added subset to readme

README.md

@@ -440,7 +440,25 @@ For the input string 'Hello World!':
 - `6,0,7`: WHo
 - `-6:`: World!
 - `-6:,:5`: World!Hello
+## Subset
 
+Subset allows for a [Python like](https://learnpython.com/blog/substring-of-string-python/) subset selection of its inputs.  
+For a filter with parents with these results:
+- First parent
+  - `zero`
+  - `one`
+- Second parent
+  - `two`
+- Third parent
+  - `three`
+  - `four`
+Then:  
+- `0`: `zero`
+- `-1`: `four`
+- `0,3`: `zero`, `three`
+- `2:4`: `two`, `three`
+- `-2:`: `three`, `four`
+- `:-2`: `zero`, `one`, `two`, `three`
 ## Contains
 
 Inputs pass if they contain the given regex.

web/scraping.go

@@ -738,10 +738,13 @@ func getFilterResultSubstring(filter *Filter) {
 					}
 				} else {
 					pos, err := strconv.ParseInt(substring, 10, 32)
-					if err != nil || pos < 0 {
+					if err != nil {
 						filter.Log("Could not parse: '", substring, "'")
 						return
 					}
+					if pos < 0 {
+						pos = int64(len(asRunes)) + pos
+					}
 					if pos < 0 || pos >= int64(len(asRunes)) {
 						filter.Log("Out of bounds:", pos)
 						continue
@@ -826,10 +829,13 @@ func getFilterResultSubset(filter *Filter) {
 			}
 		} else {
 			pos, err := strconv.ParseInt(substring, 10, 32)
-			if err != nil || pos < 0 {
+			if err != nil {
 				filter.Log("Could not parse: '", substring, "'")
 				return
 			}
+			if pos < 0 {
+				pos = int64(len(results)) + pos
+			}
 			if pos < 0 || pos >= int64(numResults) {
 				filter.Log("Out of bounds:", pos)
 				continue

web/scraping_test.go

@@ -389,6 +389,8 @@ func TestFilterSubstring(t *testing.T) {
 	}{
 		{"0123456789", "0", "0"},
 		{"0123456789", "9", "9"},
+		{"0123456789", "-1", "9"},
+
 		{"0123456789", "0,9", "09"},
 		{"0123456789", "0:3", "012"},
 		{"0123456789", ":3", "012"},
@@ -439,7 +441,6 @@ func TestFilterSubstringOutOfBounds(t *testing.T) {
 		{"01234", "0:8"},
 		{"01234", ":-6"},
 		{"01234", "-6:"},
-		{"01234", "-1"},
 		{"01234", "6"},
 	}
 
@@ -470,10 +471,11 @@ func TestFilterSubset(t *testing.T) {
 	}{
 		{[]string{"zero", "one", "two", "three", "four", "five", "six"}, "0", []string{"zero"}},
 		{[]string{"zero", "one", "two", "three", "four", "five", "six"}, "6", []string{"six"}},
-		{[]string{"zero", "one", "two", "three", "four", "five", "six"}, "-1", []string{}},
+		{[]string{"zero", "one", "two", "three", "four", "five", "six"}, "-1", []string{"six"}},
 		{[]string{"zero", "one", "two", "three", "four", "five", "six"}, "7", []string{}},
 
 		{[]string{"zero", "one", "two", "three", "four", "five", "six"}, "0,3,6", []string{"zero", "three", "six"}},
+		{[]string{"zero", "one", "two", "three", "four", "five", "six"}, "0,3,-2", []string{"zero", "three", "five"}},
 		{[]string{"zero", "one", "two", "three", "four", "five", "six"}, "0,2:5,6", []string{"zero", "two", "three", "four", "six"}},
 
 		{[]string{"zero", "one", "two", "three", "four", "five", "six"}, "3:6", []string{"three", "four", "five"}},
@@ -481,6 +483,8 @@ func TestFilterSubset(t *testing.T) {
 		{[]string{"zero", "one", "two", "three", "four", "five", "six"}, "0:7", []string{"zero", "one", "two", "three", "four", "five", "six"}},
 		{[]string{"zero", "one", "two", "three", "four", "five", "six"}, ":7", []string{"zero", "one", "two", "three", "four", "five", "six"}},
 		{[]string{"zero", "one", "two", "three", "four", "five", "six"}, "0:", []string{"zero", "one", "two", "three", "four", "five", "six"}},
+		{[]string{"zero", "one", "two", "three", "four", "five", "six"}, "-2:", []string{"five", "six"}},
+		{[]string{"zero", "one", "two", "three", "four", "five", "six"}, ":-2", []string{"zero", "one", "two", "three", "four"}},
 	}
 
 	for _, test := range tests {