implemented TextSelection.absolute_offset() and textselection_by_offs…

…et()

src/api/textselection.rs

@@ -197,8 +197,10 @@ impl<'store> ResultTextSelection<'store> {
         }
     }
 
-    /// Returns the offset of this text selection in another. Returns None if they are not embedded.
+    /// Returns the offset of this text selection in another, i.e. returning a relative offset. Returns None if they are not embedded.
     /// This also checks whether the textselections pertain to the same resource. Returns None otherwise.
+    ///
+    /// If you want to turn a relative offset into an absolute one, use [`Self.absolute_offset()`].
     pub fn relative_offset(
         &self,
         container: &ResultTextSelection<'store>,
@@ -214,6 +216,25 @@ impl<'store> ResultTextSelection<'store> {
         }
     }
 
+    /// Converts a relative offset, expressed in the coordinates of this text selection, to an absolute one
+    /// expressed in the coordinates of the resource.
+    pub fn absolute_offset(&self, offset: &Offset) -> Result<Offset, StamError> {
+        match self {
+            Self::Bound(item) => item.as_ref().absolute_offset(offset),
+            Self::Unbound(_, _, item) => item.absolute_offset(offset),
+        }
+    }
+
+    /// Return a textselection by *relative* offset.
+    /// The offset is relative to the current textselection.
+    pub fn textselection_by_offset(
+        &self,
+        offset: &Offset,
+    ) -> Result<ResultTextSelection<'store>, StamError> {
+        self.resource()
+            .textselection_by_offset(&self.absolute_offset(offset)?)
+    }
+
     pub(crate) fn store(&self) -> &'store TextResource {
         match self {
             Self::Bound(item) => item.store(),

src/textselection.rs

@@ -322,6 +322,45 @@ impl TextSelection {
         }
     }
 
+    /// Converts a relative offset, expressed in the coordinates of this text selection, to an absolute one
+    /// expressed in the coordinates of the resource.
+    pub fn absolute_offset(&self, offset: &Offset) -> Result<Offset, StamError> {
+        let textlen = self.end() - self.begin();
+        let begin = Cursor::BeginAligned(
+            self.begin()
+                + match offset.begin {
+                    Cursor::BeginAligned(x) => x,
+                    Cursor::EndAligned(x) => {
+                        if x.abs() as usize > textlen {
+                            return Err(StamError::CursorOutOfBounds(
+                                offset.begin,
+                                "(textselection_by_offset)",
+                            ));
+                        } else {
+                            textlen - (x as usize)
+                        }
+                    }
+                },
+        );
+        let end = Cursor::BeginAligned(
+            self.begin()
+                + match offset.end {
+                    Cursor::BeginAligned(x) => x,
+                    Cursor::EndAligned(x) => {
+                        if x.abs() as usize > textlen {
+                            return Err(StamError::CursorOutOfBounds(
+                                offset.end,
+                                "(textselection_by_offset)",
+                            ));
+                        } else {
+                            textlen - (x as usize)
+                        }
+                    }
+                },
+        );
+        Ok(Offset::new(begin, end))
+    }
+
     /// Resolves a relative cursor to a relative begin aligned cursor, resolving all end-aligned positions
     fn beginaligned_cursor(&self, cursor: &Cursor) -> Result<usize, StamError> {
         let textlen = self.end() - self.begin();