depth calculation functions

--Returns interpolated X value
local function intY(a,b,y)
    return a.x + ((y - a.y) * (b.x - a.x)) / (b.y - a.y)
end

--Returns interpolated Y value
local function intX(a,b,x)
    return a.y + ((b.y - a.y) * (x - a.x)) / (b.x - a.x)
end

--get the t value for a point (p) somewhere between the line formed from two give points (a,b)
local function getT(a,b,p)
    local v1 = vector.new( a.x-b.x, a.y-b.y )
    local v2 = vector.new( a.x-p.x, a.y-p.y )
    return (v1:dot(v2)) / (v1:dot(v1))
end

--use the t value of a point to interpolate between two given values (v1,v2)
--for interpolating between z's, v1 = starting z value, v2 = ending z value
local function lerp(v1,v2,t)
    return (1 - t) * v1 + t * v2
end

local function pPix(x,y,c)
    term.setCursorPos(x,y)
    term.setBackgroundColor(colors.white)
    term.setTextColor(colors.black)
    term.write(c)
    term.setBackgroundColor(colors.black)
    term.setTextColor(colors.white)
end


local function fillLerp(a,b,c)
    if a.y < b.y then a,b = b,a end
    if a.y < c.y then a,c = c,a end
    if b.x > c.x then b,c = c,b end

    for y = b.y, a.y-1 do
        local xStart = intY(a,b,y)
        local xEnd = intY(a,c,y)

        local p1 = vector.new(xStart,y)
        local p2 = vector.new(xEnd,y)

        local t1 = getT(a,b,p1)
        local t2 = getT(a,c,p2)

        local z1 = lerp(a.z,b.z,t1)
        local z2 = lerp(a.z,c.z,t2)

        for x = xStart, xEnd do
            local t3 = (x - xStart) / (xEnd - xStart)
            local z = math.floor(lerp(z1,z2,t3))
            pPix(x,y,z)
        end
    end
end

local function fillInt(a,b,c)
    if a.y < b.y then a,b = b,a end
    if a.y < c.y then a,c = c,a end
    if b.x > c.x then b,c = c,b end

    for y = b.y, a.y-1 do
        local xStart = intY(a,b,y)
        local xEnd = intY(a,c,y)

        local p1 = vector.new(xStart,y)
        local p2 = vector.new(xEnd,y)

        local t1 = getT(a,b,p1)
        local t2 = getT(a,c,p2)

        local z1 = lerp(a.z,b.z,t1)
        local z2 = lerp(a.z,c.z,t2)

        local startScanDepth = { x = xStart, y = z1}
        local endScanDepth = { x = xEnd, y = z2}

        for x = xStart, xEnd do
            local z = math.floor( intX( startScanDepth , endScanDepth, x ) )
            pPix(x,y,z)
        end
    end
end

local function fillYInt(a,b,c)
    if a.y < b.y then a,b = b,a end
    if a.y < c.y then a,c = c,a end
    if b.x > c.x then b,c = c,b end

    for y = b.y, a.y-1 do
        local xStart = intY(a,b,y)
        local xEnd = intY(a,c,y)

        local azp = vector.new(a.z,a.y)
        local bzp = vector.new(b.z,b.y)
        local czp = vector.new(c.z,c.y)

        local z1 = intY(azp,bzp,y)
        local z2 = intY(azp,czp,y)

        local startScanDepth = { x = z1, y = xStart}
        local endScanDepth = { x = z2, y = xEnd}

        for x = xStart, xEnd do
            local z = math.floor( intY( startScanDepth , endScanDepth, x ) )
            pPix(x,y,z)
        end
    end
end

local fill = fillYInt

local a = vector.new(20,20,9)
local b = vector.new(2,2,5)
local c = vector.new(30,2,8)

term.clear()
fill(a,b,c)