-- local curve25519 = require("curve25519")
-- curve25519(private_key, peer_public_value)

if not load("return (1>>1) == 0 and (1<<1) == 2 and (1&1) == 1 and (1|0) == 1") then
    error("bitwise operators not available; need Lua 5.3+")
end

local kBottom25Bits = 0x1ffffff
local kBottom26Bits = 0x3ffffff
local kTop39Bits = 0xfffffffffe000000
local kTop38Bits = 0xfffffffffc000000

local function load_3(i, offset)
    return i[1 + offset] | (i[2 + offset] << 8) | (i[3 + offset] << 16)
end

local function load_4(i, offset)
    return i[1 + offset] | (i[2 + offset] << 8) | (i[3 + offset] << 16) | (i[4 + offset] << 24)
end

local function arshift(x, n)
    n = n & 63
    if n == 0 then return x end
    if x >= 0 then
        return x >> n
    else
        return (x >> n) | (~0 << (64 - n))
    end
end

local function fe_frombytes(s)
    local h = {
        load_4(s, 0),
        load_3(s, 4) << 6,
        load_3(s, 7) << 5,
        load_3(s, 10) << 3,
        load_3(s, 13) << 2,
        load_4(s, 16),
        load_3(s, 20) << 7,
        load_3(s, 23) << 5,
        load_3(s, 26) << 4,
        (load_3(s, 29) & 0x7fffff) << 2
    }
    local carry
    carry = h[10] + (1 << 24); h[1]  = h[1]  + arshift(carry, 25) * 19; h[10] = h[10] - (carry & kTop39Bits)
    carry = h[2]  + (1 << 24); h[3]  = h[3]  + arshift(carry, 25);      h[2]  = h[2]  - (carry & kTop39Bits)
    carry = h[4]  + (1 << 24); h[5]  = h[5]  + arshift(carry, 25);      h[4]  = h[4]  - (carry & kTop39Bits)
    carry = h[6]  + (1 << 24); h[7]  = h[7]  + arshift(carry, 25);      h[6]  = h[6]  - (carry & kTop39Bits)
    carry = h[8]  + (1 << 24); h[9]  = h[9]  + arshift(carry, 25);      h[8]  = h[8]  - (carry & kTop39Bits)
    carry = h[1]  + (1 << 25); h[2]  = h[2]  + arshift(carry, 26);      h[1]  = h[1]  - (carry & kTop38Bits)
    carry = h[3]  + (1 << 25); h[4]  = h[4]  + arshift(carry, 26);      h[3]  = h[3]  - (carry & kTop38Bits)
    carry = h[5]  + (1 << 25); h[6]  = h[6]  + arshift(carry, 26);      h[5]  = h[5]  - (carry & kTop38Bits)
    carry = h[7]  + (1 << 25); h[8]  = h[8]  + arshift(carry, 26);      h[7]  = h[7]  - (carry & kTop38Bits)
    carry = h[9]  + (1 << 25); h[10] = h[10] + arshift(carry, 26);      h[9]  = h[9]  - (carry & kTop38Bits)
    return h
end

local function fe_tobytes(h)
    local q
    q = (19 * h[10] + (1 << 24)) >> 25
    q = arshift(h[1] + q, 26)
    q = arshift(h[2] + q, 25)
    q = arshift(h[3] + q, 26)
    q = arshift(h[4] + q, 25)
    q = arshift(h[5] + q, 26)
    q = arshift(h[6] + q, 25)
    q = arshift(h[7] + q, 26)
    q = arshift(h[8] + q, 25)
    q = arshift(h[9] + q, 26)
    q = arshift(h[10] + q, 25)

    h[1] = h[1] + 19 * q
    h[2] = h[2] + arshift(h[1], 26)
    h[1] = h[1] & kBottom26Bits
    h[3] = h[3] + arshift(h[2], 25)
    h[2] = h[2] & kBottom25Bits
    h[4] = h[4] + arshift(h[3], 26)
    h[3] = h[3] & kBottom26Bits
    h[5] = h[5] + arshift(h[4], 25)
    h[4] = h[4] & kBottom25Bits
    h[6] = h[6] + arshift(h[5], 26)
    h[5] = h[5] & kBottom26Bits
    h[7] = h[7] + arshift(h[6], 25)
    h[6] = h[6] & kBottom25Bits
    h[8] = h[8] + arshift(h[7], 26)
    h[7] = h[7] & kBottom26Bits
    h[9] = h[9] + arshift(h[8], 25)
    h[8] = h[8] & kBottom25Bits
    h[10] = h[10] + arshift(h[9], 26)
    h[9] = h[9] & kBottom26Bits
    h[10] = h[10] & kBottom25Bits

    return {
        (h[1] >> 0) & 0xff,
        (h[1] >> 8) & 0xff,
        (h[1] >> 16) & 0xff,
        ((h[1] >> 24) | (h[2] << 2)) & 0xff,
        (h[2] >> 6) & 0xff,
        (h[2] >> 14) & 0xff,
        ((h[2] >> 22) | (h[3] << 3)) & 0xff,
        (h[3] >> 5) & 0xff,
        (h[3] >> 13) & 0xff,
        ((h[3] >> 21) | (h[4] << 5)) & 0xff,
        (h[4] >> 3) & 0xff,
        (h[4] >> 11) & 0xff,
        ((h[4] >> 19) | (h[5] << 6)) & 0xff,
        (h[5] >> 2) & 0xff,
        (h[5] >> 10) & 0xff,
        (h[5] >> 18) & 0xff,
        (h[6] >> 0) & 0xff,
        (h[6] >> 8) & 0xff,
        (h[6] >> 16) & 0xff,
        ((h[6] >> 24) | (h[7] << 1)) & 0xff,
        (h[7] >> 7) & 0xff,
        (h[7] >> 15) & 0xff,
        ((h[7] >> 23) | (h[8] << 3)) & 0xff,
        (h[8] >> 5) & 0xff,
        (h[8] >> 13) & 0xff,
        ((h[8] >> 21) | (h[9] << 4)) & 0xff,
        (h[9] >> 4) & 0xff,
        (h[9] >> 12) & 0xff,
        ((h[9] >> 20) | (h[10] << 6)) & 0xff,
        (h[10] >> 2) & 0xff,
        (h[10] >> 10) & 0xff,
        (h[10] >> 18) & 0xff
    }
end

local function fe_add(f, g)
    local h = {}
    for i = 1, 10 do
        h[i] = (f[i] + g[i])
    end
    return h
end

local function fe_addto(f, g)
    for i = 1, 10 do
        f[i] = (f[i] + g[i])
    end
end

local function fe_sub(f, g)
    local h = {}
    for i = 1, 10 do
        h[i] = (f[i] - g[i])
    end
    return h
end

local function fe_subto(f, g)
    for i = 1, 10 do
        f[i] = (f[i] - g[i])
    end
end

local function fe_cswap(f, g, b)
    b = 0 - b
    for i = 1, 10 do
        local x = f[i] ~ g[i]
        x = x & b
        f[i] = f[i] ~ x
        g[i] = g[i] ~ x
    end
end

local function fe_mul121666(f)
    local h = {
        f[1] * 121666,
        f[2] * 121666,
        f[3] * 121666,
        f[4] * 121666,
        f[5] * 121666,
        f[6] * 121666,
        f[7] * 121666,
        f[8] * 121666,
        f[9] * 121666,
        f[10] * 121666
    }
    local carry
    carry = h[10] + (1 << 24); h[1]  = h[1]  + arshift(carry, 25) * 19; h[10] = h[10] - (carry & kTop39Bits)
    carry = h[2]  + (1 << 24); h[3]  = h[3]  + arshift(carry, 25);      h[2]  = h[2]  - (carry & kTop39Bits)
    carry = h[4]  + (1 << 24); h[5]  = h[5]  + arshift(carry, 25);      h[4]  = h[4]  - (carry & kTop39Bits)
    carry = h[6]  + (1 << 24); h[7]  = h[7]  + arshift(carry, 25);      h[6]  = h[6]  - (carry & kTop39Bits)
    carry = h[8]  + (1 << 24); h[9]  = h[9]  + arshift(carry, 25);      h[8]  = h[8]  - (carry & kTop39Bits)
    carry = h[1]  + (1 << 25); h[2]  = h[2]  + arshift(carry, 26);      h[1]  = h[1]  - (carry & kTop38Bits)
    carry = h[3]  + (1 << 25); h[4]  = h[4]  + arshift(carry, 26);      h[3]  = h[3]  - (carry & kTop38Bits)
    carry = h[5]  + (1 << 25); h[6]  = h[6]  + arshift(carry, 26);      h[5]  = h[5]  - (carry & kTop38Bits)
    carry = h[7]  + (1 << 25); h[8]  = h[8]  + arshift(carry, 26);      h[7]  = h[7]  - (carry & kTop38Bits)
    carry = h[9]  + (1 << 25); h[10] = h[10] + arshift(carry, 26);      h[9]  = h[9]  - (carry & kTop38Bits)
    return h
end

local function fe_mul(f, g)
    local h = {
        f[1] * g[1] + f[2] * g[10] * 38 + f[3] * g[9] * 19 + f[4] * g[8] * 38 + f[5] * g[7] * 19 + f[6] * g[6] * 38 + f[7] * g[5] * 19 + f[8] * g[4] * 38 + f[9] * g[3] * 19 + f[10] * g[2] * 38,
        f[1] * g[2] + f[2] * g[1] + f[3] * g[10] * 19 + f[4] * g[9] * 19 + f[5] * g[8] * 19 + f[6] & g[7] * 19 + f[7] * g[6] * 19 + f[8] * g[5] * 19 + f[9] * g[4] * 19 + f[10] * g[3] * 19,
        f[1] * g[3] + f[2] * g[2] * 2 + f[3] * g[1] + f[4] * g[10] * 38 + f[5] * g[9] * 19 + f[6] * g[8] * 38 + f[7] * g[7] * 19 + f[8] * g[6] * 38 + f[9] * g[5] * 19 + f[10] * g[4] * 38,
        f[1] * g[4] + f[2] * g[3] + f[3] * g[2] + f[4] * g[1] + f[5] * g[10] * 19 + f[6] * g[9] * 19 + f[7] * g[8] * 19 + f[8] * g[7] * 19 + f[9] * g[6] * 19 + f[10] * g[5] * 19,
        f[1] * g[5] + f[2] * g[4] * 2 + f[3] * g[3] + f[4] * g[2] * 2 + f[5] * g[1] + f[6] * g[10] * 38 + f[7] * g[9] * 19 + f[8] * g[8] * 38 + f[9] * g[7] * 19 + f[10] * g[6] * 38,
        f[1] * g[6] + f[2] * g[5] + f[3] * g[4] + f[4] * g[3] + f[5] * g[2] + f[6] * g[1] + f[7] * g[10] * 19 + f[8] * g[9] * 19 + f[9] * g[8] * 19 + f[10] * g[7] * 19,
        f[1] * g[7] + f[2] * g[6] * 2 + f[3] * g[5] + f[4] * g[4] * 2 + f[5] * g[3] + f[6] * g[2] * 2 + f[7] * g[1] + f[8] * g[10] * 38 + f[9] * g[9] * 19 + f[10] * g[8] * 38,
        f[1] * g[8] + f[2] * g[7] + f[3] * g[6] + f[4] * g[5] + f[5] * g[4] + f[6] * g[3] + f[7] * g[2] + f[8] * g[1] + f[9] * g[10] * 19 + f[10] * g[9] * 19,
        f[1] * g[9] + f[2] * g[8] * 2 + f[3] * g[7] + f[4] * g[6] * 2 + f[5] * g[5] + f[6] * g[4] * 2 + f[7] * g[3] + f[8] * g[2] * 2 + f[9] * g[1] + f[10] * g[10] * 38,
        f[1] * g[10] + f[2] * g[9] + f[3] * g[8] + f[4] * g[7] + f[5] * g[6] + f[6] * g[5] + f[7] * g[4] + f[8] * g[3] + f[9] * g[2] + f[10] * g[1]
    }
    local carry
    carry = h[1]  + (1 << 25); h[2]  = h[2]  + arshift(carry, 26);      h[1]  = h[1]  - (carry & kTop38Bits)
    carry = h[5]  + (1 << 25); h[6]  = h[6]  + arshift(carry, 26);      h[5]  = h[5]  - (carry & kTop38Bits)
    carry = h[2]  + (1 << 24); h[3]  = h[3]  + arshift(carry, 25);      h[2]  = h[2]  - (carry & kTop39Bits)
    carry = h[6]  + (1 << 24); h[7]  = h[7]  + arshift(carry, 25);      h[6]  = h[6]  - (carry & kTop39Bits)
    carry = h[3]  + (1 << 25); h[4]  = h[4]  + arshift(carry, 26);      h[3]  = h[3]  - (carry & kTop38Bits)
    carry = h[7]  + (1 << 25); h[8]  = h[8]  + arshift(carry, 26);      h[7]  = h[7]  - (carry & kTop38Bits)
    carry = h[4]  + (1 << 24); h[5]  = h[5]  + arshift(carry, 25);      h[4]  = h[4]  - (carry & kTop39Bits)
    carry = h[8]  + (1 << 24); h[9]  = h[9]  + arshift(carry, 25);      h[8]  = h[8]  - (carry & kTop39Bits)
    carry = h[5]  + (1 << 25); h[6]  = h[6]  + arshift(carry, 26);      h[5]  = h[5]  - (carry & kTop38Bits)
    carry = h[9]  + (1 << 25); h[10] = h[10] + arshift(carry, 26);      h[9]  = h[9]  - (carry & kTop38Bits)
    carry = h[10] + (1 << 24); h[1]  = h[1]  + arshift(carry, 25) * 19; h[10] = h[10] - (carry & kTop39Bits)
    carry = h[1]  + (1 << 25); h[2]  = h[2]  + arshift(carry, 26);      h[1] =  h[1]  - (carry & kTop38Bits)
    return h
end

local function fe_sq(f)
    local h = {
        f[1] * f[1] + f[2] * f[10] * 76 + f[3] * f[9] * 38 + f[4] * f[8] * 76 + f[5] * f[7] * 38 + f[6] * f[6] * 38,
        f[1] * f[2] * 2 + f[3] * f[10] * 38 + f[4] * f[9] * 38 + f[5] * f[8] * 38 + f[6] * f[7] * 38,
        f[1] * f[3] * 2 + f[2] * f[2] * 2 + f[4] * f[10] * 76 + f[5] * f[9] * 38 + f[6] * f[8] * 76 + f[7] * f[7] * 19,
        f[1] * f[4] * 2 + f[2] * f[3] * 2 + f[5] * f[10] * 38 + f[6] * f[9] * 38 + f[7] * f[8] * 38,
        f[1] * f[5] * 2 + f[2] * f[4] * 4 + f[3] * f[3] + f[6] * f[10] * 76 + f[7] * f[9] * 38 + f[8] * f[8] * 38,
        f[1] * f[6] * 2 + f[2] * f[5] * 2 + f[3] * f[4] * 2 + f[7] * f[10] * 38 + f[8] * f[9] * 38,
        f[1] * f[7] * 2 + f[2] * f[6] * 4 + f[3] * f[5] * 2 + f[4] * f[4] * 2 + f[8] * f[10] * 76 + f[9] * f[9] * 19,
        f[1] * f[8] * 2 + f[2] * f[7] * 2 + f[3] * f[6] * 2 + f[4] * f[5] * 2 + f[9] * f[10] * 38,
        f[1] * f[9] * 2 + f[2] * f[8] * 4 + f[3] * f[7] * 2 + f[4] * f[6] * 4 + f[5] * f[5] + f[10] * f[10] * 38,
        f[1] * f[10] * 2 + f[2] * f[9] * 2 + f[3] * f[8] * 2 + f[4] * f[7] * 2 + f[5] * f[6] * 2
    }
    local carry
    carry = h[1]  + (1 << 25); h[2]  = h[2]  + arshift(carry, 26);      h[1]  = h[1]  - (carry & kTop38Bits)
    carry = h[5]  + (1 << 25); h[6]  = h[6]  + arshift(carry, 26);      h[5]  = h[5]  - (carry & kTop38Bits)
    carry = h[2]  + (1 << 24); h[3]  = h[3]  + arshift(carry, 25);      h[2]  = h[2]  - (carry & kTop39Bits)
    carry = h[6]  + (1 << 24); h[7]  = h[7]  + arshift(carry, 25);      h[6]  = h[6]  - (carry & kTop39Bits)
    carry = h[3]  + (1 << 25); h[4]  = h[4]  + arshift(carry, 26);      h[3]  = h[3]  - (carry & kTop38Bits)
    carry = h[7]  + (1 << 25); h[8]  = h[8]  + arshift(carry, 26);      h[7]  = h[7]  - (carry & kTop38Bits)
    carry = h[4]  + (1 << 24); h[5]  = h[5]  + arshift(carry, 25);      h[4]  = h[4]  - (carry & kTop39Bits)
    carry = h[8]  + (1 << 24); h[9]  = h[9]  + arshift(carry, 25);      h[8]  = h[8]  - (carry & kTop39Bits)
    carry = h[5]  + (1 << 25); h[6]  = h[6]  + arshift(carry, 26);      h[5]  = h[5]  - (carry & kTop38Bits)
    carry = h[9]  + (1 << 25); h[10] = h[10] + arshift(carry, 26);      h[9]  = h[9]  - (carry & kTop38Bits)
    carry = h[10] + (1 << 24); h[1]  = h[1]  + arshift(carry, 25) * 19; h[10] = h[10] - (carry & kTop39Bits)
    carry = h[1]  + (1 << 25); h[2]  = h[2]  + arshift(carry, 26);      h[1]  = h[1]  - (carry & kTop38Bits)
    return h
end

local function fe_invert(z)
    local t0 = fe_sq(z)
    local t1 = fe_sq(t0)
    t1 = fe_sq(t1)
    t1 = fe_mul(z, t1)
    t0 = fe_mul(t0, t1)

    local t2 = fe_sq(t0)

    t1 = fe_mul(t1, t2)

    t2 = fe_sq(t1)
    for _ = 1, 4 do
        t2 = fe_sq(t2)
    end

    t1 = fe_mul(t2, t1)
    t2 = fe_sq(t1)
    for _ = 1, 9 do
        t2 = fe_sq(t2)
    end
    t2 = fe_mul(t2, t1)
    local t3 = fe_sq(t2)
    for _ = 1, 19 do
        t3 = fe_sq(t3)
    end
    t2 = fe_mul(t3, t2)

    for _ = 0, 9 do
        t2 = fe_sq(t2)
    end
    t1 = fe_mul(t2, t1)
    t2 = fe_sq(t1)
    for _ = 1, 49 do
        t2 = fe_sq(t2)
    end
    t2 = fe_mul(t2, t1)

    t3 = fe_sq(t2)

    for _ = 1, 99 do
        t3 = fe_sq(t3);
    end
    t2 = fe_mul(t3, t2)
    t2 = fe_sq(t2)

    for _ = 1, 49 do
        t2 = fe_sq(t2)
    end
    t1 = fe_mul(t2, t1)
    t1 = fe_sq(t1)
    for _ = 1, 4 do
        t1 = fe_sq(t1)
    end

    return fe_mul(t1, t0)
end

local function x25519_scalar_mult_generic(scalar, point)
    local x1 = fe_frombytes(point)
    local x2 = { 1, 0, 0, 0, 0, 0, 0, 0, 0, 0 }
    local z2 = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 }
    local x3 = { table.unpack(x1) }
    local z3 = { 1, 0, 0, 0, 0, 0, 0, 0, 0, 0 }

    local swap = 0

    local e = { table.unpack(scalar) }
    e[1] = e[1] & 248
    e[32] = e[32] & 127
    e[32] = e[32] | 64

    for pos = 254, 0, -1 do
        local b = 1 & (e[pos // 8 + 1] >> (pos & 7))
        swap = swap ~ b
        fe_cswap(x2, x3, swap)
        fe_cswap(z2, z3, swap)
        swap = b
        local tmp0 = fe_sub(x3, z3)

        local tmp1 = fe_sub(x2, z2)
        fe_addto(x2, z2)
        z2 = fe_add(x3, z3)
        z3 = fe_mul(tmp0, x2)

        z2 = fe_mul(z2, tmp1)
        tmp0 = fe_sq(tmp1)
        tmp1 = fe_sq(x2)
        x3 = fe_add(z3, z2);
        z2 = fe_sub(z3, z2);
        x2 = fe_mul(tmp1, tmp0)
        fe_subto(tmp1, tmp0);
        z2 = fe_sq(z2)
        z3 = fe_mul121666(tmp1)
        x3 = fe_sq(x3)
        fe_addto(tmp0, z3)
        z3 = fe_mul(x1, z2)
        z2 = fe_mul(tmp1, tmp0)
    end

    -- safety or something
    for i = 1, 32 do
        e[i] = 0
    end

    return fe_tobytes(fe_mul(fe_invert(z2), x2))
end

return x25519_scalar_mult_generic