1 files changed, 50 insertions, 0 deletions
diff --git a/crates/core/thin_edge_json/src/utils.rs b/crates/core/thin_edge_json/src/utils.rs
new file mode 100644
index 00000000..c4fff151
--- /dev/null
+++ b/crates/core/thin_edge_json/src/utils.rs
@@ -0,0 +1,50 @@
+/// Returns a substring of `s` starting at `line` and `column`. At most `max_chars` are returned.
+pub(crate) fn excerpt(s: &str, line: usize, column: usize, max_chars: usize) -> String {
+    let mut current_line = 1;
+    let mut chars = s.chars();
+
+    while current_line < line {
+        match chars.next() {
+            Some(ch) => {
+                if ch == '\n' {
+                    current_line += 1;
+                }
+            }
+            None => {
+                break;
+            }
+        }
+    }
+
+    // Seek forward. We cannot use `skip`, as we then can no longer call `as_str`.
+    let mut current_column = 1;
+    while current_column < column {
+        match chars.next() {
+            Some(_) => {
+                current_column += 1;
+            }
+            None => break,
+        }
+    }
+
+    // Don't use byte slicing for UTF-8 strings, e.g. `[..80]`, as this might panic in case of a
+    // wide-character at this position.
+    let mut excerpt = String::with_capacity(max_chars);
+    for _i in 1..=max_chars {
+        match chars.next() {
+            Some(ch) => excerpt.push(ch),
+            None => break,
+        }
+    }
+
+    excerpt
+}
+
+#[test]
+fn excerpt_returns_string_starting_from_line_and_column() {
+    assert_eq!(
+        "ne 2\nline 3\n",
+        excerpt("line 1\nline 2\nline 3\n", 2, 3, 80)
+    );
+    assert_eq!("n", excerpt("line 1\nline 2\nline 3\n", 2, 3, 1));
+}