import { Shader, Skia, type Uniforms } from "@shopify/react-native-skia";
import type { SharedValue } from "react-native-reanimated";

import { THRESHOLD_SAMPLE_COUNT } from "../math/threshold";
import { buildThresholdSplitShaderSource } from "./thresholdSplitShaderSource";

/**
 * Per-fragment split shader for a *time-varying* threshold. For each fragment it
 * reads the threshold's Y at the fragment's X — linearly interpolated from
 * `samples[]`, evenly spaced across `[plotLeft, plotRight]` — and paints
 * `aboveColor` above the boundary (smaller Y), `belowColor` below. This is the
 * polyline equivalent of the constant case's vertical hard-stop `LinearGradient`,
 * which can only express a *horizontal* split.
 *
 * Used for both the line stroke and the profit/loss fill band (the band passes
 * alpha-reduced colors). Output is premultiplied — Skia's shader convention, as in
 * `AreaDotsOverlay`. One GPU draw, no per-frame clip / `saveLayer`.
 */
const THRESHOLD_SPLIT_SKSL = buildThresholdSplitShaderSource(
  THRESHOLD_SAMPLE_COUNT,
);

// Compiled once. `RuntimeEffect.Make` THROWS on a compile error, so guard it —
// a shader failure must degrade to "no split" (the line keeps its plain color),
// never crash every chart screen at import. Null → the component no-ops.
let SPLIT_EFFECT: ReturnType<typeof Skia.RuntimeEffect.Make> = null;
try {
  SPLIT_EFFECT = Skia.RuntimeEffect.Make(THRESHOLD_SPLIT_SKSL);
} catch {
  SPLIT_EFFECT = null;
}

/**
 * Whether the split effect compiled. Callers must gate any `<Path>` whose ONLY
 * paint is this shader on it — without the shader child the path fills with the
 * default paint (opaque black). A fallback `color` prop can't stand in instead:
 * Skia multiplies the shader output by the paint alpha, so a transparent color
 * erases the shader's own output too.
 */
export const THRESHOLD_SPLIT_AVAILABLE = SPLIT_EFFECT !== null;

interface ThresholdSplitShaderProps {
  /**
   * Per-frame uniforms (built in `useThresholdSplitUniforms`): `sampleLeft` /
   * `sampleRight` (the gliding sample-grid span, px), `aboveColor` / `belowColor`
   * (straight-alpha `[r, g, b, a]` vec4s, channels 0..1) and `samples`
   * (`THRESHOLD_SAMPLE_COUNT` threshold pixel-Y values on the grid).
   */
  uniforms: SharedValue<Uniforms>;
}

/**
 * The split `RuntimeShader` as a paint child — drop it inside a stroked line
 * `<Path>` or a filled band `<Path>`, like a `<LinearGradient>`.
 */
export function ThresholdSplitShader({ uniforms }: ThresholdSplitShaderProps) {
  /* istanbul ignore next -- defensive: SPLIT_SKSL is static and compiles */
  if (!SPLIT_EFFECT) return null;
  return <Shader source={SPLIT_EFFECT} uniforms={uniforms} />;
}