From b21634ac613b8ff3714407f7e8a2c3b58ab32fef Mon Sep 17 00:00:00 2001
From: Shohei Jinno <18493164+sjinno@users.noreply.github.com>
Date: Mon, 11 Apr 2022 12:02:34 -0700
Subject: [PATCH] [glsl-in] Implement `MathFunction::Clamp` (#1502)

* Implement Clamp function.

* Fix the return error type.

Change `ConstantSolvingError::InvalidMathArg` to `ConstantSolvingError::NotImplemented`
for the clamp function with the input of unimplemented types.

Unimplemented types:
    - vec2
    - vec3
    - vec4

* [glsl-in] Add helpers that implement the glsl's `max` and `min` functions.

Co-authored-by: JCapucho <jcapucho7@gmail.com>

* Fix grammar.

Co-authored-by: JCapucho <jcapucho7@gmail.com>
---
 src/front/glsl/constants.rs | 95 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 94 insertions(+), 1 deletion(-)

diff --git a/src/front/glsl/constants.rs b/src/front/glsl/constants.rs
index ea81123081..e537370858 100644
--- a/src/front/glsl/constants.rs
+++ b/src/front/glsl/constants.rs
@@ -164,12 +164,20 @@ impl<'a> ConstantSolver<'a> {
 
                 self.binary_op(op, left_constant, right_constant, span)
             }
-            Expression::Math { fun, arg, arg1, .. } => {
+            Expression::Math {
+                fun,
+                arg,
+                arg1,
+                arg2,
+                ..
+            } => {
                 let arg = self.solve(arg)?;
                 let arg1 = arg1.map(|arg| self.solve(arg)).transpose()?;
+                let arg2 = arg2.map(|arg| self.solve(arg)).transpose()?;
 
                 let const0 = &self.constants[arg].inner;
                 let const1 = arg1.map(|arg| &self.constants[arg].inner);
+                let const2 = arg2.map(|arg| &self.constants[arg].inner);
 
                 match fun {
                     crate::MathFunction::Pow => {
@@ -201,6 +209,49 @@ impl<'a> ConstantSolver<'a> {
                         let inner = ConstantInner::Scalar { width, value };
                         Ok(self.register_constant(inner, span))
                     }
+                    crate::MathFunction::Clamp => {
+                        let (value, width) = match (const0, const1.unwrap(), const2.unwrap()) {
+                            (
+                                &ConstantInner::Scalar {
+                                    width,
+                                    value: value0,
+                                },
+                                &ConstantInner::Scalar { value: value1, .. },
+                                &ConstantInner::Scalar { value: value2, .. },
+                            ) => (
+                                match (value0, value1, value2) {
+                                    (
+                                        ScalarValue::Sint(a),
+                                        ScalarValue::Sint(b),
+                                        ScalarValue::Sint(c),
+                                    ) => ScalarValue::Sint(a.max(b).min(c)),
+                                    (
+                                        ScalarValue::Uint(a),
+                                        ScalarValue::Uint(b),
+                                        ScalarValue::Uint(c),
+                                    ) => ScalarValue::Uint(a.max(b).min(c)),
+                                    (
+                                        ScalarValue::Float(a),
+                                        ScalarValue::Float(b),
+                                        ScalarValue::Float(c),
+                                    ) => {
+                                        ScalarValue::Float(glsl_float_min(glsl_float_max(a, b), c))
+                                    }
+                                    _ => return Err(ConstantSolvingError::InvalidMathArg),
+                                },
+                                width,
+                            ),
+                            _ => {
+                                return Err(ConstantSolvingError::NotImplemented(format!(
+                                    "{:?} applied to vector values",
+                                    fun
+                                )))
+                            }
+                        };
+
+                        let inner = ConstantInner::Scalar { width, value };
+                        Ok(self.register_constant(inner, span))
+                    }
                     _ => Err(ConstantSolvingError::NotImplemented(format!("{:?}", fun))),
                 }
             }
@@ -506,6 +557,48 @@ impl<'a> ConstantSolver<'a> {
     }
 }
 
+/// Helper function to implement the GLSL `max` function for floats.
+///
+/// While Rust does provide a `f64::max` method, it has a different behavior than the
+/// GLSL `max` for NaNs. In Rust, if any of the arguments is a NaN, then the other
+/// is returned.
+///
+/// This leads to different results in the following example
+/// ```
+/// use std::cmp::max;
+/// std::f64::NAN.max(1.0);
+/// ```
+///
+/// Rust will return `1.0` while GLSL should return NaN.
+fn glsl_float_max(x: f64, y: f64) -> f64 {
+    if x < y {
+        y
+    } else {
+        x
+    }
+}
+
+/// Helper function to implement the GLSL `min` function for floats.
+///
+/// While Rust does provide a `f64::min` method, it has a different behavior than the
+/// GLSL `min` for NaNs. In Rust, if any of the arguments is a NaN, then the other
+/// is returned.
+///
+/// This leads to different results in the following example
+/// ```
+/// use std::cmp::min;
+/// std::f64::NAN.min(1.0);
+/// ```
+///
+/// Rust will return `1.0` while GLSL should return NaN.
+fn glsl_float_min(x: f64, y: f64) -> f64 {
+    if y < x {
+        y
+    } else {
+        x
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use std::vec;