ntt.nim

import params, reduce

# NOTE: bit-reversal table used for reordering polynomial coefficients in NTT and inverse NTT operations.
const bitrevTable: array[PARAM_N, uint16] = [
  0,512,256,768,128,640,384,896,64,576,320,832,192,704,448,960,32,544,288,800,160,672,416,928,96,608,352,864,224,736,480,992,
  16,528,272,784,144,656,400,912,80,592,336,848,208,720,464,976,48,560,304,816,176,688,432,944,112,624,368,880,240,752,496,1008,
  8,520,264,776,136,648,392,904,72,584,328,840,200,712,456,968,40,552,296,808,168,680,424,936,104,616,360,872,232,744,488,1000,
  24,536,280,792,152,664,408,920,88,600,344,856,216,728,472,984,56,568,312,824,184,696,440,952,120,632,376,888,248,760,504,1016,
  4,516,260,772,132,644,388,900,68,580,324,836,196,708,452,964,36,548,292,804,164,676,420,932,100,612,356,868,228,740,484,996,
  20,532,276,788,148,660,404,916,84,596,340,852,212,724,468,980,52,564,308,820,180,692,436,948,116,628,372,884,244,756,500,1012,
  12,524,268,780,140,652,396,908,76,588,332,844,204,716,460,972,44,556,300,812,172,684,428,940,108,620,364,876,236,748,492,1004,
  28,540,284,796,156,668,412,924,92,604,348,860,220,732,476,988,60,572,316,828,188,700,444,956,124,636,380,892,252,764,508,1020,
  2,514,258,770,130,642,386,898,66,578,322,834,194,706,450,962,34,546,290,802,162,674,418,930,98,610,354,866,226,738,482,994,
  18,530,274,786,146,658,402,914,82,594,338,850,210,722,466,978,50,562,306,818,178,690,434,946,114,626,370,882,242,754,498,1010,
  10,522,266,778,138,650,394,906,74,586,330,842,202,714,458,970,42,554,298,810,170,682,426,938,106,618,362,874,234,746,490,1002,
  26,538,282,794,154,666,410,922,90,602,346,858,218,730,474,986,58,570,314,826,186,698,442,954,122,634,378,890,250,762,506,1018,
  6,518,262,774,134,646,390,902,70,582,326,838,198,710,454,966,38,550,294,806,166,678,422,934,102,614,358,870,230,742,486,998,
  22,534,278,790,150,662,406,918,86,598,342,854,214,726,470,982,54,566,310,822,182,694,438,950,118,630,374,886,246,758,502,1014,
  14,526,270,782,142,654,398,910,78,590,334,846,206,718,462,974,46,558,302,814,174,686,430,942,110,622,366,878,238,750,494,1006,
  30,542,286,798,158,670,414,926,94,606,350,862,222,734,478,990,62,574,318,830,190,702,446,958,126,638,382,894,254,766,510,1022,
  1,513,257,769,129,641,385,897,65,577,321,833,193,705,449,961,33,545,289,801,161,673,417,929,97,609,353,865,225,737,481,993,
  17,529,273,785,145,657,401,913,81,593,337,849,209,721,465,977,49,561,305,817,177,689,433,945,113,625,369,881,241,753,497,1009,
  9,521,265,777,137,649,393,905,73,585,329,841,201,713,457,969,41,553,297,809,169,681,425,937,105,617,361,873,233,745,489,1001,
  25,537,281,793,153,665,409,921,89,601,345,857,217,729,473,985,57,569,313,825,185,697,441,953,121,633,377,889,249,761,505,1017,
  5,517,261,773,133,645,389,901,69,581,325,837,197,709,453,965,37,549,293,805,165,677,421,933,101,613,357,869,229,741,485,997,
  21,533,277,789,149,661,405,917,85,597,341,853,213,725,469,981,53,565,309,821,181,693,437,949,117,629,373,885,245,757,501,1013,
  13,525,269,781,141,653,397,909,77,589,333,845,205,717,461,973,45,557,301,813,173,685,429,941,109,621,365,877,237,749,493,1005,
  29,541,285,797,157,669,413,925,93,605,349,861,221,733,477,989,61,573,317,829,189,701,445,957,125,637,381,893,253,765,509,1021,
  3,515,259,771,131,643,387,899,67,579,323,835,195,707,451,963,35,547,291,803,163,675,419,931,99,611,355,867,227,739,483,995,
  19,531,275,787,147,659,403,915,83,595,339,851,211,723,467,979,51,563,307,819,179,691,435,947,115,627,371,883,243,755,499,1011,
  11,523,267,779,139,651,395,907,75,587,331,843,203,715,459,971,43,555,299,811,171,683,427,939,107,619,363,875,235,747,491,1003,
  27,539,283,795,155,667,411,923,91,603,347,859,219,731,475,987,59,571,315,827,187,699,443,955,123,635,379,891,251,763,507,1019,
  7,519,263,775,135,647,391,903,71,583,327,839,199,711,455,967,39,551,295,807,167,679,423,935,103,615,359,871,231,743,487,999,
  23,535,279,791,151,663,407,919,87,599,343,855,215,727,471,983,55,567,311,823,183,695,439,951,119,631,375,887,247,759,503,1015,
  15,527,271,783,143,655,399,911,79,591,335,847,207,719,463,975,47,559,303,815,175,687,431,943,111,623,367,879,239,751,495,1007,
  31,543,287,799,159,671,415,927,95,607,351,863,223,735,479,991,63,575,319,831,191,703,447,959,127,639,383,895,255,767,511,1023
]

proc bitrevVector*(poly: var array[PARAM_N, uint16]) =
  ## reorder poly array elements according to bit-reversed indices for FFT
  var r: uint16

  for i in 0 ..< PARAM_N:
    r = bitrevTable[i] # get bit-reversed index
    if uint16(i) < r: # ensure swap only happens once per pair
      swap(poly[i], poly[r])


proc mulCoefficients*(poly: var array[PARAM_N, uint16], factors: array[PARAM_N, uint16]) =
  ## multiply each coefficient of poly by corresponding factor and reduce
  for i in 0 ..< PARAM_N:
    poly[i] = montgomeryReduce(uint32(poly[i]) * uint32(factors[i]))


proc ntt*(a: var array[PARAM_N, uint16], omega: array[PARAM_N div 2, uint16]) =
  ## perform the Number Theoretic Transform (NTT) in-place on array `a`
  var
    jTwiddle: int
    distance: int
    temp: uint16
    W: uint16

  for i in countup(0, 9, 2): # loop over log2(PARAM_N) levels
    distance = 1 shl i # distance between butterfly inputs
    for start in 0 ..< distance:
      jTwiddle = 0 # twiddle factor index
      for j in countup(start, PARAM_N-1, 2*distance):
        W = omega[jTwiddle]
        inc jTwiddle
        temp = a[j]
        # even level: simple addition and subtraction
        a[j] = temp + a[j + distance]
        # odd level: use Barrett and Montgomery reductions
        a[j + distance] = montgomeryReduce(uint32(W) * (uint32(temp) + 3*PARAM_Q - uint32(a[j + distance])))
    
    distance = distance shl 1 # double distance for next level
    for start in 0 ..< distance:
      jTwiddle = 0
      for j in countup(start, PARAM_N-1, 2*distance):
        W = omega[jTwiddle]
        inc jTwiddle
        temp = a[j]
        a[j] = barrettReduce(temp + a[j + distance])
        a[j + distance] = montgomeryReduce(uint32(W) * (uint32(temp) + 3*PARAM_Q - uint32(a[j + distance])))