// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: MIT

//go:build linux || freebsd

package contactform

import (
	"fmt"
	"net/mail"
	"slices"
	"strings"
	"unicode/utf8"
)

// MinNameRunes is the default minimum allowed length of a name.
const MinNameRunes = 2

// MaxNameRunes is the default maximum allowed length of a name.
const MaxNameRunes = 100

// MinMessageRunes is the default minimum allowed length of a message body.
const MinMessageRunes = 10

// MaxMessageRunes is the default maximum allowed length of a message body.
const MaxMessageRunes = 5000

// ServiceAny is the services entry that accepts any service value.
const ServiceAny = "*"

// DefaultServices is the built-in service allow-list the contact preset
// applies when a form does not define its own list. The empty value is
// always accepted on top of whatever this list holds, because a form that
// offers no choice never sends the field at all.
var DefaultServices = []string{
	"architecture",
	"ai",
	"infrastructure",
	"software",
	"unix",
	"other",
}

// Policy is the declarative validation rule set for one form. The server
// builds it from the form's configuration; a library caller writes it
// directly. The zero value accepts any name and message, so every limit
// that matters must be set explicitly.
type Policy struct {
	// RequireName and RequireMessage switch the length checks for the two
	// free-text fields on and off. The email address is always required
	// and always checked: every form delivers mail and needs a reply-to.
	RequireName    bool
	RequireMessage bool

	MinNameRunes    int
	MaxNameRunes    int
	MinMessageRunes int
	MaxMessageRunes int

	// Services is the allow-list for the optional service field. A nil
	// list means the field is not validated at all; a non-nil list holds
	// the accepted values, with the empty value always accepted and the
	// ServiceAny entry lifting the restriction entirely.
	Services []string
}

// Preset returns the built-in policy for a form type. An empty or unknown
// type falls back to the contact preset, which is also how the server
// treats an unconfigured type. Every preset carries the default length
// limits; the newsletter preset simply leaves both free-text fields out
// of the checks, so switching them on later starts from sane limits.
func Preset(formType string) Policy {
	switch formType {
	case "newsletter":
		return Policy{
			MinNameRunes:    MinNameRunes,
			MaxNameRunes:    MaxNameRunes,
			MinMessageRunes: MinMessageRunes,
			MaxMessageRunes: MaxMessageRunes,
		}
	case "feedback", "generic":
		return Policy{
			RequireName:     true,
			RequireMessage:  true,
			MinNameRunes:    MinNameRunes,
			MaxNameRunes:    MaxNameRunes,
			MinMessageRunes: MinMessageRunes,
			MaxMessageRunes: MaxMessageRunes,
		}
	default: // contact, and the fallback for every unknown type
		return Policy{
			RequireName:     true,
			RequireMessage:  true,
			MinNameRunes:    MinNameRunes,
			MaxNameRunes:    MaxNameRunes,
			MinMessageRunes: MinMessageRunes,
			MaxMessageRunes: MaxMessageRunes,
			Services:        slices.Clone(DefaultServices),
		}
	}
}

// Validate normalises the request in place (trims whitespace from every
// text field) and checks it against p. It returns one entry per failed
// field; a nil slice means the request is valid.
func Validate(r *Request, p Policy) []FieldError {
	r.Name = strings.TrimSpace(r.Name)
	r.Email = strings.TrimSpace(r.Email)
	r.Service = strings.TrimSpace(r.Service)
	r.Message = strings.TrimSpace(r.Message)

	var errs []FieldError

	if p.RequireName {
		errs = append(errs, checkRunes("name", r.Name, p.MinNameRunes, p.MaxNameRunes)...)
	}

	if _, err := mail.ParseAddress(r.Email); err != nil {
		errs = append(errs, FieldError{Field: "email", Message: "email is invalid"})
	}

	if p.Services != nil && !serviceAllowed(p.Services, r.Service) {
		errs = append(errs, FieldError{Field: "service", Message: "service is not a recognised value"})
	}

	if p.RequireMessage {
		errs = append(errs, checkRunes("message", r.Message, p.MinMessageRunes, p.MaxMessageRunes)...)
	}

	return errs
}

// NormalizeAndValidate trims whitespace from all text fields and then
// checks the request against the built-in preset for the form type.
// Supported types: contact, feedback, newsletter, generic; an empty or
// unknown type falls back to contact. Callers that need their own limits
// or their own service allow-list build a Policy and call Validate.
func NormalizeAndValidate(r *Request, formType string) []FieldError {
	return Validate(r, Preset(formType))
}

// checkRunes reports the length errors for one field in rune counts.
func checkRunes(field, value string, minRunes, maxRunes int) []FieldError {
	n := utf8.RuneCountInString(value)
	var errs []FieldError
	if n < minRunes {
		errs = append(errs, FieldError{
			Field:   field,
			Message: fmt.Sprintf("%s must be at least %d characters", field, minRunes),
		})
	} else if n > maxRunes {
		errs = append(errs, FieldError{
			Field:   field,
			Message: fmt.Sprintf("%s must be at most %d characters", field, maxRunes),
		})
	}
	return errs
}

// serviceAllowed reports whether s passes the allow-list. The empty value
// is always legitimate: petrbalvin.org and every other frontend is free to
// post a payload without a service field. The ServiceAny entry accepts any
// non-empty value.
func serviceAllowed(list []string, s string) bool {
	if s == "" {
		return true
	}
	if slices.Contains(list, ServiceAny) {
		return true
	}
	return slices.Contains(list, s)
}