-- Module:WavelengthToRGB
-- Converts a wavelength (380–780 nm) to sRGB hex color
-- Includes smooth edge fade and blue compression for perceptually correct colors

local p = {}

-- ============================================================================
-- CIE 1931 2° CMFs and D65 illuminant (5 nm steps, 380–780 nm)
-- ============================================================================
local data = {
{380, 0.001368, 0.000039, 0.006450, 49.98},
{385, 0.002236, 0.000064, 0.010550, 52.31},
{390, 0.004243, 0.000120, 0.020050, 54.65},
{395, 0.007650, 0.000217, 0.036210, 68.70},
{400, 0.014310, 0.000396, 0.067850, 82.75},
{405, 0.023190, 0.000640, 0.110200, 87.12},
{410, 0.043510, 0.001210, 0.207400, 91.49},
{415, 0.077630, 0.002180, 0.371300, 92.46},
{420, 0.134380, 0.004000, 0.645600, 93.43},
{425, 0.214770, 0.007300, 1.039050, 90.06},
{430, 0.283900, 0.011600, 1.385600, 86.68},
{435, 0.328500, 0.016840, 1.622960, 95.77},
{440, 0.348280, 0.023000, 1.747060,104.86},
{445, 0.348060, 0.029800, 1.782600,110.94},
{450, 0.336200, 0.038000, 1.772110,117.01},
{455, 0.318700, 0.048000, 1.744100,117.41},
{460, 0.290800, 0.060000, 1.669200,117.81},
{465, 0.251100, 0.073900, 1.528100,116.34},
{470, 0.195360, 0.090980, 1.287640,114.86},
{475, 0.142100, 0.112600, 1.041900,115.39},
{480, 0.095640, 0.139020, 0.812950,115.92},
{485, 0.057950, 0.169300, 0.616200,112.37},
{490, 0.032010, 0.208020, 0.465180,108.81},
{495, 0.014700, 0.258600, 0.353300,109.35},
{500, 0.004900, 0.323000, 0.272000,109.88},
{505, 0.002400, 0.407300, 0.212300,108.65},
{510, 0.009300, 0.503000, 0.158200,107.41},
{515, 0.029100, 0.608200, 0.111700,106.18},
{520, 0.063270, 0.710000, 0.078250,104.94},
{525, 0.109600, 0.793200, 0.057250,104.74},
{530, 0.165500, 0.862000, 0.042160,104.54},
{535, 0.225750, 0.914850, 0.029840,104.33},
{540, 0.290400, 0.954000, 0.020300,104.13},
{545, 0.359700, 0.980300, 0.013400,103.94},
{550, 0.433450, 0.994950, 0.008750,103.74},
{555, 0.512050, 1.000000, 0.005750,103.54},
{560, 0.594500, 0.995000, 0.003900,103.34},
{565, 0.678400, 0.978600, 0.002750,103.13},
{570, 0.762100, 0.952000, 0.002100,102.93},
{575, 0.842500, 0.915400, 0.001800,102.73},
{580, 0.916300, 0.870000, 0.001650,102.52},
{585, 0.978600, 0.816300, 0.001400,102.32},
{590, 1.026300, 0.757000, 0.001100,102.12},
{595, 1.056700, 0.694900, 0.001000,101.92},
{600, 1.062200, 0.631000, 0.000800,101.72},
{605, 1.045600, 0.566800, 0.000600,101.52},
{610, 1.002600, 0.503000, 0.000340,101.31},
{615, 0.938400, 0.441200, 0.000240,101.11},
{620, 0.854450, 0.381000, 0.000190,100.91},
{625, 0.751400, 0.321000, 0.000100,100.71},
{630, 0.642400, 0.265000, 0.000050,100.51},
{635, 0.541900, 0.217000, 0.000030,100.31},
{640, 0.447900, 0.175000, 0.000020,100.11},
{645, 0.360800, 0.138200, 0.000010, 99.91},
{650, 0.283500, 0.107000, 0.000000, 99.71},
{655, 0.218700, 0.081600, 0.000000, 99.51},
{660, 0.164900, 0.061000, 0.000000, 99.31},
{665, 0.121200, 0.044580, 0.000000, 99.11},
{670, 0.087400, 0.032000, 0.000000, 98.91},
{675, 0.063600, 0.023200, 0.000000, 98.71},
{680, 0.046770, 0.017000, 0.000000, 98.51},
{685, 0.032900, 0.011920, 0.000000, 98.31},
{690, 0.022700, 0.008210, 0.000000, 98.11},
{695, 0.015840, 0.005723, 0.000000, 97.91},
{700, 0.011359, 0.004102, 0.000000, 97.71},
{705, 0.008111, 0.002929, 0.000000, 97.51},
{710, 0.005790, 0.002091, 0.000000, 97.31},
{715, 0.004109, 0.001484, 0.000000, 97.11},
{720, 0.002899, 0.001047, 0.000000, 96.91},
{725, 0.002049, 0.000740, 0.000000, 96.71},
{730, 0.001440, 0.000520, 0.000000, 96.51},
{735, 0.001000, 0.000361, 0.000000, 96.31},
{740, 0.000690, 0.000249, 0.000000, 96.11},
{745, 0.000476, 0.000172, 0.000000, 95.91},
{750, 0.000332, 0.000120, 0.000000, 95.71},
{755, 0.000235, 0.000085, 0.000000, 95.51},
{760, 0.000166, 0.000060, 0.000000, 95.31},
{765, 0.000117, 0.000042, 0.000000, 95.11},
{770, 0.000083, 0.000030, 0.000000, 94.91},
{775, 0.000059, 0.000021, 0.000000, 94.71},
{780, 0.000042, 0.000015, 0.000000, 94.51},
}

-- ============================================================================
-- Utilities
-- ============================================================================
local function lerp(a, b, t) return a + (b-a)*t end
local function clamp(x) if x<0 then return 0 elseif x>1 then return 1 else return x end end
local function srgb_gamma(c)
    if c <= 0.0031308 then return 12.92*c else return 1.055*(c^(1/2.4))-0.055 end
end

-- ============================================================================
-- Main conversion
-- ============================================================================
function p.fromWavelength(frame)
    local wl = tonumber(frame.args[1])
    if not wl or wl < 380 or wl > 780 then return "000000" end

    -- find surrounding samples
    local i = 1
    while i < #data and data[i+1][1] < wl do i = i + 1 end
    local a, b = data[i], data[i+1] or data[i]
    local t = (wl - a[1]) / ((b[1]-a[1]) ~= 0 and (b[1]-a[1]) or 1)

    local x = lerp(a[2], b[2], t)
    local y = lerp(a[3], b[3], t)
    local z = lerp(a[4], b[4], t)
    local d = lerp(a[5], b[5], t)

    local X, Y, Z = x*d, y*d, z*d

    -- linear sRGB
    local r = 3.2406*X - 1.5372*Y - 0.4986*Z
    local g = -0.9689*X + 1.8758*Y + 0.0415*Z
    local b = 0.0557*X - 0.2040*Y + 1.0570*Z

    -- smooth edge fade
    local f = 1
    if wl < 420 then f = 0.5*(1 - math.cos(math.pi * math.min(1, math.max(0, (wl-380)/40)))) end
    if wl > 700 then f = 0.5*(1 - math.cos(math.pi * math.min(1, math.max(0, (780-wl)/80)))) end
    r, g, b = r*f, g*f, b*f

    -- compress extreme blues
    if wl < 470 then local s = 0.8 + 0.2*(wl-380)/(470-380); r,g,b = r*s,g*s,b*s end

    -- scale to fit sRGB gamut
    local maxc = math.max(r,g,b,0)
    if maxc > 1 then r,g,b = r/maxc, g/maxc, b/maxc end

    -- gamma + clamp
    r = clamp(srgb_gamma(r))
    g = clamp(srgb_gamma(g))
    b = clamp(srgb_gamma(b))

    return string.format("%02X%02X%02X", math.floor(r*255+0.5), math.floor(g*255+0.5), math.floor(b*255+0.5))
end

return p