internal/api: add event model with validation

Supports both pageview and custom event types

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: Iaf48291cd952865ea9ec21361ae33c746a6a6964

internal/api/event.go

@@ -0,0 +1,69 @@
+package api
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+)
+
+const (
+	maxEventSize = 4 * 1024 // 4KB
+	maxPathLen   = 2048
+	maxRefLen    = 2048
+)
+
+// Represents an incoming analytics event
+type Event struct {
+	Domain   string `json:"d"` // domain
+	Path     string `json:"p"` // path
+	Referrer string `json:"r"` // referrer URL
+	Event    string `json:"e"` // custom event name (empty for pageviews)
+	Width    int    `json:"w"` // screen width for device classification
+}
+
+// Parses an event from the request body with size limits
+func ParseEvent(body io.Reader) (*Event, error) {
+	// Limit read size to prevent memory exhaustion
+	limited := io.LimitReader(body, maxEventSize+1)
+
+	data, err := io.ReadAll(limited)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read body: %w", err)
+	}
+
+	if len(data) > maxEventSize {
+		return nil, fmt.Errorf("event payload too large: %d bytes (max %d)", len(data), maxEventSize)
+	}
+
+	var event Event
+	if err := json.Unmarshal(data, &event); err != nil {
+		return nil, fmt.Errorf("invalid JSON: %w", err)
+	}
+
+	return &event, nil
+}
+
+// Validate checks if the event is valid for the given domain
+func (e *Event) Validate(expectedDomain string) error {
+	if e.Domain == "" {
+		return fmt.Errorf("domain is required")
+	}
+
+	if e.Domain != expectedDomain {
+		return fmt.Errorf("domain mismatch: got %q, expected %q", e.Domain, expectedDomain)
+	}
+
+	if e.Path == "" {
+		return fmt.Errorf("path is required")
+	}
+
+	if len(e.Path) > maxPathLen {
+		return fmt.Errorf("path too long: %d bytes (max %d)", len(e.Path), maxPathLen)
+	}
+
+	if len(e.Referrer) > maxRefLen {
+		return fmt.Errorf("referrer too long: %d bytes (max %d)", len(e.Referrer), maxRefLen)
+	}
+
+	return nil
+}