package ucp

import (
	"context"
	"net/http"
	"strings"
)

// ---------------------------------------------------------------------------
// UCP-Agent header parsing (RFC 8941 Dictionary Structured Field)
// ---------------------------------------------------------------------------

// ucpAgentProfileKey is the context key for the UCP-Agent profile URI.
type ucpAgentProfileKey struct{}

// UCPAgentProfileFromContext returns the UCP-Agent profile URI stored in the
// request context, or empty string if none was advertised.
func UCPAgentProfileFromContext(ctx context.Context) string {
	v, _ := ctx.Value(ucpAgentProfileKey{}).(string)
	return v
}

// parseUCPAgentProfile parses a single "profile" member value from an RFC 8941
// Dictionary Structured Field header. It only handles the string-value form:
//
//	UCP-Agent: profile="https://agent.example/profiles/shopping-agent.json"
//
// Returns the profile URI (without surrounding quotes) or empty string.
func parseUCPAgentProfile(header string) string {
	if header == "" {
		return ""
	}

	// Walk the dictionary entries looking for "profile".
	// RFC 8941 §3.1.2: Dict = ((im-key | key) value) *(SP "," ((im-key | key) value))
	i := 0
	bs := []byte(header)
	n := len(bs)

	for i < n {
		// Skip whitespace.
		for i < n && (bs[i] == ' ' || bs[i] == '\t') {
			i++
		}
		if i >= n {
			break
		}

		// Read key (im-key or key). Lowercase letters, digits, "_", "-", "*".
		start := i
		for i < n && isKeyChar(bs[i]) {
			i++
		}
		if i == start {
			// No key found; skip to next comma or end.
			for i < n && bs[i] != ',' {
				i++
			}
			i++ // skip comma
			continue
		}
		key := string(bs[start:i])

		// Check for "=" separator. RFC 8941 allows bare names (boolean true)
		// when "=? " follows — but we only care about key=value entries.
		if i < n && bs[i] == '=' {
			i++ // skip '='

			// Read the value. For our case it's a string (starts with '"').
			if i < n && bs[i] == '"' {
				i++ // skip opening quote
				valStart := i
				for i < n && bs[i] != '"' {
					if bs[i] == '\\' {
						i++ // skip escape
					}
					i++
				}
				val := string(bs[valStart:i])
				if i < n {
					i++ // skip closing quote
				}
				if strings.EqualFold(key, "profile") {
					return val
				}
			}
		}

		// Skip to next comma or end.
		for i < n && bs[i] != ',' {
			i++
		}
		i++ // skip comma
	}

	return ""
}

// isKeyChar reports whether b is valid in an RFC 8941 key (lc-ltr, DIGIT, "_", "-", "*").
func isKeyChar(b byte) bool {
	return (b >= 'a' && b <= 'z') || (b >= '0' && b <= '9') || b == '_' || b == '-' || b == '*'
}

// WithUCPAgent is HTTP middleware that reads the UCP-Agent header from incoming
// requests, parses it per RFC 8941, and stores the profile URI in the request
// context so downstream handlers can inspect the calling platform's identity.
//
// Usage:
//
//	mux.Handle("/ucp/v1/carts", ucp.WithUCPAgent(ucp.CartHandler(pool)))
func WithUCPAgent(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if profile := parseUCPAgentProfile(r.Header.Get("UCP-Agent")); profile != "" {
			ctx := context.WithValue(r.Context(), ucpAgentProfileKey{}, profile)
			r = r.WithContext(ctx)
		}
		next.ServeHTTP(w, r)
	})
}

// DefaultUCPAgentProfile is the profile URI sent on outgoing requests from
// this commerce platform. Override via SetDefaultUCPAgentProfile or the
// UCP_AGENT_PROFILE environment variable at init time.
var DefaultUCPAgentProfile = "https://cart.k6n.net/.well-known/ucp"

// SetDefaultUCPAgentProfile overrides the default profile URI used for outgoing
// UCP-Agent headers.
func SetDefaultUCPAgentProfile(profile string) {
	if profile != "" {
		DefaultUCPAgentProfile = profile
	}
}

// UCPAgentHeaderValue returns the RFC 8941 Dictionary value for the UCP-Agent
// header, encoding the given profile URI.
func UCPAgentHeaderValue(profile string) string {
	if profile == "" {
		profile = DefaultUCPAgentProfile
	}
	return `profile="` + profile + `"`
}

// WithUCPAgentOnRequest adds the UCP-Agent header to an outgoing HTTP request.
// It sets the header to the default profile unless a custom profile is provided.
func WithUCPAgentOnRequest(req *http.Request, profile ...string) {
	p := DefaultUCPAgentProfile
	if len(profile) > 0 && profile[0] != "" {
		p = profile[0]
	}
	req.Header.Set("UCP-Agent", `profile="`+p+`"`)
}