Initial implementation of a SkColorSpace_A2B xform

src/core/SkColorSpaceXformPriv.h

+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkColorSpaceXformPriv_DEFINED
+#define SkColorSpaceXformPriv_DEFINED
+
+#include "SkColorSpace_Base.h"
+#include "SkHalf.h"
+#include "SkSRGB.h"
+
+#define AI SK_ALWAYS_INLINE
+
+#define SkCSXformPrintfDefined 0
+#define SkCSXformPrintf(...)
+
+// Interpolating lookup in a variably sized table.
+static AI float interp_lut(float input, const float* table, int tableSize) {
+    float index = input * (tableSize - 1);
+    float diff = index - sk_float_floor2int(index);
+    return table[(int) sk_float_floor2int(index)] * (1.0f - diff) +
+           table[(int) sk_float_ceil2int(index)] * diff;
+}
+
+// Inverse table lookup.  Ex: what index corresponds to the input value?  This will
+// have strange results when the table is non-increasing.  But any sane gamma
+// function will be increasing.
+static float inverse_interp_lut(float input, const float* table, int tableSize) {
+    if (input <= table[0]) {
+        return table[0];
+    } else if (input >= table[tableSize - 1]) {
+        return 1.0f;
+    }
+
+    for (int i = 1; i < tableSize; i++) {
+        if (table[i] >= input) {
+            // We are guaranteed that input is greater than table[i - 1].
+            float diff = input - table[i - 1];
+            float distance = table[i] - table[i - 1];
+            float index = (i - 1) + diff / distance;
+            return index / (tableSize - 1);
+        }
+    }
+
+    // Should be unreachable, since we'll return before the loop if input is
+    // larger than the last entry.
+    SkASSERT(false);
+    return 0.0f;
+}
+
+#undef AI
+
+#endif