tests.nixpkgs-check-by-name: LineIndex functionality, semantics, tests

- `fromlinecolumn` is added to be almost the reverse of `line`.
  This is needed in the future to get from `builtins.unsafeGetAttrPos`
  locations to the index for rnix
- Previously the semantics for newline indices were not really defined,
  now they are, and make more sense.
- Now there's a unit test for these functions

pkgs/test/nixpkgs-check-by-name/src/utils.rs

@@ -35,12 +35,13 @@ impl LineIndex {
         // the vec
         for split in s.split_inclusive('\n') {
             index += split.len();
-            newlines.push(index);
+            newlines.push(index - 1);
         }
         LineIndex { newlines }
     }
 
-    /// Returns the line number for a string index
+    /// Returns the line number for a string index.
+    /// If the index points to a newline, returns the line number before the newline
     pub fn line(&self, index: usize) -> usize {
         match self.newlines.binary_search(&index) {
             // +1 because lines are 1-indexed
@@ -48,4 +49,47 @@ impl LineIndex {
             Err(x) => x + 1,
         }
     }
+
+    /// Returns the string index for a line and column.
+    pub fn fromlinecolumn(&self, line: usize, column: usize) -> usize {
+        // If it's the 1th line, the column is the index
+        if line == 1 {
+            // But columns are 1-indexed
+            column - 1
+        } else {
+            // For the nth line, we add the index of the (n-1)st newline to the column,
+            // and remove one more from the index since arrays are 0-indexed.
+            // Then add the 1-indexed column to get not the newline index itself,
+            // but rather the index of the position on the next line
+            self.newlines[line - 2] + column
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn line_index() {
+        let line_index = LineIndex::new("a\nbc\n\ndef\n");
+
+        let pairs = [
+            (0, 1, 1),
+            (1, 1, 2),
+            (2, 2, 1),
+            (3, 2, 2),
+            (4, 2, 3),
+            (5, 3, 1),
+            (6, 4, 1),
+            (7, 4, 2),
+            (8, 4, 3),
+            (9, 4, 4),
+        ];
+
+        for (index, line, column) in pairs {
+            assert_eq!(line_index.line(index), line);
+            assert_eq!(line_index.fromlinecolumn(line, column), index);
+        }
+    }
 }