From 82e0d85da08731d465056ddc4f2b3ed5c46b76f4 Mon Sep 17 00:00:00 2001
From: Finn Bear <finnbearlabs@gmail.com>
Date: Tue, 12 Sep 2023 03:34:17 -0700
Subject: [PATCH] Bugfix - avoid panics when displaying error snippets. (#2674)

---
 lib/src/sql/error/mod.rs    | 29 ++++++++++++++++-------------
 lib/src/sql/error/render.rs |  8 ++++----
 2 files changed, 20 insertions(+), 17 deletions(-)

diff --git a/lib/src/sql/error/mod.rs b/lib/src/sql/error/mod.rs
index 67a25c21..ca7773a8 100644
--- a/lib/src/sql/error/mod.rs
+++ b/lib/src/sql/error/mod.rs
@@ -137,32 +137,35 @@ impl<I: Clone> ParseError<I> {
 #[derive(Clone, Copy, Debug)]
 pub struct Location {
 	pub line: usize,
+	/// In chars.
 	pub column: usize,
 }
 
 impl Location {
-	/// Returns the location of a substring in the larger string.
-	pub fn of_in(substr: &str, s: &str) -> Self {
-		let offset = s
-			.len()
-			.checked_sub(substr.len())
+	/// Returns the location of the start of substring in the larger input string.
+	///
+	/// Assumption: substr must be a subslice of input.
+	pub fn of_in(substr: &str, input: &str) -> Self {
+		// Bytes of input before substr.
+		let offset = (substr.as_ptr() as usize)
+			.checked_sub(input.as_ptr() as usize)
 			.expect("tried to find location of substring in unrelated string");
-		let lines = s.split('\n').enumerate();
-		let mut total = 0;
-		for (idx, line) in lines {
+		// Bytes of input prior to line being iteratated.
+		let mut bytes_prior = 0;
+		for (line_idx, line) in input.split('\n').enumerate() {
 			// +1 for the '\n'
-			let new_total = total + line.len() + 1;
-			if new_total > offset {
+			let bytes_so_far = bytes_prior + line.len() + 1;
+			if bytes_so_far > offset {
 				// found line.
-				let line_offset = offset - total;
+				let line_offset = offset - bytes_prior;
 				let column = line[..line_offset].chars().count();
 				// +1 because line and column are 1 index.
 				return Self {
-					line: idx + 1,
+					line: line_idx + 1,
 					column: column + 1,
 				};
 			}
-			total = new_total;
+			bytes_prior = bytes_so_far;
 		}
 		unreachable!()
 	}
diff --git a/lib/src/sql/error/render.rs b/lib/src/sql/error/render.rs
index 253af87b..ec6fcf3f 100644
--- a/lib/src/sql/error/render.rs
+++ b/lib/src/sql/error/render.rs
@@ -40,7 +40,7 @@ pub struct Snippet {
 	truncation: Truncation,
 	/// The location of the snippet in the orignal source code.
 	location: Location,
-	/// The offset into the snippet where the location is.
+	/// The offset, in chars, into the snippet where the location is.
 	offset: usize,
 	/// A possible explanation for this snippet.
 	explain: Option<String>,
@@ -58,7 +58,7 @@ impl Snippet {
 		explain: Option<&'static str>,
 	) -> Self {
 		let line = source.split('\n').nth(location.line - 1).unwrap();
-		let (line, truncation, offset) = Self::truncate_line(line, location.column);
+		let (line, truncation, offset) = Self::truncate_line(line, location.column - 1);
 
 		Snippet {
 			source: line.to_owned(),
@@ -71,9 +71,9 @@ impl Snippet {
 
 	/// Trims whitespace of an line and additionally truncates a string if it is too long.
 	fn truncate_line(mut line: &str, around_offset: usize) -> (&str, Truncation, usize) {
-		let full_line_length = line.len();
+		let full_line_length = line.chars().count();
 		line = line.trim_start();
-		let mut offset = around_offset - (full_line_length - line.len());
+		let mut offset = around_offset - (full_line_length - line.chars().count());
 		line = line.trim_end();
 		let mut truncation = Truncation::None;