(This post is partly self-plagiarized.)

Objective

Given a Hangul syllable, toggle its vowel harmony.

Introduction to Hangul syllables

Hangul(한글) is the Korean writing system invented by Sejong the Great. Hangul syllables are allocated in Unicode point U+AC00 – U+D7A3. A Hangul syllable consists of an initial consonant, a vowel, and an optional final consonant.

The initial consonants are:

ᄀ ᄁ ᄂ ᄃ ᄄ ᄅ ᄆ ᄇ ᄈ ᄉ ᄊ ᄋ ᄌ ᄍ ᄎ ᄏ ᄐ ᄑ ᄒ

The vowels are:

ᅡ ᅢ ᅣ ᅤ ᅥ ᅦ ᅧ ᅨ ᅩ ᅪ ᅫ ᅬ ᅭ ᅮ ᅯ ᅰ ᅱ ᅲ ᅳ ᅴ ᅵ

The final consonants are:

(none) ᄀ ᄁ ᆪ ᄂ ᆬ ᆭ ᄃ ᄅ ᆰ ᆱ ᆲ ᆳ ᆴ ᆵ ᄚ ᄆ ᄇ ᄡ ᄉ ᄊ ᄋ ᄌ ᄎ ᄏ ᄐ ᄑ ᄒ

For example, 뷁 has initial consonant ᄇ, vowel ᅰ, and final consonant ᆰ.

South Korean dictionary order

The consonants and vowels above are sorted in South Korean dictionary order. The syllables are firstly sorted by initial consonants, secondly by vowels, and finally by (optional) final consonants.

The Unicode block for Hangul syllables contains every consonant/vowel combinations, and is entirely sorted in South Korean dictionary order.

The Unicode block can be seen here, and the first 256 characters are shown for illustrative purpose:

가각갂갃간갅갆갇갈갉갊갋갌갍갎갏감갑값갓갔강갖갗갘같갚갛개객갞갟갠갡갢갣갤갥갦갧갨갩갪갫갬갭갮갯갰갱갲갳갴갵갶갷갸갹갺갻갼갽갾갿걀걁걂걃걄걅걆걇걈걉걊걋걌걍걎걏걐걑걒걓걔걕걖걗걘걙걚걛걜걝걞걟걠걡걢걣걤걥걦걧걨걩걪걫걬걭걮걯거걱걲걳건걵걶걷걸걹걺걻걼걽걾걿검겁겂것겄겅겆겇겈겉겊겋게겍겎겏겐겑겒겓겔겕겖겗겘겙겚겛겜겝겞겟겠겡겢겣겤겥겦겧겨격겪겫견겭겮겯결겱겲겳겴겵겶겷겸겹겺겻겼경겾겿곀곁곂곃계곅곆곇곈곉곊곋곌곍곎곏곐곑곒곓곔곕곖곗곘곙곚곛곜곝곞곟고곡곢곣곤곥곦곧골곩곪곫곬곭곮곯곰곱곲곳곴공곶곷곸곹곺곻과곽곾곿

Vowel Harmony

Korean vowels express vowel harmony as positive-negative pairs. They're paired like the followings:

(Positive) - (Negative)
ᅡ - ᅥ
ᅢ - ᅦ
ᅣ - ᅧ
ᅤ - ᅨ
ᅩ - ᅮ
ᅪ - ᅯ
ᅫ - ᅰ
ᅬ - ᅱ
ᅭ - ᅲ

Note that ᅳ, ᅴ, and ᅵ lack counterparts. More accurately, ᅵ is neither positive nor negative. ᅳ and ᅴ are negative, but their positive counterparts have vanished historically. As such, Hangul syllables whose vowel is ᅳ, ᅴ, or ᅵ are considered to be an invalid input.

I/O format

Flexible. In particular, I/O in Unicode codepoints are okay.

Examples

뷁 → 봵
냥 → 녕
멍 → 망
망 → 멍