/*
 * Copyright (c) 2008-2009 Bjoern Hoehrmann <[email protected]>
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy of this software
 * and associated documentation files (the "Software"), to deal in the Software without
 * restriction, including without limitation the rights to use, copy, modify, merge, publish,
 * distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all copies or
 * substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING
 * BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
 * DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 */

// See https://bjoern.hoehrmann.de/utf-8/decoder/dfa/ for more information on how this works.

/// Decoder ground state.
const OK: u8 =   0;
/// Decoder error state.
const ER: u8 =  96;
/// Decoder error state. Offending byte should be passed in again ("rewind")
const RW: u8 = 108;

const UTF8_TABLE: [u8; 256+96] = [
    // Maps bytes to character classes
     0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0x00
     0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, // 0x80
     9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, // 0x90
     7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, // 0xA0
     7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
     8, 8, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, // 0xC0
     2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
    10, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 3, 3, // 0xE0
    11, 6, 6, 6, 5, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, // 0xF0

    // Maps state (row) + character class (column) to next state
    OK, ER, 12, 24, 48, 84, 72, ER, ER, ER, 36, 60, //  0 - OK
    RW, OK, RW, RW, RW, RW, RW, OK, RW, OK, RW, RW, // 12 - 1 byte  needed
    RW, 12, RW, RW, RW, RW, RW, 12, RW, 12, RW, RW, // 24 - 2 bytes needed
    RW, RW, RW, RW, RW, RW, RW, 12, RW, RW, RW, RW, // 36 - 2 bytes needed, E0 lead
    RW, 12, RW, RW, RW, RW, RW, RW, RW, 12, RW, RW, // 48 - 2 bytes needed, ED lead
    RW, RW, RW, RW, RW, RW, RW, 24, RW, 24, RW, RW, // 60 - 3 bytes needed, F0 lead
    RW, 24, RW, RW, RW, RW, RW, 24, RW, 24, RW, RW, // 72 - 3 bytes needed
    RW, 24, RW, RW, RW, RW, RW, RW, RW, RW, RW, RW, // 84 - 3 bytes needed, F4 lead
];

#[derive(Copy, Clone, Debug, Default)]
pub struct UTF8Decoder {
    code_point: u32,
    state: u8
}

#[derive(Copy, Clone, Eq, PartialEq, Debug)]
pub enum DecodeState {
    Done(char),
    Continue,
    Error,
    Rewind
}

impl UTF8Decoder {    
    #[inline]
    pub fn reset(&mut self) {
        self.state = 0;
    }
    
    pub fn decode_byte(&mut self, byte: u8) -> DecodeState {
        let class = UTF8_TABLE[byte as usize];
        
        self.code_point =
            if self.state == OK {
                // The character class values for leading bytes simultaneously form a bitmask.
                // For class 0, this is a no-op
                // For classes > 7, this is 0 (continuations & invalid bytes)
                (0xFF >> class) & byte as u32
            } else {
                // Standard continuation byte extraction.
                // It's okay if this is gibberish due to invalid input,
                // errors reset state to OK, and code_point gets cleared on the next input.
                (self.code_point << 6) | (byte as u32 & 0x3F)
            };
        
        unsafe {
            // The compiler can't verify this access is always in bounds, but it is, I promise.
            self.state = *UTF8_TABLE.get_unchecked(256 + self.state as usize + class as usize);
        
            match self.state {
                // Surrogate or out of bounds code points will be rejected, so this is safe.
                OK => DecodeState::Done(std::char::from_u32_unchecked(self.code_point)),
                ER => { self.reset(); DecodeState::Error  },
                RW => { self.reset(); DecodeState::Rewind },
                _  => DecodeState::Continue
            }
        }
    }
}