Fix typo in enum CPDF_Function::Type.

"kType2ExponentialInterpolation" is the proper spelling. Also add a
comment to reference the PDF spec and indicate these enum values are not
arbitrary.

Change-Id: Ifba299dca7205ddf54a9772126005763bc956b36
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/67851
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

core/fpdfapi/page/cpdf_expintfunc.cpp

diff --git a/core/fpdfapi/page/cpdf_expintfunc.cpp b/core/fpdfapi/page/cpdf_expintfunc.cpp
index 7194f97..5be0ac6 100644
--- a/core/fpdfapi/page/cpdf_expintfunc.cpp
+++ b/core/fpdfapi/page/cpdf_expintfunc.cpp
@@ -13,7 +13,7 @@
 #include "third_party/base/stl_util.h"
 
 CPDF_ExpIntFunc::CPDF_ExpIntFunc()
-    : CPDF_Function(Type::kType2ExpotentialInterpolation) {}
+    : CPDF_Function(Type::kType2ExponentialInterpolation) {}
 
 CPDF_ExpIntFunc::~CPDF_ExpIntFunc() = default;
 

core/fpdfapi/page/cpdf_function.cpp

diff --git a/core/fpdfapi/page/cpdf_function.cpp b/core/fpdfapi/page/cpdf_function.cpp
index 055a979..75e7710 100644
--- a/core/fpdfapi/page/cpdf_function.cpp
+++ b/core/fpdfapi/page/cpdf_function.cpp
@@ -64,7 +64,7 @@
   Type type = IntegerToFunctionType(iType);
   if (type == Type::kType0Sampled)
     pFunc = pdfium::MakeUnique<CPDF_SampledFunc>();
-  else if (type == Type::kType2ExpotentialInterpolation)
+  else if (type == Type::kType2ExponentialInterpolation)
     pFunc = pdfium::MakeUnique<CPDF_ExpIntFunc>();
   else if (type == Type::kType3Stitching)
     pFunc = pdfium::MakeUnique<CPDF_StitchFunc>();
@@ -168,7 +168,7 @@
 }
 
 const CPDF_ExpIntFunc* CPDF_Function::ToExpIntFunc() const {
-  return m_Type == Type::kType2ExpotentialInterpolation
+  return m_Type == Type::kType2ExponentialInterpolation
              ? static_cast<const CPDF_ExpIntFunc*>(this)
              : nullptr;
 }

core/fpdfapi/page/cpdf_function.h

diff --git a/core/fpdfapi/page/cpdf_function.h b/core/fpdfapi/page/cpdf_function.h
index 91fa31e..fd68b6b 100644
--- a/core/fpdfapi/page/cpdf_function.h
+++ b/core/fpdfapi/page/cpdf_function.h
@@ -18,10 +18,11 @@
 
 class CPDF_Function {
  public:
+  // Valid values are from PDF 32000-1:2008 spec, table 38. DO NOT CHANGE.
   enum class Type {
     kTypeInvalid = -1,
     kType0Sampled = 0,
-    kType2ExpotentialInterpolation = 2,
+    kType2ExponentialInterpolation = 2,
     kType3Stitching = 3,
     kType4PostScript = 4,
   };