--Make Gradient library for TIC-80. Useful for effects such as changing color
--on scanline or palette animation, all in just over 1600 bytes (except this info).
--Two functions you need to be aware of: make_gradient and make gradient_direct.
--Both take the same parameters (r1,g1,b1,r2,g2,b2,steps) where first six are two
--sets of rgb values of opposite colors and the final is how long the gradient will
--be (in pixels) and both return an array of rgb values (as records, usable by
--out[index].r,etc.). However make_gradient also first converts color to HSV, which is
--usable when you want a rainbow-like pattern.

function lerp(t,a,b)
    return a+(b-a)*t
end

function rgbToHsv(r, g, b)
  r,g,b=r/255,g/255,b/255
  local max,min=math.max(r,g,b),math.min(r,g,b)
  local h,s,v
  v=max
  local d=max-min
  if max==0 then s=0 else s=d/max end
  if max==min then
    h=0
  else
    if max==r then
    h=(g-b)/d
    if g<b then h=h+6 end elseif max==g then h=(b-r)/d+2  elseif max==b then h=(r-g)/d+4 end
    h=h/6
  end
  return h,s,v
end

function hsvToRgb(h,s,v)
  local r,g,b
  local i=math.floor(h*6);
  local f=h*6-i;
  local p=v*(1-s);
  local q=v*(1-f*s);
  local t=v*(1-(1-f)*s);
        i=i%6
        if i==0 then r,g,b=v,t,p elseif i==1 then r,g,b=q,v,p elseif i==2 then r,g,b=p,v,t elseif i==3 then r,g,b=p,q,v elseif i==4 then r,g,b=t,p,v elseif i==5 then r,g,b=v,p,q end
  return math.floor(r*255),math.floor(g*255),math.floor(b*255)
end

function make_gradient(r1,g1,b1,r2,g2,b2,steps)
steps = math.abs(steps)
local out = {}
local h1=0
local s1=0
local v1=0
h1,s1,v1=rgbToHsv(r1,g1,b1)
local h2=0
local s2=0
local v2=0
h2,s2,v2=rgbToHsv(r2,g2,b2)
local stepamount = 1/steps
local prog =0
local i=0
    for i=1,steps,1 do
        local temph=lerp(prog,h1,h2)
        local temps=lerp(prog,s1,s2)
        local tempv=lerp(prog,v1,v2)
        out[i]={}
        out[i].r,out[i].g,out[i].b=hsvToRgb(temph,temps,tempv)
        prog=prog+stepamount
    end
return out
end
function make_gradient_direct(r1,g1,b1,r2,g2,b2,steps)
steps = math.abs(steps)
local out = {}
local stepamount = 1/steps
local prog =0
local i=0
    for i=1,steps,1 do
        out[i]={}
        out[i].r=lerp(prog,r1,r2)
        out[i].g=lerp(prog,g1,g2)
        out[i].b=lerp(prog,b1,b2)
        prog=prog+stepamount
    end
return out
end