Stress is determined based on the syllables, not the vowels, so the
diphthongs need to count as a single syllable. If there is a diphthong
at the end of a word, the penultimate syllable can still be unstressed
if it is light. (The name of Arvedui, last king of Arthedain, is one
example of this that I could think of, but I’m sure it’s not the only
one.)

Both are copied from the Finnish phonemes, since Finnish was a major
inspiration for Quenya. This means that the ⟨iu⟩ diphthong is a
“falling” one – according to Appendix E of The Lord of the Rings, this
is the original pronunciation, but by the Third Age (the time in which
The Lord of the Rings is set) it had become a “rising” one, so I may
change the phoneme later, not sure.

According to Appendix E of The Lord of the Rings, ⟨ph⟩ stands for /f/
when final (because ⟨f⟩ is pronounced as /v/ in that position), and
otherwise is used instead of ⟨f⟩ either because it’s derived from ⟨p⟩
(in which case it’s presumably pronounced just like ⟨f⟩), or to
represent an especially long /f/. We can’t really tell which case we
have, but from the Omikhleia Sindarin dictionary [1], it appears that
all the long ⟨ph⟩’s are between two vowels, and all the short ones have
at least one adjacent consonant, so let’s use that as a rule and hope it
works out. (The Ambar Eldaron Quenya dictionary [2] is less easily
searchable, so I’m just hoping that this rule works reasonably well for
both languages.)

[1]: https://www.jrrvf.com/hisweloke/sindar/index.html
[2]: https://ambar-eldaron.com/telechargements/quenya-engl-A4.pdf

Not all of the diphthongs in Quenya and Sindarin are defined in the
Latin phonemes, and for now we’re sticking to those, so some diphthongs
just get TODOs for now. Also, we’re temporarily using the same phoneme
for ⟨ae⟩/⟨ai⟩ and ⟨oe⟩/⟨oi⟩, which should really be different (though
Appendix E of The Lord of the Rings notes that ⟨ae⟩ and ⟨oe⟩ don’t have
close English equivalents, and that they “may be pronounced as ai, oi”).

A circumflex “specially prolonged” vowels; according to Appendix E of
The Lord of the Rings, only in Sindarin, but the Ambar Eldaron Quenya
Dictionary [1] also has some very few circumflex words (sû, lîs), so
let’s support the circumflex in Quenya as well. Marking extra-long
vowels with two colons seems to work well and is also done in several
other languages.

[1]: https://ambar-eldaron.com/telechargements/quenya-engl-A4.pdf

Long vowels, marked with an acute accent or a circumflex, are longer
than short vowels (duh) and always make a heavy syllable (i.e. we don’t
include the rules to move stress to the previous syllable). In Quenya,
⟨é⟩ and ⟨ó⟩ are “tenser and ‘closer’” than the short vowels, according
to Appendix E of The Lord of the Rings, while in Sindarin they’re
supposed to be the same; the phonemes we inherit from Latin seem to
reproduce this reasonably well for Quenya, and for now we use them for
Sindarin too, which works nicely for the most common Sindarin word with
a long o, “Lothlórien” (because Lórien is actually a Quenya name, and
therefore I assume *that* ⟨ó⟩ should actually be /o/ and not /ɔ/). I
might adjust the phonemes later (at which point Lothlórien will
presumably have to go in sjn_list).

In Elvish languages, ⟨ch⟩, ⟨dh⟩, and ⟨th⟩ count as single consonants for
the purposes of stress, since they represent single letters in the
original scripts. The easiest way to implement this is to replace them
with single letters at the beginning – ⟨ð⟩ for ⟨dh⟩ and ⟨þ⟩ for ⟨th⟩ are
natural, and ⟨x⟩ for ⟨ch⟩ also makes some sense, though it means we need
to replace real ⟨x⟩ first (it’s not mentioned in Appendix E of The Lord
of the Rings, but does occur in some Quenya words, notably Helcaraxë).
Real ⟨x⟩ is pronounced like /ks/, but of course we need to spell that as
⟨cs⟩, since ⟨k⟩ does not occur in Elvish languages.

The Elvish languages have the same general stress rule as Latin [1]:
stress falls on the penultimate syllable if that is heavy (contains a
long vowel, a diphthong, or a vowel followed by two or more consonants),
otherwise on the antepenultimate syllable. For Latin, espeak-ng
implements this by declaring “penultimate syllable” as the general
stress rule in espeak-ng-data/, and then adding rules in dictsource/
that match light syllables and move the primary stress to the previous
syllable, i.e. the antepenultimate one. We use the same basic principle
for the Elvish languages here (but using the terms “heavy” and “light”
rather than “weak” and “strong” like in the Latin files).

Note that this doesn’t fully implement the stress rules yet: we have no
concept of diphthongs, long vowels aren’t really properly handled yet,
and we also still count ⟨ch⟩, ⟨dh⟩, ⟨th⟩ as two consonants rather than
one as it should be. This will be improved separately (I prefer doing
this in small incremental steps).

[1]: https://menegroth.github.io/stress-in-sindarin.html

Copied from Sindarin with only a few changes so far: initial ⟨i⟩ is not
special, ⟨qu⟩ exists, and ⟨y⟩ is a consonant.

This prepares the languages of Quenya and Sindarin, setting up their
infrastructure without declaring a lot of rules yet – just enough for
“Eä” (a Quenya word, but I can’t think of a similarly simple one for
Sindarin). Phonemes are inherited from Esperanto for now.