"""Literal NFC character codec for the experimental compact TrOCR decoder.

Tokenizer BOS0 is serialization only; training targets exclude it. Generation
starts with EOS2 and suppresses BOS/PAD/UNK. Unknown outputs remain visible as
replacement characters when decoded, never silently removed.
"""
import unicodedata


class CharacterCodec:
    def __init__(self, characters):
        if len(characters) != len(set(characters)) or any(len(c) != 1 for c in characters):
            raise ValueError('Require ordered, unique Unicode codepoints')
        self.characters = list(characters)
        self.native = set(characters)
        self.character_ids = {c: i + 4 for i, c in enumerate(characters)}
        self.id_characters = {i: c for c, i in self.character_ids.items()}
        self.classes = len(characters) + 4

    def encode(self, text):
        if not text or text != unicodedata.normalize('NFC', text).strip():
            raise ValueError('Require nonempty, stripped NFC text')
        if any(c not in self.native for c in text):
            raise ValueError('Text outside the fixed OCR alphabet')
        return [0, *[self.character_ids[c] for c in text], 2], 'characters'

    def decode(self, ids):
        pieces = []
        for index in ids:
            if index in [0, 1, 2]:
                continue
            if index == 3:
                pieces.append('\ufffd')
            elif index in self.id_characters:
                pieces.append(self.id_characters[index])
            else:
                raise ValueError('Unassigned character output ID')
        return unicodedata.normalize('NFC', ''.join(pieces))

    def manifest(self):
        return {'version': 1, 'kind': 'literal_NFC_characters', 'classes': self.classes,
                'special_ids': {'BOS_serialization': 0, 'PAD': 1, 'EOS_and_decoder_start': 2, 'UNK': 3},
                'character_ids': self.character_ids, 'generation_suppressed_ids': [0, 1, 3],
                'training_target': 'Drop serialization BOS0 only; retain every text codepoint and EOS2',
                'unknown_output': 'Decode3 as U+FFFD, never silently remove it',
                'scope': 'Experimental codec, not recognition quality or Paddle qualification.'}