import { Color } from "./Color"
import type { Framebuffer } from "./Framebuffer"
import { Camera } from "../scene/Camera"
import { Vec3 } from "../math/Vec3"

/** Procedural sky: a vertical gradient plus a sun disc. No texture needed. */
export type SkyConfig = {
  zenith: Color
  horizon: Color
  sun: Color
  /** World-space direction toward the sun (need not be normalized). */
  sunDir: Vec3
  /** Angular radius of the sun's core, in radians. */
  sunSize: number
}

const UP: Vec3 = { x: 0, y: 1, z: 0 }

export namespace Sky {
  /**
   * Fill the whole framebuffer with the sky and reset depth to 0. Run first each
   * frame in place of Framebuffer.clear; opaque geometry then overwrites the sky
   * wherever it is nearer.
   *
   * Per pixel it reconstructs the view ray from the camera basis, shades a
   * horizon->zenith gradient by the ray's elevation (so it pans with pitch and
   * yaw), and brightens toward `sun` where the ray points near `sunDir`.
   */
  export function render(fb: Framebuffer, camera: Camera, sky: SkyConfig): void {
    const { width, height, color, depth } = fb
    const forward = Camera.forward(camera)
    const right = Vec3.normalize(Vec3.cross(forward, UP))
    const up = Vec3.cross(right, forward)
    const tanY = Math.tan(camera.fov / 2)
    const tanX = tanY * (width / height)
    const sun = Vec3.normalize(sky.sunDir)
    const cosSun = Math.cos(sky.sunSize)
    for (let y = 0; y < height; y++) {
      const ndcY = 1 - ((y + 0.5) / height) * 2
      for (let x = 0; x < width; x++) {
        const ndcX = ((x + 0.5) / width) * 2 - 1
        // View ray = forward + right*ndcX*tanX + up*ndcY*tanY, then normalized.
        let dx = forward.x + right.x * ndcX * tanX + up.x * ndcY * tanY
        let dy = forward.y + right.y * ndcX * tanX + up.y * ndcY * tanY
        let dz = forward.z + right.z * ndcX * tanX + up.z * ndcY * tanY
        const inv = 1 / Math.hypot(dx, dy, dz)
        dx *= inv
        dy *= inv
        dz *= inv
        // dy is the ray elevation: 0 at the horizon, 1 straight up.
        const t = Math.max(0, Math.min(1, dy))
        let c = Color.lerp(sky.horizon, sky.zenith, t)
        const facing = dx * sun.x + dy * sun.y + dz * sun.z
        if (facing > cosSun) {
          const glow = Math.min(1, ((facing - cosSun) / (1 - cosSun)) * 1.5)
          c = Color.lerp(c, sky.sun, glow)
        }
        const i = y * width + x
        color[i] = c
        depth[i] = 0
      }
    }
  }
}