[WIP] new functions: startswith and endswith

.travis.yml

@@ -1,8 +1,8 @@
 language: go
 
 go:
-  - 1.11.x
-  - 1.12.x
+  - oldstable
+  - stable
   - tip
 
 before_install:

cty/function/stdlib/string.go

@@ -96,6 +96,54 @@ var StrlenFunc = function.New(&function.Spec{
 	},
 })
 
+var StartsWithFunc = function.New(&function.Spec{
+	Params: []function.Parameter{
+		{
+			Name:             "str",
+			Type:             cty.String,
+			AllowDynamicType: true,
+		},
+		{
+			Name:             "prefix",
+			Type:             cty.String,
+			AllowDynamicType: true,
+		},
+	},
+	Type: function.StaticReturnType(cty.Bool),
+	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
+		str := []byte(args[0].AsString())
+		prefix := []byte(args[1].AsString())
+
+		// Empty prefix always matches
+		prefixLenNum, err := Strlen(args[1])
+		if err != nil {
+			// should never happen
+			panic("Stdlen returned an error")
+		}
+		var prefixLen int
+		err = gocty.FromCtyValue(prefixLenNum, &prefixLen)
+		if err != nil {
+			// should never happen
+			panic("Stdlen returned a non-int number")
+		}
+		if prefixLen == 0 {
+			return cty.BoolVal(true), nil
+		}
+
+		// For each character of prefix, check the matching character of str.
+		// If they don't match, fail
+		var i int
+		for i = 0; i < prefixLen; {
+			if str[i] != prefix[i] {
+				return cty.BoolVal(false), nil
+			}
+		}
+
+		// We do match
+		return cty.BoolVal(true), nil
+	},
+})
+
 var SubstrFunc = function.New(&function.Spec{
 	Params: []function.Parameter{
 		{
@@ -151,7 +199,6 @@ var SubstrFunc = function.New(&function.Spec{
 			return cty.StringVal(""), nil
 		}
 
-
 		sub := in
 		pos := 0
 		var i int
@@ -473,6 +520,10 @@ func Strlen(str cty.Value) (cty.Value, error) {
 	return StrlenFunc.Call([]cty.Value{str})
 }
 
+func StartsWith(str cty.Value, prefix cty.Value) (cty.Value, error) {
+	return StartsWithFunc.Call([]cty.Value{str, prefix})
+}
+
 // Substr is a Function that extracts a sequence of characters from another
 // string and creates a new string.
 //

cty/function/stdlib/string_test.go

@@ -268,6 +268,64 @@ func TestStrlen(t *testing.T) {
 	}
 }
 
+func TestStartsWith(t *testing.T) {
+	tests := []struct {
+		String cty.Value
+		Prefix cty.Value
+		Want   cty.Value
+	}{
+		{
+			cty.StringVal("hello"),
+			cty.StringVal("h"),
+			cty.BoolVal(true),
+		},
+		{
+			cty.StringVal("HELLO"),
+			cty.StringVal("h"),
+			cty.BoolVal(false),
+		},
+		{
+			cty.StringVal(""),
+			cty.StringVal("foo"),
+			cty.BoolVal(true),
+		}
+		{
+			cty.StringVal("foo"),
+			cty.StringVal(""),
+			cty.BoolVal(true),
+		},
+		{
+			cty.StringVal(""),
+			cty.StringVal(""),
+			cty.BoolVal(true),
+		},
+		{
+			cty.StringVal("short1"),
+			cty.StringVal("short1extra"),
+			cty.BoolVal(false),
+		},
+		{
+			cty.StringVal("short2"),
+			cty.StringVal("longerprefix"),
+			cty.BoolVal(false),
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.String.GoString(), func(t *testing.T) {
+			got, err := StartsWith(test.String, test.Prefix)
+
+			if err != nil {
+				t.Fatalf("unexpected error: %s", err)
+			}
+
+			if !got.RawEquals(test.Want) {
+				t.Errorf("wrong result\ngot:  %#v\nwant: %#v", got, test.Want)
+			}
+		})
+	}
+
+}
+
 func TestSubstr(t *testing.T) {
 	tests := []struct {
 		Input  cty.Value