pixa/internal/imgcache/imgcache.go

// Package imgcache provides interfaces and types for the image caching proxy.
package imgcache

import (
	"context"
	"errors"
	"io"
	"net/url"
	"time"
)

// ErrInvalidFitMode is returned when an invalid fit mode is provided.
var ErrInvalidFitMode = errors.New("invalid fit mode")

// ImageFormat represents supported output image formats.
type ImageFormat string

// Supported image output formats.
const (
	FormatOriginal ImageFormat = "orig"
	FormatJPEG     ImageFormat = "jpeg"
	FormatPNG      ImageFormat = "png"
	FormatWebP     ImageFormat = "webp"
	FormatAVIF     ImageFormat = "avif"
	FormatGIF      ImageFormat = "gif"
)

// Size represents requested image dimensions
type Size struct {
	Width  int
	Height int
}

// OriginalSize returns true if this represents "keep original size"
func (s Size) OriginalSize() bool {
	return s.Width == 0 && s.Height == 0
}

// FitMode represents how to fit 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"
)

// ValidateFitMode checks if the given fit mode is valid.
// Returns ErrInvalidFitMode for unrecognized fit modes.
func ValidateFitMode(fit FitMode) error {
	switch fit {
	case FitCover, FitContain, FitFill, FitInside, FitOutside, "":
		return nil
	default:
		return ErrInvalidFitMode
	}
}

// ImageRequest represents a request for a processed image
type ImageRequest struct {
	// SourceHost is the origin host (e.g., "cdn.example.com")
	SourceHost string
	// SourcePath is the path on the origin (e.g., "/photos/cat.jpg")
	SourcePath string
	// SourceQuery is the optional query string for the origin URL
	SourceQuery string
	// Size is the requested output dimensions
	Size Size
	// Format is the requested output format
	Format ImageFormat
	// Quality is the output quality (1-100) for lossy formats
	Quality int
	// FitMode is how to fit the image into requested dimensions
	FitMode FitMode
	// Signature is the HMAC signature for non-allowlisted hosts
	Signature string
	// Expires is the signature expiration timestamp
	Expires time.Time
	// AllowHTTP indicates whether HTTP (non-TLS) is allowed for this request
	AllowHTTP bool
}

// SourceURL returns the full upstream URL to fetch.
// Uses http:// scheme when AllowHTTP is true, otherwise https://.
func (r *ImageRequest) SourceURL() string {
	scheme := "https"
	if r.AllowHTTP {
		scheme = "http"
	}

	url := scheme + "://" + r.SourceHost + r.SourcePath
	if r.SourceQuery != "" {
		url += "?" + r.SourceQuery
	}

	return url
}

// ImageResponse represents a processed image ready to serve
type ImageResponse struct {
	// Content is the image data reader
	Content io.ReadCloser
	// ContentLength is the size in bytes (-1 if unknown)
	ContentLength int64
	// ContentType is the MIME type of the response
	ContentType string
	// ETag is the entity tag for caching
	ETag string
	// LastModified is when the content was last modified
	LastModified time.Time
	// CacheStatus indicates HIT, MISS, or STALE
	CacheStatus CacheStatus
	// FetchedBytes is the number of bytes fetched from upstream (0 if cache hit)
	FetchedBytes int64
}

// CacheStatus indicates whether the response was served from cache.
type CacheStatus string

// Cache status values for response headers.
const (
	CacheHit   CacheStatus = "HIT"
	CacheMiss  CacheStatus = "MISS"
	CacheStale CacheStatus = "STALE"
)

// ImageCache is the main interface for the image caching proxy
type ImageCache interface {
	// Get retrieves a processed image, fetching and processing if necessary
	Get(ctx context.Context, req *ImageRequest) (*ImageResponse, error)

	// Warm pre-fetches and caches an image without returning it
	Warm(ctx context.Context, req *ImageRequest) error

	// Purge removes a cached image
	Purge(ctx context.Context, req *ImageRequest) error

	// Stats returns cache statistics
	Stats(ctx context.Context) (*CacheStats, error)
}

// CacheStats contains cache statistics
type CacheStats struct {
	// TotalItems is the number of cached items
	TotalItems int64
	// TotalSizeBytes is the total size of cached content
	TotalSizeBytes int64
	// HitCount is the number of cache hits
	HitCount int64
	// MissCount is the number of cache misses
	MissCount int64
	// HitRate is HitCount / (HitCount + MissCount)
	HitRate float64
}

// SignatureValidator validates request signatures
type SignatureValidator interface {
	// Validate checks if the signature is valid for the request
	Validate(req *ImageRequest) error
	// Generate creates a signature for a request
	Generate(req *ImageRequest) string
}

// Allowlist checks if a URL is allowlisted (no signature required)
type Allowlist interface {
	// IsAllowlisted returns true if the URL doesn't require a signature
	IsAllowlisted(u *url.URL) bool
}

// Storage handles persistent storage of cached content
type Storage interface {
	// Store saves content and returns its hash
	Store(ctx context.Context, content io.Reader) (hash string, err error)
	// Load retrieves content by hash
	Load(ctx context.Context, hash string) (io.ReadCloser, error)
	// Delete removes content by hash
	Delete(ctx context.Context, hash string) error
	// Exists checks if content exists
	Exists(ctx context.Context, hash string) (bool, error)
}