bsnes/nall/cd/efm.hpp

#pragma once

//eight-to-fourteen modulation:
//separates each 1-bit by at least two 0-bits and at most ten 0-bits

namespace nall::CD::EFM {

//the algorithm to generate this table is unknown
inline auto lookup(uint8_t index) -> uint16_t {
  static const uint16_t lookup[256] = {
    0x1220, 0x2100, 0x2420, 0x2220, 0x1100, 0x0110, 0x0420, 0x0900,
    0x1240, 0x2040, 0x2440, 0x2240, 0x1040, 0x0040, 0x0440, 0x0840,
    0x2020, 0x2080, 0x2480, 0x0820, 0x1080, 0x0080, 0x0480, 0x0880,
    0x1210, 0x2010, 0x2410, 0x2210, 0x1010, 0x0210, 0x0410, 0x0810,
    0x0020, 0x2108, 0x0220, 0x0920, 0x1108, 0x0108, 0x1020, 0x0908,
    0x1248, 0x2048, 0x2448, 0x2248, 0x1048, 0x0048, 0x0448, 0x0848,
    0x0100, 0x2088, 0x2488, 0x2110, 0x1088, 0x0088, 0x0488, 0x0888,
    0x1208, 0x2008, 0x2408, 0x2208, 0x1008, 0x0208, 0x0408, 0x0808,
    0x1224, 0x2124, 0x2424, 0x2224, 0x1124, 0x0024, 0x0424, 0x0924,
    0x1244, 0x2044, 0x2444, 0x2244, 0x1044, 0x0044, 0x0444, 0x0844,
    0x2024, 0x2084, 0x2484, 0x0824, 0x1084, 0x0084, 0x0484, 0x0884,
    0x1204, 0x2004, 0x2404, 0x2204, 0x1004, 0x0204, 0x0404, 0x0804,
    0x1222, 0x2122, 0x2422, 0x2222, 0x1122, 0x0022, 0x1024, 0x0922,
    0x1242, 0x2042, 0x2442, 0x2242, 0x1042, 0x0042, 0x0442, 0x0842,
    0x2022, 0x2082, 0x2482, 0x0822, 0x1082, 0x0082, 0x0482, 0x0882,
    0x1202, 0x0248, 0x2402, 0x2202, 0x1002, 0x0202, 0x0402, 0x0802,
    0x1221, 0x2121, 0x2421, 0x2221, 0x1121, 0x0021, 0x0421, 0x0921,
    0x1241, 0x2041, 0x2441, 0x2241, 0x1041, 0x0041, 0x0441, 0x0841,
    0x2021, 0x2081, 0x2481, 0x0821, 0x1081, 0x0081, 0x0481, 0x0881,
    0x1201, 0x2090, 0x2401, 0x2201, 0x1090, 0x0201, 0x0401, 0x0890,
    0x0221, 0x2109, 0x1110, 0x0121, 0x1109, 0x0109, 0x1021, 0x0909,
    0x1249, 0x2049, 0x2449, 0x2249, 0x1049, 0x0049, 0x0449, 0x0849,
    0x0120, 0x2089, 0x2489, 0x0910, 0x1089, 0x0089, 0x0489, 0x0889,
    0x1209, 0x2009, 0x2409, 0x2209, 0x1009, 0x0209, 0x0409, 0x0809,
    0x1120, 0x2111, 0x2490, 0x0224, 0x1111, 0x0111, 0x0490, 0x0911,
    0x0241, 0x2101, 0x0244, 0x0240, 0x1101, 0x0101, 0x0090, 0x0901,
    0x0124, 0x2091, 0x2491, 0x2120, 0x1091, 0x0091, 0x0491, 0x0891,
    0x1211, 0x2011, 0x2411, 0x2211, 0x1011, 0x0211, 0x0411, 0x0811,
    0x1102, 0x0102, 0x2112, 0x0902, 0x1112, 0x0112, 0x1022, 0x0912,
    0x2102, 0x2104, 0x0249, 0x0242, 0x1104, 0x0104, 0x0422, 0x0904,
    0x0122, 0x2092, 0x2492, 0x0222, 0x1092, 0x0092, 0x0492, 0x0892,
    0x1212, 0x2012, 0x2412, 0x2212, 0x1012, 0x0212, 0x0412, 0x0812,
  };
  return lookup[index];
}

//

inline auto encode(uint8_t data) -> uint16_t {
  return lookup(data);
}

//

inline auto decode(uint16_t data) -> maybe<uint8_t> {
  static uint16_t table[1 << 14];
  static bool once = true;
  if(once) {
    once = false;
    for(uint n : range(1 << 14)) table[n] = 0xffff;
    for(uint n : range(1 <<  8)) table[lookup(n)] = n;
  }
  uint16_t result = table[data & 0x3fff];
  if(result == 0xffff) return {};
  return (uint8_t)result;
}

}