Define step timing function for the cc animation framework.

cc/animation/timing_function.cc

Index: cc/animation/timing_function.cc
diff --git a/cc/animation/timing_function.cc b/cc/animation/timing_function.cc
index 2531cc69f36457ccac33f807ad39ce960002f147..94bf12874d30eeba4b6c65352e39bd6078e6caaf 100644
--- a/cc/animation/timing_function.cc
+++ b/cc/animation/timing_function.cc
@@ -5,6 +5,7 @@
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
 #include "cc/animation/timing_function.h"
+#include "cc/base/math_util.h"
 
 namespace cc {
 
@@ -63,4 +64,41 @@ scoped_ptr<TimingFunction> EaseInOutTimingFunction::Create() {
   return CubicBezierTimingFunction::Create(0.42, 0.0, 0.58, 1);
 }
 
+scoped_ptr<StepsTimingFunction> StepsTimingFunction::Create(
+    int steps,
+    float steps_start_offset) {
+  return make_scoped_ptr(new StepsTimingFunction(steps, steps_start_offset));
+}
+
+StepsTimingFunction::StepsTimingFunction(int steps, float steps_start_offset)
+    : steps_(steps), steps_start_offset_(steps_start_offset) {
+  // Restrict it to CSS presets: step_start, step_end and step_middle.
+  // See the Web Animations specification, 3.12.4. Timing in discrete steps.
+  DCHECK(steps_start_offset_ == 0 || steps_start_offset_ == 1 ||
+         steps_start_offset_ == 0.5);
+}
+
+StepsTimingFunction::~StepsTimingFunction() {
+}
+
+float StepsTimingFunction::GetValue(double t) const {
+  const double steps = static_cast<double>(steps_);
+  const double value = MathUtil::ClampToRange(
+      std::floor((steps * t) + steps_start_offset_) / steps, 0.0, 1.0);
+  return static_cast<float>(value);
+}
+
+scoped_ptr<TimingFunction> StepsTimingFunction::Clone() const {
+  return make_scoped_ptr(new StepsTimingFunction(*this));
+}
+
+void StepsTimingFunction::Range(float* min, float* max) const {
+  *min = 0.0f;
+  *max = 1.0f;
+}
+
+float StepsTimingFunction::Velocity(double x) const {
+  return 0.0f;
+}
+
 }  // namespace cc