#version 330 core

in vec2 vUV;
in vec3 vWorldPos;
out vec4 FragColor;

uniform sampler2D uHeightTex;
uniform float uMinV;
uniform float uMaxV;
uniform float uHeightScale;
uniform vec2 uTexelSize;
uniform vec2 uPlaneSize;
uniform vec3 uCameraPos;
uniform vec3 uLightDir;

float saturate(float x) { return clamp(x, 0.0, 1.0); }

float value01(float v) {
    return saturate((v - uMinV) / max(uMaxV - uMinV, 1e-6));
}

vec3 colorRamp(float t) {
    t = saturate(t);
    vec3 c0 = vec3(0.10, 0.15, 0.85);
    vec3 c1 = vec3(0.05, 0.75, 0.95);
    vec3 c2 = vec3(0.10, 0.90, 0.35);
    vec3 c3 = vec3(0.95, 0.90, 0.20);
    vec3 c4 = vec3(0.95, 0.25, 0.12);
    if (t < 0.25) return mix(c0, c1, t / 0.25);
    if (t < 0.50) return mix(c1, c2, (t - 0.25) / 0.25);
    if (t < 0.75) return mix(c2, c3, (t - 0.50) / 0.25);
    return mix(c3, c4, (t - 0.75) / 0.25);
}

void main() {
    float vC = texture(uHeightTex, vUV).r;
    float t = value01(vC);
    vec3 base = colorRamp(t);

    float vL = texture(uHeightTex, vUV - vec2(uTexelSize.x, 0.0)).r;
    float vR = texture(uHeightTex, vUV + vec2(uTexelSize.x, 0.0)).r;
    float vD = texture(uHeightTex, vUV - vec2(0.0, uTexelSize.y)).r;
    float vU = texture(uHeightTex, vUV + vec2(0.0, uTexelSize.y)).r;

    float hL = value01(vL) * uHeightScale;
    float hR = value01(vR) * uHeightScale;
    float hD = value01(vD) * uHeightScale;
    float hU = value01(vU) * uHeightScale;

    float dx = max(1e-6, uPlaneSize.x * uTexelSize.x);
    float dy = max(1e-6, uPlaneSize.y * uTexelSize.y);
    float dHx = (hR - hL) / (2.0 * dx);
    float dHy = (hU - hD) / (2.0 * dy);

    vec3 N = normalize(vec3(-dHx, -dHy, 1.0));
    vec3 L = normalize(uLightDir);
    vec3 V = normalize(uCameraPos - vWorldPos);

    float diff = saturate(dot(N, L));
    float spec = pow(saturate(dot(reflect(-L, N), V)), 20.0);

    vec3 col = base * (0.50 + 0.50 * diff);
    col += vec3(1.0) * spec * 0.20;
    FragColor = vec4(clamp(col, 0.0, 1.0), 1.0);
}