Add a logarithm function

Fixes hashicorp#9498. Adds a `log` function for calculating logarithms.

config/interpolate_funcs.go

@@ -81,6 +81,7 @@ func Funcs() map[string]ast.Function {
 		"jsonencode":   interpolationFuncJSONEncode(),
 		"length":       interpolationFuncLength(),
 		"list":         interpolationFuncList(),
+		"log":          interpolationFuncLog(),
 		"lower":        interpolationFuncLower(),
 		"map":          interpolationFuncMap(),
 		"max":          interpolationFuncMax(),
@@ -489,6 +490,17 @@ func interpolationFuncCeil() ast.Function {
 	}
 }
 
+// interpolationFuncLog returns the logarithnm.
+func interpolationFuncLog() ast.Function {
+	return ast.Function{
+		ArgTypes:   []ast.Type{ast.TypeFloat, ast.TypeFloat},
+		ReturnType: ast.TypeFloat,
+		Callback: func(args []interface{}) (interface{}, error) {
+			return math.Log(args[0].(float64)) / math.Log(args[1].(float64)), nil
+		},
+	}
+}
+
 // interpolationFuncChomp removes trailing newlines from the given string
 func interpolationFuncChomp() ast.Function {
 	newlines := regexp.MustCompile(`(?:\r\n?|\n)*\z`)

config/interpolate_funcs_test.go

@@ -370,6 +370,34 @@ func TestInterpolateFuncCeil(t *testing.T) {
 	})
 }
 
+func TestInterpolateFuncLog(t *testing.T) {
+	testFunction(t, testFunctionConfig{
+		Cases: []testFunctionCase{
+			{
+				`${log(1, 10)}`,
+				"0",
+				false,
+			},
+			{
+				`${log(10, 10)}`,
+				"1",
+				false,
+			},
+
+			{
+				`${log(0, 10)}`,
+				"-Inf",
+				false,
+			},
+			{
+				`${log(10, 0)}`,
+				"-0",
+				false,
+			},
+		},
+	})
+}
+
 func TestInterpolateFuncChomp(t *testing.T) {
 	testFunction(t, testFunctionConfig{
 		Cases: []testFunctionCase{

website/source/docs/configuration/interpolation.html.md

@@ -278,6 +278,8 @@ The supported built-in functions are:
       * `${list("a", "b", "c")}` returns a list of `"a", "b", "c"`.
       * `${list()}` returns an empty list.
 
+  * `log(x, base)` - Returns the logarithm of `x`.
+
   * `lookup(map, key, [default])` - Performs a dynamic lookup into a map
       variable. The `map` parameter should be another variable, such
       as `var.amis`. If `key` does not exist in `map`, the interpolation will