internal: implement path normalization w/ configurable rules

Strips query strings and URL fragmenets, prevents unbounded Prometheus
metrics by normalizing paths like:

- `/users/12345/profile -> /users/:id/profile`
- `/page?utm_source=twitter -> /page`
- `/a/../b -> /b`

etc.

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I72f2fa2452f4666567143d052b5716476a6a6964

internal/normalize/path.go

@@ -0,0 +1,121 @@
+package normalize
+
+import (
+	"strings"
+
+	"notashelf.dev/watchdog/internal/config"
+)
+
+const maxPathLength = 2048
+
+type PathNormalizer struct {
+	cfg config.PathConfig
+}
+
+func NewPathNormalizer(cfg config.PathConfig) *PathNormalizer {
+	return &PathNormalizer{cfg: cfg}
+}
+
+func (n *PathNormalizer) Normalize(path string) string {
+	// Return as-is if path is too long
+	if len(path) > maxPathLength {
+		return path
+	}
+
+	if path == "" {
+		return "/"
+	}
+
+	// Strip query string
+	if n.cfg.StripQuery {
+		if idx := strings.IndexByte(path, '?'); idx != -1 {
+			path = path[:idx]
+		}
+	}
+
+	// Strip fragment
+	if n.cfg.StripFragment {
+		if idx := strings.IndexByte(path, '#'); idx != -1 {
+			path = path[:idx]
+		}
+	}
+
+	// Ensure leading slash
+	if !strings.HasPrefix(path, "/") {
+		path = "/" + path
+	}
+
+	// Split into segments, first element is *always* empty for paths starting with '/'
+	segments := strings.Split(path, "/")
+	if len(segments) > 0 && segments[0] == "" {
+		segments = segments[1:]
+	}
+
+	// Remove empty segments (from double slashes)
+	filtered := make([]string, 0, len(segments))
+	for _, seg := range segments {
+		if seg != "" {
+			filtered = append(filtered, seg)
+		}
+	}
+	segments = filtered
+
+	// Resolve . and .. segments
+	resolved := make([]string, 0, len(segments))
+	for _, seg := range segments {
+		if seg == "." {
+			// Skip current directory
+			continue
+		} else if seg == ".." {
+			// Go up one level if possible
+			if len(resolved) > 0 {
+				resolved = resolved[:len(resolved)-1]
+			}
+			// If already at root, skip ..
+		} else {
+			resolved = append(resolved, seg)
+		}
+	}
+	segments = resolved
+
+	// Collapse numeric segments
+	if n.cfg.CollapseNumericSegments {
+		for i, seg := range segments {
+			if isNumeric(seg) {
+				segments[i] = ":id"
+			}
+		}
+	}
+
+	// Limit segments
+	if n.cfg.MaxSegments > 0 && len(segments) > n.cfg.MaxSegments {
+		segments = segments[:n.cfg.MaxSegments]
+	}
+
+	// Reconstruct path
+	var result string
+	if len(segments) == 0 {
+		result = "/"
+	} else {
+		result = "/" + strings.Join(segments, "/")
+	}
+
+	// Strip trailing slash if configured (except root)
+	if n.cfg.NormalizeTrailingSlash && result != "/" && strings.HasSuffix(result, "/") {
+		result = strings.TrimSuffix(result, "/")
+	}
+
+	return result
+}
+
+func isNumeric(s string) bool {
+	if s == "" {
+		return false
+	}
+	for _, c := range s {
+		if c < '0' || c > '9' {
+			return false
+		}
+	}
+	return true
+}