat(data)
-	if err != nil {
-		t.Fatalf("DetectFormat() error = %v", err)
-	}
-
-	if mime != MIMETypeJPEG {
-		t.Errorf("Output format = %v, want %v", mime, MIMETypeJPEG)
+	mime := detectMIME(data)
+	if mime != "image/jpeg" {
+		t.Errorf("Output format = %v, want image/jpeg", mime)
 	}
 }
 
@@ -123,7 +142,7 @@ func TestImageProcessor_ConvertToPNG(t *testing.T) {
 
 	input := createTestJPEG(t, 200, 150)
 
-	req := &ImageRequest{
+	req := &Request{
 		Size:    Size{Width: 200, Height: 150},
 		Format:  FormatPNG,
 		FitMode: FitCover,
@@ -140,13 +159,9 @@ func TestImageProcessor_ConvertToPNG(t *testing.T) {
 		t.Fatalf("failed to read result: %v", err)
 	}
 
-	mime, err := DetectFormat(data)
-	if err != nil {
-		t.Fatalf("DetectFormat() error = %v", err)
-	}
-
-	if mime != MIMETypePNG {
-		t.Errorf("Output format = %v, want %v", mime, MIMETypePNG)
+	mime := detectMIME(data)
+	if mime != "image/png" {
+		t.Errorf("Output format = %v, want image/png", mime)
 	}
 }
 
@@ -156,7 +171,7 @@ func TestImageProcessor_OriginalSize(t *testing.T) {
 
 	input := createTestJPEG(t, 640, 480)
 
-	req := &ImageRequest{
+	req := &Request{
 		Size:    Size{Width: 0, Height: 0}, // Original size
 		Format:  FormatJPEG,
 		Quality: 85,
@@ -186,7 +201,7 @@ func TestImageProcessor_FitContain(t *testing.T) {
 	// Should result in 400x200 (maintaining aspect ratio)
 	input := createTestJPEG(t, 800, 400)
 
-	req := &ImageRequest{
+	req := &Request{
 		Size:    Size{Width: 400, Height: 400},
 		Format:  FormatJPEG,
 		Quality: 85,
@@ -213,7 +228,7 @@ func TestImageProcessor_ProportionalScale_WidthOnly(t *testing.T) {
 	// Should scale proportionally to 400x300
 	input := createTestJPEG(t, 800, 600)
 
-	req := &ImageRequest{
+	req := &Request{
 		Size:    Size{Width: 400, Height: 0},
 		Format:  FormatJPEG,
 		Quality: 85,
@@ -243,7 +258,7 @@ func TestImageProcessor_ProportionalScale_HeightOnly(t *testing.T) {
 	// Should scale proportionally to 400x300
 	input := createTestJPEG(t, 800, 600)
 
-	req := &ImageRequest{
+	req := &Request{
 		Size:    Size{Width: 0, Height: 300},
 		Format:  FormatJPEG,
 		Quality: 85,
@@ -271,7 +286,7 @@ func TestImageProcessor_ProcessPNG(t *testing.T) {
 
 	input := createTestPNG(t, 400, 300)
 
-	req := &ImageRequest{
+	req := &Request{
 		Size:    Size{Width: 200, Height: 150},
 		Format:  FormatPNG,
 		FitMode: FitCover,
@@ -292,11 +307,6 @@ func TestImageProcessor_ProcessPNG(t *testing.T) {
 	}
 }
 
-func TestImageProcessor_ImplementsInterface(t *testing.T) {
-	// Verify ImageProcessor implements Processor interface
-	var _ Processor = (*ImageProcessor)(nil)
-}
-
 func TestImageProcessor_SupportedFormats(t *testing.T) {
 	proc := New(Params{})
 
@@ -319,7 +329,7 @@ func TestImageProcessor_RejectsOversizedInput(t *testing.T) {
 	// This should be rejected before processing to prevent DoS
 	input := createTestJPEG(t, 10000, 100)
 
-	req := &ImageRequest{
+	req := &Request{
 		Size:    Size{Width: 100, Height: 100},
 		Format:  FormatJPEG,
 		Quality: 85,
@@ -343,7 +353,7 @@ func TestImageProcessor_RejectsOversizedInputHeight(t *testing.T) {
 	// Create an image with oversized height
 	input := createTestJPEG(t, 100, 10000)
 
-	req := &ImageRequest{
+	req := &Request{
 		Size:    Size{Width: 100, Height: 100},
 		Format:  FormatJPEG,
 		Quality: 85,
@@ -365,10 +375,9 @@ func TestImageProcessor_AcceptsMaxDimensionInput(t *testing.T) {
 	ctx := context.Background()
 
 	// Create an image at exactly MaxInputDimension - should be accepted
-	// Using smaller dimensions to keep test fast
 	input := createTestJPEG(t, MaxInputDimension, 100)
 
-	req := &ImageRequest{
+	req := &Request{
 		Size:    Size{Width: 100, Height: 100},
 		Format:  FormatJPEG,
 		Quality: 85,
@@ -388,7 +397,7 @@ func TestImageProcessor_EncodeWebP(t *testing.T) {
 
 	input := createTestJPEG(t, 200, 150)
 
-	req := &ImageRequest{
+	req := &Request{
 		Size:    Size{Width: 100, Height: 75},
 		Format:  FormatWebP,
 		Quality: 80,
@@ -407,13 +416,9 @@ func TestImageProcessor_EncodeWebP(t *testing.T) {
 		t.Fatalf("failed to read result: %v", err)
 	}
 
-	mime, err := DetectFormat(data)
-	if err != nil {
-		t.Fatalf("DetectFormat() error = %v", err)
-	}
-
-	if mime != MIMETypeWebP {
-		t.Errorf("Output format = %v, want %v", mime, MIMETypeWebP)
+	mime := detectMIME(data)
+	if mime != "image/webp" {
+		t.Errorf("Output format = %v, want image/webp", mime)
 	}
 
 	// Verify dimensions
@@ -436,7 +441,7 @@ func TestImageProcessor_DecodeAVIF(t *testing.T) {
 	}
 
 	// Request resize and convert to JPEG
-	req := &ImageRequest{
+	req := &Request{
 		Size:    Size{Width: 2, Height: 2},
 		Format:  FormatJPEG,
 		Quality: 85,
@@ -455,13 +460,9 @@ func TestImageProcessor_DecodeAVIF(t *testing.T) {
 		t.Fatalf("failed to read result: %v", err)
 	}
 
-	mime, err := DetectFormat(data)
-	if err != nil {
-		t.Fatalf("DetectFormat() error = %v", err)
-	}
-
-	if mime != MIMETypeJPEG {
-		t.Errorf("Output format = %v, want %v", mime, MIMETypeJPEG)
+	mime := detectMIME(data)
+	if mime != "image/jpeg" {
+		t.Errorf("Output format = %v, want image/jpeg", mime)
 	}
 }
 
@@ -477,7 +478,7 @@ func TestImageProcessor_RejectsOversizedInputData(t *testing.T) {
 		t.Fatalf("test JPEG must exceed %d bytes, got %d", limit, len(input))
 	}
 
-	req := &ImageRequest{
+	req := &Request{
 		Size:    Size{Width: 100, Height: 75},
 		Format:  FormatJPEG,
 		Quality: 85,
@@ -502,7 +503,7 @@ func TestImageProcessor_AcceptsInputWithinLimit(t *testing.T) {
 	proc := New(Params{MaxInputBytes: limit})
 	ctx := context.Background()
 
-	req := &ImageRequest{
+	req := &Request{
 		Size:    Size{Width: 10, Height: 10},
 		Format:  FormatJPEG,
 		Quality: 85,
@@ -536,7 +537,7 @@ func TestImageProcessor_EncodeAVIF(t *testing.T) {
 
 	input := createTestJPEG(t, 200, 150)
 
-	req := &ImageRequest{
+	req := &Request{
 		Size:    Size{Width: 100, Height: 75},
 		Format:  FormatAVIF,
 		Quality: 85,
@@ -555,13 +556,9 @@ func TestImageProcessor_EncodeAVIF(t *testing.T) {
 		t.Fatalf("failed to read result: %v", err)
 	}
 
-	mime, err := DetectFormat(data)
-	if err != nil {
-		t.Fatalf("DetectFormat() error = %v", err)
-	}
-
-	if mime != MIMETypeAVIF {
-		t.Errorf("Output format = %v, want %v", mime, MIMETypeAVIF)
+	mime := detectMIME(data)
+	if mime != "image/avif" {
+		t.Errorf("Output format = %v, want image/avif", mime)
 	}
 
 	// Verify dimensions

internal/imgcache/imgcache.go

@@ -199,36 +199,6 @@ type FetchResult struct {
 	TLSCipherSuite string
 }
 
-// Processor handles image transformation (resize, format conversion)
-type Processor interface {
-	// Process transforms an image according to the request
-	Process(ctx context.Context, input io.Reader, req *ImageRequest) (*ProcessResult, error)
-	// SupportedInputFormats returns MIME types this processor can read
-	SupportedInputFormats() []string
-	// SupportedOutputFormats returns formats this processor can write
-	SupportedOutputFormats() []ImageFormat
-}
-
-// ProcessResult contains the result of image processing
-type ProcessResult struct {
-	// Content is the processed image data
-	Content io.ReadCloser
-	// ContentLength is the size in bytes
-	ContentLength int64
-	// ContentType is the MIME type of the output
-	ContentType string
-	// Width is the output image width
-	Width int
-	// Height is the output image height
-	Height int
-	// InputWidth is the original image width before processing
-	InputWidth int
-	// InputHeight is the original image height before processing
-	InputHeight int
-	// InputFormat is the detected input format (e.g., "jpeg", "png")
-	InputFormat string
-}
-
 // Storage handles persistent storage of cached content
 type Storage interface {
 	// Store saves content and returns its hash

internal/imgcache/service.go

@@ -11,13 +11,14 @@ import (
 	"time"
 
 	"github.com/dustin/go-humanize"
+	"sneak.berlin/go/pixa/internal/imageprocessor"
 )
 
 // Service implements the ImageCache interface, orchestrating cache, fetcher, and processor.
 type Service struct {
 	cache           *Cache
 	fetcher         Fetcher
-	processor       Processor
+	processor       *imageprocessor.ImageProcessor
 	signer          *Signer
 	whitelist       *HostWhitelist
 	log             *slog.Logger
@@ -82,7 +83,7 @@ func NewService(cfg *ServiceConfig) (*Service, error) {
 	return &Service{
 		cache:           cfg.Cache,
 		fetcher:         fetcher,
-		processor:       New(Params{MaxInputBytes: maxResponseSize}),
+		processor:       imageprocessor.New(imageprocessor.Params{MaxInputBytes: maxResponseSize}),
 		signer:          signer,
 		whitelist:       NewHostWhitelist(cfg.Whitelist),
 		log:             log,
@@ -300,7 +301,14 @@ func (s *Service) processAndStore(
 	// Process the image
 	processStart := time.Now()
 
-	processResult, err := s.processor.Process(ctx, bytes.NewReader(sourceData), req)
+	processReq := &imageprocessor.Request{
+		Size:    imageprocessor.Size{Width: req.Size.Width, Height: req.Size.Height},
+		Format:  imageprocessor.Format(req.Format),
+		Quality: req.Quality,
+		FitMode: imageprocessor.FitMode(req.FitMode),
+	}
+
+	processResult, err := s.processor.Process(ctx, bytes.NewReader(sourceData), processReq)
 	if err != nil {
 		return nil, fmt.Errorf("image processing failed: %w", err)
 	}