package mailx

import (
    "cmp"
    "net/textproto"

    "pkg.jfrech.com/brief/internal/crumbs"
)

// Field represents an RFC 822 header field (cf. [RFC 822, 3.2])
//
// [RawField.Decode] does not canonicalize:
// Field.Name may not be in textproto.CanonicalMIMEHeaderKey form
// and neither in "fixed" form (i.e. it may contain odd characters;
// cf. [Field.CanonicalName]).
//
// [Field.Equal] and [Field.Compare] compare according to [Field.CanonicalName].
// Use [Field.Is] rather than testing Field.Name as a string.
//
// [RFC 822, 3.2]: https://www.rfc-editor.org/rfc/rfc822.html#section-3.2
type Field struct {
    Name, Body string
}

// Field.CanonicalName returns the canoncialized,
// fixed (that is, bytes !(c>'\x20'&&c!=':'&&c<='~') become '-') name.
func (field Field) CanonicalName() string {
    return textproto.CanonicalMIMEHeaderKey(fix(field.Name))
}

// Equality in the /[Field.CanonicalName] coset.
func (field Field) Equal(gield Field) bool {
    return field.Compare(gield) == 0
}

// Is reports whether field's [Field.CanonicalName] is name modulo canoncialization.
//
//   (Field{"\xfe", "xfe"}).Is("") == true
func (field Field) Is(name string) bool {
    return field.Equal(Field{name, field.Body})
}

// Order in the /[Field.CanonicalName] coset.
//
// Body bytes are ordered according to their bytes
// (matters only for non-UTF8-valid bodies).
//
// Field.Compare returns {-1,0,+1} with semantics of [cmp.Compare].
func (field Field) Compare(gield Field) int {
    return cmp.Or(
        cmp.Compare(field.CanonicalName(), gield.CanonicalName()),
        cmp.Compare(field.Body, gield.Body),
    )
}

// fix maps the empty string to "-" and
// replaces ever byte !(c>'\x20'&&c!=':'&&c<='~') by '-'.
func fix(s string) string {
    if s == "" {
        return "-"
    }

    valid := func(c byte) bool { return c > ' ' && c != ':' && c <= '~' }

    // fast path
    if crumbs.All(valid, []byte(s)) {
        return s
    }

    // slow path
    buf := make([]byte, len(s))
    for j, c := range []byte(s) {
        if !valid(c) {
            c = '-'
        }
        buf[j] = c
    }
    return string(buf)
}

// fixedName returns fix(field.Name).
func (field Field) fixedName() string {
    return fix(field.Name)
}