PoC

src/line.rs

@@ -9,24 +9,35 @@ pub struct Line(pub u64);
 
 /// Constructors.
 impl Line {
-    pub fn from_index(index: u64) -> Self {
+    pub const fn from_index(index: u64) -> Self {
         Self(index)
     }
 
-    pub fn from_linenr(linenr: NonZeroU64) -> Self {
+    pub const fn from_linenr(linenr: NonZeroU64) -> Self {
         Self(linenr.get() - 1)
     }
+
+    pub const fn next(self) -> Self {
+        Self::from_index(self.index() + 1)
+    }
+
+    /// Panics if self is line index 0.
+    pub const fn prev(self) -> Self {
+        Self::from_index(self.index() - 1)
+    }
 }
 
 /// Getters.
 impl Line {
     /// 0-indexed
-    pub fn index(self) -> u64 {
+    pub const fn index(self) -> u64 {
         self.0
     }
 
     /// 1-indexed
-    pub fn linenr(self) -> u64 {
+    pub const fn linenr(self) -> u64 {
         self.0 + 1
     }
 }
+
+pub struct Lines(Vec<Line>);