template: add tests, fix up easy holes

should cope with double-braced blocks now (treating them the same as
single-braced. templateEmail now returns an error, which should not be
seen as catastrophic, but reports likely mistakes.

template.go

@@ -1,6 +1,9 @@
 package main
 
-import "fmt"
+import (
+	"fmt"
+	"slices"
+)
 
 func truthy(val interface{}) bool {
 	switch v := val.(type) {
@@ -18,43 +21,72 @@ func truthy(val interface{}) bool {
 // Variables should be written as {varName}.
 // If statements should be written as {if (!)varName}...{endif}.
 // Strings are true if != "", ints are true if != 0.
-func templateEmail(content string, variables []string, conditionals []string, values map[string]interface{}) string {
+// Errors returned are likely warnings only.
+func templateEmail(content string, variables []string, conditionals []string, values map[string]interface{}) (string, error) {
+	// minimum length for templatable content (albeit just "{}" -> "")
+	if len(content) < 2 {
+		return content, nil
+	}
 	ifStart, ifEnd := -1, -1
 	ifTrue := false
 	invalidIf := false
 	previousEnd := -2
-	cStart, cEnd := -1, -1
+	blockRawStart := -1
+	blockContentStart, blockContentEnd := -1, -1
 	varStart, varEnd := -1, -1
 	varName := ""
 	out := ""
+	var err error = nil
+
+	oob := func(i int) bool { return i < 0 || i >= len(content) }
+
 	for i, c := range content {
 		if c == '{' {
-			cStart = i + 1
-			for content[cStart] == ' ' {
-				cStart++
+			blockContentStart = i + 1
+			blockRawStart = i
+			if content[i+1] == '{' {
+				err = fmt.Errorf(`double braces ("{{") at position %d, use single brace only`, i)
+				blockContentStart++
 			}
-			if content[cStart:cStart+3] == "if " {
-				varStart = cStart + 3
+			for !oob(blockContentStart) && content[blockContentStart] == ' ' {
+				blockContentStart++
+			}
+			if oob(blockContentStart) {
+				continue
+			}
+			if !oob(blockContentStart+3) && content[blockContentStart:blockContentStart+3] == "if " {
+				varStart = blockContentStart + 3
 				for content[varStart] == ' ' {
 					varStart++
 				}
 			}
-			if ifStart == -1 {
+			if ifStart == -1 && (oob(i-1) || content[i-1] != '{') {
 				out += content[previousEnd+2 : i]
 			}
-			if content[cStart:cStart+5] != "endif" || invalidIf {
+			if invalidIf || oob(blockContentStart+5) || content[blockContentStart:blockContentStart+5] != "endif" {
 				continue
 			}
 			ifEnd = i - 1
 			if ifTrue {
-				out += templateEmail(content[ifStart:ifEnd+1], variables, conditionals, values)
+				toAppend, subErr := templateEmail(content[ifStart:ifEnd+1], variables, conditionals, values)
+				out += toAppend
+				if subErr != nil {
+					err = subErr
+				}
 				ifTrue = false
 			}
 		} else if c == '}' {
+			doubleBraced := !oob(i+1) && content[i+1] == '}'
+			if doubleBraced {
+				err = fmt.Errorf(`double braces ("}}") at position %d, use single brace only`, i)
+			}
+			if !oob(i-1) && content[i-1] == '}' {
+				continue
+			}
 			if varStart != -1 {
 				ifStart = i + 1
 				varEnd = i - 1
-				for content[varEnd] == ' ' {
+				for !oob(varEnd) && content[varEnd] == ' ' {
 					varEnd--
 				}
 				varName = content[varStart : varEnd+1]
@@ -63,14 +95,8 @@ func templateEmail(content string, variables []string, conditionals []string, va
 					positive = false
 					varName = varName[1:]
 				}
-				validVar := false
 				wrappedVarName := "{" + varName + "}"
-				for _, v := range conditionals {
-					if v == wrappedVarName {
-						validVar = true
-						break
-					}
-				}
+				validVar := slices.Contains(conditionals, wrappedVarName)
 				if validVar {
 					ifTrue = positive == truthy(values[varName])
 				} else {
@@ -79,27 +105,26 @@ func templateEmail(content string, variables []string, conditionals []string, va
 				}
 				varStart, varEnd = -1, -1
 			}
-			cEnd = i - 1
-			for content[cEnd] == ' ' {
-				cEnd--
+			blockContentEnd = i - 1
+			for content[blockContentEnd] == ' ' {
+				blockContentEnd--
 			}
 			previousEnd = i - 1
-			if content[cEnd-4:cEnd+1] == "endif" && !invalidIf {
+			// Skip the extra brace
+			if doubleBraced {
+				previousEnd++
+			}
+			if !oob(blockContentEnd-4) && !oob(blockContentEnd+1) && content[blockContentEnd-4:blockContentEnd+1] == "endif" && !invalidIf {
 				continue
 			}
-			validVar := false
-			varName = content[cStart : cEnd+1]
-			cStart, cEnd = -1, -1
+			varName = content[blockContentStart : blockContentEnd+1]
+			blockContentStart, blockContentEnd = -1, -1
+			blockRawStart = -1
 			if ifStart != -1 {
 				continue
 			}
 			wrappedVarName := "{" + varName + "}"
-			for _, v := range variables {
-				if v == wrappedVarName {
-					validVar = true
-					break
-				}
-			}
+			validVar := slices.Contains(variables, wrappedVarName)
 			if !validVar {
 				out += wrappedVarName
 				continue
@@ -107,11 +132,17 @@ func templateEmail(content string, variables []string, conditionals []string, va
 			out += fmt.Sprint(values[varName])
 		}
 	}
+	if blockContentStart != -1 && blockContentEnd == -1 {
+		err = fmt.Errorf(`incomplete block (single "{") near position %d`, blockContentStart)
+		// Include the brace, maybe the user wants it.
+		previousEnd = blockRawStart - 2
+	}
 	if previousEnd+1 != len(content)-1 {
 		out += content[previousEnd+2:]
+
 	}
 	if out == "" {
-		return content
+		return content, err
 	}
-	return out
+	return out, err
 }