added tests for substring filter and fixed a bunch of bugs in substring filter

scraping.go

@@ -69,23 +69,45 @@ func getFilterResultSubstring(s string, filter *Filter) string {
 	asRunes := []rune(s)
 
 	for _, substring := range substrings {
-		if strings.Contains(substring, "-") {
-			from_to := strings.Split(substring, "-")
+		if strings.Contains(substring, ":") {
+			from_to := strings.Split(substring, ":")
 			if len(from_to) != 2 {
 				return s
 			}
-			from, err := strconv.ParseInt(from_to[0], 10, 32)
-			if err != nil {
-				return s
+			fromStr := from_to[0]
+			var hasFrom bool = true
+			if fromStr == "" {
+				hasFrom = false
 			}
-			to, err := strconv.ParseInt(from_to[1], 10, 32)
-			if err != nil {
+			from64, err := strconv.ParseInt(fromStr, 10, 32)
+			var from = int(from64)
+			if hasFrom && err != nil {
 				return s
+			} else if from < 0 {
+				from = len(asRunes) + from
+			}
+			toStr := from_to[1]
+			var hasTo bool = true
+			if toStr == "" {
+				hasTo = false
+			}
+			to64, err := strconv.ParseInt(toStr, 10, 32)
+			var to = int(to64)
+			if hasTo && err != nil {
+				return s
+			} else if to < 0 {
+				to = len(asRunes) + to
+			}
+			if hasFrom && hasTo {
+				sb.WriteString(string(asRunes[from:to]))
+			} else if hasFrom {
+				sb.WriteString(string(asRunes[from:]))
+			} else if hasTo {
+				sb.WriteString(string(asRunes[:to]))
 			}
-			sb.WriteString(string(asRunes[from:to]))
 		} else {
 			pos, err := strconv.ParseInt(substring, 10, 32)
-			if err != nil {
+			if err != nil || pos < 0 {
 				return s
 			}
 			sb.WriteRune(asRunes[pos])

scraping_test.go

@@ -0,0 +1,53 @@
+package main
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestFilterSubstring(t *testing.T) {
+	var tests = []struct {
+		Input string
+		Query string
+		Want  string
+	}{
+		{"0123456789", "0", "0"},
+		{"0123456789", "9", "9"},
+		{"0123456789", "0,9", "09"},
+		{"0123456789", "0:3", "012"},
+		{"0123456789", ":3", "012"},
+		{"0123456789", "3:", "3456789"},
+		{"0123456789", "-3:", "789"},
+		{"0123456789", ":-3", "0123456"},
+		{"0123456789", ":-1", "012345678"},
+
+		{"0123456789", "0,3,7,9", "0379"},
+		{"0123456789", "0:3,7,9", "01279"},
+
+		{"世界", "1", "界"},
+		{"世界日本語", ":3", "世界日"},
+		{"世界日本語", ":-1", "世界日本"},
+		{"世界日本語", "-1:", "語"},
+
+		{"0123456789", "A", "0123456789"},
+		{"0123456789", "1:A", "0123456789"},
+		{"0123456789", "A:1", "0123456789"},
+		{"0123456789", "A:B", "0123456789"},
+		{"0123456789", "A:B:C", "0123456789"},
+	}
+
+	for _, test := range tests {
+		testname := fmt.Sprintf("%s %s", test.Input, test.Query)
+		t.Run(testname, func(t *testing.T) {
+			result := getFilterResultSubstring(
+				test.Input,
+				&Filter{
+					From: test.Query,
+				},
+			)
+			if result != test.Want {
+				t.Errorf("Got %s, want %s", result, test.Want)
+			}
+		})
+	}
+}