Improve structured errors

src/errors.rs

@@ -1,5 +1,7 @@
 //! Errors that can occur during encoding / decoding of Polylines
 
+use geo_types::Coord;
+
 #[derive(Debug, PartialEq)]
 #[non_exhaustive]
 pub enum PolylineError {
@@ -23,26 +25,38 @@ pub enum PolylineError {
         /// The string index of the character that caused the decoding error
         idx: usize,
     },
-    DecodeCharError,
+    EncodeToCharError,
+    CoordEncodingError {
+        coord: Coord<f64>,
+        /// The array index of the coordinate error
+        idx: usize,
+    },
 }
 
 impl std::error::Error for PolylineError {}
 impl std::fmt::Display for PolylineError {
     fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
         match self {
             PolylineError::LongitudeCoordError { coord, idx } => {
-                write!(f, "invalid longitude: {} at position {}", coord, idx)
+                write!(f, "longitude out of bounds: {} at position {}", coord, idx)
             }
             PolylineError::LatitudeCoordError { coord, idx } => {
-                write!(f, "invalid latitude: {} at position {}", coord, idx)
+                write!(f, "latitude out of bounds: {} at position {}", coord, idx)
             }
             PolylineError::DecodeError { idx } => {
                 write!(f, "cannot decode character at index {}", idx)
             }
             PolylineError::NoLongError { idx } => {
                 write!(f, "no longitude to go with latitude at index: {}", idx)
             }
-            PolylineError::DecodeCharError => write!(f, "couldn't decode character"),
+            PolylineError::EncodeToCharError => write!(f, "couldn't encode character"),
+            PolylineError::CoordEncodingError { coord, idx } => {
+                write!(
+                    f,
+                    "the coordinate {:?} at index: {} could not be encoded",
+                    coord, idx
+                )
+            }
         }
     }
 }

src/lib.rs

@@ -47,11 +47,11 @@ fn encode(delta: i64, output: &mut String) -> Result<(), PolylineError> {
     }
     while value >= 0x20 {
         let from_char = char::from_u32(((0x20 | (value & 0x1f)) + 63) as u32)
-            .ok_or(PolylineError::DecodeCharError)?;
+            .ok_or(PolylineError::EncodeToCharError)?;
         output.push(from_char);
         value >>= 5;
     }
-    let from_char = char::from_u32((value + 63) as u32).ok_or(PolylineError::DecodeCharError)?;
+    let from_char = char::from_u32((value + 63) as u32).ok_or(PolylineError::EncodeToCharError)?;
     output.push(from_char);
     Ok(())
 }
@@ -95,8 +95,18 @@ where
             x: scale(next.x, factor),
             y: scale(next.y, factor),
         };
-        encode(scaled_next.y - previous.y, &mut output)?;
-        encode(scaled_next.x - previous.x, &mut output)?;
+        encode(scaled_next.y - previous.y, &mut output).map_err(|_| {
+            PolylineError::CoordEncodingError {
+                coord: next,
+                idx: i,
+            }
+        })?;
+        encode(scaled_next.x - previous.x, &mut output).map_err(|_| {
+            PolylineError::CoordEncodingError {
+                coord: next,
+                idx: i,
+            }
+        })?;
         previous = scaled_next;
     }
     Ok(output)