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// Package captokens provides canonical token serialization and validation
// for capability-based authorization tokens.
package captokens

import (
	"bytes"
	"fmt"
	"sort"
	"strconv"
	"strings"
)

// KnownMetadataFields lists all allowed metadata fields (exhaustive).
// Fields with $ or _ prefix must be in this list.
var KnownMetadataFields = map[string]bool{
	"$tok_id":  true, // Token type identifier
	"_session": true, // Session UUID
	"_issued":  true, // ISO8601 timestamp
	"_expires": true, // ISO8601 timestamp
	"_sig":     true, // Base64-encoded signature (not hashed)
}

// IsValidFieldName checks if a field name follows naming rules.
// Allowed characters: [_$\-a-zA-Z0-9]
// Must be non-empty.
func IsValidFieldName(name string) bool {
	if len(name) == 0 {
		return false
	}

	for _, c := range name {
		if !((c >= 'a' && c <= 'z') ||
			(c >= 'A' && c <= 'Z') ||
			(c >= '0' && c <= '9') ||
			c == '_' || c == '$' || c == '-') {
			return false
		}
	}
	return true
}

// IsKnownMetadataField checks if a field name is a known metadata field.
func IsKnownMetadataField(name string) bool {
	return KnownMetadataFields[name]
}

// CanonicalTokenBytes generates canonical bytes for token signing/verification.
// The _sig field is excluded from canonicalization.
// Returns deterministic bytes with sorted keys and validated field names.
func CanonicalTokenBytes(token map[string]any) ([]byte, error) {
	// Extract and validate all fields except "_sig"
	fields := make(map[string]string)
	for key, value := range token {
		if key == "_sig" {
			continue
		}

		// Validate field name
		if !IsValidFieldName(key) {
			return nil, fmt.Errorf("invalid field name: %s", key)
		}

		// Validate reserved fields
		if strings.HasPrefix(key, "$") || strings.HasPrefix(key, "_") {
			if !IsKnownMetadataField(key) {
				return nil, fmt.Errorf("unknown metadata field: %s", key)
			}
		}

		// Serialize value (validates nested keys recursively)
		serialized, err := serializeValue(value)
		if err != nil {
			return nil, fmt.Errorf("field %s: %w", key, err)
		}
		fields[key] = serialized
	}

	// Sort keys by ASCII lexicographic order
	keys := make([]string, 0, len(fields))
	for key := range fields {
		keys = append(keys, key)
	}
	sort.Strings(keys)

	// Concatenate: key=value\n
	var buf bytes.Buffer
	for _, key := range keys {
		buf.WriteString(key)
		buf.WriteString("=")
		buf.WriteString(fields[key])
		buf.WriteString("\n")
	}

	return buf.Bytes(), nil
}

// serializeValue serializes a value to canonical string format.
// Handles: string, number, bool, null, array, object.
// Validates nested object keys recursively.
func serializeValue(value any) (string, error) {
	switch v := value.(type) {
	case string:
		return v, nil

	case float64:
		// Canonical number format
		if v == float64(int64(v)) {
			// Integer value - no decimal point
			return strconv.FormatInt(int64(v), 10), nil
		}
		// Float value - use shortest representation
		return strconv.FormatFloat(v, 'g', -1, 64), nil

	case int:
		return strconv.FormatInt(int64(v), 10), nil

	case int64:
		return strconv.FormatInt(v, 10), nil

	case bool:
		if v {
			return "true", nil
		}
		return "false", nil

	case nil:
		return "null", nil

	case []any:
		// Array: [elem1,elem2,elem3]
		var buf bytes.Buffer
		buf.WriteString("[")
		for i, item := range v {
			if i > 0 {
				buf.WriteString(",")
			}
			serialized, err := serializeValue(item)
			if err != nil {
				return "", fmt.Errorf("array element %d: %w", i, err)
			}
			buf.WriteString(serialized)
		}
		buf.WriteString("]")
		return buf.String(), nil

	case map[string]any:
		// Object: {key1=val1,key2=val2} with sorted keys
		// Validate all keys follow naming rules
		keys := make([]string, 0, len(v))
		for key := range v {
			if !IsValidFieldName(key) {
				return "", fmt.Errorf("invalid nested key: %s", key)
			}
			keys = append(keys, key)
		}
		sort.Strings(keys)

		var buf bytes.Buffer
		buf.WriteString("{")
		for i, key := range keys {
			if i > 0 {
				buf.WriteString(",")
			}
			buf.WriteString(key)
			buf.WriteString("=")

			serialized, err := serializeValue(v[key])
			if err != nil {
				return "", fmt.Errorf("nested field %s: %w", key, err)
			}
			buf.WriteString(serialized)
		}
		buf.WriteString("}")
		return buf.String(), nil

	default:
		return "", fmt.Errorf("unsupported value type: %T", value)
	}
}