HCL relative_url() function

eval/context.go

@@ -498,6 +498,7 @@ func newFunctionsMap() map[string]function.Function {
 		"json_decode":   stdlib.JSONDecodeFunc,
 		"json_encode":   stdlib.JSONEncodeFunc,
 		"merge":         lib.MergeFunc,
+		"relative_url":  lib.RelativeUrlFunc,
 		"to_lower":      stdlib.LowerFunc,
 		"to_upper":      stdlib.UpperFunc,
 		"unixtime":      lib.UnixtimeFunc,

eval/lib/relative_url.go

@@ -0,0 +1,50 @@
+package lib
+
+import (
+	"fmt"
+	"net/url"
+	"regexp"
+	"strings"
+
+	"github.com/zclconf/go-cty/cty"
+	"github.com/zclconf/go-cty/cty/function"
+)
+
+var (
+	// https://datatracker.ietf.org/doc/html/rfc3986#page-50
+	regexParseURL   = regexp.MustCompile(`^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?`)
+	RelativeUrlFunc = newRelativeUrlFunction()
+)
+
+func newRelativeUrlFunction() function.Function {
+	return function.New(&function.Spec{
+		Params: []function.Parameter{{
+			Name: "s",
+			Type: cty.String,
+		}},
+		Type: function.StaticReturnType(cty.String),
+		Impl: func(args []cty.Value, _ cty.Type) (ret cty.Value, err error) {
+			absURL := strings.TrimSpace(args[0].AsString())
+
+			if !strings.HasPrefix(absURL, "/") && !strings.HasPrefix(absURL, "http://") && !strings.HasPrefix(absURL, "https://") {
+				return cty.StringVal(""), fmt.Errorf("invalid url given: '%s'", absURL)
+			}
+
+			// Do not use the result of url.ParseRequestURI() above,
+			// to preserve the URL encoding, e.g. in the query.
+			if _, err := url.Parse(absURL); err != nil {
+				return cty.StringVal(""), err
+			}
+
+			// The regexParseURL garanties the len of 10 in the result.
+			urlParts := regexParseURL.FindStringSubmatch(absURL)
+
+			// The path must begin w/ a slash.
+			if !strings.HasPrefix(urlParts[5], "/") {
+				urlParts[5] = "/" + urlParts[5]
+			}
+
+			return cty.StringVal(urlParts[5] + urlParts[6] + urlParts[8]), nil
+		},
+	})
+}