diff --git a/core/binary.mk b/core/binary.mk
index 7f6f2fb..6413153 100644
--- a/core/binary.mk
+++ b/core/binary.mk
@@ -103,9 +103,7 @@
   # missing API levels to existing ones where necessary, but we're not doing
   # that for the generated libraries. Clip the API level to the minimum where
   # appropriate.
-  my_ndk_api := \
-    $(shell if [ $(LOCAL_SDK_VERSION) -lt $(my_min_sdk_version) ]; then \
-        echo $(my_min_sdk_version); else echo $(LOCAL_SDK_VERSION); fi)
+  my_ndk_api := $(call math_max,$(LOCAL_SDK_VERSION),$(my_min_sdk_version))
 
   # Traditionally this has come from android/api-level.h, but with the libc
   # headers unified it must be set by the build system since we don't have
diff --git a/core/definitions.mk b/core/definitions.mk
index 83f7efc..61e0461 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -718,15 +718,6 @@
 endef
 
 ###########################################################
-## Run rot13 on a string
-## $(1): the string.  Must be one line.
-###########################################################
-define rot13
-$(shell echo $(1) | tr 'a-zA-Z' 'n-za-mN-ZA-M')
-endef
-
-
-###########################################################
 ## Returns true if $(1) and $(2) are equal.  Returns
 ## the empty string if they are not equal.
 ###########################################################
@@ -3170,6 +3161,55 @@
 endef
 
 ###########################################################
+# Basic math functions for positive integers <= 100
+#
+# (SDK versions for example)
+###########################################################
+__MATH_NUMBERS :=  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 \
+                  21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 \
+                  41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 \
+                  61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 \
+                  81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100
+
+# Returns true if $(1) is a positive integer <= 100, otherwise returns nothing.
+define math_is_number
+$(strip \
+  $(if $(1),,$(error Argument missing)) \
+  $(if $(word 2,$(1)),$(error Multiple words in a single argument: $(1))) \
+  $(if $(filter $(1),$(__MATH_NUMBERS)),true))
+endef
+
+#$(warning true == $(call math_is_number,2))
+#$(warning == $(call math_is_number,foo))
+#$(call math_is_number,1 2)
+#$(call math_is_number,no 2)
+
+define _math_check_valid
+$(if $(call math_is_number,$(1)),,$(error Only positive integers <= 100 are supported (not $(1))))
+endef
+
+#$(call _math_check_valid,0)
+#$(call _math_check_valid,1)
+#$(call _math_check_valid,100)
+#$(call _math_check_valid,101)
+#$(call _math_check_valid,)
+#$(call _math_check_valid,1 2)
+
+# Returns the greater of $1 or $2.
+# If $1 or $2 is not a positive integer <= 100, then an error is generated.
+define math_max
+$(strip $(call _math_check_valid,$(1)) $(call _math_check_valid,$(2)) \
+  $(lastword $(filter $(1) $(2),$(__MATH_NUMBERS))))
+endef
+
+#$(call math_max)
+#$(call math_max,1)
+#$(call math_max,1 2,3)
+#$(warning 1 == $(call math_max,1,1))
+#$(warning 42 == $(call math_max,5,42))
+#$(warning 42 == $(call math_max,42,5))
+
+###########################################################
 ## Other includes
 ###########################################################
 
diff --git a/core/product.mk b/core/product.mk
index ad9b022..7193cf7 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -301,7 +301,7 @@
 #
 define stash-product-vars
 $(foreach v,$(_product_stash_var_list), \
-        $(eval $(strip $(1))_$(call rot13,$(v)):=$$($$(v))) \
+        $(eval $(strip $(1))_rot26_$(v):=$$($$(v))) \
  )
 endef
 
@@ -313,7 +313,7 @@
 $(strip \
   $(eval changed_variables:=)
   $(foreach v,$(_product_stash_var_list), \
-    $(if $(call streq,$($(v)),$($(strip $(1))_$(call rot13,$(v)))),, \
+    $(if $(call streq,$($(v)),$($(strip $(1))_rot26_$(v))),, \
         $(eval $(warning $(v) has been modified: $($(v)))) \
         $(eval $(warning previous value: $($(strip $(1))_$(call rot13,$(v))))) \
         $(eval changed_variables := $(changed_variables) $(v))) \