LibPDF: Do less work in SampledFunction::evaluate()'s inner loop

Instead of recomputing the left index and the float amount in that
interval for each coordinate all the time, do it once when we
preprocess the input coordinates.

One line less, faster, and arguably easier to read.

No behavior change.

Userland/Libraries/LibPDF/Function.cpp

@@ -68,6 +68,7 @@ private:
     ReadonlyBytes m_sample_data;
 
     Vector<float> mutable m_inputs;
+    Vector<unsigned> mutable m_left_index;
     Vector<float> mutable m_outputs;
 };
 
@@ -167,6 +168,7 @@ SampledFunction::create(Document* document, Vector<Bound> domain, Optional<Vecto
     function->m_encode = move(encode);
     function->m_decode = move(decode);
     function->m_inputs.resize(function->m_domain.size());
+    function->m_left_index.resize(function->m_domain.size());
     function->m_outputs.resize(function->m_range.size());
     return function;
 }
@@ -189,8 +191,11 @@ PDFErrorOr<ReadonlySpan<float>> SampledFunction::evaluate(ReadonlySpan<float> xs
     for (size_t i = 0; i < m_domain.size(); ++i) {
         float x = clamp(xs[i], m_domain[i].lower, m_domain[i].upper);
         float e = interpolate(x, m_domain[i].lower, m_domain[i].upper, m_encode[i].lower, m_encode[i].upper);
-        float ec = clamp(e, 0.0f, static_cast<float>(m_sizes[i] - 1));
-        m_inputs[i] = ec;
+
+        unsigned n = m_sizes[i] - 1;
+        float ec = clamp(e, 0.0f, static_cast<float>(n));
+        m_left_index[i] = min(ec, n - 1);
+        m_inputs[i] = ec - m_left_index[i];
     }
 
     for (size_t r = 0; r < m_range.size(); ++r) {
@@ -207,20 +212,14 @@ PDFErrorOr<ReadonlySpan<float>> SampledFunction::evaluate(ReadonlySpan<float> xs
         Vector<int> coordinates;
         coordinates.resize(m_domain.size());
         for (size_t mask = 0; mask < (1u << m_domain.size()); ++mask) {
-            for (size_t i = 0; i < m_domain.size(); ++i) {
-                unsigned n = m_sizes[i] - 1;
-                unsigned left_index = min(m_inputs[i], n - 1);
-                coordinates[i] = left_index + ((mask >> i) & 1u);
-            }
+            for (size_t i = 0; i < m_domain.size(); ++i)
+                coordinates[i] = m_left_index[i] + ((mask >> i) & 1u);
             samples[mask] = sample(coordinates, r);
         }
 
         for (int i = static_cast<int>(m_domain.size() - 1); i >= 0; --i) {
-            unsigned n = m_sizes[i] - 1;
-            float ec = m_inputs[i];
-            unsigned e0 = min(static_cast<unsigned>(ec), n - 1);
             for (size_t mask = 0; mask < (1u << i); ++mask)
-                samples[mask] = mix(samples[mask], samples[mask | (1 << i)], ec - e0);
+                samples[mask] = mix(samples[mask], samples[mask | (1 << i)], m_inputs[i]);
         }
 
         float result = interpolate(samples[0], 0.0f, 255.0f, m_decode[r].lower, m_decode[r].upper);